Week 14
Community Ecology
BIO182 - 29460 • Spring 2026 Dr. Rivera, CGCC
Learning Objectives
At the end of this week students should be able to,
- explain the difference between population density and dispersion and describe different types of dispersion.
- name and explain the factors that affect population size.
- explain the differences between S-shaped and J-shaped curves.
- explain and provide examples of density-dependent and density-independent factors that affect population size.
- explain the different survivorship curves and provide examples of organisms that exhibit each curve.
- explain how the human population has changed over time.
- interpret age structure diagrams and provide examples of countries in each scenario.
- estimate population size using a variety of techniques.
- different types of ecological niches
- explain the types of interactions that play a role in how an organism relates to the niche.
- explain different types of mimicry.
- summarize the main determinants of species richness in a community.
Levels of Biological Organization
- Ecology - the study of orgasnims' interactions with the living (biotic) and non-living (abiotic) components of their environment.
- Ecology is not conservation!
Ecology
- Organisms/Individual – how are organisms adapted to their environment to increase their fitness
- Population – how do the numbers of individuals/conspecifics change over time and why?
- Community – how do different species interact with one another and what are the consequences?
- Ecosystem – how do nutrients and energy between the living and non-living components of an ecosystem?
Tradeoffs
- Tradeoffs – a situation where one thing increasing means the other is decreasing
- Why do organisms face tradeoffs?
- Living organisms are faced with limited resources, time, and energy.
- As such they face tradeoffs, investing resources into one aspect will come at the cost of investment into another aspect of their life.
- Understanding how organisms “choose” in tradeoffs can help us understand organisms’ distributions, interactions, life cycles, and more!
- Sometimes evolution tips the scales!
Life History
- Organisms face a number of decisions in how to allocate resources across their life.
- How they allocate resources shapes their life history.
- Life History - Patterns and timing of events relating to an organism’s growth, development, reproduction, and survival; including
Life History Traits
- Life span
- Age and size at maturity
- Frequency of reproducing
- Number of offspring produced
- Parental care
- Mortality rates
Populations are Shaped by Life Histories
- Populations are Shaped by Life Histories
- Features of populations
- Population density
- Population dispersion
- Birth and death rates
- Growth rates
- Survivorship
- Age structure
Population Density
- The number of individuals of as pieces per unit of area or volume at a given time
- Researchers measure it by sampling a subset and estimating the whole populations density
Population Dispersion
- Individuals in a population may exhibit characteristic patterns of spacing relative to one another
- Provides more detail on interactions than density measurements alone
3 Types
- Random - individuals are spaced in a manner that is unrelated to the presence of others (plants dispersed by the wind)
- Clumped - Individuals are concentrated in specific parts of the habitat (most common), Results from patchy distribution of resources in the environment, Advantageous when social animals benefit from their association (e.g., schooling fish)
- Uniform - Individuals are more evenly spaced, Competition among individuals is severe; Plant roots or leaves that have been shed produce toxic substances that inhibit growth of nearby plants, Animals establish feeding or mating territories
Population Growth Rate
- Growth rate of a population = change in # of individuals over time
- The growth rate(r) of a local population must take into account:
- birth rate (b)
- death rate (d)
- immigration rate (i)
- emigration rate (e)
- r = (b − d) + (i − e)
Intrinsic Rate of Increase
- The maximum rate at which a population of a given species could increase under ideal conditions
- Influenced by,
- age at which reproduction begins
- fraction of lifespan during which an individual reproduces
- number of reproductive periods
- number of offspring during each period of reproduction
- Graphically represented by a tangent line of the growth rate
Exponential Growth
- J shaped curve - Growth rate under ideal conditions
- Accelerating growth rate occurs when optimal conditions allow constant per capita growth rate – the larger the population gets, the faster it grows
- Populations will increase exponentially as long as their per capita growth rates remain constant
- This is unsustainable
Carrying Capacity
- The largest population that can be maintained for an indefinite period by a particular environment, assuming no changes in the environment
- Takes into account annual reproduction and population losses
- In nature, carrying capacity is dynamic and changes in response to environmental changes
Logistic Growth
- Exponential growth limited by the carrying capacity
- The closer the population gets to carrying capacity, the slower the population growth
- If the population is over the carrying capacity, population growth rate is negative!
- A population regulated by environmental limits over long periods of time
- Initial exponential increase is followed by a leveling out as carrying capacity of the environment is approached
Population Crashes
- A population rarely stabilizes at carrying capacity, but may temporarily rise higher, then drop back to, or below it
- Sometimes a population that overshoots capacity and will crash.
- Observed in bacterial cultures, zooplankton, and other populations whose resources have been exhausted
What Limits Population Growth?
- Factors that affect population size fall into two categories and can be biotic or abiotic:
- density-dependent factors
- density-independent factors
Density Dependent Factors
- Density dependent factors change in intensity as the population size changes
- Density-dependent factors act as negative feedback systems
- Examples, predation, disease, competition
- As the density of a population increases:
- Predators are more likely to find prey
- The chance of transmitting parasites and infectious disease organisms increases
- Competition for resources such as living space, food, cover, water, minerals, and sunlight increases
- As population density increases, density-dependent factors tend to slow population growth by causing an increase in death rate and/or a decrease in birth rate
Density Dependent Factors
- Predator prey dynamics are a classic example of density dependent factors
- As prey population increases competition for resources slow growth
- Predator population follows the increase in their resoruce (prey) then falls as prey decreases
Density Independent Factors
- Any environmental factor that affects the size of a population but is not influenced by changes in population density
- Typically abiotic, such as random weather events
Survivorship Curves
- Survivorship: the probability that a given individual in a population or cohort will survive to a particular age
- Determined by plotting the logarithm of the number of surviving individuals against age, from birth to the maximum age reached by any individual
- Ecologists recognize three main types of survivorship curves
- Type I - mortality is greatest later in life
- Type II - death is equally likely across age
- Type III - mortaliity is greatest earlier in life
Human Population
- Recent increase in human population is due to a decrease in death rate (d), not an increase in birth rate (b)
- Greater food production, better medical care, and improved sanitation practices have increased life expectancies
- Carrying capacity of Earth is debated- We may level out at 10 Billion
- Increases have slowed due to lower fertility rates
Age Structures
- Age structure is the proportion of a population at different age ranges.
- Age structure allows better prediction of population growth, plus the ability to associate this growth with the level of economic development in the region.
