An oft-touted premise in ecology, evolution and biodiversity conservation asserts that diversity begets diversity. Many positive and even negative species interactions have been both posited and demonstrated to contribute to such a relationship. However, some individual species, especially herbivores, can suppress plant species abundance. Here we take advantage of a multi-layered exclosure experiment to examine the separate and combined effects of three different guilds of wild and domestic large mammalian herbivores on woody plant abundance and diversity. We also conduct a meta-analysis to ask how diversity may beget diversity in the context of trophic relationships and interactions.
Results/Conclusions
After 16 years, the exclusion of all three guilds (mega-herbivores, other wild ungulates, and cattle) nearly tripled the species richness of woody plants. Each of the guilds contributed to the suppression of woody species, and species richness and diversity declined monotonically with the number of herbivores guilds present. The woody species had multiple patterns of response to release from herbivory. Woody plant diversity across the exclosure plots was positively correlated with previously measured bird diversity, suggesting a positive diversity cascade back up the food chain that extended the diversity suppression by large herbivores to additional trophic levels. Many of the woody species that appeared in the exclosure plots did not occur elsewhere in the vicinity of the experiment, strongly suggesting that considerable long distance dispersal occurs, but remains cryptic because of the local failure of seedlings to survive herbivory. These results are part of a broader emerging pattern in which increased species diversity on one trophic level may increase species diversity at the level(s) above it, but suppress species diversity at the level directly below it. Our meta-analysis shows that this is a pattern broadly applicable to other ecosystems.