Causes and consequences of dominance and diversity in grasslands

Understanding how species coexist and differ in abundance is central to ecology. Theory predicts competitively superior species should dominate systems and suppress diversity, yet many natural communities characterized by dominants are species rich. Understanding how shifts in dominance among species, and the inherent diversity of communities affect ecosystem function is a second central theme in ecology. For example, decreasing the number of species in local communities can reduce the ability of the community to respond to disturbance. These ideas have captured ecologists\u27 attention because anthropogenic pressures are causing many systems to lose species. In my dissertation I focused on processes that determine dominance and diversity and the consequences of changes in dominance and diversity on the ability of communities to respond to exotic invasion and disturbance. In chapter 1 I asked: does competitive ability correspond with large differences in plant species abundance found in field surveys of grasslands in western Montana? In a garden experiment I found that intraspecific competition appears to promote coexistence, but differences in abundance were not related to inherent interspecific competitive abilities in common garden experiments. In chapter 2 I asked: how important is competition relative to other mechanisms of coexistence, such as dispersal limitation and seed predation? Experimental manipulations of natural grassland communities showed that dispersal limitation caused a greater constraint on local diversity than competition from a single dominant species or from several common species. Seed predation, in contrast, did not influence diversity two years after the treatments were applied. In chapter 3 I asked: does invader impact on natives vary with disturbance and diversity? Burning a subset of experimentally invaded assemblages showed that the buffering effect of diversity on invader impact was lost after a form of disturbance that is commonly experienced by the native system. An important historical focus in plant ecology was on how species could coexist despite competition for limiting and shared resources, and in this context I found that competition did not correspond well with the ability of species to coexist or with the relative abundance of species. I also found that increasing native species diversity did not buffer the relative responses of natives and invaders to disturbance. Overall I found that by studying ecological processes together instead of in isolation we can gain a better understanding of the complexity of ecosystems