Populations --> Communities
- A community is an association of populations of different species that live and interact in the same place at the same time
- A biological community and its abiotic environment together compose an ecosystem
- Community ecology focuses on understanding how different species interact with one another (biotic interactions)
- Organisms live in communities with other species, which leads to interactions that affect individuals, populations, and communities
Species Interactions
- Species interaction - direct or indirect relationships among individuals of different species
- Often leads to evolutionary change (a species exerts selection on another)
- Often interactions are trophic (related to feeding) but can be non-trophic as well.
- Can involve behavior but also other types of interactions, chemical interactions, and physical interactions.
Species Interactions
- Let’s focus on pairwise interactions, between 2 individuals of different species.
- We can classify interactions based on the effect they have on each species.
- What is the effect on fitness? Can be positive (+), negative (–), or neutral (0) for either species.
Predation
- Predation—individuals of one species (predators) kill and/or consume individuals (or parts) of another species (its prey).
- Carnivory—predator and prey usually animals.
- Herbivory—predator is an animal, prey are plants or algae.
- Parasitism—predator (a parasite) lives on or in the prey (its host) and consumes some tissues; may not kill the host. Some parasites are pathogens that cause disease.
Carnivores
- Predator and prey are usually* animals
- Carnivores typically kill their prey
- Prey defenses are usually behavioral or morphological
- Most carnivores are generalists, consuming a variety of prey species
Carnivores - Finding & Capturing Prey
- Active hunters – many carnivores forage by moving in search of prey (wolves, sharks, hawks)
- Ambush predators – remain in a single place and attack prey that come within striking distance (moray eels, some snakes)
- Trap-setters – some set passive traps to ensnare prey (spider webs, carnivorous plants)
Carnivore - Prey Defense
- Physical defenses: Large size, body plan designed for rapid or agile movement; body armor.
- Warning coloration (aposematic): Predators learn not to eat organisms that have toxins.
- Mimicry: Prey resembles another organism that is toxic or very fierce.
- Crypsis: The prey is camouflaged or resembles its background.
Herbivores
- Prey are plants or algae, predators are usually animals
- Herbivores typically don’t kill their prey
- Prey defenses are usually chemical or morphological (plants can’t run away)
- Most herbivores are specialists, consuming a variety of prey species
Herbivores
- Most specialize in feeding on species of plants and specific parts of the plant (leaves, stems, roots, seeds, sap)
- Seeds and leaves are generally the most nutritious (lowest fiber content)
- Herbivores can reduce the growth and reproduction of plants, but only some (large herbivores) kill plants outright
Herbivores - Specificity
- Insect herbivores mostly feed on few species (some are generalist like grasshoppers)
- Large browsers often focus on a single species at a time (temporal specialization)
Herbivores - Plant Defenses
- Structural defenses: tough leaves, spines, thorns, saw-like edges, pernicious (nearly invisible) hairs that can pierce the skin.
- Chemical defenses: Toxic secondary compounds make digesting the plant difficult/dangerous for herbivores
- Induced defenses—produced in response to herbivore attack, chemical/physical
Herbivores - Plant Defenses
Secondary Compound Case Study
- Researchers measured lower levels of external parasites (ectoparasites) in birds whose nests contain more cigarett butts.
Carnivore vs. Herbivore
- Animal prey are energetically more valuable but less abundant and harder to capture. Being a generalist allows them to take advantage of whatever prey they encounter.
- Plant prey is less valuable (more fibrous) but much more abundant and easier to capture. Herbivores can spend the time specializing on a select type of plant.
Parasitism
- Parasitism—predator (a parasite) lives symbiotically on or in the prey (its host) and consumes certain tissues; may not kill the host. Some parasites are pathogens that cause disease.
- More than half of Earth’s species are parasites—organisms that live in or on other organisms and have a negative effect on the host
Types of Parasites
- Macroparasites: Large species such as arthropods and worms.
- Microparasites: Microscopic, such as bacteria.
- Parasitoids: Insects whose larvae feed on a single host and almost always kill it.
Plasmodium falciparum - Malaria
Ixodes sp. - Deer tick
Parasitoid
Resources
- Organisms require resources to survive, both abiotic and biotic, but these vary in quality and quantity
- Abiotic
- Wagter
- Light
- Space (nesting, basking sites)
- Nutrients/minerals
- Biotic
- Food
- Mates
- Biotic habitats (parasite hosts)
- Mutualistic partners*
- Limiting resource - A resource that determines the carrying capacity of a species
Competition
- Competition: Non-trophic interaction between individuals of two or more species in which all species are negatively affected by their shared use of a resource.
- Can occur between or across species (even distantly related species), as long as
- between closely related or distantly related resources, as long as they use the same resource!
- A competitor causes you to 1) have fewer resources, or 2) expend more energy to maintain access to the resource
- Competitive ability can depend on environmental conditions
Mechanism of Competition
- If 2 species are using the same resources, there are two potential mechanisms by which they can compete.
Exploitation (indirect competition)
Interference (direct competition)
Obtain the resources quicker
Take & mantain control of the resource
Outcomes of Competition
- Competitive exclusion – dominant species prevents another species from using an essential resource driving it to local extinction
- Competitive coexistence – 2 species are able to coexist despite sharing a limited resource, most commonly observed
Exclusion
Coexistence
Competitive Exclusion
- Requires,
- One species is a superior competitor to the other
- Resource under competition is a limiting resource
- The resource is used the same way by both species
- Natrual selection can favor traits to bypass competition
Competitive Coexistence
- The main mechanism allowing for competitive coexistence is resource partitioning – 2 or more species use a resource in different ways (or to different degrees)
- The same resource, with different uses, reduces the amount of competition!
- Partitioning can occur with any resource
- Food – foraging in different locations or times, foraging on a subset of potential food items
- Space – preferentially choosing parts of the habitat to avoid competition
Mutualisms
- Mutualism: Mutually beneficial interaction between individuals of two species (+/+ relationship).
- Benefits to a species can include a service and/or resource.
- Resource – a resource the other species needs to survive (food, shelter)
- Service – a benefit that the other species needs which would be difficult to do without partner species (seed dispersal, pollination, cleaning/parasite removal, predator defense)
Mutualisms Come at a Cost
- There is often a cost to one or both partners (energetic, resource) but the net benefit on fitness is positive; the interaction increases the fitness of each species.
- In positive interactions it’s important to separate the interests of each species, why are they doing the interaction?
- The benefits each partner receives must be greater than the cost provided to the other partner.
