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Predation on Animals
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Article Contents
Kathleen L Prudic, Yale University, New Haven, Connecticut, USA
. Introduction
. Classification and Diversity of Predator Lifestyles
. Prey Specialization and Trophic Structure
. Predicting the Population Effects: Ecological Modelling of
Predator–Prey Dynamics
. Predation and Community Dynamics
. Evolution of Prey Defences and Predator Counterdefences
. Predation and Macro-evolution in the Fossil Record
. Summary and Conclusions
Online posting date: 15th December 2009
The word predator often invokes a vision of a fierce and
cruel animal the world would be better without. However,
predation is a major ecological process controlling both
the structure and function of communities. Predation
affects the distribution and abundance of species, the
strength and direction of energy flow within a system and
the diversity and composition of communities. Predators
play an essential role in evolution. Traits that decrease the
likelihood of being predated and traits that increase the
efficacy of the predating are under strong selection. This
process has resulted in a vast array of prey defences and
predator counter-defences. Also, according to recent
studies of the fossil record, predation has played a central
role in determining the history of life on earth. Thus,
predators are not to be viewed as cold-blooded killers;
instead, predation is a critical process for maintaining
species diversity in both ecological and evolutionary time.
Introduction
All organisms must procure energy to grow and reproduce.
Some animals hunt and others are hunted. Predation is a
class of ecological interactions where one species actively
pursues and consumes another animal. The consequence of
this interaction is certain death for the prey. Interactions
between prey and predator affect both population
dynamics and community structure in most, if not all,
habitats. The threat of predation is also a major driver in
evolution and it has long shaped animal form and function.
ELS subject area: Ecology
How to cite:
Prudic, Kathleen L (December 2009) Predation on Animals. In: Encyclopedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester.
DOI: 10.1002/9780470015902.a0003284
Prey species have evolved specific defensive traits and
behaviours to reduce the likelihood of being predated.
Predators, in turn, have countered by evolving new ways to
detect, capture and consume prey. This coevolutionary
arms race between the prey and their predators has led
to substantial behavioural and morphological changes
through time and adaptive radiations in the fossil record
(Vermeij, 2002; Kelley and Kowalski Hansen, 2003).
Our current framework for understanding predator–
prey interactions stems mainly from a discovery of regular
cycles of abundance in arctic mammals and birds (Elton,
1924). These cycles are thought to reflect basic properties of
all predator–prey interactions (Elton, 1924). Lotka (1925)
and Volterra (1926) developed mathematical models that
describe the conditions for population oscillations in
predator–prey interactions thus lay the foundation for the
study of population dynamics and community structure.
These ideas about present day predator–prey interactions
have been applied to longer time scales in the fossil
record with similar outcomes (Vermeij, 2002; Kelley and
Kowalski Hansen, 2003).
The study of predation is broad and includes activities
such as searching, handling and consuming prey, adaptations of the prey and predator and processes that result in
their coexistence. Thus, predation integrates the fields
of animal behaviour, ecology, evolution, neurobiology,
paleontology, physiology and psychology. This article
highlights the role predation plays in ecological functioning
and evolutionary pattern. As it stands, surprisingly little is
known about predator and prey relationships in nature, past
and present. What we currently know is a complicated
mixture of field observations, experiments with laboratory
systems, mathematical models of population processes and
ancient and current patterns of evolution.
Classification and Diversity of Predator
Lifestyles
Predation, in the broad sense, is the consumption of
another living organism and it includes herbivores,
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Predation on Animals
omnivores, carnivores, parisitoids and parasites. However,
here we narrow our scope and only consider animals which
actively catch and consume other animals by killing them.
