16.1: Predation

Introduction

Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey (Figure \(\PageIndex{1}\)).

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Figure \(\PageIndex{1}\): (Left) A solitary predator: a polar bear feeds on a bearded seal it has killed. (Right) Social predators: meat ants cooperate to feed on a cicada far larger than themselves.

It is one of a family of common feeding behaviors that includes parasitism and micropredation (which usually do not kill the host) and parasitoidism (which always does, eventually) (Figure \(\PageIndex{2}\)). It is distinct from scavenging on dead prey, though many predators also scavenge. Predation and herbivory overlap because seed predators and destructive frugivores kill their “prey”.

Figure \(\PageIndex{2}\): Relation of predation to other feeding strategies

The concept of predation is broad, defined differently in different contexts, and includes a wide variety of feeding methods; and some relationships that result in the prey's death are not generally called predation. A parasitoid, such as an ichneumon wasp, lays its eggs in or on its host; the eggs hatch into larvae, which eat the host, and it inevitably dies (Figure \(\PageIndex{3}\)). Zoologists generally call this a form of parasitism, though conventionally parasites are thought not to kill their hosts. A predator can be defined to differ from a parasitoid in that it has many prey, captured over its lifetime, whereas a parasitoid's larva has just one, or at least has its food supply provisioned for it on just one occasion.

Figure \(\PageIndex{3}\): Spider wasps paralyze and eventually kill their hosts, but are considered parasitoids, not predators.

There are other difficult and borderline cases. Micropredators are small animals that, like predators, feed entirely on other organisms; they include fleas and mosquitoes that consume blood from living animals, and aphids that consume sap from living plants. However, since they typically do not kill their hosts, they are now often thought of as parasites. Animals that graze on phytoplankton or mats of microbes are predators, as they consume and kill their food organisms; but herbivores that browse leaves are not, as their food plants usually survive the assault. Scavengers, organisms that only eat organisms found already dead, are not predators, but many predators such as the jackal and the hyena scavenge when the opportunity arises. Among invertebrates, social wasps (yellowjackets) are both hunters and scavengers of other insects.

Predators are adapted and often highly specialized for hunting, with acute senses such as vision, hearing, or smell. Many predatory animals, both vertebrate and invertebrate, have sharp claws or jaws to grip, kill, and cut up their prey. Other adaptations include stealth and aggressive mimicry that improve hunting efficiency.

When prey is detected, the predator assesses whether to attack it. Predators may actively search for or pursue prey (pursuit predation) or sit and wait for prey (ambush predation), often concealed, prior to attack. If the attack is successful, the predator kills the prey, removes any inedible parts like the shell or spines, and eats it.

Predation has a powerful selective effect on prey, and the prey develop antipredator adaptations such as warning coloration, alarm calls and other signals, camouflage, mimicry of well-defended species, and defensive spines and chemicals. Sometimes predator and prey find themselves in an evolutionary arms race, a cycle of adaptations and counter-adaptations.

Foraging

To feed, a predator must search for, pursue and kill its prey. These actions form a foraging cycle (Figure \(\PageIndex{4}\)). The predator must decide where to look for prey based on its geographical distribution; and once it has located prey, it must assess whether to pursue it or wait for a better choice. If it chooses pursuit, its physical capabilities determine the mode of pursuit (e.g., ambush or pursuit). Having captured the prey, it may also need to expend energy handling it (e.g., killing it, removing any shell or spines, and ingesting it). 

Figure \(\PageIndex{4}\): A basic foraging cycle for a predator, with some variations indicated (Kramer 2001).

Search

Predators have a choice of search modes ranging from sit-and-wait to active or widely foraging. The sit-and-wait method is most suitable if the prey are dense and mobile, and the predator has low energy requirements (Figure \(\PageIndex{5}\)). Wide foraging expends more energy, and is used when prey is sedentary or sparsely distributed. There is a continuum of search modes with intervals between periods of movement ranging from seconds to months. Sharks, sunfish, insectivorous birds and shrews are almost always moving while web-building spiders, aquatic invertebrates, praying mantises and kestrels rarely move. In between, plovers and other shorebirds, freshwater fish including crappies, and the larvae of coccinellid beetles (ladybirds), alternate between actively searching and scanning the environment. 

