Genetic population structure of the fairy shrimp Branchinecta coloradensis (Anostraca) in the Rocky Mountains of Colorado

Dispersal rates for freshwater invertebrates are often inferred from population genetic data. Although genetic approaches can indicate the amount of isolation in natural populations, departures from an equilibrium between drift and gene flow often lead to biased gene flow estimates. I investigated the genetic population structure of the pond-dwelling fairy shrimp Branchinecta coloradensis in the Rocky Mountains of Colorado, U.S.A., using allozymes. Glaciation in this area and the availability of direct dispersal estimates from previous work permit inferences regarding the relative impacts of history and contemporary gene flow on population structure. Hierarchical F statistics were used to quantify differentiation within and between valleys (thetaSV and thetaVT, respectively). Between valleys separated by 5-10 km, a high degree of differentiation (thetaVT = 0.77) corresponds to biologically reasonable gene flow estimates of 0.07 individuals per generation, although it is possible that this value represents founder effects and nonequilibrium conditions. On a local scale (<=110 m), populations are genetically similar (thetaSV = 0.13) and gene flow is estimated to be 1.7 individuals exchanged between ponds each generation. This is very close to an ecological estimate of dispersal for B. coloradensis via salamanders. Gene flow estimates from previous studies on other Anostraca are also similar on comparable geographic scales. Thus, population structure in B. coloradensis appears to be at or near equilibrium on a local scale, and possibly on a regional scale as well.

Seasonal movement patterns in a subalpine population of the tiger salamander, Ambystoma tigrinum nebulosum

Seasonal movements of a subalpine population of metamorphic tiger salamanders, Ambystoma tigrinum nebulosum, were explored from 1990 to 1992. Metamorphic adults bred in permanent and semipermanent habitats during June of each year. After breeding, soma individuals returned to the terrestrial environment, but many remained and congregated in nonpermanent ponds. Metamorphic adults in nonpermanent ponds included those that moved from permanent ponds, those that bred and remained in semipermanent ponds, and those that did not breed but migrated from the terrestrial environment after the breeding season. Dietary analyses indicated that metamorphic adults in nonpermanent ponds have significantly greater numbers, biomass, and calories of prey in their gut than the few metamorphic adults remaining in permanent ponds. This difference was due to the presence of fairy shrimp (Branchinecta coloradensis), which composed 91 % of metamorphic adult diets in nonpermanent ponds and accounted for three times the average caloric value of all prey in gut samples from metamorphic adults in permanent ponds. Intraspecific competition also may have contributed to movement into nonpermanent ponds: post-breeding densities of metamorphic adults in permanent ponds were inversely related to the densities of other morphs (paedo-morphic adults and large larvae). We conclude that metamorphic A. t. nebulosum in this population utilize nonpermanent ponds after the breeding season because of the abundance of high-quality aquatic prey and reduced competition from con-specific morphs.