The consequences of resource heterogeneity for stream ecosystems

Stream ecosystems occupy topographic low points in the landscape, and thus integrate the land-use processes occurring in the watershed. Detritus, in the form of senesced leaf litter, comprises the major energy source for stream food webs, defining a critical ecological link between terrestrial and aquatic environments. While there exists extensive work on the impact loss of deciduous riparian vegetation has on in-stream food webs, the impact of species loss of this vegetation has received less study. This is despite the effects disease, invasive herbivores and heterogeneous deforestation has on the distribution of tree species along stream margins. In this dissertation I show that while the loss of species not always translates into altered consumer-resource dynamics in stream ecosystems, loss of key tree species does alter the patterns of in-stream decomposition of leaf litter, detritivore secondary production and feeding patterns, and the temporal stability of consumers. A combination of fieldwork, laboratory experiments and computer simulations was used to establish that species-specific properties translate into altered palatability for stream detritivores, altering the growth and feeding activity of these organisms when leaf species are present in mixtures. This translates into slower rates of leaf decomposition, which is partially explained by preferential feeding by these detritivores. That is, refractory leaf species are consumed at a faster rate compared to when other leaf species are available for consumption in mixtures. Computer simulations of consumer colonization dynamics, at varying levels of leaf species richness of 11 common forest communities, revealed that mixing leaf species also alters the temporal stability of consumer abundance. Comparison of this result with field observations highlighted some discrepancies, suggesting the interaction between stream detritivores and their resources is complex, and may depend on other food web components (e.g., predators). The conclusion from my research is that resource heterogeneity, in the form of leaf species richness, has implications for stream detritivore dynamics and organic matter decomposition. Given that leaf species richness reflects the species richness of deciduous vegetation occupying stream margins, community structure of riparian plant communities, and not merely the presence of trees, alters the magnitude of in-stream ecosystem processes.