Effects of species identity and functional group richness on community structure and function: Are species functionally redundant?

Background/Question/Methods

Efforts to elucidate the mechanisms underlying the assembly and diversity of ecological communities have increasingly incorporated the ecosystem concept of the functional group, a suite of trophically similar species with high resource use overlap. Niche based models, for example, predict that communities assemble toward specific relative abundances of different functional groups. Inherent to the use of functional groups is the assumption of functional redundancy or ecological equivalence among species within a group. This assumption has not been adequately tested, however, and has profound implications for the assembly of communities and the functioning of ecosystems. Here we use a long-term study of desert rodents involving experimental removal of the dominant functional group, the large granivores (mean mass > 30g), to gain insight into the roles of species and functional groups in the structure and function of the community. Experimental treatment and a recent colonization event yielded plots that contain only one species (Chaetodipus baileyi) in the large granivore group, in contrast to the three species (Dipodomys merriami, D. ordii, C. baileyi) found on control plots.

Results/Conclusions
Comparison of these treatments reveals that 1) the relative abundance of functional groups is consistent across treatments, 2) the competitive effect of large granivores on the small granivores, as indicated by changes in abundance, richness, and colonization and extinction probabilities, is equivalent whether one or all species of the functional group is present, and, 3) total resource use, as measured by energy use, of the granivorous rodent community tends to be lower in the absence of kangaroo rats (Dipodomys spp.). These results support predictions of niche-based processes of community assembly and lend insights into the fundamental and realized niches of both species and functional groups and to the distinct roles of species richness and functional group richness in the structure and function of communities.