Plant attribute diversity, resilience, and ecosystem function: The nature and significance of dominant and minor species

This study tested an hypothesis concerning patterns in species abundance in ecological communities. Why do the majority of species occur in low abunda nce, with just a few making up the bulk of the biomass? We propose that man y of the minor species are analogues of the dominants in terms of the ecosy stem functions they perform, but differ in terms of their capabilities to r espond to environmental stresses and disturbance. They thereby confer resil ience on the community with respect to ecosystem function. Under changing c onditions, ecosystem function is maintained when dominants decline or are l ost because functionally equivalent minor species are able to substitute fo r them. We have tested this hypothesis with respect to ecosystem functions relating to global change. Ln particular, we identified five plant function al attributes-height, biomass, specific leaf area, longevity, and leaf litt er quality-that determine carbon and water fluxes. We assigned values for t hese functional attributes to each of the graminoid species in a lightly gr azed site and in a heavily grazed site in an Australian rangeland. Our resi lience proposition was cast in the form of three specific hypotheses in rel ation to expected similarities and dissimilarities between dominant and min or species, within and between sites. Functional similarity-or ecological d istance-was determined as the euclidean distance between species in functio nal attribute space. The analyses provide evidence in support of the resili ence hypothesis. Specifically, within the lightly grazed community, dominan t species were functionally more dissimilar to one another, and functionall y similar species more widely separated in abundance rank, than would be ex pected on the basis of average ecological distances in the community. Betwe en communities, depending on the test used, two of three, or three of four minor species in the lightly grazed community that were predicted to increa se in the heavily grazed community did in fact do so. Although there has be en emphasis on the importance of functional diversity in supporting the flo w of ecosystem goods and services, the evidence from this study indicates t hat functional similarity (between dominant and minor species, and among mi nor species) may be equally important in ensuring persistence (resilience) of ecosystem function under changing environmental conditions.