Predator-induced cyclical changes in lateral plate frequencies of Gasterosteus

This study investigates ecological aspects to intrapopulation variability in the lateral plates of threespine stickleback (Gasterosteus aculeatus) at a coastal lake in western Canada. The frequency distribution of the plates, which was slightly skewed to the right and leptokurtic, varied spatially and seasonally. Cyclical frequency changes in plates were most pronounced on sub-adult size classes and these closely tracked (multiple r = 0.77) seasonal differences in the relative proportion of the two major predator groups, trout and diving birds. Trout predation was correlated with an increase in plate number while bird predation was correlated with a reduction in plate number. Such divergent selection pressures, which are probably common in natural populations, would account for the presence of multiple phenotypes and allow for rapid shifts in defensive morphology among different populations where predation regime differs.