Figure \(\PageIndex{5}\): Seven-spot ladybirds select plants of good quality for their aphid prey.

Prey distributions are often clumped, and predators respond by looking for patches where prey is dense and then searching within patches (Kramer 2001). Where food is found in patches, such as rare shoals of fish in a nearly empty ocean, the search stage requires the predator to travel for a substantial time, and to expend a significant amount of energy, to locate each food patch. For example, the black-browed albatross regularly makes foraging flights to a range of around 700 kilometers (430 miles), up to a maximum foraging range of 3,000 kilometers (1,860 miles) for breeding birds gathering food for their young (Figure \(\PageIndex{6}\)). With static prey, some predators can learn suitable patch locations and return to them at intervals to feed. 

Figure \(\PageIndex{5}\): The black-browed albatross regularly flies hundreds of kilometers across the nearly empty ocean to find patches of food.

Assessment

Having found prey, a predator must decide whether to pursue it or keep searching. The decision depends on the costs and benefits involved. A bird foraging for insects spends a lot of time searching but capturing and eating them is quick and easy, so the efficient strategy for the bird is to eat every palatable insect it finds. By contrast, a predator such as a lion or falcon finds its prey easily but capturing it requires a lot of effort. In that case, the predator is more selective (Pianka 2011). 

One of the factors to consider is size. Prey that is too small may not be worth the trouble for the amount of energy it provides. Too large, and it may be too difficult to capture. For example, a mantid captures prey with its forelegs and they are optimized for grabbing prey of a certain size. Mantids are reluctant to attack prey that is far from that size. There is a positive correlation between the size of a predator and its prey. 

Capture

To capture prey, predators have a spectrum of pursuit modes that range from overt chase (pursuit predation) to a sudden strike on nearby prey (ambush predation).

Ambush

Ambush, or sit-and-wait predators, are carnivorous animals that capture prey by stealth or surprise. In animals, ambush predation is characterized by the predator's scanning the environment from a concealed position until prey is spotted, and then rapidly executing a fixed surprise attack. Vertebrate ambush predators include frogs, fish such as the angel shark, the northern pike, and the eastern frogfish. Among the many invertebrate ambush predators are trapdoor spiders and Australian Crab spiders on land and mantis shrimps in the sea. Ambush predators often construct a burrow in which to hide, improving concealment at the cost of reducing their field of vision. Some ambush predators also use lures to attract prey within striking range. The capturing movement has to be rapid to trap the prey, given that the attack is not modifiable once launched. 

Figure \(\PageIndex{7}\): (Left) A western green lizard ambushes its grasshopper prey. (Right) A trapdoor spider waiting in its burrow to ambush its prey

Pursuit

In pursuit predation, predators chase fleeing prey. If the prey flees in a straight line, capture depends only on the predator's being faster than the prey. If the prey maneuvers by turning as it flees, the predator must react in real-time to calculate and follow a new intercept path as it closes on the prey. Many pursuit predators use camouflage to approach the prey as close as possible unobserved (stalking) before starting the pursuit. Pursuit predators include terrestrial mammals such as humans, African wild dogs, spotted hyenas and wolves; marine predators such as dolphins, orcas, and many predatory fishes, such as tuna; predatory birds (raptors) such as falcons; and insects such as dragonflies (Figure \(\PageIndex{8}\)). Pursuit predators may be social, like the lion and wolf that hunt in groups, or solitary. 

An extreme form of pursuit is endurance or persistence hunting, in which the predator tires out the prey by following it over a long distance, sometimes for hours at a time. The method is used by human hunter-gatherers and by canids such as African wild dogs and domestic hounds. The African wild dog is an extreme persistence predator, tiring out individual prey by following them for many miles at relatively low speed. 

