Why Australian terrestrial vertebrates increase mycorrhizal spore diversity while decreasing tree seedling diversity.
Theimer, Tad1, Gehring, Catherine1, 1 Department of Biological Sciences, Flagstaff, AZ, USA
ABSTRACT- Vertebrates can impact species-rich, tropical plant communities by acting as seed dispersers, seed predators and seedling herbivores. The effects of vertebrates are most often determined through studies of interacting subsets of the community, while community-level tests of vertebrate impacts are rarer. We have excluded terrestrial vertebrates from 14 small (6 x 7.5m) plots of Australian tropical rain forest since 1996 and have monitored the response of the tree seedling community. Five years of excluding terrestrial vertebrates resulted in significantly higher seedling recruitment and survival. Seedling species richness and species diversity was also significantly higher on exclosure plots, although this difference was driven by higher seedling densities inside exclosures. These results contrasted with those of an earlier study of diversity and abundance of mycorrhizal spores censused in the same plots after three years of vertebrate exclusion (Gehring et al. 2002). We developed a conceptual model to explain differences in mycorrhizal spore and tree seedling responses to vertebrate exclusion. According to this model, when vertebrates act primarily as agents of seed/spore dispersal, as they do for mycorrhizal spores in our system, they increase local species richness by increasing the rate of local colonization. When vertebrates act primarily as agents of random mortality, as they do for seeds and seedlings in our system, they increase the rate of local extinction and act to depress local species richness. In the latter case, vertebrates increase recruitment limitation and could potentially maintain diversity on larger spatial scales, although this was not the case in our system.
Key words: recruitment limitation, tropical rain forest
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