- Mechanisms to punish cheaters, withdraw reward or from the interaction.
- In high-temperature environments, algae in coral produce toxic compounds, and the coral expel them from their tissue.
Putting it together
- The Sonoran Desert contains a species of flowering cacti that produces nectar to attract pollinators. A species of bee feeds on the nectar and, in the process, transfers pollen between cacti.
- Recently, a new fly species has been introduced that also feeds on the nectar, but it does not transfer pollen. At the same time, a parasitic mite begins infecting the bees, reducing their population size.
Last slide of this lecture
Ecological Niche
- Niche: an animal’s ecological role within the structure and function of a community
- An ecological niche takes into account all biotic and abiotic factors (physical, chemical, and biological) that a species needs to survive and reproduce
Here are 2 factors (temperature and salinity) which can determine an oragnsims's niche. This fish can survive any environment with the conditions in blue area.
Ecological Niche
- Fundamental niche:The potential ecological niche of a species
- Various factors, such as competition, may exclude it from part of its fundamental niche
- Realized niche:The lifestyle that a species actually pursues and the resources it actually uses
Community Biodiversity
- Species diversity: a measure of both the number of species within a community and the relative importance of each species
- Species richness: the number of species in a community, is determined by counting the species of interest
- Both areas have the same number of species, but Area 1 is more diverse (Area 2 is dominated by a single species)
Species Richness
- Species richness is influenced by:
- Structural complexity of habitats
- Geographic isolation (distance effect)
- Habitat stress (pollution, extreme conditions)
- Latitude (species richness–energy hypothesis)
- Closeness to the margins of adjacent communities (edge effect)
- Dominance of one species over others
- Geologic disturbances
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
When we talk about life history more specifically, we refer to life history traits. These include characteristics like lifespan, the age and size at which an organism reaches maturity, how frequently it reproduces, how many offspring it produces, how much parental care it provides, and its mortality rates.Different species show very different combinations of these traits. For example, some organisms produce many offspring with little parental care, while others produce few offspring but invest heavily in each one. These differences aren’t random—they reflect evolutionary strategies shaped by environmental pressures and resource availability.
The most nutritious parts are leaves, seeds, and fruit. They are the most at risk from herbivores and benefit more from chemical defense.
At its core, ecology is about interactions. It asks questions like: how do organisms affect each other, and how do they respond to their environment? These interactions can be between individuals of the same species, between different species, or between organisms and abiotic factors like temperature or water availability. What makes ecology particularly interesting is that small-scale processes—like how an individual feeds or reproduces—can scale up to influence entire populations and ecosystems. So as we move through this lecture, keep in mind that everything is connected. Individual traits influence populations, and populations influence ecosystems.”
blank
blank
blank
Finally, we can gain even more insight into population growth by looking at age structure, which is the proportion of individuals in different age groups within a population.Age structure is important because it helps us predict future population trends. For example, a population with a large proportion of young individuals is likely to continue growing, even if birth rates start to decline, because many individuals are entering reproductive age. In contrast, a population with a larger proportion of older individuals may experience slower growth or even decline.
blank
Finally, we look at biodiversity at the community level. Species diversity includes both the number of species present and how evenly distributed they are. Species richness specifically refers to the number of species. Species richness is influenced by several factors, including habitat complexity, geographic isolation, environmental stress, and latitude. For example, more structurally complex habitats tend to support more species because they provide more niches. All of these factors, combined with species interactions, shape the incredible diversity of life we see across different ecosystems. And ultimately, that diversity emerges from the balance of competition, cooperation, and environmental constraints
Organisms don’t live in isolation—they exist in communities where they constantly interact with other species. These interactions can affect individuals directly, but they also scale up to influence population sizes and even the structure of entire communities. One of the most important ideas here is that these interactions often drive evolutionary change. When one species affects the fitness of another, it can act as a selective pressure. Over time, this can lead to adaptations and even coevolution, where two species evolve in response to each other. Many interactions are trophic, meaning they involve feeding relationships, but not all are. Some involve competition, cooperation, or other forms of interaction. To simplify things, we’ll focus on pairwise interactions between two species
Now let’s switch gears to positive interactions—mutualisms. In a mutualistic interaction, both species benefit. These benefits can involve resources, like food or shelter, or services, like pollination, seed dispersal, or protection from predators. Mutualisms are incredibly widespread and often essential for ecosystem function. But it’s important to recognize that they are not purely cooperative in the way we might think. Each species is still acting in its own self-interest, and the interaction persists because it increases the fitness of both partners
In some cases, populations overshoot their carrying capacity, meaning they grow beyond what the environment can support. When this happens, resources can become depleted, leading to a rapid decline or crash in population size.This pattern is often observed in systems like bacterial cultures or certain aquatic organisms, where growth can be rapid but unsustainable
blank
blank
blank
When species compete, there are two primary outcomes. One is competitive exclusion, where one species outcompetes the other and drives it to local extinction. The other is competitive coexistence, where both species persist despite sharing a resource. Competitive exclusion tends to occur under very specific conditions—when one species is consistently superior, the resource is limiting, and both species use it in exactly the same way. Under these conditions, there’s no room for sharing. However, in nature, coexistence is actually much more common. That’s because species often find ways to reduce competition rather than eliminate it entirely.”
blank
Density-dependent factors are those whose effects become stronger as population density increases. As a population becomes more crowded, individuals interact more frequently, and that increases the impact of certain limiting factors. For example, predation often becomes more intense because predators can find prey more easily when they are concentrated. Disease transmission also increases, because individuals are in closer contact with each other, making it easier for pathogens to spread. Competition is another major density-dependent factor. As population size increases, individuals must compete more for limited resources like food, water, space, and sunlight. These factors tend to act as a kind of negative feedback system. As the population grows, these pressures increase, which slows growth by increasing death rates or decreasing birth rates. So density-dependent factors are one of the main mechanisms that keep populations from growing indefinitely.
Herbivory is another form of predation, but here the prey are plants or algae. Unlike animal prey, plants cannot move to escape, so their defenses are typically structural or chemical rather than behavioral. Herbivores often do not kill the plants they consume, but they can still significantly reduce growth and reproduction. Many herbivores are specialists, focusing on specific plant species or even specific parts of a plant, like leaves, seeds, or roots. This specialization reflects the challenges of digesting plant material, which is often fibrous and chemically defended, and it leads to very tight evolutionary relationships between herbivores and their food sources
Another way we study populations is by looking at survivorship, which is the probability that an individual will survive to a particular age. We often visualize this using survivorship curves, which track how many individuals remain alive at different ages. These curves help us understand patterns of mortality across the lifespan of a species. To create these curves, ecologists typically plot the logarithm of the number of surviving individuals against age, which allows us to compare patterns across very different species. What’s important here is not just how long individuals live, but when mortality is most likely to occur during their life There are three general types of survivorship curves. Type I curves are characterized by high survival early and middle in life, followed by a steep decline in older age. Humans are a classic example of this pattern, where most individuals survive to adulthood, but mortality increases later in life. Type II curves show a constant probability of death across all ages. In this case, the likelihood of dying is roughly the same whether the individual is young or old. This pattern is less common but can be seen in some birds and small mammals. Type III curves are the opposite of Type I. In these populations, there is very high mortality early in life, but those individuals that survive the early stages tend to live much longer. Many organisms that produce large numbers of offspring with little parental care, like oysters, follow this pattern.
a parasite lives on or inside another organism, called the host, and consumes its tissues or body fluids. Unlike typical predators, parasites usually don’t kill their host right away. In fact, from an evolutionary perspective, it’s often beneficial for the parasite to keep the host alive as long as possible, because the host is essentially its habitat and food source combined. What’s really striking is just how common parasitism is. It’s estimated that more than half of all species on Earth are parasitic in some form. That means parasitism is not some rare ecological oddity—it’s actually one of the dominant lifestyles on the planet. Some parasites are also pathogens, meaning they cause disease. But not all parasites immediately cause obvious harm. Many exist in a kind of balance with their host, slowly extracting resources over time. This creates a very different dynamic compared to something like carnivory, where the interaction is fast and lethal.
The intrinsic rate of increase represents the maximum potential growth rate of a population under ideal conditions, where resources are unlimited and environmental constraints are minimal.This rate depends on several life history traits, including how early reproduction begins, how long individuals reproduce, and how many offspring are produced during each reproductive event. This concept is useful because it provides a baseline—what a population could do under perfect conditions—even though real populations rarely experience those conditions.
We can break parasites into a few major categories based on their size and life strategy. Macroparasites are relatively large—things like worms or arthropods—that you could often see with the naked eye. Microparasites, on the other hand, are microscopic, including bacteria and many pathogens.Then there are parasitoids, which are particularly interesting. These are often insects whose larvae develop inside a single host and almost always end up killing it. So they sit somewhere between parasitism and predation—they rely on the host like a parasite, but ultimately kill it like a predator. Most parasites are highly specialized. They are adapted to a very specific host species, or even a specific part of that host. This specialization is one of the reasons we see such incredible diversity in parasites—each host species can support many different parasite species, each occupying its own niche
blank
blank
When conditions are ideal, populations can grow exponentially, producing a characteristic J-shaped curve. In exponential growth, the population increases at a constant per capita rate, meaning that as the population gets larger, it grows faster and faster.This happens because more individuals are reproducing, leading to even more offspring. However, exponential growth is not sustainable in real environments, because resources are always limited. So while it’s useful as a model, it’s not something we typically see long-term in nature
blank
blank
blank
Plants have evolved a wide range of defenses against herbivores. Structural defenses include things like thorns, tough leaves, and protective hairs, which make it physically difficult to consume the plant. Chemical defenses are even more interesting. Plants produce secondary compounds that can be toxic, reduce digestibility, or deter feeding. Some of these compounds are always present, while others are induced only when the plant is attacked.
When we scale up from individuals to populations, life history traits start to shape population-level characteristics. Populations are described by features like density, dispersion, birth and death rates, growth rates, survivorship, and age structure.All of these are directly influenced by life history decisions. For example, a species that reproduces early and produces many offspring will tend to have faster population growth. In contrast, a species that reproduces later and invests heavily in fewer offspring may have slower growth but higher survival rates. So understanding populations really starts with understanding the biology of individuals
To understand what limits population growth, we introduce the concept of carrying capacity. This is the maximum population size that an environment can sustain over time, given the available resources. Importantly, carrying capacity is not fixed—it can change with environmental conditions like food availability, climate, or habitat quality. So populations are constantly adjusting in response to shifting limits
All of these interactions—competition, predation, mutualism—feed into the concept of the ecological niche. A niche describes the role a species plays in its community, including all the biotic and abiotic conditions it requires to survive and reproduce. We distinguish between the fundamental niche, which is the full range of conditions a species could theoretically occupy, and the realized niche, which is what it actually occupies in the presence of other species. Interactions like competition often shrink the realized niche compared to the fundamental niche, because species are excluded from certain resources or habitats.
Of course, prey are not passive participants in this interaction. They have evolved a wide range of defenses to avoid being eaten. These include physical defenses like armor or speed, as well as behavioral strategies like vigilance or escape responses. Some species use warning coloration to signal that they are toxic, while others use mimicry to resemble those toxic species. And many rely on camouflage, or crypsis, to blend into their environment and avoid detection altogether. These adaptations illustrate the concept of coevolution—predators and prey are constantly influencing each other’s evolution, leading to increasingly sophisticated strategies on both sides.
Density tells us how many individuals are present, but it doesn’t tell us how they’re arranged in space. That’s where dispersion comes in. Dispersion describes the pattern of spacing among individuals in a population, and it can tell us a lot about how individuals interact with each other and their environment.There are three main patterns. Random dispersion occurs when individuals are spaced independently of each other, which is relatively rare. Clumped dispersion, which is the most common, occurs when individuals aggregate in areas where resources are concentrated or where social behavior provides benefits. Uniform dispersion occurs when individuals are evenly spaced, often due to competition or territorial behavior. So dispersion gives us insight into underlying ecological processes like resource distribution and social interactions
The key to coexistence is resource partitioning. This occurs when species divide a resource in some way—by using it at different times, in different places, or in different forms. For example, two species might feed on the same type of food but at different times of day, or they might occupy slightly different habitats. Even small differences can reduce competition enough to allow both species to persist. Context also matters. Environmental conditions can shift which species has the advantage. A drought-tolerant species might dominate during dry periods but be outcompeted when water is abundant. This dynamic balance helps maintain diversity within communities
blank
In contrast, density-independent factors affect populations regardless of how large or small they are. These are typically abiotic factors, like weather events, natural disasters, or sudden changes in environmental conditions.For example, a drought, flood, wildfire, or extreme temperature event can reduce a population dramatically, whether the population was large or small to begin with. These factors don’t respond to population size—they act independently of it. Because of that, they can cause sudden and sometimes unpredictable changes in population size. So while density-dependent factors tend to regulate populations gradually, density-independent factors can cause abrupt shifts
Carnivores have evolved different strategies for finding and capturing prey. Some are active hunters, like wolves or sharks, that move through their environment searching for prey. Others are ambush predators, like certain snakes or eels, that remain hidden and strike when prey comes close. There are also trap-setters, like spiders, that create structures to capture prey without actively pursuing it. Each of these strategies represents a different solution to the same problem—how to obtain energy efficiently.
