Hunting for Nothing

In an increasingly human-dominated world, many predators are making due without the resource they needed most: their prey. Featured image source:

Reference: M.A. Parsons, T.M. Newsome, J.K. Young. (2022). The consequences of predators without prey. Frontiers in Ecology and the Environment, 20(1): 31-39. doi: 10.1002/fee.2419

A lizard stages a dramatic escape from a snake-infested valley. A wolf reintroduction in Yellowstone brings harmony to a long-disrupted ecosystem. These viral videos highlight the inherent drama of the predator-prey relationship, one of the most fundamental concepts in understanding the balance of nature. As humanity’s ecological footprint grows, however, this relationship is beginning to change. Many studies have evaluated what happens to prey species when their predators are gone, but recent efforts are asking a new question: what happens when the predators are the only ones left?

Heavy is the head

A rift between predators and their prey can form through a variety of mechanisms, such as habitat loss, urbanization, climate change, or overhunting. While starving polar bears may dominate National Geographic covers, the loss of prey is not necessarily the death sentence one would assume. Rather, many top predators search for what researchers call “non-prey resources,” or food that doesn’t require being hunted and killed (think of a black bear rummaging through trash, or hyenas fighting over a carcass). These resources are often found close to civilization, a setup that lends itself to dangerous and sometimes fatal encounters with humans.

Food sources near civilization, like trash and roadkill, are often less healthy for predators. Food found through foraging in urban areas tends to have low nutritional value despite its high caloric content. Malnutrition can make predators bolder in their search for food, which increases the odds of making riskier decisions that threaten their survival. Similarly, shifting to a carcass-heavy diet increases the odds of contracting diseases, and it can disrupt their natural cycles: bears, for example, have shorter hibernation periods when they have easy access to human food.

With greater urbanization, interactions between apex predators and humans have become common. Image source:
A bloodless cascade

trophic cascade is a phenomenon that occurs when one level of a food chain disappears, disrupting many other species in the system. This concept is usually expressed through the loss of a “keystone species,” or a species whose ecological role preserves the balance among other species. For example, the loss of otters in the north Pacific caused sea urchin populations to skyrocket, which in turn destroyed the kelp forests upon which many other organisms depend.

A trophic cascade could occur when a top predator exits the predator-prey relationship even when prey is abundant simply because it is easier to meet caloric demands through non-hunting strategies. This situation often occurs when a predator takes an initial risk out of desperation,such as an exploration into a city where it finds ample food. With an increase in urbanization, some predators like bears and dingoes have an easier time eating their fill by foraging through dumps or searching for carcasses. If not properly deterred, these animals will understand urban areas as places full of easily-accessible resources, which again leads to unsafe situations for both animals and humans.

Cascades are usually explained through the loss of a top predator, but these cascades can occur from the bottom up as well. Loss of a prey species can prove devastating to predators, especially in organisms that have evolved to get most of their calories from a specific species. However, research has rarely focused on what happens to predator numbers in the absence of prey. A predator shifting to foraging or scavenging may well be due to an inability to find its natural prey, but the consequences of this loss have yet to be fully understood.

The future of foraging

If there is hope to be found in these situations, it is in the recognition that severing the link between predator and prey is not always an immediate death sentence for the species involved. Silver linings could be discovered through research, and some may already be known: some predators that shift to foraging in anthropogenic sources have greater reproduction rates, and it is possible that these organisms do not have to invest as much energy rearing their young before they are ready to fend for themselves. Predators-turned-foragers also seem to be less territorial and more willing to occupy the same space, which may change their social structures.

The Anthropocene will continue to bring animals of all trophic levels in closer contact with humanity. How they adapt to us, and we to them, will determine their odds of survival.

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