A specialized form of pursuit predation is the lunge feeding of baleen whales. These very large marine predators feed on plankton, especially krill, diving and actively swimming into concentrations of plankton, and then taking a huge gulp of water and filtering it through their feathery baleen plates. 

Figure \(\PageIndex{8}\): (Left) Humpback whales are lunge feeders, filtering thousands of krill from seawater and swallowing them alive.
(Right) Dragonflies, like this common clubtail with captured prey, are invertebrate pursuit predators.

Handling

Once the predator has captured the prey, it has to handle it: very carefully if the prey is dangerous to eat, such as if it possesses sharp or poisonous spines, as in many prey fish. Some catfish such as the Ictaluridae have spines on the back (dorsal) and belly (pectoral) which lock in the erect position; as the catfish thrashes about when captured, these could pierce the predator's mouth, possibly fatally (Figure \(\PageIndex{9}\)). Some fish-eating birds like the osprey avoid the danger of spines by tearing up their prey before eating it. Some prey may also be time-consuming to handle, such as clams or other bivalves that are difficult for predators to crush or open (Figure \(\PageIndex{9}\)). 

Figure \(\PageIndex{9}\): (Left) Catfish has sharp dorsal and pectoral spines which it holds erect to discourage predators such as herons which swallow prey whole. (Right) Osprey tears its fish prey apart, avoiding dangers such as sharp spines.

Predator Adaptations

Physical adaptations

Under the pressure of natural selection, predators have evolved a variety of physical adaptations for detecting, catching, killing, and digesting prey. These include speed, agility, stealth, sharp senses, claws, teeth, filters, and suitable digestive systems (Figure \(\PageIndex{10}\)).

For detecting prey, predators have well-developed vision, smell, or hearing. Predators as diverse as owls and jumping spiders have forward-facing eyes, providing accurate binocular vision over a relatively narrow field of view, whereas prey animals often have less acute all-round vision. Animals such as foxes can smell their prey even when it is concealed under 2 feet (60 cm) of snow or earth. Many predators have acute hearing, and some such as echolocating bats hunt exclusively by active or passive use of sound. 

Predators including big cats, birds of prey, and ants share powerful jaws, sharp teeth, or claws which they use to seize and kill their prey. Some predators such as snakes and fish-eating birds like herons and cormorants swallow their prey whole; some snakes can unhinge their jaws to allow them to swallow large prey, while fish-eating birds have long spear-like beaks that they use to stab and grip fast-moving and slippery prey. Fish and other predators have developed the ability to crush or open the armored shells of molluscs.

Many predators are powerfully built and can catch and kill animals larger than themselves; this applies as much to small predators such as ants and shrews as to big and visibly muscular carnivores like the cougar and lion. 

Figure \(\PageIndex{10}\): a - Skull of brown bear has large pointed canines for killing prey, and self-sharpening carnassial teeth at rear for cutting flesh with a scissor-like action; b - Large compound eyes, sensitive antennae, and powerful jaws (mandibles) of jack jumper ant; c - Crab spider, an ambush predator with forward-facing eyes, catching another predator, a field digger wasp; d - Red-tailed hawk uses sharp hooked claws and beak to kill and tear up its prey; e - Specialist: a great blue heron with a speared fish; f - Indian python unhinges its jaw to swallow large prey like this chital

Camouflage and mimicry

Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Predators may use camouflage to more effectively capture prey, while prey may use camouflage to more effectively evade predation. Members of the cat family such as the snow leopard (treeless highlands), tiger (grassy plains, reed swamps), ocelot (forest), fishing cat (waterside thickets), and lion (open plains) are camouflaged with coloration and disruptive patterns suiting their habitats (Figure \(\PageIndex{11}\)).

