Community Ecology Learning Objectives:
- Describe resource, resource partitioning, character displacement, and the niche concept
- Identify factors responsible for species to occupy defined niches; distinguish between fundamental and realized niches and explain an example of this concept.
- Recognize the positive, neutral, and negative pairwise effects that a species can have on another species and be able to name and define the following interspecific interactions competition, predation/parasitism/herbivory, and mutualism.
- Explain how competition reduces fitness for both species involved and explain the long-term consequences of competition: coexistence, competitive exclusion, resource partitioning, and character displacement
- Give a biological example of a mutualism and describe how mutualisms are vulnerable to cheating.
- Explain how predators can exert strong selection on prey.
Resources and Niche Concept
As we consider how organisms interact with organisms of other species in their environment, we need to officially define some ideas that we’ve been bandying around in class. Organisms need many things to survive, including food, water, sunlight if you photosynthesize, etc. Many species need oxygen and carbon dioxide, but there are entire groups of bacteria and archaea that live in anoxic conditions and don’t use or require oxygen. Not all of these required elements are called resources. A resource in ecology is a thing or factor that causes population growth and that is reduced by use. By this definition, oxygen isn’t a resource for humans unless we are living in space or underwater or somewhere requiring we get our oxygen from a finite tank. Likewise, sunlight is not considered a resource. The resource that is reduced the most by use, for example water in a desert, and thereby limits population growth first is specially designated as a “limiting resource.”
The full range of resources that a species can use combined with the range of conditions a species can tolerate (now we can add sunlight and oxygen back into the mix) defines the ecological niche for that species. Niches have many dimensions, one for each requirement of the species, but we usually reduce a complex niche down to the one or two dimensions deemed most important for the question we are testing. The figure below shows a range of seed sizes to define the niche breadth on the x-axis, with seed consumption on the y-axis.
The figure above shows the fundamental niche, the full resource axis of seed sizes that a finch species is theoretically able to use. Frequently in nature we observe that a species only uses part of its fundamental niche; the part they use is called the realized niche.
- Where is the realized niche for species a?
- Is species a or species b the stronger competitor?
Competition between species for resources is strongest where their niches overlap. In the zone of overlap, intense competition can result in the species using a realized niche smaller than its fundamental niche. In communities with many similar species, the result is resource partitioning, where realized niches pack together for several similar species.
These ecological interactions can exert such strong selection pressure that the trait to use the resource (like the beak of the finch to eat seeds) can adapt over generations to a distinctly new character state (like a smaller or larger beak better matched to the available seed food supply). The shift in the trait value is called character displacement. Character displacement is an evolutionary adaptation in a heritable trait, caused by resource partitioning.
Competitive interactions have negative effects on both species
The competition for resources that drives reductions in niche overlap is called interspecific competition, and it has a negative effect on both species in the interaction. That is, individuals of each species would have more success (more resource access with less energy expenditure = greater growth, survival and reproduction) in the absence of the other species. We can consider this mathematically with a slight alteration of the logistic equation to include the detrimental effect of species 2 on species 1, where the two species are represented with subscripts.
The new term in this equation is “minus alpha.” Alpha is the competition coefficient, and you can think of it as converting units of species 2 into units of species 1. For more individuals of species 2 or larger values of alpha, species 1 sees lower population growth. You can create the complementary population growth equation for species 2 by swapping the 1’s for 2’s and vice versa. Competition is a lose-lose interaction. It’s also very very common because resources are finite and necessary for survival and reproduction.
Predation, Parasitism, and Herbivory are detrimental to one species while beneficial to the other
Another class of interspecific interactions has a negative impact on one species while providing benefits to the other: predator-prey interactions, parasite-host interactions, and herbivory. Predator-prey interactions, or predation, involve one species consuming (eating) the other. A well-studied example is the lynx-hare interaction in the Canadian arctic.
Strong predation on the hare populations reduces hare numbers, which in turn reduces lynx numbers. Predation results in continual co-evolution of traits for agility and speed in both species, as well as winter cryptic fur color in the hare against the snowy arctic background. Predation is in part responsible for the stable cycling of the population sizes of these two species, shown below.
