Tyler J. Pilger

Consecutive wildfires affect stream biota in cold- and warmwater dryland river networks

Climate change and fire suppression have altered fire regimes globally, leading to larger, more frequent, and more severe wildfires. Responses of coldwater stream biota to single wildfires are well studied, but measured responses to consecutive wildfires in warmwater systems that often include mixed assemblages of native and nonnative taxa are lacking. We quantified changes in physical habitat, resource availability, and biomass of cold- and warmwater oligochaetes, insects, crayfish, fishes, and tadpoles following consecutive megafires (covering >100 km2) in the upper Gila River, New Mexico, USA. We were particularly interested in comparing responses of native and nonnative fishes that might have evolved under different disturbance regimes. Changes in habitat and resource availability were related to cumulative fire effects, fire size, and postfire precipitation. The 2nd of 2 consecutive wildfires in the basin was larger and, coupled with moderate postfire discharge, resulted in increased siltation and decreased algal biomass. Several insect taxa responded to these fires with reduced biomass, whereas oligochaete biomass was unaffected. Biomass of 6 of 7 native fish species decreased after the fires, and decreases were associated with site proximity to fire. Nonnative fish decreases after fire were most pronounced for coldwater salmonids, and warmwater nonnative fishes exhibited limited responses. All crayfish and tadpoles collected were nonnative and were unresponsive to fire disturbance. More pronounced responses of native insects and fishes to fires indicate that increasing fire size and frequency threatens the persistence of native fauna and suggests that management activities promoting ecosystem resilience might help ameliorate wildfire effects.

River network architecture, genetic effective size, and distributional patterns predict differences in genetic structure across species in a dryland stream fish community

Dendritic ecological network (DEN) architecture can be a strong predictor of spatial genetic patterns in theoretical and simulation studies. Yet, interspecific differences in dispersal capabilities and distribution within the network may equally affect species’ genetic structuring. We characterized patterns of genetic variation from up to ten microsatellite loci for nine numerically dominant members of the upper Gila River fish community, New Mexico, USA. Using comparative landscape genetics, we evaluated the role of network architecture for structuring populations within species (pairwise FST) while explicitly accounting for intraspecific demographic influences on effective population size (Ne). Five species exhibited patterns of connectivity and/or genetic diversity gradients that were predicted by network structure. These species were generally considered to be small‐bodied or habitat specialists. Spatial variation of Ne was a strong predictor of pairwise FST for two species, suggesting patterns of connectivity may also be influenced by genetic drift independent of network properties. Finally, two study species exhibited genetic patterns that were unexplained by network properties and appeared to be related to nonequilibrium processes. Properties of DENs shape community‐wide genetic structure but effects are modified by intrinsic traits and nonequilibrium processes. Further theoretical development of the DEN framework should account for such cases.

Spatio-temporal variation in parasite communities maintains diversity at the major histocompatibility complex class IIβ in the endangered Rio Grande Silvery Minnow

Climate change will strongly impact aquatic ecosystems particularly in arid and semi‐arid regions. Fish–parasite interactions will also be affected by predicted altered flow and temperature regimes, and other environmental stressors. Hence, identifying environmental and genetic factors associated with maintaining diversity at immune genes is critical for understanding species’ adaptive capacity. Here, we combine genetic (MHC class IIβ and microsatellites), parasitological and ecological data to explore the relationship between these factors in the remnant wild Rio Grande silvery minnow (Hybognathus amarus) population, an endangered species found in the southwestern United States. Infections with multiple parasites on the gills were observed and there was spatio‐temporal variation in parasite communities and patterns of infection among individuals. Despite its highly endangered status and chronically low genetic effective size, Rio Grande silvery minnow had high allelic diversity at MHC class IIβ with more alleles recognized at the presumptive DAB1 locus compared to the DAB3 locus. We identified significant associations between specific parasites and MHC alleles against a backdrop of generalist parasite prevalence. We also found that individuals with higher individual neutral heterozygosity and higher amino acid divergence between MHC alleles had lower parasite abundance and diversity. Taken together, these results suggest a role for fluctuating selection imposed by spatio‐temporal variation in pathogen communities and divergent allele advantage in maintenance of high MHC polymorphism. Understanding the complex interaction of habitat, pathogens and immunity in protected species will require integrated experimental, genetic and field studies.

Microsatellite markers for Longfin Dace, Agosia chrysogaster, a sentinel fish species in imperiled arid-land rivers of the Sonoran Desert

We isolated and characterized 16 microsatellite DNA loci in Longfin Dace, Agosia chrysogaster, a minnow native to Sonoran Desert streams (southwestern US and northwestern Mexico). After optimization, all primer pairs produced consistently scorable products. Genetic diversity metrics were determined for each locus using 50 individuals from two populations in the Gila River basin, New Mexico. Allelic richness ranged from 2 to 37 and observed heterozygosity ranged from 0.08 to 0.95 across loci. These microsatellites offer a powerful tool to study effects of habitat fragmentation, dewatering, and climate change on population connectivity and genetic diversity in this species. Moreover, Longfin Dace co-occurs with more geographically restricted and endangered desert fish species. Genetic information for Longfin Dace could provide an important comparative dataset to assist conservation and management of other imperiled fishes in the region.

Complex patterns of genetic and phenotypic divergence in populations of the Lake Malawi cichlid Maylandia zebra

Ecological speciation, which relies heavily on selection driving the emergence of new species, has become the primary paradigm through which rapid species radiations are understood. In this way, selection, particularly ecological selection, is assumed to be the driver of most species radiations. However, in many radiations, such as the radiation of Lake Malawi’s cichlids, the assumption of selection as the driver of speciation has rarely been explicitly tested, and drift, often, has completely been ignored as potential factor. In order to understand the forces driving the divergence of Lake Malawi cichlids at the microevolutionary level, we studied the genetic and phenotypic divergence of ten allopatric populations of Maylandia zebra. We estimated effective population sizes as proxy for drift. Further, we compared neutral genetic differentiation to divergence in three phenotypic traits: body size, body shape, and melanophore counts. We found small, yet significant, population differentiation in all the studied traits across most populations. Population sizes were small rendering the potential for drift to be high. However, phenotypic differentiation exceeded neutral expectations for all traits suggesting divergent local selection. Our data suggest that natural, and potentially also sexual, selection may be the dominant force driving population differentiation in Lake Malawi’s rock-dwelling cichlids, despite the potential for drift in small populations.

Variation in Unionid Assemblages between Streams and a Reservoir within the Kansas River Basin

Abstract North American freshwater mussels of the Order Unionoida are critically imperiled, primarily due to stream habitat modifications and fragmentation by reservoirs. Whereas many species respond negatively to impoundments, some species benefit by increases in lentic habitat. During winter drawdown of Tuttle Creek Reservoir, KS in 2006–2007, we collected freshwater mussel shells to characterize spatial variation in assemblage structure within the reservoir and compare reservoir assemblages to stream assemblages within the surrounding drainage basin. Of the 22 unionid species that occurred in the basin, six were found in Tuttle Creek Reservoir. Species richness in the reservoir did not differ from that found in both small and large streams. Species composition in streams varied along a gradient from small to large (1st-7th) order streams, and mussel assemblages in the reservoir were most similar to that of large order streams. This study identified the subset of stream-dwelling unionid species that are habitat generalists and capable of persisting in reservoirs.