If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Community structure

Species richness and species diversity. Why more diverse ecosystems may be more stable. Roles of foundation and keystone species.

Key points:

  • A community's structure can be described by its species richness, which is the number of species present, and species diversity, which is a measure of both species richness and species evenness (relative numbers).
  • Community structure is influenced by many factors, including abiotic factors, species interactions, level of disturbance, and chance events.
  • Some species, such as foundation species and keystone species, play particularly important roles in determining their communities' structure.


Different ecological communities can be pretty different in terms of the types and numbers of species they contain. For instance, some Arctic communities include just a few species, while some tropical rainforest communities have huge numbers of species packed into each cubic meter.
One way to describe this difference is to say that the communities have different structures. Community structure is essentially the composition of a community, including the number of species in that community and their relative numbers1. It can also be interpreted more broadly, to include all of the patterns of interaction between these different species2.
In this article, we'll look at some of the ways that community structure can be quantified (measured numerically). Then, we'll examine factors that shape community structure, focusing especially on foundation and keystone species.

How do we measure community structure?

Two important measures ecologists use to describe the composition of a community are species richness and species diversity.

Species richness

Species richness is the number of different species in a particular community. If we found 30 species in one community, and 300 species in another, the second community would have much higher species richness than the first.
Communities with the highest species richness tend to be found in areas near the equator, which have lots of solar energy (supporting high primary productivity), warm temperatures, large amounts of rainfall, and little seasonal change. Communities with the lowest species richness lie near the poles, which get less solar energy and are colder, drier, and less amenable to life. This pattern is illustrated below for mammalian species richness (species richness calculated only for mammal species, not for all species). Many other factors in addition to latitude can also affect a community's species-richness.
Global species richness as calculated for mammal species. Image credit: "Community ecology: Figure 14," by OpenStax College, Biology, CC BY 4.0. Modification of work by NASA, CIESIN, Columbia University.

Species diversity

Species diversity is a measure of community complexity. It is a function of both the number of different species in the community (species richness) and their relative abundances (species evenness). Larger numbers of species and more even abundances of species lead to higher species diversity. For example:
  • A forest community with 20 different kinds of trees would have greater species diversity than a forest community with only 5 kinds of trees (assuming that the tree species were even in abundance in both cases).
  • A forest community with 20 different kinds of trees in even abundances would have greater species diversity than a forest community with the same number of species in very uneven abundances (for instance, with 90% of the trees belonging to a single species).
In general, ecologists think that more diverse ecological communities are more stable (that is, more able to recover after a disturbance) than less diverse communities. You can explore why this is the case in the video on ecological networks. However, the diversity-stability relationship isn't a universal rule, and there are some cases where other factors (besides species diversity) are more important in determining community and ecosystem stability5,6.

What factors shape community structure?

The structure of a community is the result of many interacting factors, both abiotic (non-living) and biotic (living organism-related). Here are some important factors that influence community structure:
  • The climate patterns of the community's location.
  • The geography of the community's location.
  • The heterogeneity (patchiness) of the environment7.
  • The frequency of disturbances, or disruptive events.
  • Interactions between organisms.
A community's structure can also be shaped by the chance events that happened during its history. For instance, suppose that a single seed blows into the dirt of a particular area. If it happens to take root, the species may establish itself and, after some period of time, become dominant (excluding similar species). If the seed fails to sprout, another similar species may instead be the lucky one to establish itself and become dominant.

Foundation and keystone species

Some species have unusually strong impacts on community structure, preserving the balance of the community or even making its existence possible. These "special" species include foundation and keystone species.

Foundation species

A foundation species plays a unique, essential role in creating and defining a community. Often, foundation species act by modifying the environment so that it can support the other organisms that form the community9,10,11.
Kelp (brown algae) is a foundation species that forms the basis of the kelp forests off the coast of California. Kelps create environments that allow the survival of other organisms that make up the kelp forest community9. The corals of a coral reef are another foundation species. The exoskeletons of living and dead coral make up most of the reef structure, which protects other species from waves and ocean currents. Beavers, which modify their environment by building dams, can also be seen as a foundation species9,11.
A coral reef. Image credit: "Community ecology: Figure 13," by OpenStax College, Biology, CC BY 4.0. Original work by Jim E. Maragos, USFWS.

Keystone species

A keystone species is a species that has a disproportionately large effect on community structure relative to its biomass or abundance. Keystone species differ from foundation species in two main ways: they are more likely to belong to higher trophic levels (to be top predators), and they act in more diverse ways than foundation species, which tend to modify their environment10,11.
The intertidal sea star Pisaster ochraceus, which is found in the northwestern United States, is perhaps the most famous example of a keystone species. In a classic experiment of community ecology, the sea stars were experimentally removed from the intertidal zone where they lived. As a result, populations of their prey (mussels) increased, altering the species composition of the community and sharply reducing species diversity. When the sea stars were present, about 25 species of barnacles and algae were found in the lower part of the intertidal zone, but when they were missing, the mussel population expanded downward and almost entirely replaced these other species12.
This type of sharp reduction in diversity or collapse of community structure commonly occurs when a keystone species is removed. In this case, the loss of diversity happened because the mussels crowded out other species, which could normally persist because the sea stars kept the mussels in check.
Image credit: "Community ecology: Figure 15," by OpenStax College, Biology, CC BY 4.0. Original work by Jerry Kirkhart.

Explore outside of Khan Academy

Do you want to learn more about the classic keystone species experiment described above? Check out this scrollable interactive from LabXchange.
LabXchange is a free online science education platform created at Harvard’s Faculty of Arts and Sciences and supported by the Amgen Foundation.

Want to join the conversation?