Forms of Predation 14.1
Predation is defined generally as the consumption of all or part of
one living organism by another. Forms of predation include carnivory,
parasitoidism, cannibalism, and herbivory.
Model of Predation 14.2
A mathematical model that links the two populations through the
processes of birth and death can describe interactions between predator
and prey. Predation represents a source of mortality for the prey
population, whereas the reproduction of the predator population is
linked to the consumption of prey.
Population Cycles 14.3
The models of predator–prey interactions predict oscillations of
predator and prey populations, with the predator population lagging
behind that of the prey population.
Mutual Population Regulation 14.4
The results of the models assume mutual regulation of predator and
prey populations. The growth rate of the prey population is influenced
by the per capita consumption of prey by the predator population. The
relationship between the per capita rate of consumption and the number
of prey is referred to as the predator’s functional response. This
increased consumption of prey results in an increase in predator
reproduction referred to as the predator’s numerical response.
Functional Response 14.5
There are three types of functional responses. In Type I, the number
of prey affected increases linearly. In Type II, the number of prey
affected increases at a decreasing rate toward a maximum value. The Type
II response is a function of allocation of feeding time by predators
between the activities of searching for prey and handling prey (chasing,
capturing, killing, consuming, etc.). In Type III, the number of prey
consumed increases sigmoidally as the density of prey increases.
Numerical Response 14.6
A numerical response is the increase of predators with an increased
food supply. Numerical response may involve an aggregative response: the
influx of predators to a food-rich area. More important, a numerical
response involves a change in the growth rate of a predator population
through changes in fecundity.
Optimal Foraging 14.7
Central to the study of predation is the concept of optimal foraging.
This approach to understanding the foraging behavior of animals assumes
that natural selection favors “efficient” foragers, that is,
individuals that maximize their energy or nutrient intake per unit of
effort. Decisions are based on the relative profitability of alternative
prey types, defined as the energy gained per unit of handling time. An
optimal diet includes the most efficient size of prey for handling and
net energy return.
Foraging Behavior and Risk of Predation 14.8
Most predators are also prey to other predatory species and thus face
the risk of predation while involved in their routine activities, such
as foraging. If predators are about, it may be to the forager’s
advantage not to visit a most profitable but predator-prone area and to
remain in a less profitable but more secure part of the habitat.
Coevolution of Predator and Prey 14.9
Prey species evolve characteristics to avoid being caught by
predators. Predators have evolved their own strategies for overcoming
these prey defenses. This process represents a coevolution of predator
and prey in which each functions as an agent of natural selection on the
other.
Predator Defenses 14.10
Chemical defense in animals usually takes the form of distasteful or
toxic secretions that repel, warn, or inhibit would-be attackers.
Cryptic coloration and behavioral patterns enable prey to escape
detection. Warning coloration declares that the prey is distasteful or
disagreeable. Some palatable species mimic unpalatable species for
protection. Armor and aggressive use of toxins defend some prey. Alarms
and distraction displays help others. Another form of defense is
predator satiation wherein prey species produce many young at once so
that predators can take only a fraction of them. Predator defenses can
be classified as permanent or induced.
Predator Evolution 14.11
Predators have evolved different methods of hunting that include
ambush, stalking, and pursuit. Predators also employ cryptic coloration
for hiding and aggressive mimicry for imitating the appearance of prey.
Herbivory 14.12
Herbivory is a form of predation. The amount of plant or algal
biomass actually eaten by herbivores varies between communities. Plants
respond to defoliation with a flush of new growth, which draws down
nutrient reserves. Such drawdown can weaken plants, especially woody
ones, making them more vulnerable to insects and disease. Moderate
grazing may stimulate leaf growth in grasses up to a point. By removing
older leaves less active in photosynthesis, grazing stimulates the
growth of new leaves.
Herbivore Defenses 14.13
Plants affect herbivores by denying them palatable or digestible food
or by producing toxic substances that interfere with growth and
reproduction. Certain specialized herbivores are able to breach the
chemical defenses. They detoxify the secretions, block their flow, or
sequester them in their own tissues as a defense against predators.
Defenses can be either permanent (constitutive) or induced by damage
inflicted by herbivores.
Vegetation–Herbivore–Carnivore Systems 14.14
Plant–herbivore and herbivore–carnivore systems are closely related.
An example of a three-level feeding interaction is the cycle of
vegetation, hares, and their predators. Malnourished hares fall quickly
to predators. Recovery of hares follows recovery of plants and decline
in predators.
Lethal and Nonlethal Influences 14.15
Besides influencing prey population directly through mortality,
predators can cause changes in prey characteristics by inducing defense
responses in prey morphology, physiology, or behavior. Reduced activity
by prey in the presence of predators can reduce foraging time and food
intake, subsequently delaying growth and development. The net result can
be a reduction in the growth rate of the prey population.
Fisheries Management Ecological Issues & Applications
The harvesting of natural fish populations often leads to
overexploitation and population decline. Management practices based on
sustainable yield attempt to limit harvests to levels at which natural
recruitment (reproduction) offsets mortality resulting from fishing
activities.