When we incorporate carrying capacity into population growth, we get logistic growth, which produces an S-shaped curve. Early on, growth looks exponential because resources are abundant. But as the population approaches carrying capacity, growth slows down due to increased competition and limited resources. Eventually, the population stabilizes around the carrying capacity, although in reality, it often fluctuates rather than staying perfectly stable. This model is much more realistic than exponential growth because it accounts for environmental limits
Population density refers to the number of individuals of a species per unit area or volume at a given time. This is one of the simplest ways to describe a population, but it’s also very informative.In practice, it’s often difficult to count every individual in a population, so researchers typically estimate density by sampling a smaller area and scaling up.
blank
In carnivory, both predator and prey are typically animals, and the predator usually kills the prey outright. Carnivores often face the challenge of locating and capturing prey, which can be rare, mobile, and capable of defending itself. Because of this, many carnivores are generalists—they consume a variety of prey species rather than specializing on just one. This flexibility allows them to adapt to changing conditions and increases their chances of finding food. At the same time, prey species evolve defenses to avoid being eaten, setting up a dynamic interaction where both predator and prey are under strong selective pressure
blank
blank
blank
blank
To understand how populations change over time, we need to consider population growth rate. Growth rate depends on four main factors: births, deaths, immigration, and emigration. Births and immigration add individuals to the population, while deaths and emigration remove them. So the overall growth rate reflects the balance between these processes. If more individuals are being added than removed, the population grows. If more are being removed than added, the population declines. This framework allows us to quantify and predict changes in population size over time
blank
Predation is one of the most familiar types of species interactions. It occurs when one organism, the predator, kills and consumes another organism, the prey. But predation actually includes several subcategories. Carnivory involves predators consuming other animals. Herbivory involves animals consuming plants or algae. And parasitism, which we’ll discuss more later, involves organisms living on or in a host and consuming its tissues. These interactions are incredibly important because they shape both predator and prey populations, and they often lead to evolutionary arms races, where each species evolves adaptations to outcompete the other.”
A community is defined as an association of populations of different species that live and interact in the same place at the same time. So unlike a population, which focuses on a single species, a community includes all the species in a given area and the ways they interact. If we take it one step further, when we include the abiotic environment—things like temperature, water, nutrients—we get an ecosystem. So a community is the biological component, and the ecosystem is the biological plus the physical environment. This distinction is important because many of the patterns we see in nature emerge not just from individual species, but from how species interact with each other and with their environment
Competition occurs when two or more individuals use the same limited resource, and as a result, all individuals involved experience a reduction in fitness. This is a non-trophic interaction, meaning it’s not about one organism eating another—it’s about access to resources.Competition can occur between closely related species or completely unrelated ones, as long as they rely on the same resource. And importantly, competition doesn’t require direct interaction. If another organism is using a resource you need, it’s affecting you—even if you never encounter it directly. From a functional perspective, a competitor either reduces the amount of resources available to you or forces you to expend more energy to access those resources.
So at this point, we’ve seen how populations can grow and what happens when they approach or exceed carrying capacity, but now we need to ask the key question: what actually limits population growth in real ecosystems? All limiting factors fall into two broad categories: density-dependent and density-independent factors, and these can include both biotic, meaning living, and abiotic, meaning nonliving, influences. The distinction between these two types of factors is really important because it tells us whether the effect on a population changes as the population size changes. In other words, does the impact of the factor depend on how crowded the population is, or does it act regardless of population size? That difference helps us understand how populations are regulated over time
blank
To study ecology effectively, we need to think in terms of levels of organization. At the most basic level, we have the individual organism. This is where behavior, physiology, and life history decisions occur. Next is the population, which is a group of individuals of the same species living in a specific area. This is where we start looking at patterns like population growth and density.Above that is the community, which includes all the different species interacting in an area—this is where things like predation, competition, and symbiosis come into play. And finally, we have the ecosystem, which includes both the biological community and the abiotic environment, such as climate, soil, and water.
There are two main ways competition occurs. The first is exploitation competition, which is indirect. Organisms compete simply by consuming shared resources. Whoever is faster or more efficient at finding and using the resource gains an advantage.The second is interference competition, which is direct. Here, organisms actively prevent others from accessing resources—through aggression, territorial behavior, or other forms of direct interaction. These two mechanisms select for very different traits. Exploitation favors efficiency, speed, and detection, while interference favors strength, dominance, and control. So an interesting question to consider is whether a species that is good at one type of competition is also good at the other—or whether there are tradeoffs between these strategies
Even though both species benefit, mutualisms come with costs. Each partner is investing energy or resources into the interaction, so the net benefit must outweigh those costs. This creates the potential for conflict. For example, one partner might ‘cheat’ by taking benefits without providing its share. As a result, many mutualisms have evolved mechanisms to discourage cheating, such as withholding rewards or terminating the interaction. A great example comes from coral and algae. Under stressful conditions, like high temperatures, the algae can produce harmful compounds, and the coral responds by expelling them
All organisms require resources to survive, and these resources can be either abiotic or biotic. Abiotic resources include things like water, light, space, and nutrients, while biotic resources include food, mates, hosts, and even mutualistic partners.One key point is that resources are not evenly distributed. They vary across space and time, both in quantity and quality. This uneven distribution sets the stage for competition, because organisms are often trying to access the same limited, high-quality resources. When a particular resource directly limits the growth or survival of a population, we call it a limiting resource. Identifying limiting resources is critical in ecology because they often determine population size and distribution.
blank
blank
blank
This table is one of the most important tools for organizing species interactions. Each interaction is classified based on its effect on the fitness of the two species involved. For example, in predation, one species benefits while the other is harmed. In competition, both species are negatively affected. In mutualism, both species benefit
Week 14 - Community Ecology
Michael
Created on April 18, 2026
Start designing with a free template
Discover more than 1500 professional designs like these:
View
Customer Service Course
View
Dynamic Visual Course
View
Dynamic Learning Course
View
Akihabara Course
Explore all templates
Transcript
Week 14
Community Ecology
BIO182 - 29460 • Spring 2026 Dr. Rivera, CGCC
Learning Objectives
At the end of this week students should be able to,
Levels of Biological Organization
Ecology
Tradeoffs
Life History
Life History Traits
Populations are Shaped by Life Histories
Population Density
Population Dispersion
3 Types
Population Growth Rate
Intrinsic Rate of Increase
Exponential Growth
Carrying Capacity
Logistic Growth
Population Crashes
What Limits Population Growth?