Predation is a clear win-lose scenario that sets us up for food web dynamics when we read about ecosystems on the next webpage. Another win-lose scenario is the parasite-host interaction, or parasitism, where a species (the parasite) lives on or in another species (the host) that it harms over time, sometimes resulting in host death. Parasitism is a win for parasite and a loss for the host. Herbivory is similar to predation in the sense that a grazer eats a plant. However, some grazing interactions can be argued to provide a benefit because grazing can promote the growth of new plant material. To determine if an herbivore-plant interaction is a win-lose, the cost and benefit to the plant would need to be measured.
Mutualisms benefit both players in the interspecific interaction
Mutualism is the final class of interactions we will consider by name. In a mutualism, both species benefit from the interaction. For example, plants have fungi that live on or in their roots, called mycorrhizae. The fungus gains access to sugars like glucose and sucrose from the root system, and the plants use the phosphorus and nitrogen that the fungi “fix,” or process into a biologically usable form. Mutualisms are vulnerable to cheating, where one pair in the interaction saves time or energy and circumvents the benefit to the other species. For example, pollinating insects transfer pollen (sperm) from one plant to another as they forage for nectar in the plant’s flowers. Exact matches between the flower structure and the pollinator’s mouth and tongue structure show evidence of strong co-evolution. However, some insects cheat on this interaction, such as the bumblebee in the images below:
Interspecific interactions are not static but can evolve
All biological interactions are dynamic, and interspecific interactions are no exception. Mutualisms, for instance, are susceptible to cheating by one member of the interaction, which can shift the interaction to parasitism. Likewise, a parasitic interaction, such as Wolbachia bacterial infection in Drosophila that reduces female fecundity in the flies, can evolve rapidly from parasitism into mutualism (Weeks et al. 2007. From Parasite to Mutualist: Rapid Evolution of Wolbachia in Natural Populations of Drosophila. PLoS Biol 5(5): e114. doi:10.1371/journal.pbio.0050114).
Community Ecology Learning Objectives:
- Recognize that species diversity (richness and evenness) is important for community function.
- Describe the simple model of island biogeography, and roles of island size and distance from a mainland source in determining number of species at equilibrium.
- Describe the concept of keystone species, provide examples of how a keystone species is responsible for community species composition, and appreciate that the keystone species is not necessarily strongest competitor.
Community composition can be quantified for species richness and evenness
Take all those different interactions between species we just read about–competition, predation, mutualism, parasitism–mix them together in different assemblages, and the picture begins to seem much more complicated. How many species are in a community, are some more common and others more rare, and does that matter? Let’s think about two metrics of species diversity in a community assemblage: richness and evenness. Species richness, S, is simply a count of the number of species in a community. The skew, if any, in how common versus rare those different species are is captured by species evenness, which we’ll calculate using the Shannon diversity index, , where pi is the proportion of each species i in the community. Imagine that each box in the diagram below is an individual in a community, with the colors representing individuals of different species:
- What is S? (HINT: Count the number of species in the grid above)
- What is H’? (HINT: Open Excel and calculate the proportion of each species, the natural log of that proportion, then plug in to the H’ equation.
- What if there were 10 of each color instead of the skew shown above?
- Which of these species might be a predator versus its prey?
Community stability is disproportionately controlled by two types of species: keystone species and dominant competitors
Communities are not random assemblages of species because of interspecific interactions. Sometimes a single species serves as the unintentional lynchpin for stability of a complex set of community interactions. For example, the banded tetra, a fish in tropical streams, provides phosphorus to other species in the community (Source: OpenStax Biology). We’ll learn another example in class. In most cases, the keystone species isn’t the most numerous one, nor the best competitor for resources in the ecosystem.
The Theory of Island Biogeography
MacArthur and Wilson (1963) put forth a theory to explain why ecologists could predict species richness on islands but could not predict the composition of those species. They started with the observation that there’s a very consistent log-linear relationship between habitat area and the number of (similar) species that tend to pack into that area, as can be seen here for reptiles and amphibians in the West Indies.
This species-area pattern is consistent for birds, mammals, and lots of other groups. MacArthur and Wilson assumed that
- more isolated islands have lower immigration rates (a in figure below)
- larger islands have lower “extinction” rates, and by extinction they meant loss of the species to the island. One way that a species can remain on the island is to re-migrate, which is more likely to happen by chance on larger islands (b in figure below)
The intersection of the immigration and extinction rates for an island predict an equilibrium number of species, S-hat, assuming island size and island distance from mainland.
Here’s a short video that walks through basic island biogeography theory. The rescue and target effect concepts at the end provide a little more detail than we expect for this course:
Here’s Hank Green’s Crash Course review of Community Ecology: