On the determination of local species richness and abundance

A goal of community ecology is to understand the processes controlling local species richness and abundance. I used two experimental systems to examine this: a moss-microarthropod system and the larval insect community of a water-filled bromeliad tank. In chapter 2, I test the effects of local disturbance frequency and undisturbed habitat loss on local species richness of moss microarthropods. Diversity declined linearly with disturbance rate, and the decline depended on the surrounding region: it was strongest in local patches disconnected from the region. Fragmentation resulted in loss of system resilience, with a threshold effect in the local patches of the smallest, unconnected region size. All treatments had similar gamma diversity: reduced alpha diversity was offset by increased beta diversity. Dispersal from undisturbed habitat maintains local richness and abundance, though this can be overridden by decreased region size below a threshold level, as well as fragmentation, and edge effects. In Chapter 3, I examined the impact of the number of species present in regions on the community in local patches. Over 16 months, local richness was independent of regional richness. The number of local species varied with time, due to competition and dispersal from the region, but most strongly because of seasonality. Even when local richness did not change, community composition did. Thus, regional composition was more important than regional species richness, and the imprint of biogeography (the composition of regional species pools), is only seasonally evident. Chapter 4 examined positive and negative interactions in determining species’ abundances in bromeliad phytotelmata. The top predator reduces the abundance of detritivorous insect larvae, but an intermediate predator has little effect. Without the top predator, a processing chain takes place between several detritivore species, and these positive interactions increase the emergence rate of chironomids at the end of the chain. The mechanism of facilitation is likely the processing, by tipulids and scirtids, of detritus into small particles which the detritivore chironomids can feed on. This also increases the surface:volume ratio of detritus, and may indirectly benefit chironomids by increasing their food, bacteria, algae, and fungi.Science, Faculty ofZoology, Department ofGraduat