The phenotypes, or observable traits, of wild populations are continually shaped and reshaped by the environment and numerous agents of natural selection, including predators. In stark contrast with most predators, human fishers, hunters, and plant harvesters typically exploit high proportions of prey populations and target large, reproductive-aged adults. Consequently, organisms subject to such ‘harvest selection’ may be expected to show dramatic and rapid changes in phenotype over the period of exploitation. Here, we report the magnitude of phenotypic changes in 40 human-harvested systems. We then use the ‘Darwin’ – an evolutionary rate metric – to compare the average rate of change in these human-harvested systems with those calculated from studies examining phenotypic change in not only natural systems (n = 20) but also in systems subject to other human-driven perturbations in the wild (e.g. pollution, introductions, n= 25 systems). Finally, in 25 human-harvested systems for which detailed allochronic data exist (n > 200 time series), we use a model fitting approach to examine whether rates of phenotypic change remain constant over the period of exploitation.
Results/Conclusions
Trait changes were dramatic in human-harvested systems. Morphological traits like body/horn size declined on average by nearly 20%. Shifts in life history traits (e.g., reproduction at earlier ages/smaller sizes) averaged about 25%. Our Darwin analysis, which controlled for time period, showed that these changes were remarkably abrupt, especially in commercially harvested systems, and outpaced those in natural and other human-driven systems by >300% and 50%, respectively. Finally, we found that more than 85% of the time series we assessed better fit a two-stage linear model compared with a one stage linear model. Such a pattern is consistent with two different regimes of trait change speed over the period of exploitation. Collectively, these results suggest the potential for transitively large, rapid and varied effects on size- and life history-mediated ecological dynamics that could imperil populations, industries, and ecosystems.
