Demography Predicts Genetic Effective Size in a Desert Stream Fish Community.

AbstractPatterns of genetic diversity and effective size should be predicted by life history traits (intrinsic), landscape properties (extrinsic), and population dynamics. Theoretical models portray complicated relationships among population subdivision, rates of extirpation and recolonization, and metapopulation genetic effective size (metaNe) but make simplifying assumptions about effects of intrinsic and extrinsic factors. Using previously published data sets, we compared estimates of genetic effective size to demographic time-series data gathered for nine dominant species in a desert stream fish community. These species occupy a common desert stream network and experience the same disturbance regime but differ in abundance, distribution, and life history traits that should affect reproduction, dispersal, local persistence, and genetic diversity patterns. Measures of genetic effective size were positively related to network-wide abundance and mean adult density across species and were negatively related to extirpation probability. Comparative data supported the theoretical prediction that population subdivision decreases metapopulation genetic effective size relative to panmictic populations of the same size. Estimates of metaNe reflected differences in intrinsic traits and population dynamics across species measured over ecological timescales. This comparative study exemplifies why ecological differences are important considerations when seeking to preserve genetic diversity.

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.

Natural flow regimes, nonnative fishes, and native fish persistence in arid-land river systems.

Escalating demands for water have led to substantial modifications of river systems in arid regions, which coupled with the widespread invasion of nonnative organisms, have increased the vulnerability of native aquatic species to extirpation. Whereas a number of studies have evaluated the role of modified flow regimes and nonnative species on native aquatic assemblages, few have been conducted where the compounding effects of modified flow regimes and established nonnatives do not confound interpretations, particularly at spatial and temporal scales that are relevant to conservation of species at a range-wide level. By evaluating a 19-year data set across six sites in the relatively unaltered upper Gila River basin, New Mexico, USA, we tested how natural flow regimes and presence of nonnative species affected long-term stability of native fish assemblages. Overall, we found that native fish density was greatest during a wet period at the beginning of our study and declined during a dry period near the end of the study. Nonnative fishes, particularly predators, generally responded in opposite directions to these climatic cycles. Our data suggested that chronic presence of nonnative fishes, coupled with naturally low flows reduced abundance of individual species and compromised persistence of native fish assemblages. We also found that a natural flow regime alone was unlikely to ensure persistence of native fish assemblages. Rather, active management that maintains natural flow regimes while concurrently suppressing or excluding nonnative fishes from remaining native fish strongholds is critical to conservation of native fish assemblages in a system, such as the upper Gila River drainage, with comparatively little anthropogenic modification.