Answers to Review Questions

1. A community is a group of interacting species that exist together at the same place and time. Interactions among multiple species and their physical environment give communities their character and function.

2. Species richness is the number of species in a community, but that measure tells us nothing about the relative abundances of those species. If two communities had a similar number of species, but great differences in species evenness (as in Figure 16.6), species richness would not reflect this difference, but species diversity indices would. Rank abundance curves (as in Figure 16.8) allow hypotheses to be generated about how those species may be interacting in the community based on their abundances.

3. Foundation species have a large effect on other species due to their large size and high abundance. For example, kelp and trees have a large influence on species diversity by virtue of providing their communities with habitat, food, and other services that are directly related to their size. Keystone species have a large effect despite their small size and low abundance, because of the important role they play in their communities. For example, sea otters have large effects on their communities by preying on herbivores (sea urchins), which, in turn, eat primary producers (kelp). This indirect interaction can allow primary producers to have higher abundances. Finally, ecosystem engineers are able to create, modify, or maintain physical habitat for themselves and other species. Trees and kelp are examples of ecosystem engineers that are foundation species, and beavers are an example of a keystone species that is also an ecosystem engineer.