Predators range in form from speedy cheetahs to sessile anemones. Swimming, flying, running and waiting,
predators attack from both above and below. Predators
have a diverse arsenal ranging from physical strength to
poisonous venom, to capture and consume prey. Some
species of birds, mammals and fish are active predators,
patrolling a territory searching for prey. These predators
benefit by procuring large amounts of nutrients in a short
amount of time. However, all that chasing and hunting is
energetically expensive and it exposes the predator to a
greater likelihood of predation. Other species such as orb
weaving spiders, pit viper snakes and anemones adopt an
ambush approach letting the prey come to them, usually in
a location where the prey are foraging or mating. These
predators spend less energy capturing and consuming prey
and are exposed to fewer predators themselves. The cost of
this approach is a lower rate of nutrient acquisition.
Regardless of form, predators have evolved a variety of
adaptations to capture their prey across habitats. See also:
Parasitism: Life Cycles and Host Defences against Parasites; Predation (Including Parasites and Disease) and
Herbivory
Prey Specialization and Trophic
Structure
Predators have several constraints affecting what they eat
and when they eat. Prey varies in both quantity and quality.
The classic models of optimal foraging use prey energy as a
predictor of predator feeding behaviour; however, this
behaviour is affected by more than just potential calories
(Sih and Christenson, 2001). Predators themselves may be
prey for other predators, and hunting, in particular, places
a predator at risk for becoming a meal for another predator. Thus, predators, like prey, trade-off between the
benefits of energy intake and procuring a mate against the
reproductive costs of an early demise. Generalist predators
have a broader resource base and suffer less competition
among and within species. Although even with the broader
prey selection, generalist predators, such as the Northern
goshawk, are sometimes food limited (Rutz and Bijlsma,
2006). This generalist strategy also trades-off against an
increase in exposure to predators and a decrease in capture
success.
Predators employ other strategies to reduce costs. They
may evolve prey specialization, meaning the predator feeds
on a limited type, or suite, of prey. For example, snail kites
specialize on apple snails and concentrate their foraging
efforts on areas with the highest density of snails (Bourne,
1985). Other predators are less specialized and are able
to switch their choice to more available or abundant
prey through time or space. For example, ruddy turnstones demonstrate flexible specialization based on the
2
dominance hierarchy within the flock and available
resources (Whitfield, 1990). Specialization is also affected
by other factors such as ability to detect, capture and
handle prey. Finding, catching and consuming prey might
be an acquired technique taking extensive experience to
develop. As such, switching among multiple prey types is
not always efficient or even feasible for the predator.
Predators can also be restricted to a certain habitat based
on their own physiological, reproductive or ecological
requirements, and as a consequence they hunt and consume a certain suite of prey. Predators can be specialized
based on specific nutritional needs or digestive abilities.
Predator and prey may have coevolved different adaptations making switching between other prey types not
efficient or effective. See also: Foraging
Trophic structure from specialization results in energy
flow through a community. Simply organized, a predator
occupies one trophic level and its prey another. However in
nature, most predators eat at several trophic levels, sometimes even scavenging (Polis and Strong, 1996). As a consequence, prey experiences multiple predator effects and
predators experience multiple prey effects (Sih et al., 1998).
These multiple effects certainly muddy the waters of identifying the role of predation in ecological patterns, but both
empirical and theoretical studies suggest that the structure
and the strength of the trophic links have strong influences
on the abundance, productivity and stability of the populations in the community. For example, predation has a
positive influence on the efficiency of energy flow. High
predation levels by birds and mammals in desert ecosystems results in more effective energy flow from primary
productivity to the top predators (Ayal, 2007). Trophic
interactions and predation affect every level of energy
transfer all the way down to primary productivity. See also:
Food Webs; Nonconsumptive Effects of Predators and
Trait-Mediated Indirect Effects
Predicting the Population Effects:
Ecological Modelling of Predator–Prey
Dynamics
A major task in ecology is to understand how both prey and
predator populations grow and persist through time. This
includes documenting the patterns of predator and prey
distribution and abundance, and explaining why there are
differences between different systems. These populations
are more than simple collections of individuals; their life
history and ecological interactions are also important.
Central to this understanding is explaining how prey and
predators coexist in populations. If predators are completely efficient and always have the upper hand, then they
would eliminate all prey and both parties would go extinct.