Density Dependent Factors
Density Dependent Factors
Density Independent Factors
Survivorship Curves
Human Population
Age Structures
Populations --> Communities
Species Interactions
Species Interactions
Predation
Carnivores
Carnivores - Finding & Capturing Prey
Carnivore - Prey Defense
Herbivores
Herbivores
Herbivores - Specificity
Herbivores - Plant Defenses
Herbivores - Plant Defenses
Secondary Compound Case Study
Carnivore vs. Herbivore
Parasitism
Types of Parasites
Plasmodium falciparum - Malaria
Ixodes sp. - Deer tick
Parasitoid
Resources
Competition
Mechanism of Competition
Exploitation (indirect competition)
Interference (direct competition)
Obtain the resources quicker
Take & mantain control of the resource
Outcomes of Competition
Exclusion
Coexistence
Competitive Exclusion
Competitive Coexistence
Mutualisms
Mutualisms Come at a Cost
Putting it together
Last slide of this lecture
Ecological Niche
Here are 2 factors (temperature and salinity) which can determine an oragnsims's niche. This fish can survive any environment with the conditions in blue area.
Ecological Niche
Community Biodiversity
Species Richness
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
blank
When we talk about life history more specifically, we refer to life history traits. These include characteristics like lifespan, the age and size at which an organism reaches maturity, how frequently it reproduces, how many offspring it produces, how much parental care it provides, and its mortality rates.Different species show very different combinations of these traits. For example, some organisms produce many offspring with little parental care, while others produce few offspring but invest heavily in each one. These differences aren’t random—they reflect evolutionary strategies shaped by environmental pressures and resource availability.
The most nutritious parts are leaves, seeds, and fruit. They are the most at risk from herbivores and benefit more from chemical defense.
At its core, ecology is about interactions. It asks questions like: how do organisms affect each other, and how do they respond to their environment? These interactions can be between individuals of the same species, between different species, or between organisms and abiotic factors like temperature or water availability. What makes ecology particularly interesting is that small-scale processes—like how an individual feeds or reproduces—can scale up to influence entire populations and ecosystems. So as we move through this lecture, keep in mind that everything is connected. Individual traits influence populations, and populations influence ecosystems.”
blank
blank
blank
Finally, we can gain even more insight into population growth by looking at age structure, which is the proportion of individuals in different age groups within a population.Age structure is important because it helps us predict future population trends. For example, a population with a large proportion of young individuals is likely to continue growing, even if birth rates start to decline, because many individuals are entering reproductive age. In contrast, a population with a larger proportion of older individuals may experience slower growth or even decline.
blank
Finally, we look at biodiversity at the community level. Species diversity includes both the number of species present and how evenly distributed they are. Species richness specifically refers to the number of species. Species richness is influenced by several factors, including habitat complexity, geographic isolation, environmental stress, and latitude. For example, more structurally complex habitats tend to support more species because they provide more niches. All of these factors, combined with species interactions, shape the incredible diversity of life we see across different ecosystems. And ultimately, that diversity emerges from the balance of competition, cooperation, and environmental constraints
Organisms don’t live in isolation—they exist in communities where they constantly interact with other species. These interactions can affect individuals directly, but they also scale up to influence population sizes and even the structure of entire communities. One of the most important ideas here is that these interactions often drive evolutionary change. When one species affects the fitness of another, it can act as a selective pressure. Over time, this can lead to adaptations and even coevolution, where two species evolve in response to each other. Many interactions are trophic, meaning they involve feeding relationships, but not all are. Some involve competition, cooperation, or other forms of interaction. To simplify things, we’ll focus on pairwise interactions between two species
Now let’s switch gears to positive interactions—mutualisms. In a mutualistic interaction, both species benefit. These benefits can involve resources, like food or shelter, or services, like pollination, seed dispersal, or protection from predators. Mutualisms are incredibly widespread and often essential for ecosystem function. But it’s important to recognize that they are not purely cooperative in the way we might think. Each species is still acting in its own self-interest, and the interaction persists because it increases the fitness of both partners
In some cases, populations overshoot their carrying capacity, meaning they grow beyond what the environment can support. When this happens, resources can become depleted, leading to a rapid decline or crash in population size.This pattern is often observed in systems like bacterial cultures or certain aquatic organisms, where growth can be rapid but unsustainable
blank
blank
blank
When species compete, there are two primary outcomes. One is competitive exclusion, where one species outcompetes the other and drives it to local extinction. The other is competitive coexistence, where both species persist despite sharing a resource. Competitive exclusion tends to occur under very specific conditions—when one species is consistently superior, the resource is limiting, and both species use it in exactly the same way. Under these conditions, there’s no room for sharing. However, in nature, coexistence is actually much more common. That’s because species often find ways to reduce competition rather than eliminate it entirely.”
blank
Density-dependent factors are those whose effects become stronger as population density increases. As a population becomes more crowded, individuals interact more frequently, and that increases the impact of certain limiting factors. For example, predation often becomes more intense because predators can find prey more easily when they are concentrated. Disease transmission also increases, because individuals are in closer contact with each other, making it easier for pathogens to spread. Competition is another major density-dependent factor. As population size increases, individuals must compete more for limited resources like food, water, space, and sunlight. These factors tend to act as a kind of negative feedback system. As the population grows, these pressures increase, which slows growth by increasing death rates or decreasing birth rates. So density-dependent factors are one of the main mechanisms that keep populations from growing indefinitely.
