The lynx and the snowshoe hare is the best-known example of predator-prey population relationships. If the hare numbers rise, so do those of the lynx that feed on them. This means that hare numbers must fall, bringing lynx numbers down also.
The key conclusion is that the lynx can never wipe out its prey and both populations are held in a dynamic equilibrium. Humans provide a conspicuous exception to this rule. Our development of effective weapons for killing at a distance tipped the balance against our large prey animals.
The Pleistocene Overkill
There are many possible reasons that large grazing animals would go extinct. Possible causes include climate change, large volcano eruptions that blot out the sun for years, and asteroid collisions. Certainly, many such explanations were floated for the disappearance of dinosaurs.
Large grazers disappeared around the globe beginning about 100 thousand to 40 thousand years ago. A role of human activity is indicated by the fact that the arrival of humans on a landmass was soon followed by mass extinctions of these species (1). If a suspect is always present at the scene of the crime and carries the murder weapon, they are probably guilty.
Interestingly, some species survived the human onslaught, including American bison, and a large number of herd animals that still teem on African plains. At least some of these survivors migrate over large distances that may have helped them to elude human hunters.
Chimpanzees are another primate having the dubious distinction of being so good at hunting that they depress the population of prey animals.
The Chimps of Kibale
Chimps derive about a tenth of their nourishment from meat. The population at Kibale, Uganda, hunts red colobus monkeys. They use effective group strategies that catch the prey more than half of the time, which is much better than the success rate of other cooperative hunters like wolves or lions. Chimps are so effective as hunters that they greatly depress the population of red colobus monkeys and might well wipe them out within their home range.
It is worth pointing out that chimps occupy fairly small home ranges where they are safe from human activities. This means that their pressure on the colobus monkeys is probably exaggerated in those areas. A similar narrative applies to killer whales (a type of dolphin) that also use highly successful cooperative hunting strategies.
The waters around Alaska experienced a collapse of many species of large animals, such as seals and sea lions. Killer whales are believed to be responsible.
These highly intelligent animals use complex group hunting strategies that are not possible for other marine animals. One stunt they pull is to swim in a coordinated fashion around an ice flow where a seal has found refuge. The killer whales create a large wave that tilts the ice flow tossing the hapless seal back into the water.
Researchers concluded that the extinctions are due to killer whale predatory behavior. This means that we are not the only species capable of overkill. Yet, the killer whales would probably not have wiped out so many marine animals if humans were not around.
The reason: We have killed so many whales. Because these larger prey animals are so scarce, killer whales are forced to target smaller animals, such as sea lions. Once again our fingerprints are all over the crime.
From around a hundred thousand years ago, humans were a destabilizing force in global ecosystems. Hunting drove most large grazing animals to extinction. With the demise of these large animals, our species switched to smaller prey. These were exploited with the help of dogs as described in a recent post.
The Pleistocene Overkill was the first convincing example of humans as an ecologically destabilizing force.
With the switch to settled farming, humans produced much much more food, permitting a hundredfold increase in our population. Our domestic animals also dominate the globe, pushing many wild vertebrates to extinction.
With the industrialization of the economy, each of these destabilizing trends went into overdrive, including the population explosion, and the poisoning of natural ecosystems with industrial wastes that poses an existential crisis for our own species. Our activities warm the oceans, collapsing polar ice shelves and bleaching coral reefs. No other species matches us in wiping out prey animals and many others to boot.