1210
A community is a group of species that occur together at the same place and time. Communities are often defined by a subset of species based on similar taxonomy, resource use (guild), ecological functions (functional group), or trophic and energetic connections (food webs). Species diversity and composition are descriptors of community structure. Species diversity is a measure that combines the number of species (species richness) and their relative abundances (species evenness). Biodiversity is a term that describes multiple scales of diversity, from genes to species to communities.
learning outcomes
You should be able to:
Give examples of ways communities can be subdivided, and explain how these might also be used to understand communities.
Define food web, and analyze types of information it might provide about community structure.
Use the equation for the Shannon index to determine the species diversity of a community, given appropriate data.
Explain the significance of the value of H in the Shannon index.
Why are subsets of species used to define communities? What type of community subset was used to define the frogs and toads in ponds on Mount St. Helens?
Subsets of species are used to describe communities because it is impractical to account for every species in a community. There are at least two reasons for this: (1) communities can vary in spatial and temporal scale, making it hard to designate boundaries around them, and (2) it is impossible to identify every species in a community because many are not described. For example, many bacteria, small microbes, and small invertebrates are hard to see, let alone identify.
The community subset used for frogs and toads on Mount St. Helens was a taxonomic one—
What types of species interactions are missing from the food web depicted in Figure 56.3?
Nontrophic interactions, such as competition and facilitation, are not depicted in the food web.
Refer to Table 56.1. Replace the abundance of amphibians in Pond A with the following values (Pacific tree frog = 6, western toad = 8, northern red-
Pond A
| Abundance | Proportion (pi) | ln (pi) | pi ln(pi) | |
|---|---|---|---|---|
| Pacific tree frog | 6 | 0.3 | –1.20 | –0.36 |
| Western toad | 8 | 0.4 | –0.92 | –0.37 |
| Northern red- |
4 | 0.2 | –1.61 | –0.32 |
| Cascades frog | 2 | 0.1 | –2.30 | –0.23 |
| H | 1.28 |
The new Shannon index value for pond A is H = 1.28. Pond B, with H = 1.39, still has slightly higher species diversity than Pond A.
Measuring community structure prompts us to consider many interesting questions about communities. Why do communities differ in the number and composition of species? What processes determine community membership? In the next section we will explore how community membership is shaped by which individuals get to the community, deal with the environmental conditions there, and form interactions with other individuals.