Herbivory is another form of predation, but here the prey are plants or algae. Unlike animal prey, plants cannot move to escape, so their defenses are typically structural or chemical rather than behavioral. Herbivores often do not kill the plants they consume, but they can still significantly reduce growth and reproduction. Many herbivores are specialists, focusing on specific plant species or even specific parts of a plant, like leaves, seeds, or roots. This specialization reflects the challenges of digesting plant material, which is often fibrous and chemically defended, and it leads to very tight evolutionary relationships between herbivores and their food sources
Another way we study populations is by looking at survivorship, which is the probability that an individual will survive to a particular age. We often visualize this using survivorship curves, which track how many individuals remain alive at different ages. These curves help us understand patterns of mortality across the lifespan of a species. To create these curves, ecologists typically plot the logarithm of the number of surviving individuals against age, which allows us to compare patterns across very different species. What’s important here is not just how long individuals live, but when mortality is most likely to occur during their life There are three general types of survivorship curves. Type I curves are characterized by high survival early and middle in life, followed by a steep decline in older age. Humans are a classic example of this pattern, where most individuals survive to adulthood, but mortality increases later in life. Type II curves show a constant probability of death across all ages. In this case, the likelihood of dying is roughly the same whether the individual is young or old. This pattern is less common but can be seen in some birds and small mammals. Type III curves are the opposite of Type I. In these populations, there is very high mortality early in life, but those individuals that survive the early stages tend to live much longer. Many organisms that produce large numbers of offspring with little parental care, like oysters, follow this pattern.
a parasite lives on or inside another organism, called the host, and consumes its tissues or body fluids. Unlike typical predators, parasites usually don’t kill their host right away. In fact, from an evolutionary perspective, it’s often beneficial for the parasite to keep the host alive as long as possible, because the host is essentially its habitat and food source combined. What’s really striking is just how common parasitism is. It’s estimated that more than half of all species on Earth are parasitic in some form. That means parasitism is not some rare ecological oddity—it’s actually one of the dominant lifestyles on the planet. Some parasites are also pathogens, meaning they cause disease. But not all parasites immediately cause obvious harm. Many exist in a kind of balance with their host, slowly extracting resources over time. This creates a very different dynamic compared to something like carnivory, where the interaction is fast and lethal.
The intrinsic rate of increase represents the maximum potential growth rate of a population under ideal conditions, where resources are unlimited and environmental constraints are minimal.This rate depends on several life history traits, including how early reproduction begins, how long individuals reproduce, and how many offspring are produced during each reproductive event. This concept is useful because it provides a baseline—what a population could do under perfect conditions—even though real populations rarely experience those conditions.
We can break parasites into a few major categories based on their size and life strategy. Macroparasites are relatively large—things like worms or arthropods—that you could often see with the naked eye. Microparasites, on the other hand, are microscopic, including bacteria and many pathogens.Then there are parasitoids, which are particularly interesting. These are often insects whose larvae develop inside a single host and almost always end up killing it. So they sit somewhere between parasitism and predation—they rely on the host like a parasite, but ultimately kill it like a predator. Most parasites are highly specialized. They are adapted to a very specific host species, or even a specific part of that host. This specialization is one of the reasons we see such incredible diversity in parasites—each host species can support many different parasite species, each occupying its own niche
blank
blank
When conditions are ideal, populations can grow exponentially, producing a characteristic J-shaped curve. In exponential growth, the population increases at a constant per capita rate, meaning that as the population gets larger, it grows faster and faster.This happens because more individuals are reproducing, leading to even more offspring. However, exponential growth is not sustainable in real environments, because resources are always limited. So while it’s useful as a model, it’s not something we typically see long-term in nature
blank
blank
blank
Plants have evolved a wide range of defenses against herbivores. Structural defenses include things like thorns, tough leaves, and protective hairs, which make it physically difficult to consume the plant. Chemical defenses are even more interesting. Plants produce secondary compounds that can be toxic, reduce digestibility, or deter feeding. Some of these compounds are always present, while others are induced only when the plant is attacked.
When we scale up from individuals to populations, life history traits start to shape population-level characteristics. Populations are described by features like density, dispersion, birth and death rates, growth rates, survivorship, and age structure.All of these are directly influenced by life history decisions. For example, a species that reproduces early and produces many offspring will tend to have faster population growth. In contrast, a species that reproduces later and invests heavily in fewer offspring may have slower growth but higher survival rates. So understanding populations really starts with understanding the biology of individuals
To understand what limits population growth, we introduce the concept of carrying capacity. This is the maximum population size that an environment can sustain over time, given the available resources. Importantly, carrying capacity is not fixed—it can change with environmental conditions like food availability, climate, or habitat quality. So populations are constantly adjusting in response to shifting limits
All of these interactions—competition, predation, mutualism—feed into the concept of the ecological niche. A niche describes the role a species plays in its community, including all the biotic and abiotic conditions it requires to survive and reproduce. We distinguish between the fundamental niche, which is the full range of conditions a species could theoretically occupy, and the realized niche, which is what it actually occupies in the presence of other species. Interactions like competition often shrink the realized niche compared to the fundamental niche, because species are excluded from certain resources or habitats.
Of course, prey are not passive participants in this interaction. They have evolved a wide range of defenses to avoid being eaten. These include physical defenses like armor or speed, as well as behavioral strategies like vigilance or escape responses. Some species use warning coloration to signal that they are toxic, while others use mimicry to resemble those toxic species. And many rely on camouflage, or crypsis, to blend into their environment and avoid detection altogether. These adaptations illustrate the concept of coevolution—predators and prey are constantly influencing each other’s evolution, leading to increasingly sophisticated strategies on both sides.
Density tells us how many individuals are present, but it doesn’t tell us how they’re arranged in space. That’s where dispersion comes in. Dispersion describes the pattern of spacing among individuals in a population, and it can tell us a lot about how individuals interact with each other and their environment.There are three main patterns. Random dispersion occurs when individuals are spaced independently of each other, which is relatively rare. Clumped dispersion, which is the most common, occurs when individuals aggregate in areas where resources are concentrated or where social behavior provides benefits. Uniform dispersion occurs when individuals are evenly spaced, often due to competition or territorial behavior. So dispersion gives us insight into underlying ecological processes like resource distribution and social interactions
The key to coexistence is resource partitioning. This occurs when species divide a resource in some way—by using it at different times, in different places, or in different forms. For example, two species might feed on the same type of food but at different times of day, or they might occupy slightly different habitats. Even small differences can reduce competition enough to allow both species to persist. Context also matters. Environmental conditions can shift which species has the advantage. A drought-tolerant species might dominate during dry periods but be outcompeted when water is abundant. This dynamic balance helps maintain diversity within communities
blank
In contrast, density-independent factors affect populations regardless of how large or small they are. These are typically abiotic factors, like weather events, natural disasters, or sudden changes in environmental conditions.For example, a drought, flood, wildfire, or extreme temperature event can reduce a population dramatically, whether the population was large or small to begin with. These factors don’t respond to population size—they act independently of it. Because of that, they can cause sudden and sometimes unpredictable changes in population size. So while density-dependent factors tend to regulate populations gradually, density-independent factors can cause abrupt shifts
Carnivores have evolved different strategies for finding and capturing prey. Some are active hunters, like wolves or sharks, that move through their environment searching for prey. Others are ambush predators, like certain snakes or eels, that remain hidden and strike when prey comes close. There are also trap-setters, like spiders, that create structures to capture prey without actively pursuing it. Each of these strategies represents a different solution to the same problem—how to obtain energy efficiently.
