Print PageThe traditional view of species as static units is being reexamined due to increasing evidence that ecological and evolutionary processes may overlap in time. Contemporary evolution can have ecological consequences if selection is shaping the morphologies of trophic traits (e.g. beak depth, gill raker spacing). However; selection can also act on life-history traits in ways that result in different development rates and densities achieved for the divergent phenotypes. Therefore, contemporary evolution can alter ecological processes through either 1) divergence in per capita performance, or 2) shifts in equilibrium density. To examine the ecological consequences of rapid ecotype evolution in the facultatively anadromous steelhead/rainbow trout (Oncorhynchus mykiss) we make use of a natural experiment where fish from a predominantly anadromous population were introduced above a barrier waterfall in 1910 and have subsequently evolved a resident life-history.
Results/Conclusions
We present results from field observation and common garden growth and development experiments which provide evidence that hard selection against anadromy in the above falls population has 1) resulted in lower propensity to migrate for a given growth rate relative to the below falls population, and 2) led to a 5:1 difference in density between the below falls and above falls populations. We test the ecological consequences of these two evolutionary responses by performing an in-stream mesocosm study, using F1 generation fish to neutralize maternal environment effects. In addition to no-fish treatments, mesocosms were stocked with either above or below falls origin fish at high and low densities. Community and ecosystem properties were measured over the two month study and results were analyzed by planned contrasts to assess the effects of ecology (presence/absence of fish) and evolution (phenotype, density, and their interaction). There was a significant effect of fish, regardless of density or phenotype, in decreasing the accumulation of total solids; however, the proportion of substrate covered in silt was additionally reduced in high density treatments regardless of fish phenotype. Additionally, the rate of leaf litter decomposition was accelerated in high density treatments. There was no evidence of top-down effects of fish on algal production, though the high incidence of armored grazers may have buffered any potential top down control. Finally, mortalities of above falls fish at high density were significantly greater than those of below falls fish suggesting that while we did not detect direct ecosystem consequences of phenotype, the evolutionary trade-off of lower densities in resident populations will indirectly alter ecosystem processes.
