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16.1: Predation

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    Figure \(\PageIndex{1}\): Polar bears (top photo) are solitary predators, here shown feeding on a beaded seal. Other species, like meat ants (photo below), cooperate to feed and are called social predators. Here, meat ants feed socially on a cicada far larger than themselves.


    Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. 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). 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”.

    Venn-diagram shows how scavenging, herbivory, and parasitism relate to predation using overlapping circles. The scavenging circle with text reading “prey already dead” overlaps with predation in a region labeled “opportunistic scavenging”. Parasitism labeled with the text “prey stays alive” and containing the smaller circle of micropredation overlaps with predations under the label “parasitoidism: eventually kills prey”. Herbivory includes browsing, defined as “parts of plant eaten” and overlaps predation with grazing or whole plants eaten.

    Figure \(\PageIndex{2}\): The relationship 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. 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, where a parasitoid's larva has just one, or at least has its food supply provisioned for it on just one occasion (Lafferty & Kuris 2002, Gurr & Wratten 2012).


    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 (Poulin et al. 2011, Poulin & Randhawa 2015). 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 (Bengston 2002). 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 (Kruuk 1972, Kane et al. 2017). Among invertebrates, social wasps (yellowjackets) are both hunters and scavengers of other insects (Schmidt 2009).

    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.



    16.1.1 Foraging

    The foraging cycle is represented by the four actions in the foraging cycle: search, assess, pursue, and handle. “Search” includes travel to a food patch via the prey found arrow leads to “assess”. From assessment, the “no” arrow leads back to “Search”, while the “yes” arrow leads to “Pursue”. Pursuit includes stalking, sitting, and waiting and either leads back to search by abandoning prey, or “Hande” if the prey is captured. Handling includes killing, removing shells, and other processes that lead to eating, as well as the action of eating itself. After handling, a dotted line leads back to “Search” labeled “time for next meal”.

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

    To feed, a predator must search for, pursue and kill its prey. These actions form a foraging cycle (Griffiths 1980; Wetzel and Liken 2000). 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 to wait for a better choice. If it chooses pursuit, its physical capabilities determine the mode of pursuit (e.g., ambush or pursuit) (MacArthur 1984; Pianka 2011). 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) (Griffiths 1980; Kramer 2001). 



    Predators have a choice of search modes ranging from sit-and-wait to active or widely foraging (Perry 1999; Kramer 2011; Bell 2012; Eastman and Thiel 2015). The sit-and-wait method is most suitable if the prey are dense and mobile, and the predator has low energy requirements (Bell 2012). Wide foraging expends more energy, and is used when prey is sedentary or sparsely distributed (Pianka 2011; Bell 2012). 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 (Bell 2012). 



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

    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 (Bell 2012). 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 (Gremillet et al. 2000). With static prey, some predators can learn suitable patch locations and return to them at intervals to feed (Bell 2012). 




    Figure \(\PageIndex{6}\): Seven-spot ladybirds select plants of good quality for their aphid prey.

    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 (Pianka 2011). 



    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) (Stevens 2010; Kramer 2011; Williams et al. 2012)





    Figure \(\PageIndex{7}\): A western green lizard ambushes its grasshopper prey (top photo) and a trapdoor spider waits in its burrow to ambush its prey (bottom photo).


    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 a prey is spotted, and then rapidly executing a fixed surprise attack (deVries et al. 2012; Moore and Biewener 2015). Vertebrate ambush predators include frogs, fish such as the angel shark, the northern pike and the eastern frogfish (Canadian Wildlife Service 2007; Bray 2014; Moore and Biewener 2015; Indiana Division of Fish and Wildlife 2018). Among the many invertebrate ambush predators are trapdoor spiders and Australian Crab spiders on land and mantis shrimps in the sea (deVries et al. 2012; BBC 2014; Arizona-Sonora Desert Museum). 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 (Moore and Biewener 2015). The capturing movement has to be rapid to trap the prey, given that the attack is not modifiable once launched (Moore and Biewener 2015). 