However, in many natural systems, we observe predators
and prey coexisting through time. The sheer number of
ecological factors which may contribute to the continued
existence of predators and prey makes it challenging to
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Predation on Animals
The Type 1 functional response is a direct relationship in
which the predator consumes all of the prey available up to
a certain saturation point (Figure 1a). After saturation, the
prey density continues to increase linearly with no effect on
how much prey is being eaten. The Type 2 functional
response incorporates predator handling time. Handling
time is the time needed for pursuing, subduing and
40
30
20
10
5
Individual predation rate (prey killed per unit time)
determine which factors are of primary importance. Hence,
mathematical modelling approaches have helped identify
the key ecological parameters important in generating
these observed patterns. See also: Coexistence
Predator–prey coexistence reflects a balance between
stabilizing and destabilizing factors. Predation directly
affects prey death rate and predator birth rate. With many
predators, the prey population shrinks; conversely with
many prey, the predator population grows. Below a certain
number of predators, the prey population always increases,
and above that number of predators the prey population
always decreases. There can be coupled oscillations with a
time lag between prey abundance and predator abundance
as seen in the Canadian lynx and the snowshoe hare (Krebs
et al., 1995).
This cyclic pattern of predator prey dynamics was first
mathematically described by the Lotka–Volterra model, a
pair of differential equations that describe predator–prey
dynamics in their simplest case (Lotka, 1925; Volterra,
1926). This model is characterized by oscillations in the
population size of both predator and prey, with the peak of
the predator’s oscillation lagging slightly behind the peak
of the prey’s oscillation. The model makes several simplifying assumptions: first, the prey population will grow
exponentially when the predator is absent; second, the
predator population will starve in the absence of the prey
population as opposed to switch to another type of prey;
third, predators can consume infinite quantities of prey and
fourth, there is no environmental complexity so both
predator and prey populations are moving randomly
through a homogeneous environment. Many examples of
cyclical relationships between predator and prey populations have been demonstrated in the laboratory or
observed in nature (Krebs et al., 1995; Yoshida et al., 2003).
But in general, more complex models incorporating terms
that represent predator and prey carrying capacity and
environmental complexity provided better explanations of
population dynamics (Rosenzweig and McArthur, 1963).
At least three factors are likely to promote stability and
coexistence: refuges for prey, alternative prey for predators
and other predator–predator interactions. Additionally, as
both predator and prey can undergo genetic change during
these oscillations, the effects of selection should be considered in the understanding of predator–prey population
dynamics (Yoshida et al., 2003). See also: Population
Dynamics: Introduction; Nonlinear Dynamics and Chaos
Other mathematical model approaches suggest different
mechanisms for how predator and prey coexist and which
prey defences are most beneficial at different prey densities
(Solomon, 1949; Holling, 1965). The Lotka–Volterra
model assumes the predator population increases numerically when predators consume more prey, which may not
always be the case (Hassel, 1978; Abrams and Ginzburg,
2000). One common approach categorizes different functional responses based on prey density and the amount of
prey consumed. It assumes an increase in the number of
prey consumed per unit time by each individual predator in
relation to prey density.
(a) 0
40
30
20
10
5
(b) 0
40
30
20
10
5
0
0
(c)
25
50
75
100
Prey density
Figure 1 Functional responses of predator–prey interactions (a) Type 1,
(b) Type 2 and (c) Type 3.
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Predation on Animals
consuming the prey, and then preparing for further search.
In this type of response, the relationship between prey
density and consumption is not linear. At low prey densities, the consumption rate increases with density, but
eventually, as prey density continues to increase, there is a
decline in the rate at which prey consumption increases
until a maximum level is reached. This gradual deceleration
of consumption reflects the handling time parameter
(Figure 1b). The Type 3 functional response is similar to
Type 2 at high prey densities, but includes the additional
parameter of very little or no prey consumption when prey
is at low densities (Figure 1c). This pattern results from a
combination of three factors: the ability of prey to hide, a
search image for the predator and prey switching by the
predator. In this scenario, prey at low densities increase
irrespective of predator density, and the predator abundance is largely independent of the prey abundance.
