Contemporary trait change in a classic ecological experiment: rapid decrease in alewife gill-raker spacing following introduction to an inland lake

1. Ecological experiments showing large effects are predicted to drive contemporary trait changes resulting from evolution and phenotypic plasticity. However, few classic ecological experiments conducted in the wild have been investigated for evidence of contemporary trait change. 2. We examined a classic experiment in trophic ecology, the introduction of alewife (Alosa pseudoharengus) to Crystal Lake, Connecticut, U.S.A. (Brooks & Dodson, 1965), for the presence of contemporary trait change. 3. Alewife were introduced to Crystal Lake, an inland lake isolated from the coastal ocean, from an anadromous (migratory sea-run) source population. We utilised museum specimens collected soon after introduction and modern samples to measure changes in gill-raker morphology for the Crystal Lake alewife population since its introduction. We compared the gill-rakers of the Crystal Lake population to those of other nearby anadromous and landlocked (freshwater resident) alewife populations. 4. At introduction, the Crystal Lake population showed gill-raker spacing (GRS) similar to that of anadromous populations. Following introduction, we found evidence for a rapid decrease in GRS, an important trait for size-selective prey capture in fishes. This decrease occurred alongside a dramatic decline in zooplankton size, shown by Brooks and Dodson (1965) to be caused by the onset of alewife predation. After 45 years of isolation in freshwater, the Crystal Lake population showed GRS typical of landlocked populations. 5. Brooks and Dodson's study is a classic example of the strong effects predators can have on prey communities. Our study shows that such community effects of predators may feed back to shape predator trophic morphology. The rate of trait change observed for the Crystal Lake alewife population is comparable to rates observed from evolutionary experiments conducted explicitly to examine trait changes over contemporary time scales. 6. We conclude that strong ecological effects of introduced populations may be important drivers of contemporary trait change. We propose that classic ecological experiments represent underutilised resources for examining interactions between contemporary trait change and ecological effects in the wild.