Predation is classified as natural mortality because it is one of the oldest and most fundamental forces shaping life on Earth. In ecology, natural mortality includes deaths from predation, disease, parasitism, starvation, aging, and cannibalism, all causes that exist independent of human activity. Predation stands out among these because it doesn’t just end one life; it actively sustains the health of entire ecosystems, from the genetic fitness of prey species to the cycling of nutrients through soil and water.
What Ecologists Mean by Natural Mortality
In wildlife biology, natural mortality refers to any death not caused directly or indirectly by human activities like hunting, fishing, vehicle strikes, or habitat destruction. The main components are predation, disease, starvation, aging, parasitism, and cannibalism. These forces have shaped animal populations for hundreds of millions of years, long before humans had any ecological footprint at all.
Predation is often the single largest source of natural mortality for many species, particularly earlier in life when animals are small and vulnerable. As prey animals grow larger, their risk of being killed by a predator typically declines, which means predation acts as a selective filter that shapes which individuals survive to reproduce. This is fundamentally different from human-caused mortality, which often kills indiscriminately regardless of an animal’s age, health, or fitness.
Predation Keeps Ecosystems in Balance
The most compelling reason predation is considered natural is that ecosystems evolved to depend on it. When predators limit the number or change the behavior of their prey, the effects ripple downward through the food web in what ecologists call trophic cascades. Predators eat grazers, grazers eat plants, and by controlling the middle link, predators indirectly protect vegetation. This three-level interaction is why most healthy ecosystems stay green rather than being stripped bare by herbivores.
The sea otter example illustrates this clearly. Where sea otters are present in healthy numbers, they keep sea urchin populations in check, allowing kelp forests to thrive. At sites where sea otters have been absent for long periods, urchin populations swell to enormous densities and create what marine biologists call “urchin barrens,” areas with almost no kelp left. When otters return, the kelp forests recover, demonstrating that predators can trigger whole-ecosystem restoration.
A similar dynamic plays out on land. GPS tracking studies of elk have shown that herds avoid areas with high wolf densities, spending more time in alternate habitats where wolves are scarce. This behavioral shift relaxes browsing pressure on young aspen trees in the riskier zones, allowing forests to regenerate. The predator doesn’t even need to kill the prey animal to reshape the landscape. Fear alone redistributes grazing pressure across the environment.
Predators Remove the Sick and Weak
Predation is selective in ways that other forms of death are not. Predators disproportionately take individuals that are old, young, injured, or diseased, because these animals are easier to catch. This selective pressure has real consequences for the health of prey populations over time.
The “healthy herds hypothesis” describes how predators reduce the prevalence of infection by culling sick individuals before they can spread disease further. Some parasites even make their hosts more conspicuous to predators. In one well-studied freshwater system, infected prey become more opaque and visually obvious, making them easier targets for predators that hunt by sight. The result is that predation preferentially removes the most heavily parasitized animals, lowering infection rates across the surviving population.
This isn’t just good for the prey species in the abstract. It directly improves the odds that the healthiest, most genetically fit individuals survive to reproduce. Over generations, this selective culling strengthens the population’s resilience to disease, environmental stress, and competition. No other form of mortality exerts this kind of targeted evolutionary pressure.
Fewer Predators Means More Disease
When predator populations decline, the consequences extend beyond overgrazed landscapes. Fewer predators can directly increase the risk of infectious disease, including diseases that jump to humans.
Foxes in North America help control Lyme disease risk by preying on rodents, the primary reservoir hosts for the bacteria that infected ticks carry. Raptors and snakes serve a similar function, keeping rodent and bat populations in check, populations that harbor pathogens dangerous to people. In India, leopards living near Mumbai’s Sanjay Gandhi National Park appear to reduce stray dog densities in surrounding neighborhoods, potentially lowering the risk of rabies transmission to humans.
One of the most dramatic examples involves India’s vulture collapse. Between 1990 and 2000, vulture populations dropped by 92%. Vultures are scavengers rather than active predators, but they fill a closely related ecological role by consuming carcasses before they become breeding grounds for disease. With vultures gone, carrion accumulated, stray dog populations surged to fill the scavenging niche, and human exposure to dog bites and rabies spiked. The loss of one group of meat-eating animals cascaded into a public health crisis.
Predation Recycles Nutrients Faster
When a predator kills and consumes prey, it accelerates the return of nutrients to the ecosystem. Decomposition of organic matter is essential for cycling energy and nutrients through every ecosystem on Earth, and predation speeds this process up compared to an animal simply dying of old age in place.
A carcass left on the landscape attracts scavengers that break it down, extracting sodium, magnesium, phosphorus, potassium, and calcium from skin, hair, and bones. When only a few large scavenger species handle decomposition, the process moves quickly and feeds nutrients back into the soil faster. When a more diverse community of scavengers is involved, decomposition slows down but supports greater biodiversity among the decomposer species themselves. Either way, the dead animal becomes fuel for the living ecosystem. Predation is the engine that creates this cycle consistently rather than leaving it to chance.
How Human-Caused Death Differs
The contrast with human-caused mortality helps explain why predation feels “more natural” even intuitively. A long-term study tracking 590 radio-collared mountain lions across California from 1974 to 2020 documented 263 deaths during the monitoring period. The annual rate of human-caused mortality (0.13) was more than double the rate of natural mortality (0.06). Critically, human-caused deaths did not compensate for or replace natural deaths. They stacked on top of them, reducing overall survival at the population level.
This is a key distinction. In a system governed only by natural forces, predation, disease, and starvation tend to balance each other. If predation removes many individuals, fewer die of disease or starvation because resources are less scarce. Human-caused mortality breaks this balance because it adds a death toll the population never evolved to absorb. Vehicle strikes, poisoning, and habitat fragmentation don’t selectively remove the weakest animals. They kill randomly with respect to fitness, which means they erode the population without providing any of the selective benefits that predation does.
Predation co-evolved with prey species over millions of years. Prey animals developed speed, camouflage, herding behavior, and heightened senses specifically in response to predators. These adaptations are among the most impressive features of wildlife. Human-caused threats arrived too recently and change too quickly for evolutionary adaptation to keep pace, which is precisely why they are classified separately from the natural mortality that ecosystems are built to handle.