When we incorporate carrying capacity into population growth, we get logistic growth, which produces an S-shaped curve. Early on, growth looks exponential because resources are abundant. But as the population approaches carrying capacity, growth slows down due to increased competition and limited resources. Eventually, the population stabilizes around the carrying capacity, although in reality, it often fluctuates rather than staying perfectly stable. This model is much more realistic than exponential growth because it accounts for environmental limits
Population density refers to the number of individuals of a species per unit area or volume at a given time. This is one of the simplest ways to describe a population, but it’s also very informative.In practice, it’s often difficult to count every individual in a population, so researchers typically estimate density by sampling a smaller area and scaling up.
blank
In carnivory, both predator and prey are typically animals, and the predator usually kills the prey outright. Carnivores often face the challenge of locating and capturing prey, which can be rare, mobile, and capable of defending itself. Because of this, many carnivores are generalists—they consume a variety of prey species rather than specializing on just one. This flexibility allows them to adapt to changing conditions and increases their chances of finding food. At the same time, prey species evolve defenses to avoid being eaten, setting up a dynamic interaction where both predator and prey are under strong selective pressure
blank
blank
blank
blank
To understand how populations change over time, we need to consider population growth rate. Growth rate depends on four main factors: births, deaths, immigration, and emigration. Births and immigration add individuals to the population, while deaths and emigration remove them. So the overall growth rate reflects the balance between these processes. If more individuals are being added than removed, the population grows. If more are being removed than added, the population declines. This framework allows us to quantify and predict changes in population size over time
blank
Predation is one of the most familiar types of species interactions. It occurs when one organism, the predator, kills and consumes another organism, the prey. But predation actually includes several subcategories. Carnivory involves predators consuming other animals. Herbivory involves animals consuming plants or algae. And parasitism, which we’ll discuss more later, involves organisms living on or in a host and consuming its tissues. These interactions are incredibly important because they shape both predator and prey populations, and they often lead to evolutionary arms races, where each species evolves adaptations to outcompete the other.”
A community is defined as an association of populations of different species that live and interact in the same place at the same time. So unlike a population, which focuses on a single species, a community includes all the species in a given area and the ways they interact. If we take it one step further, when we include the abiotic environment—things like temperature, water, nutrients—we get an ecosystem. So a community is the biological component, and the ecosystem is the biological plus the physical environment. This distinction is important because many of the patterns we see in nature emerge not just from individual species, but from how species interact with each other and with their environment
Competition occurs when two or more individuals use the same limited resource, and as a result, all individuals involved experience a reduction in fitness. This is a non-trophic interaction, meaning it’s not about one organism eating another—it’s about access to resources.Competition can occur between closely related species or completely unrelated ones, as long as they rely on the same resource. And importantly, competition doesn’t require direct interaction. If another organism is using a resource you need, it’s affecting you—even if you never encounter it directly. From a functional perspective, a competitor either reduces the amount of resources available to you or forces you to expend more energy to access those resources.
So at this point, we’ve seen how populations can grow and what happens when they approach or exceed carrying capacity, but now we need to ask the key question: what actually limits population growth in real ecosystems? All limiting factors fall into two broad categories: density-dependent and density-independent factors, and these can include both biotic, meaning living, and abiotic, meaning nonliving, influences. The distinction between these two types of factors is really important because it tells us whether the effect on a population changes as the population size changes. In other words, does the impact of the factor depend on how crowded the population is, or does it act regardless of population size? That difference helps us understand how populations are regulated over time
blank
To study ecology effectively, we need to think in terms of levels of organization. At the most basic level, we have the individual organism. This is where behavior, physiology, and life history decisions occur. Next is the population, which is a group of individuals of the same species living in a specific area. This is where we start looking at patterns like population growth and density.Above that is the community, which includes all the different species interacting in an area—this is where things like predation, competition, and symbiosis come into play. And finally, we have the ecosystem, which includes both the biological community and the abiotic environment, such as climate, soil, and water.
There are two main ways competition occurs. The first is exploitation competition, which is indirect. Organisms compete simply by consuming shared resources. Whoever is faster or more efficient at finding and using the resource gains an advantage.The second is interference competition, which is direct. Here, organisms actively prevent others from accessing resources—through aggression, territorial behavior, or other forms of direct interaction. These two mechanisms select for very different traits. Exploitation favors efficiency, speed, and detection, while interference favors strength, dominance, and control. So an interesting question to consider is whether a species that is good at one type of competition is also good at the other—or whether there are tradeoffs between these strategies
Even though both species benefit, mutualisms come with costs. Each partner is investing energy or resources into the interaction, so the net benefit must outweigh those costs. This creates the potential for conflict. For example, one partner might ‘cheat’ by taking benefits without providing its share. As a result, many mutualisms have evolved mechanisms to discourage cheating, such as withholding rewards or terminating the interaction. A great example comes from coral and algae. Under stressful conditions, like high temperatures, the algae can produce harmful compounds, and the coral responds by expelling them
All organisms require resources to survive, and these resources can be either abiotic or biotic. Abiotic resources include things like water, light, space, and nutrients, while biotic resources include food, mates, hosts, and even mutualistic partners.One key point is that resources are not evenly distributed. They vary across space and time, both in quantity and quality. This uneven distribution sets the stage for competition, because organisms are often trying to access the same limited, high-quality resources. When a particular resource directly limits the growth or survival of a population, we call it a limiting resource. Identifying limiting resources is critical in ecology because they often determine population size and distribution.
blank
blank
blank
This table is one of the most important tools for organizing species interactions. Each interaction is classified based on its effect on the fitness of the two species involved. For example, in predation, one species benefits while the other is harmed. In competition, both species are negatively affected. In mutualism, both species benefit