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

    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 (Moore and Biewener 2015). 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 (Moore and Biewener 2015). Many pursuit predators use camouflage to approach the prey as close as possible unobserved (stalking) before starting the pursuit (Moore and Biewener 2015). 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 (Gazda et al. 2005; Tyus 2011); predatory birds (raptors) such as falcons; and insects such as dragonflies (Combes et al. 2013). Pursuit predators may be social, like the lion and wolf that hunt in groups, or solitary (Lafferty and Kuris 2002). 

    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 (Hubel et al. 2016). 

    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 (Goldbogen et al. 2006; Sanders et al. 2015). 





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

    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. Some fish-eating birds like the osprey avoid the danger of spines by tearing up their prey before eating it (Forbes 1989).] Some prey may also be time-consuming to handle, such as clams or other bivalves that are difficult for predators to crush or open. 


    16.1.2 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 (Bar-Yam 2018).

    For detecting prey, predators have well-developed vision, smell, or hearing (Stevens et al. 2010). 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 (Royal Saskatchewan Museum 2012). 

    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 (Royal Saskatchewan Museum 2012). Fish and other predators have developed the ability to crush or open the armored shells of molluscs (Vermeij 1993).

    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 (Lafferty and Kuris 2002; Getz 2011; Royal Saskatchewan Museum 2012). 



    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

    A brown spotted snow leopard blends in to spotted rocks in a sparsely vegetated outcrop.


    Figure \(\PageIndex{11}\): A camouflaged snow leopard in Ladakh (top photo). Striated frogfish uses camouflage and aggressive mimicry in the form of a fishing rod-like lure on its head to attract prey (bottom photo).


    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 (Cott 1940).

    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 (Lloyd 1965). Flower mantises are ambush predators; camouflaged as flowers, such as orchids, they attract prey and seize it when it is close enough (Forbes 2009). 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 (Bester 2017).

    Other Adaptations

    • Venom - Many smaller predators such as the box jellyfish use venom to subdue their prey (Ruppert et al. 2005), and venom can also aid in digestion (as is the case for rattlesnakes and some spiders) (Cetaruk 2005; Barceloux 2008). 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 inactives it. These changes are explained by the fact that its prey does not need to be subdued (Li et al. 2005).

    • Electric fields - Several groups of predatory fish have the ability to detect, track, and sometimes, as in the electric ray, to incapacitate their prey by generating electric fields using electric organs (Castello et al. 2009; Feulner et al. 2009; Catania 2015). The electric organ is derived from modified nerve or muscle tissue (Kramer 1996).

    • 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 (Jurkevitch and Davidov 2006). 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 (Karasov et al. 1988).


    Predator/Prey Coevolution


    Figure \(\PageIndex{12}\): Bats use echolocation to hunt moths at night.


    To counter predation, prey have evolved defenses for use at each stage of an attack (Ruxton 2004). They can try to avoid detection (Ruxton 2004), such as by using camouflage and mimicry (Merilaita  et al. 2017). They can detect predators (Caro 2005) and warn others of their presence (Bergstrom and Lachmann 2001; Getty 2002). If detected, they can try to avoid being the target of an attack, for example, by signaling that they are toxic or unpalatable (Cott 1940;  Bowers et al. 1985), by forming groups (Beauchamp 2012; Krause et al. 2002), or that a chase would be unprofitable (Ruxton et al. 2004; Caro 2005). If they become a target, they can try to fend off the attack with defenses such as armor, quills, unpalatability, or mobbing (Dominey 1983; Ruxton et al. 2004; Brodie 2009) and they can escape an attack in progress by startling the predator (Cott 1940; Merilaita et al. 2011; Edmunds 2012), playing dead, shedding body parts such as tails, or simply fleeing (Caro 2005).


    A fly with a long narrow body and black and yellow wasp-like stripes sits on a pink flower.

    Figure \(\PageIndex{13}\): Dead leaf mantis's camouflage makes it less visible to both predators and prey (top photo). Syrphid hoverfly misdirects predators by mimicking a wasp, but has no sting (bottom photo).