Predation and Community Dynamics
Predator–prey interactions have long captured the attention of community ecologists for good reason. Predators
remove prey from the community which directly affects
community composition in both species abundance and
richness. In New Zealand, a non-native brown trout decimated the native aquatic arthropod fauna, greatly reducing
both species abundance and richness (Townsend, 1996).
Changes in predator abundance can alter the distribution
and abundance of primary producers on a communitywide basis, an effect known as a trophic cascade (Carpenter
et al., 1985). Trophic cascades have been shown to occur
following the removal of predators preying upon herbivores (Schmitz et al., 2000), but controversy still exists
whether community-level trophic cascades might be
widespread and strong in terrestrial ecosystems (Polis and
Strong, 1996; Schmitz et al., 2000). See also: Trophic
Cascades
There are several indirect responses of predation on
community dynamics. Removing a predatory starfish from
the community decreases species richness and diversity due
to competition among prey species (Paine, 1966). The
effects of predation may be negligible relative to other
biotic or abiotic factors; for instance, large mammalian
diversity on the Serengeti is unaffected by the active
removal of predators (Bertram, 1978). Prey respond to
predator presence and species richness in costs to prey
survival, growth, body condition and reproduction. For
example, predator presence increases glucocorticoid concentrations in female snowshoe hares causing a decline in
their reproductive output both in both number and size of
offspring (Sherrif et al., 2009).
Community dynamics and predation have economical
implications for invasive species biology and conservation management of natural and agricultural systems.
The strength of predator–prey interactions varies tremendously within and between communities. Communities dominated by weak interactions may be more
4
resilient to non-native invasions (Lonsdale, 1999). Communities with strong interactions may be more responsive
to integrative pest management mediated by introduced
predators (Zhang et al., 2008). See also: Community
Ecology: An Introduction; Species Richness: Small Scale
Evolution of Prey Defences and
Predator Counter-defences
Natural selection has shaped prey to balance safety from
predation against the acts of foraging and reproducing.
Prey species have multiple approaches, or adaptations, to
avoid being consumed, whereas predators counteract these
defences with different ways to find and capture prey. In the
broadest sense, these prey strategies can be divided into two
classifications: limiting prey detection and limiting prey
consumption. Prey that limits exposure to or detection by
predators are never discovered by predators, and as a
consequence have a lower probability of predation. This
could be as simple as avoiding predator environments
either in time or space, usually at the cost of foraging success. For example, garden skinks avoid foraging in areas
where predators are present (Downes, 2001), and leafcutting ants are active at night when their parasitoids are
absent (Curie et al., 1999). A more complex prey defence is
camouflage, or cryptic coloration in relation to the local
background. To counter, some predators such as blue jays
have evolved better edge detection to see the prey outline
even when the prey is matched with its background (Bond
and Kamil, 2006). Other predators counter by being cryptic
themselves such as crab spiders (Figure 2). An African
assassin bug conceals itself from prey with bits of sand and
the remnants of small insects from previous meals (Brandt
and Mahsberg, 2002). Other predators lure cryptic prey to
a more conspicuous location because camouflage is
dependant on a particular background. The bolas spider
does not use a typical spider web to capture its prey;
instead, it produces a single thread of silk with a sticky
substance on one end which contains a chemical cue that
mimics a female moth pheromone (Eberhard, 1980). Male
moths leave their cryptic backgrounds and fly towards the
spider, expecting to find a mate. The spider uses its sticky
silk strand to catch the now conspicuous moth.
