In the past, I've never thought about how my hunting hobby shapes the evolution of the organism I harvest.
A study in PNAS this week quantified the impact of harvesting selection on evolution in nature. Harvesting selection is the uniquely human predatory behavior in which we pick the strongest, healthiest individuals and remove them from the prey population. This type of selection stands in stark contrast to that of coyotes, wolves, lions, and pretty much every other predator, who take the young, small and weak from the population. Harvesting selection is practiced by trophy hunters and fishers who only take the biggest game. It's also practiced on an industrial scale by the fishing industry.
The theory of evolution through harvesting selection is pretty straightforward. By killing the biggest and strongest, we make it more likely that the small and weak survive to produce offspring. This type of selection also rewards individuals who reach reproductive age prematurely and can reproduce before they become big enough to get snagged in the fisherman's net. This pressure, over several generations, produces a smaller, younger prey population. This is bad because smaller, younger individuals are generally less able to rear a large number of healthy offspring.
Thus, through harvesting selection, the numbers, quality and health of a population decay over time. It's the opposite effect of natural predation, which thins a herd to make in stronger and healthier.
So how strong is this effect? In the paper, the researchers quantify change over time in traits such as body size and time to reproductive age in 29 species. They find that relative to natural selection, harvesting selection increases the rate of change by 300%. Harvesting selection makes humans one of the strongest evolutionary forces in nature, perhaps second only to asteroids and comets.
This research quantifies the fact that trophy hunting is a huge problem for natural ecosystems. For example, another study found that the biggest, best bighorn sheep are often killed before they reach reproductive age. Trophy hunting deprives a prey population of its best genetic material. Unfortunately, hunting isn't the biggest problem. This study found that commercial harvests, like fishing, are significantly more detrimental than recreational or scientific harvests. Industrial-scale harvesting selection is likely to blame for the collapse of an Atlantic Cod population.
So what are we, as responsible stewards of the planet, supposed to do? I have no idea. Perhaps we should limit our large scale consumption of food to include only domesticated sources. Then we aren't preying on natural populations, but we are still altering the environment and destroying habitat. Also, diseases are very good at developing in a crowded domesticated situation and then jumping over into a wild population. Alternatively, we could start taking the small and weak in our harvest, but that's not very lucrative.
What about me? Do I need to worry about my turkey hunting habit damaging the long term prospects of the species' survival? Probably not. There are lots of hunters who hold their shot for an entire season, waiting for the big trophy to come into range. I, on the other hand, shoot the first legal bird that crosses my path. It comes with the territory of being a poor scientist who needs the meat.
For hunters to be responsible ecologists, it turns out more of us may have become like other self-respecting natural predators and start shooting the babies and weaklings.