    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 the echolocation calls (Jacobs and Bastian 2017; Barbosa and Castellanos 2005). Many pursuit predators that run on land, such as wolves, have evolved long limbs in response to the increased speed of their prey (Janis and Wilhelm 1993). Their adaptations have been characterized as an evolutionary arms race, an example of the coevolution of two species (Dawkins and Krebs 1979). In a gene centered view of evolution, the genes of predator and prey can be thought of as competing for the prey's body (Dawkins and Krebs 1979). 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 (Dawkins and Krebs 1979). 



    Figure \(\PageIndex{14}\): 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 learnt.


    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 (Abrams 1986), 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 (Brodie 1999).



    16.1.3 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 (Preisser et al. 2005; Peckarsky et al. 2008; Sheriff et al. 2020). 


    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 (Bond 2012). 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 (Bond 2012).  

    A side by side comparison of photos of the same site in panel A in 2002 and panel B in 2015 shows the transition of a small depression from mostly sparse small herbaceous plants and some larger shrubs to larger shrub-like plants and trees that cover more of the depression.

    Figure \(\PageIndex{15}\): Riparian willow recovery at Blacktail Creek, Yellowstone National Park, after reintroduction of wolves, the local keystone species and apex predator (Ripple and Beschta 2004). 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 (Ripple and Beschta 2004). 


    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 (Nelson et al. 2004). 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 (Krebs et al. 2001; Peckarsky et al. 2008; Krebs 2014; BBC 2015).


    A line graph with time on the x-axis and population on the y-axis shows prey populations oscillating between very high and very low populations, recovering slower than they decline, on regular intervals. Predator populations, represented by a dotted red line, oscillate on the same interval, with lower peak populations and a lag. When predators increase sharply, prey numbers decrease sharply, leading to a decrease in predators.

    Figure \(\PageIndex{16}\): 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 (Goel et al. 1971). 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 (Levin et al. 2009).

    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 (Murdoch et al. 2009; Nowak and May 2000). 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 (Levin et al. 2009). 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 (Genovart et al. 2010).

    Many factors can stabilize predator and prey populations (Rockwood 2009). 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 (Rockwood 2009). As a result, population cycles tend to be found in northern temperate and subarctic ecosystems because the food webs are simpler (Rockwood 2009). 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 (Rockwood 2009).

    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 an age structure, so that only some individuals reproduce; to live in a varying environment, such as with changing seasons (Cushing 2005; Thieme 2003); and analyzing the interactions of more than just two species at once. Such models predict widely differing and often chaotic predator-prey population dynamics (Cushing 2005; Kozlov and Vakulenko 2013). The presence of refuge areas, where prey are safe from predators, may enable prey to maintain larger populations but may also destabilize the dynamics (Sih 1987; McNair 1986; Berryman et al. 2006; Ross and Garay 2009).  


    Alternative Stable States and Predator-Prey Dynamics \(\PageIndex{1}\)

    Case study based on the CC-BY Paper By: 
    Glaspie, C. N., Seitz, R. D., & Lipcius, R. N. (2020). Are predator-prey model predictions supported by empirical data? Evidence for a storm-driven shift to an alternative stable state in a crab-clam system. Marine Ecology Progress Series, 645, 83-90.


    Predators play a key role in ecosystem stability and function by consuming dominant competitors (Lubchenco & Gaines 1981, Boudreau & Worm 2012). Predators can also destabilize ecosystems or collapse food webs if they become too abundant (Estes et al. 2009), or if their prey do not have natural defenses against predation (Johnston et al. 2015). One of the ways the balance between predator and prey adaptations manifests itself in nature is through density-dependent predation. Predators can exhibit a numerical response to prey densities by increasing reproduction rates due to an overabundance of prey (demographic response) or by gathering in areas with relatively high densities of prey (aggregative response) (Holling 1959). An individual predator may also adjust its predation rate to prey density through a ‘functional response’ (changes in a predator’s consumption rate in response to prey density). 


