As humans alter the environmental landscape, ecosystems become increasingly imperiled due to habitat alteration and the associated species extinctions and extirpations. Consequently, recent research has often focused on how human altered landscape processes influence the distribution of species and the structure of biological communities. Further, recent research has also addressed how biodiversity itself is an important cog in the performance of ecosystem processes by biological communities. While both fields of research have yielded many important insights, they have both been limited by their scope. For example, research into how landscape processes influences species distributions and overall biodiversity often fail to recognize that biodiversity itself has the potential to feedback and influence landscape processes. Further, research into how biodiversity influences ecosystem function are often conducted on trophic processes within a single habitat, and fail to acknowledge that biodiversity might affect the physical transport of materials and resources across landscapes. My dissertation research aims to merge these areas of research to better integrate biodiversity into landscape ecosystem processes.

In my first chapter, I examine how high flow events in rivers, an important landscape process modified by humans via regulated releases by dams, influences the biodiversity of mussel communities. I sampled mussels and measured sediment and hydraulic variables (at low and high flows) at sites on the Little River, Oklahoma. To test which variables were most limiting mussel species richness, I evaluated univariate and multivariate 95th-, 90th-, and 85th-quantile regression models using an information theoretic model selection approach. I found that models using hydraulic variables related to substrate stability at high flows most limited mussel species richness. Models using the same variables estimated at low flows performed poorly. These results demonstrate that substrate stability at high flows is an important factor governing mussel distributions.

In my second chapter, I take the inverse view of my first chapter to test how mussel biodiversity itself influences substrate stability. I conducted a flume experiment, manipulating mussel species richness in a classic "biodiversity and ecosystem-function" design. Using three mussel species, I measured how species by themselves ("monocultures") influenced erosion of the streambed, and compared their performance to those of species mixtures ("polycultures" of 2 and 3 species). Mussel species vary in traits that should modify their effects on substrate erosion, such as shell morphology and burrowing behavior. Further, I crossed these mussel species treatments with two density treatments (high and low). I found that mussel species richness was associated with an increase in gravel erosion at both low and high densities. Planned contrasts showed that the erosion observed in species mixtures was purely additive at low density, as erosion in a polyculture could be routinely predicted by monoculture performance. However, at high density certain combinations of species showed non-additive effects on erosion, suggesting that organism abundance can fundamentally alter biodiversity effects. Further, this experiment shows that biodiversity can modify the physical transport of materials across landscapes.

In my third chapter, I investigate how mussel biodiversity can increase the flow of resources from aquatic to terrestrial ecosystems via a complex trophic cascade. Mussel biodiversity increases algae production in streams, which is followed by increases in abundance of grazing aquatic insect larvae. Because aquatic insects are an important prey subsidy to terrestrial predators, I conducted experiments to see if mussel biodiversity increases the flux of aquatic insect prey subsidies to terrestrial predators. In a mesocosm experiment I found that mussel species richness was associated with an increase in algae production rates, aquatic insect emergence rates, and spider standing crop biomass. Effects of mussel polycultures on algae production could be predicted additively from monocultures, and mussel effects were linked through stable isotope analyses to mussel-derived nitrogen subsidies. In contrast, certain mussel species mixtures had non-additive effects on insect emergence. Mussel polycultures were associated with a more evenly distributed algae community than mussel monocultures, and aquatic insect emergence rates were higher in these more mixed algal assemblages. Finally spider standing crop biomass weakly tracked increases in aquatic insect emergence. In a field study of mussel communities on 2 rivers we found that sites with greater mussel species richness had higher aquatic insect emergence rates. These results show that because food webs in adjacent ecosystems are linked, effects of biodiversity losses on ecosystem functioning are not limited to the ecosystem in which extinctions occur.