Figure \(\PageIndex{11}\): A camouflaged predator: snow leopard in Ladakh

In aggressive mimicry, certain predators, including insects and fishes, make use of coloration and behavior to attract prey. Female Photuris fireflies, for example, copy the light signals of other species, thereby attracting male fireflies, which they capture and eat. Flower mantises are ambush predators; camouflaged as flowers, such as orchids, they attract prey and seize it when it is close enough. Frogfishes are extremely well camouflaged, and actively lure their prey to approach using an esca, a bait on the end of a rod-like appendage on the head, which they wave gently to mimic a small animal, gulping the prey in an extremely rapid movement when it is within range.

Figure \(\PageIndex{12}\): Striated frogfish uses camouflage and aggressive mimicry in the form of a fishing rod-like lure on its head to attract prey.

Other Adaptation Examples

• Venom - Many smaller predators such as the box jellyfish use venom to subdue their prey, and venom can also aid in digestion (as is the case for rattlesnakes and some spiders). The marbled sea snake that has adapted to egg predation has atrophied venom glands, and the gene for its three-finger toxin contains a mutation (the deletion of two nucleotides) that inactivates it. These changes are explained by the fact that its prey does not need to be subdued.

• Electric fields - Several groups of predatory fish have the ability to detect, track, and sometimes, as in the electric ray, incapacitate their prey by generating electric fields using electric organs. The electric organ is derived from modified nerve or muscle tissue.

• Physiology - Physiological adaptations to predation include the ability of predatory bacteria to digest the complex peptidoglycan polymer from the cell walls of the bacteria that they prey upon. Carnivorous vertebrates of all five major classes (fishes, amphibians, reptiles, birds, and mammals) have lower relative rates of sugar to amino acid transport than either herbivores or omnivores, presumably because they acquire plenty of amino acids from the animal proteins in their diet.

• Echolocation - Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them (Figure \(\PageIndex{13}\)). They use these echoes to locate and identify the objects. Echolocation is used for navigation, foraging, and hunting in various environments.

Figure \(\PageIndex{13}\): Bats use echolocation to hunt moths at night.

Predator/Prey Coevolution

To counter predation, prey have evolved defenses for use at each stage of an attack. They can try to avoid detection, such as by using camouflage and mimicry. They can detect predators and warn others of their presence. If detected, they can try to avoid being the target of an attack, for example, by signaling that they are toxic or unpalatable, by forming groups, or that a chase would be unprofitable. If they become a target, they can try to fend off the attack with defenses such as armor, quills, unpalatability, or mobbing and they can escape an attack in progress by startling the predator, playing dead, shedding body parts such as tails, or simply fleeing.

Dead leaf mantis's camouflage makes it less visible to both predators and prey.

Syrphid hoverfly misdirects predators by mimicking a wasp, but has no sting.

Predators and prey are natural enemies, and many of their adaptations seem designed to counter each other. For example, bats have sophisticated echolocation systems to detect insects and other prey, and insects have developed a variety of defenses including the ability to hear echolocation calls. Many pursuit predators that run on land, such as wolves, have evolved long limbs in response to the increased speed of their prey. Their adaptations have been characterized as an evolutionary arms race, an example of the coevolution of two species. In a gene-centered view of evolution, the genes of predator and prey can be thought of as competing for the prey's body. Though criticized, the "life-dinner" principle of Dawkins and Krebs predicts that this arms race is asymmetric: if a predator fails to catch its prey, it loses its dinner, while if it succeeds, the prey loses its life. 

 

Eastern coral snake, itself a predator, is venomous enough to kill predators that attack it, so when they avoid it, this behavior must be inherited, not learned.

The metaphor of an arms race implies ever-escalating advances in attack and defense. However, these adaptations come with a cost; for instance, longer legs have an increased risk of breaking, while the specialized tongue of the chameleon, with its ability to act like a projectile, is useless for lapping water, so the chameleon must drink dew off vegetation.

