Species richness and composition: The varying importance of plant colonization along resource gradients

Ecologists have generally sought to explain patterns of species diversity by determining how habitat conditions affect "local" competitive species interactions and extinction. However, it is increasingly recognized that the number of available species in the regional pool can also have large effects on species diversity. In this dissertation I examine colonization from the available species pool under numerous habitat conditions, and I seek to understand how the pool of available species interacts with habitat conditions to affect species diversity.; My dissertation consists of three northeast Kansas manipulative field experiments with the following objectives: (1) to determine how cool-season grassland plant species diversity is affected by productivity levels, disturbance, and the number of available species in the regional pool; (2) to isolate the role of light limitation in the observed decrease in plant species richness and colonization at high productivity; and (3) to determine the importance of C4 grass seed sowing density on the establishment of forb species and conservative species in a Conservation Reserve Program prairie restoration begun on bare soil.; The results from my first chapter show that almost no species in the available pool are able to colonize into cool-season grassland habitat of high productivity and minimal disturbance, which suggests local competitive species interactions largely control species diversity under these habitat conditions. Many species in the available pool were able to colonize into habitat of moderate productivity and moderate disturbance, suggesting that the size of the available species pool primarily controls species diversity under these habitat conditions. In my second chapter I found that species richness and colonization decline at high productivity largely independent of measured light levels, and that the role of light limitation changes depending on drought conditions. The results of my third chapter show that high grass seed sowing densities decreased the establishment of forbs and conservative species.; In aggregate, the results presented in this dissertation show that the relative importance of colonization from the available species pool and local competitive processes on species diversity can change along habitat resource gradients. I discuss the general significance of these findings in regards to current ecological theory.