Environmental and Spatial Regulators of Fish Assemblage Structure and Population Size in Semi-Arid Riverscapes

Abstract

Anthropogenically-driven climate change combined with existing ecosystem degradation is projected to cause future losses of global freshwater biodiversity, particularly in arid areas within temperate climate regions. Efficient fisheries management and conservation strategies have been hindered by lack of a greater, broad-scale inclusion of freshwater ecosystems and their response to climate change. Our current understanding of when and how climatic disruptions to biodiversity patterns will occur focuses on one or a few species to represent a snapshot of shifting environments— rarely entire assemblages across broad landscapes. Furthermore, the use of a multi-scaled, “continuous view” of the riverscape is essential when utilizing space-for-time substitutions to provide a comprehensive context for ecological processes. Here, I have presented a transferrable framework that provides monitoring and conservation guidance that can be used to delineate highly suitable or ecologically important habitat such as cool-water refugia in a changing climate and to prioritize individual species, populations, and assemblages most vulnerable to abrupt climatic disturbances. At the assemblage scale, I focused on fish assemblages in the Colorado River basin of central Texas. My results suggest that freshwater fish assemblages are vulnerable to future climate change and projections can be effectively developed utilizing an existing aridity spatial gradient. Shifts towards warm-water, invasive assemblages and the potential loss of endemic, spring-dependent species are expected to occur under both mild and high emissions scenarios. At the population-scale, I highlighted the Conchos pupfish (Cyprinodon eximius) in the Devils River of southwestern Texas. Results provide empirical evidence for previous qualitative descriptions of Conchos pupfish abundance, distribution, and range. Conchos pupfish persist at habitat “margins” across multiple spatial scales, with highest abundance along shallow, non-flowing transects with abundant flocculent deposition and algae growth, particularly in areas with high hydrologic variability. Distribution modelling for the species predicted highest occurrences in areas of dry wintertime conditions and intermediate minimum air temperatures within their previously described range. By analyzing changes in both assemblage structure and population abundance and distribution, I demonstrated how to identify vulnerable habitats and fishes at multiple scales to prioritize future fisheries management and conservation strategies.