Successful recovery and sustainability of threatened and exploited species depends in part on retention and maintenance of genetic diversity. Theory indicates that genetic diversity is lost at a rate inversely proportional to the genetically effective population size (N e ), which is roughly equal to one-half the adult census size (N ) in many organisms. However, N e has been reported to be up to five orders of magnitude lower than N in species with life histories that result in type III survivorship (high fecundity, but heavy mortality in early life stages, e.g. bony fishes), prompting speculation that low values of N e may be a general feature of such organisms despite sometimes vast abundances. Here, we compared N e and the ratio N e /N across three ecologically similar fish species from the arid southwestern United States, all with type III life histories but with differing expectations of egg and larval survivorship that correlate with the degree of human-imposed habitat fragmentation. Our study indicates that type III life history may be necessary, but this alone is insufficient to account for extraordinarily low values of N e /N. Rather, life history interacts with environmentally imposed mortality to determine the rate and magnitude of change in genetic diversity in these desert fish species.
SynopsisA sweepstakes-mismatch process whereby reproduction is poorly coordinated with appropriate resources for larval development and recruitment can result in large variance in reproductive success among individuals and spawning aggregations. This process has been proposed to explain low ratio of genetic effective population size (N e ) to adult census size (N) ratios in marine species with high fecundity, pelagic spawning, and extensive mortality in early life stages. This process is also hypothesized to also account for very low N e /N ([ 0.001) observed in the federally endangered Rio Grande silvery minnow, Hybognathus amarus. This species is a freshwater fish that shares life-history features with marine pelagic spawners. We tested two key predictions of the sweepstakes-mismatch hypothesis using molecular data: (i) that temporally distinct samples of eggs differ in genetic composition and, (ii) that egg samples do not comprise a random subset of potential adult breeders. We present genetic data that supports both predictions and that are consistent with the hypothesis that high variance in reproductive success among adult breeders is an important factor that lowers N e /N in H. amarus. This study highlights the importance of understanding the interaction of early life history and fragmentation in devising conservation plans for endangered aquatic organisms.
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