Consequences of Predation

Predators affect their ecosystems not only directly by eating their own prey, but by indirect means such as reducing predation by other species, or altering the foraging behavior of an herbivore, as with the biodiversity effect of wolves on riverside vegetation or sea otters on kelp forests. This may explain population dynamics effects such as the cycles observed in lynx and snowshoe hares. 

Biodiversity maintained by apex predation

Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Such predators are known as keystone species and may have a profound influence on the balance of organisms in a particular ecosystem. Introduction or removal of this predator, or changes in its population density, can have drastic cascading effects on the equilibrium of many other populations in the ecosystem. For example, grazers of a grassland may prevent a single dominant species from taking over.  

Riparian willow recovery at Blacktail Creek, Yellowstone National Park, after reintroduction of wolves, the local keystone species and apex predator. Left, in 2002; right, in 2015

The elimination of wolves from Yellowstone National Park had profound impacts on the entire ecosystem’s structure and function. In Yellowstone, wolves are both keystone species and apex predators. Without predation, herbivores began to over-graze many woody browse species, affecting the area's plant populations. In addition, wolves often kept animals from grazing near streams, protecting the beavers' food sources. The removal of wolves had a direct effect on the beaver population, as their habitat became territory for grazing. Increased browsing on willows and conifers along Blacktail Creek due to a lack of predation caused channel incision because the reduced beaver population was no longer able to slow the water down and keep the soil in place. The predators were thus demonstrated to be of vital importance in the ecosystem. 

Population dynamics

In the absence of predators, the population of a species can grow exponentially until it approaches the carrying capacity of the environment (Neal 2004). Predators limit the growth of prey both by consuming them and by changing their behavior. Increases or decreases in the prey population can also lead to increases or decreases in the number of predators, for example, through an increase in the number of young they bear.

Cyclical fluctuations have been seen in populations of predator and prey, often with offsets between the predator and prey cycles. A well-known example is that of the snowshoe hare and lynx. Over a broad span of boreal forests in Alaska and Canada, the hare populations fluctuate in near synchrony with a 10-year period, and the lynx populations fluctuate in response. This was first seen in historical records of animals caught by fur hunters for the Hudson Bay Company over more than a century.

Predator-prey population cycles in a Lotka-Volterra model

A simple model of a system with one species each of predator and prey, the Lotka–Volterra equations, predicts population cycles. However, attempts to reproduce the predictions of this model in the laboratory have often failed; for example, when the protozoan Didinium nasutum is added to a culture containing its prey, Paramecium caudatum, the latter is often driven to extinction.

The Lotka-Volterra equations rely on several simplifying assumptions, and they are structurally unstable, meaning that any change in the equations can stabilize or destabilize the dynamics. For example, one assumption is that predators have a linear functional response to prey: the rate of kills increases in proportion to the rate of encounters. If this rate is limited by time spent handling each catch, then prey populations can reach densities above which predators cannot control them. Another assumption is that all prey individuals are identical. In reality, predators tend to select young, weak, and ill individuals, leaving prey populations able to regrow.

Many factors can stabilize predator and prey populations. One example is the presence of multiple predators, particularly generalists that are attracted to a given prey species if it is abundant and look elsewhere if it is not. As a result, population cycles tend to be found in northern temperate and subarctic ecosystems because the food webs are simpler. The snowshoe hare-lynx system is subarctic, but even this involves other predators, including coyotes, goshawks and great horned owls, and the cycle is reinforced by variations in the food available to the hares.

A range of mathematical models have been developed by relaxing the assumptions made in the Lotka-Volterra model; these variously allow animals to have geographic distributions, or to migrate; to have differences between individuals, such as sexes and and age structure, so that only some individuals reproduce; to live in a varying environment, such as with changing seasons; and analyzing the interactions of more than just two species at once. Such models predict widely differing and often chaotic predator-prey population dynamics. The presence of refuge areas, where prey are safe from predators, may enable prey to maintain larger populations but may also destabilize the dynamics.  

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