    Many mathematical models can be used to predict predator−prey dynamics (Briggs & Hoopes 2004). These models contain nonlinear functions describing the density-dependent interactions between predator and prey. Due to nonlinearities, model behavior often includes shifts to alternative stable states (Drake & Griffen 2010). The states may include extinction of one or both species, or coexistence steady states where both predator and prey are able to coexist at densities predicted by the model. The theory of alternative stable states predicts that ecosystems can exist under multiple "states" (sets of unique biotic and abiotic conditions). These alternative states are non-transitory and therefore considered stable over ecologically-relevant timescales. Ecosystems may transition from one stable state to another, in what is known as a state shift or regime shift, when perturbed. Due to ecological feedbacks, ecosystems display resistance to state shifts and therefore tend to remain in one state unless perturbations are large enough.


    In their study, Glaspie et al. (2020) examined evidence for a storm-driven shift to a low-density state for the soft-shell clam Mya arenaria, which was once a biomass-dominant species in Chesapeake Bay, USA, in the face of predation by the blue crab Callinectes sapidus. Tropical Storm Agnes, which reached and remained in the Chesapeake Bay watershed from 21 to 23 June 1972, has long been suspected of causing long-term changes to the Bay (Orth & Moore 1983). Tropical Storm Agnes was a ‘100-year storm’ that caused sustained, extremely low salinities and increased sedimentation throughout Chesapeake Bay (Schubel 1976, Schubel et al. 1976). This storm has been blamed for accelerating the loss of seagrass Zostera marina (Orth & Moore 1983) and oysters Crassostrea virginica (Haven et al. 1976) in Chesapeake Bay, although these losses were already in motion before the storm occurred. Unlike seagrass and oysters, declines in abundance of the soft-shell clam M. arenaria were uniquely related to the storm. M. arenaria was abundant enough to support a major commercial fishery throughout Chesapeake Bay prior to 1972 (Haven 1970). Widespread mass mortality of M. arenaria occurred after the storm (Cory & Redding 1976), and the fishery never recovered in lower Chesapeake Bay (Virginia) (Glaspie et al. 2018).


    A time series with a double x-axis shows the years 1960-1990 and the y-axes show “clam landings in 1000 bushels” from 0 to 800 on one side and “adult female crab index of abundance” from -1 to 2 on the other. Initially, the blue dashed C. sapidus crab line showed peaks parallel to the red M. arenaria clam line, lagging by about 1 year. Both species saw a drastic decline at the vertical 1972 “Tropical Storm Agnes” line. After the storm, peaks in the clam line fell about 1 year behind that of the crabs.

    Figure \(\PageIndex{17}\): Though the boom-and-bust cycle of the species continued after the Tropical Storm Agnes, the storm clearly impacted the abundance of the populations and disrupted the pattern of their population cycles.

    Using a 30-year dataset on soft-shell clams and blue crabs, Glaspie et al. (2020) found that Mya arenaria was subjected to a storm-driven shift to a low-density alternative stable state, which has been maintained by blue crab predation in Chesapeake Bay. Before the storm, clams were likely prey for juvenile crabs that entered the fishery at 1 yr of age, resulting in a positive correlation between crab and clam abundance with a lag of 1 yr (Fig. 2a). After the storm, crabs were likely consuming juvenile clams that would have been large enough to enter the fishery a year later, resulting in a negative correlation between clams and crab abundance with a lag of 1 yr (Fig. 2b). Both M. arenaria and Callinectes sapidus enter their respective fisheries after 1−1.5 yr (Newell & Hidu 1986, Lipcius & van Engel 1990), providing an explanation for the 1 yr lag. This is consistent with a shift from a system controlled from the bottom up (i.e., by the abundance of prey resources), to a system controlled from the top down by predation pressure on bivalves.


    Both panels show scatter plots with “clam landings in 1000 bushels” on the x-axis and “adult female cram index of abundance” on the y axis. Panel A, titled “Before the storm'', shows data points that have a positive trendline with the equation “y equals 0.019 plus 0.002 x”, and R squared value of 0.43 and a P value of 0.01. Panel B, titled “After the storm'', shows data points that have a negative trendline with the equation “y equals 1.379 minus 0.005 x”, and R squared value of 0.22 and a P value of 0.04.

    Figure \(\PageIndex{18}\): The change in trendlines from a positive correlation before the storm to a negative correlation after the storm again shows the impact natural disasters can have on the patterns of population cycles.

    16.1: Predation is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.