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Community structure

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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 numbersstart superscript, 1, end superscript. It can also be interpreted more broadly, to include all of the patterns of interaction between these different speciessquared.
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.
Map shows the spatial distribution of mammal species richness in North and South America. The highest number of mammal species, 179-228 per square kilometer, occurs in the Amazon region of South America. Species richness is generally highest in tropical latitudes, and then decreases to the north and south, with zero species in the Arctic regions.
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, percent 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 stabilitystart superscript, 5, comma, 6, end superscript.

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:
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 communitystart superscript, 9, comma, 10, comma, 11, end superscript.
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 communitystart superscript, 9, end superscript. 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 speciesstart superscript, 9, comma, 11, end superscript.
Photograph of coral reef.
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 environmentstart superscript, 10, comma, 11, end superscript.
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 speciesstart superscript, 12, end superscript.
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.
Photograph of sea star.
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?

  • piceratops ultimate style avatar for user mario.rosales
    Is it possible for there to be more than one keystone species in an area, as well as foundation species?
    (7 votes)
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  • aqualine seed style avatar for user manishve86
    How does climate shape a biological community?
    (1 vote)
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    • leaf red style avatar for user Seolfor
      Climate is one of the abiotic factors that can introduce variation. According to the videos on niches and competition, no two species can coexist indefinitely if they use the same survival strategy. For example, both wolves and foxes may hunt rabbits, but wolves can run faster so they will eventually outcompete the foxes, driving them to extinction and reducing overall biodiversity. With temperature variation, the foxes' strategy might actually prove better in some years. For example, foxes might have better vision and find it easier to spot rabbits against snow, so they would do better in years with more snow, while wolves will do better in years with less snow. The constant change of climate allows both species to find a niche in otherwise identical conditions and therefore coexist.

      Another example would be tree populations - taller trees grow better because they get more sunlight, so if a single tree was to grow much taller than all the others and grow in an umbrella shape, blocking out sunlight and killing all the trees around it, giving other tress 0 chance of survival. If a tree would be much taller that all other trees, it would be more susceptible to breaking due to wind as it would have to take all the wind resistance by itself. A forest full of trees roughly the same height help each other break wind thus climate (wind intensity or lightning bolts) prevent one tree from pushing out all the others.
      (8 votes)
  • blobby green style avatar for user Clara
    Can you explain how, for example, an area with only 7 types of species and a total 27 individuals has greater biodiversity than an area with 10 different species and a total 48 individuals?
    (2 votes)
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  • mr pants teal style avatar for user Arvyn De
    What about the other types of species like other than keystone and foundation species. Someone should add that.
    (3 votes)
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  • blobby green style avatar for user maeganresuello
    Explain why we would expect low species diversity when disturbance/predation occur with a high frequency.
    (2 votes)
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  • leaf blue style avatar for user christina_potter26
    Is it possible for there to be more than one keystone species in an area, as well as foundation species?
    (2 votes)
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  • blobby green style avatar for user 😊
    what are Dominant species?
    (1 vote)
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  • blobby green style avatar for user deusera
    why can ecological communities be pretty different in terms of the types and numbers of species they contain.
    (1 vote)
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    • leafers seedling style avatar for user ✮Fiona Frison❁
      It all depends on the habitat. For example, a spot with optimal living conditions, such as a jungle, will have more species diversity than an Arctic tundra.
      Additionally, consider the variety of habitats on Earth. There are different communities adapted to different ecosystems. A polar bear would not do very well in the Sahara...
      In short, harsher climates often mean less diversity, while richer ones generally have more species.
      Hope this helps.
      (1 vote)
  • blobby green style avatar for user 001303405
    What is a social structure examples?
    (1 vote)
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  • blobby green style avatar for user amyyun1218
    How do you know that this a related of the biodiversity for the whole entire things? Can you explain clearly about the stuff.
    (1 vote)
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    • winston baby style avatar for user Ivana - Science trainee
      Because you know that something larger is composed of something smaller.

      How do you know the fact that you drink glass fo water will help you actually retain water in your body? Because you have cells and aquaporins.

      Something you do not expect from your body and examine under the microscope on daily basis. (I just made analogy).

      Everything is structured in nature, even formation of planets in the Solar System (ugh I cannot go further relating Astrophysics because I am just merely life scientists).

      Do you need help with something specific?
      (1 vote)