Novel resource inputs represent an increasingly common phenomenon in ecological systems as global change alters environmental factors and species distributions. In semiarid riparian areas, hydric pioneer tree species are being replaced by drought-tolerant species as water availability decreases. Additionally, introduced omnivorous crayfish, which feed upon primary producers, allochthonous detritus, and benthic invertebrates, can impact communities at multiple levels through both direct and indirect effects. In arid and semiarid systems of the American Southwest, crayfish may be especially important as detrital processors due to the lack of specialized detritivores. I tested the impact of virile crayfish (Orconectes virilis) on benthic invertebrates and detrital resources across a gradient of riparian vegetation drought-tolerance using field cages with leaf litter bags in the San Pedro River in Southeastern Arizona. Virile crayfish increased breakdown rate of drought-tolerant saltcedar (Tamarix ramosissima), but did not impact breakdown of Fremont cottonwood (Populus fremontii), Gooding's willow (Salix goodingii), or seepwillow (Baccharis salicifolia). The density and composition of the invertebrate community colonizing leaf litter bags were both heavily influenced by litter species but not directly by crayfish presence. As drought-tolerant species become more abundant in riparian zones, their litter will become a larger component of the organic matter budget of desert streams. By increasing breakdown rates of saltcedar, crayfish shift the composition of leaf litter in streams, which in turn may affect the composition and biomass of colonizing invertebrate communities. More research is needed to determine the full extent to which these alterations change community composition over time.

Infectious diseases have emerged as a significant threat to wildlife. Environmental change is often implicated as an underlying factor driving this emergence. With this recent rise in disease emergence and the acceleration of environmental change, it is important to identify the environmental factors that alter host-pathogen dynamics and their underlying mechanisms. The emerging pathogen Batrachochytrium dendrobatidis (Bd) is a clear example of the negative effects infectious diseases can have on wildlife. Bd is linked to global declines in amphibian diversity and abundance. However, there is considerable variation in population-level responses to Bd, with some hosts experiencing marked declines while others persist. Environmental factors may play a role in this variation. This research used populations of pond-breeding chorus frogs (Pseudacris maculata) in Arizona to test if three rapidly changing environmental factors nitrogen (N), phosphorus (P), and temperature influence the presence, prevalence, and severity of Bd infections. I evaluated the reliability of a new technique for detecting Bd in water samples and combined this technique with animal sampling to monitor Bd in wild chorus frogs. Monitoring from 20 frog populations found high Bd presence and prevalence during breeding. A laboratory experiment found 85% adult mortality as a result of Bd infection; however, estimated chorus frog densities in wild populations increased significantly over two years of sampling despite high Bd prevalence. Presence, prevalence, and severity of Bd infections were not correlated with aqueous concentrations of N or P. There was, however, support for an annual temperature-induced reduction in Bd prevalence in newly metamorphosed larvae. A simple mathematical model suggests that this annual temperature-induced reduction of Bd infections in larvae in combination with rapid host maturation may help chorus frog populations persist despite high adult mortality. These results demonstrate that Bd can persist across a wide range of environmental conditions, providing little support for the influence of N and P on Bd dynamics, and show that water temperature may play an important role in altering Bd dynamics, enabling chorus frogs to persist with this pathogen. These findings demonstrate the importance of environmental context and host life history for the outcome of host-pathogen interactions.