Answers to Review Questions

1. Yes. Community membership is dependent on dispersal, environmental factors, and biological interactions. Given all the introductions of non­native species that have occurred worldwide, it is clear that “getting there” has been an important constraint on the entrance of species into communities.

2. Resource partitioning is the idea that coexistence among species is possible if the species in a community use its resources in slightly different ways. Other models, such as the intermediate disturbance hypothesis, rely on population fluctuations due to disturbance, stress, or predation as the mechanism of coexistence. These models suggest that as long as populations of species never reach their carrying capacities, competitive exclusion will not occur, and coexistence will be possible. Lottery or neutral models assume that resources made available by disturbance, stress, or predation are captured at random by recruits from a larger pool of colonists, all of which have an equal chance of obtaining those resources.

3. Lottery and neutral models best support the tropical rainforest data set. These models assume that resources made available by the deaths of individuals are captured at random by recruits from a larger pool of colonists such that no one species has an advantage, and that species diversity is maintained as a result.

4. Species diversity-community function relationships can differ depending on two variables: the degree of overlap in the ecological functions of species, and variation in the strength of ecological functions of species. Graph A is best described by the complementarity hypothesis, which proposes that as species richness increases, there will be a linear increase in community function. This linear relationship occurs because each species added to the community has a unique and equally incremental effect on community function. Graph B is best described by the redundancy hypothesis, in which there is an upper limit on the effect of species richness on community function. This curvilinear relationship occurs because the unique functional contributions of species reach a threshold due to their overlap. Graph C best describes the idiosyncratic hypothesis, which suggests that the strengths of the effects of species' functions vary dramatically. Dominant species have a large effect on community function such that when they are present, they increase community function, but when they are absent, it declines. This produces a variable species richness and community function pattern.