Genetic influences on experimental population dynamics of the least killifish

When natural populations differ in density or in the dynamic fluctuations o f population size, some of those differences may result from their differen t ecological conditions, and some may originate from genetically based diff erences in life history expression. Natural populations of the live-bearing poeciliid fish, Heterandria formosa, vary considerably in their population dynamics, with densities that differ between populations by as much as sev enfold. This system offers an excellent opportunity to explore the potentia l role of genetically based differences in life history expression in creat ing different dynamic patterns in a common environment. We created five dif ferent genetic stocks of H. formosa by carrying out a series of crosses usi ng fish from two North Florida populations (the Wacissa River and Trout Pon d) and used them to initiate replicate experimental populations in artifici al ponds. The five stocks consisted of two "controls," which were pure Waci ssa River and Trout Pond stocks, and three types of hybrid stocks. The hybr id stocks differed in a regular way in the proportion of genes from one pop ulation or the other. The crossing scheme was designed so that each hybrid stock would have the same proportion of heterozygous (or "heterodemic") loc i but would differ in the proportion and/or identity of homozygous (or "hom odemic") loci from the Wacissa River and Trout Pond populations. These popu lations were chosen because a previous study had found that population dens ities in the Wacissa River greatly exceeded those of Trout Pond and exhibit ed a higher range of population fluctuation during the breeding season. We addressed four questions in this experiment: (1) Are there genetically base d differences in life history traits of fish from the two populations? (2) If so, do differences in life history expression produce differences in pop ulation dynamics in a common environment? (3) Which traits have the greates t influence on population dynamics? (4) How do changes in density affect th e phenotypes of individual traits that govern the rates of birth and death in a population? We followed experimental populations of the five genetic s tocks from their initiation at low density through 4-6 generations of popul ation growth and decline. The mean offspring size differed among stocks by as much as 50%. At low densities, offspring size exhibited a trade-off with brood size: Trout Pond alleles were associated with more, smaller offsprin g. At higher densities, offspring sizes were similar among stocks, and the trade-off with offspring number was not evident. Stocks differed in realize d population growth rate by as much as 70%; the rank order differences amon g stocks with respect to population growth rate appeared to match the genet ic relatedness among stocks based on the expected percentage of Trout Pond alleles. Differences in population growth rate appeared to be due to differ ences in brood size among stocks at low density. Stocks did not differ in t he equilibrium population size, which indicated the absence of a trade-off between population growth rate and carrying capacity in this environment. A dult survival and recruitment of juveniles into the adult population both d eclined linearly with increasing density; and stocks did not generally diff er in those rates after the effects of density had been taken into account. The stocks differed in their response to the depressant effects of density on life history trait expression. The offspring size of the pure Wacissa Ri ver stock was much more sensitive to density than was the offspring size of the pure Trout Pond stock. However, the brood sizes of the Wacissa River s tock were reduced much less than those of the Trout Pond stock when exposed to the same high density. These results suggest that life history distinct ions among populations, both in the mean values and plasticity of traits, p lay a role in creating different dynamics. We discuss the ways in which phe notypic plasticity in reproductive traits potentially acts as a mechanism t o stabilize population dynamics in this species.