River and riparian areas are important foraging habitat for insectivorous bats. Numerous studies have shown that aquatic insects provide an important trophic resource to terrestrial consumers, including bats, and are key in regulating population size and species interactions in terrestrial food webs. Yet these studies have generally ignored how structural characteristics of the riverine landscape influence trophic resource availability or how terrestrial consumers respond to ensuing spatial and temporal patterns of trophic resources. Moreover, few studies have examined linkages between a stream's hydrologic regime and the timing and magnitude of aquatic insect availability. The main objective of my dissertation is to understand the causes of bat distributions in space and time. Specifically, I examine how trophic resource availability, structural components of riverine landscapes (channel confinement and riparian vegetation structure), and hydrologic regimes (flow permanence and timing of floods) mediate spatial and temporal patterns in bat activity. First, I show that river channel confinement determines bat activity along a river's longitudinal axis (directly above the river), while trophic resources appear to have stronger effects across a river's lateral (with distance from the river) axis. Second, I show that flow intermittency affects bat foraging activity indirectly via its effects on trophic resource availability. Seasonal river drying appears to have complex effects on bat foraging activity, initially causing imperfect tracking by consumers of localized concentrations of resources but later resulting in disappearance of both insects and bats after complete river drying. Third, I show that resource tracking by bats varies among streams with contrasting patterns of trophic resource availability and this variation appears to be in response to differences in the timing of aquatic insect emergence, duration and magnitude of emergence, and adult body size of emergent aquatic insects. Finally, I show that aquatic insects directly influence bat activity along a desert stream and that riparian vegetation composition affects bat activity, but only indirectly, via effects on aquatic insect availability. Overall, my results show river channel confinement, riparian vegetation structure, flow permanence, and the timing of floods influence spatial and temporal patterns in bat distributions; but these effects are indirect by influencing the ability of bats to track trophic resources in space and time.

Non-native consumers can significantly alter processes at the population, community, and ecosystem level, and they are a major concern in many aquatic systems. Although the community-level effects of non-native anuran tadpoles are well understood, their ecosystem-level effects have been less studied. Here, I tested the hypothesis that natural densities of non-native bullfrog tadpoles (Lithobates catesbeianus) and native Woodhouse's toad tadpoles (Anaxyrus woodhousii) have dissimilar effects on aquatic ecosystem processes because of differences in grazing and nutrient recycling (excretion and egestion). I measured bullfrog and Woodhouse's carbon, nitrogen, and phosphorus nutrient recycling rates. Then, I determined the impact of tadpole grazing on periphyton biomass (chlorophyll a) during a 39-day mesocosm experiment. Using the same experiment, I also quantified the effect of tadpole grazing and nutrient excretion on periphyton net primary production (NPP). Lastly I measured how dissolved and particulate nutrient concentrations and respiration rates changed in the presence of the two tadpole species. Per unit biomass, I found that bullfrog and Woodhouse's tadpoles excreted nitrogen and phosphorus at similar rates, though Woodhouse's tadpoles egested more carbon, nitrogen, and phosphorus. However, bullfrogs recycled nutrients at higher N:C and N:P ratios. Tadpole excretion did not cause a detectable change in dissolved nutrient concentrations. However, the percent phosphorus in mesocosm detritus was significantly higher in both tadpole treatments, compared to a tadpole-free control. Neither tadpole species decreased periphyton biomass through grazing, although bullfrog nutrient excretion increased areal NPP. This result was due to higher biomass, not higher biomass-specific productivity. Woodhouse's tadpoles significantly decreased respiration in the mesocosm detritus, while bullfrog tadpoles had no effect. This research highlights functional differences between species by showing non-native bullfrog tadpoles and native Woodhouse's tadpoles may have different effects on arid, aquatic ecosystems. Specifically, it indicates bullfrog introductions may alter primary productivity and particulate nutrient dynamics.