EI - Community Structure Lesson

Community Structure

Welcome to the Community!

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 numbers, which includes all of the patterns of interaction between different species.

Biodiversity

Biodiversity is the variety of life and its processes; including the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur. Within an ecosystem biodiversity boosts ecosystem productivity because every species, no matter how small, all have a role to play in the ecosystem. In general, biodiversity increases interactions among organisms and helps ensures natural sustainability of the ecosystem.

Remember that ecosystems include all of the interaction, biotic and abiotic, within a defined area.

A community includes all of the interactions of the biotic factors in a defined area. Ecosystems can be composed of several different communities each contributing the biodiversity of a specific ecosystem.

To ensure that biodiversity is kept to a maximum, scientist look at community structures. Use the presentation below to learn about these community structures.

2 Types of 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:

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 community.	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 environment.

Take a Closer Look at the Foundation Species

Remember that foundation species act by modifying the environment so that it can support the other organisms that form the community.

Take a look at the examples given:

Take a Closer Look at the Keystone Species

Remember that 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 environment.

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 species.

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.

Ecosystems and Ecological Networks

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. 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 stability. 

Community Structure

Before You Go, You Need to Know

The following key points are from this explore section of the lesson. You must know the following information before moving to the next lesson. This is just a summary of the key points.

• Biodiversity is t he variety of life and its processes; in an ecosystem and in general the more diverse an ecosystem is, the more stable it will be.
• 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).
• Some species, such as foundation species and keystone species, play particularly important roles in determining their communities' structure.

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