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Ephemeral pools occur worldwide, provide habitat for organisms with a variety of life history strategies, and may have served as evolutionary refugia for some taxa since Mesozoic times . Yet, our understanding of the ecology and evolutionary history of ephemeral pool communities is hampered by a paucity of such basic data as the species composition of pool assemblages . We surveyed 58 vernal (ephemeral spring-time) pools from 14 sites in northern California for crustaceans, and found diverse assemblages composed largely of endemic and rare species . Sixty-seven species of crustaceans were found, and as many as 30 of these may be new, undescribed species . Differences in species composition among pools correspond with physical and chemical aspects of the habitat (depth, solutes concentration, elevation, biogeographic region), and with existing geologic/floristic-based habitat descriptions . Species richness is positively correlated with both depth and surface area . This relationship can be explained in terms of hydroperiod (accommodation of species with slower developmental rates in long-lived pools, greater time for temporal resource partitioning) and size (spatial habitat heterogeneity) . High species richness and numerous co-occurrences of congeneric species in temporary pools may be due to super-abundant resources, low levels of predation, and annual truncation of the community which prevents ecological interactions from going to completion . The results of this survey underscore the need for conservation of the vernal pool habitat and endemic vernal pool species in California . The best preservation strategy will include many pools at each site, multiple sites of each habitat type, and all identified habitat types .

Abstract. The fitness consequences of hybridization critically affect the speciation process. When hybridization is costly, selection favors the evolution of prezygotic isolating mechanisms (e.g., mating behaviors) that reduce heterospecific matings and, consequently, enhance reproductive isolation between species (a process termed reinforcement). If, however, selection to avoid hybridization differs between species, reinforcement may be impeded. Here, we examined both the frequency and fitness effects of hybridization between plains spadefoot toads (Spea bombifrons) and New Mexico spadefoot toads (S. multiplicata). Hybridization was most frequent in smaller breeding ponds that tend to be ephemeral, and heterospecific pairs consisted almost entirely of S. bombifrons females and S. multiplicata males. Moreover, in controlled experimental crosses, hybrid offspring from crosses in which S. multiplicata was maternal had significantly lower survival and longer development time than pure S. multiplicata offspring. By contrast, hybrid offspring from crosses in which S. bombifrons was maternal outperformed pure S. bombifrons offspring by reaching metamorphosis faster. These data suggest that, although S. multiplicata females are under selection to avoid hybridization, selection might favor those S. bombifrons females that hybridize with S. multiplicata if their breeding pond is highly ephemeral. Generally, the strength of selection to avoid hybridization may differ for hybridizing species, possibly impeding reinforcement. Speciation by reinforcement is a potentially important means by which populations become reproductively isolated. When hybridization by two sympatric species produces offspring that have lower fitness than pure-species types, selection can lead to the evolution of premating isolating barriers (e.g., mating behaviors) that reduce hybridization

Abstract.— The fitness consequences of hybridization critically affect the speciation process. When hybridization is costly, selection favors the evolution of prezygotic isolating mechanisms (e.g., mating behaviors) that reduce heter‐ospecific matings and, consequently, enhance reproductive isolation between species (a process termed reinforcement). If, however, selection to avoid hybridization differs between species, reinforcement may be impeded. Here, we examined both the frequency and fitness effects of hybridization between plains spadefoot toads (Spea bombifrons) and New Mexico spadefoot toads (S. multiplicata). Hybridization was most frequent in smaller breeding ponds that tend to be ephemeral, and heterospecific pairs consisted almost entirely of S. bombifrons females and S. multiplicata males. Moreover, in controlled experimental crosses, hybrid offspring from crosses in which S. multiplicata was maternal had significantly lower survival and longer development time than pure S. multiplicata offspring. By contrast, hybrid offspring from crosses in which S. bombifrons was maternal outperformed pure S. bombifrons offspring by reaching metamorphosis faster. These data suggest that, although S. multiplicata females are under selection to avoid hybridization, selection might favor those S. bombifrons females that hybridize with S. multiplicata if their breeding pond is highly ephemeral. Generally, the strength of selection to avoid hybridization may differ for hybridizing species, possibly impeding reinforcement.

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