Another prey strategy is to limit consumption by
predators once detected. This can be a physical defence
such as weapons (e.g. horns, antlers or teeth) or ability to
escape (e.g. speed or agility). Some prey such as rabbits or
songbirds use conspicuous, loud screams in an attempt to
attract other predators to increase the likelihood of interference by other predators, allowing the prey to escape
(Wise et al., 1999). Predators counter this defence with
having more weapons themselves or dispatching their prey
quickly (Wise et al., 1999). Many insects, amphibians and
marine invertebrates employ toxic chemicals to deter
predators. These toxins are either produced by the prey
de novo or sequestered from food sources; virtually every
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Predation on Animals
Trials to criteria
8
6
4
2
0
(b)
Days to resampling
Predator behavioural response
(a)
20
15
10
5
0
Cryptic prey
Low luminance contrast
(c)
Figure 2 The prey defence and predator counter-defence. A warningly
coloured wasp is being consumed by a camouflaged crab spider. Reprinted
with permission of Jeffrey C Oliver http://commons.wikimedia.org/wiki/
File:Crab_spider.JPG.
class of chemical compound is used by some prey species to
defend against predation. Predators counter chemical
defences by developing resistance, eating the chemically
undefended parts of the prey or eating only a few chemically defended individuals. For example, black-backed
orioles eat large amounts of monarch butterflies by consuming only the palatable parts of the monarch body.
However, another predator, the black-headed grosbeak,
has developed insensitivity to the toxic cardenolides found
in the monarch. Both bird species periodically reduce their
consumption of monarchs to avoid harmful levels of toxic
accumulation (Brower, 1988).
In either physical or chemical defence, prey may advertise their unprofitability to predators. For example,
Thompson’s gazelles leap several feet into the air while
flaring their white rump patches when they detect a
predator. This conspicuous behaviour communicates to
the predator that they are hard to capture and the predators
are more likely to abandon the hunt (Caro, 1986). Aposematism, also known as warning coloration, is a way prey
communicate to predators that they are toxic and not good
energy sources. This bright colour pattern signal is not only
easier for the predator to detect, but it has also been shown
to be easier to learn and less likely to be forgotten (Alatalo
and Mappes, 1996). Warning coloration has two components: hue, the wavelength of the signal, and luminance,
the grey scale brightness of the signal. Luminance alone can
provide enough information from prey to predator for the
Conspicuous prey
High luminance contrast
Unpalatable prey treatment
Figure 3 Mantids are colour-limited predators with only one known opsin.
However, even with limited visual capabilities they still learn to avoid bright
conspicuous prey. Warning coloration is not a signal limited to only birds or
other predators with complex visual systems. Mantid photo reprinted with
permission of Alex Wild khttp://www.alexanderwild.coml. Reprinted with
permisison of the Oxford University Press from Prudic et al. (2007).
functional benefits of increased predator learning and
memory retention (Prudic et al., 2007). Thus, warning
coloration can even evolve as an effective signal to colourblind or colour-limited predators (Figure 3).
Social defences are another way prey combat predators
once detected. This can be either as social aggregation or
group vigilance. The former strategy uses the dilution effect
against attacking predators; the more prey available, the
less likely the individual will be eaten. This is a well-known
approach in large herbivorous mammals and roosting
birds, but it also occurs in many insects. The more female
mayflies that emerge together on a single evening, the less
likely a single mayfly is eaten by a predator and the more
likely a female mayfly will reproduce (Sweeney and Vannote, 1982). Group vigilance is another social defence used
by many prey in multiple environments. Fathead minnows
communicate to each other about potential predators via
water-borne alarm pheromones (Chivers and Smith, 1994).
Other groups of prey fight back against predator attack.
African honey bees, for example, are renowned for their
ferocity to creatures that threaten the hive and sting much
more frequently as compared to their European counterparts (Guzmán-Novoa and Page, 1994). Predators can
counter prey social defences by being social hunters or
attacking quickly before being detected. See also: Mimicry;
Predator Avoidance; Signalling and Reception
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Predation on Animals
Predation and Macro-evolution
in the Fossil Record
The role of predation in generating macro-evolutionary
patterns is more contentious. At the core of the argument is
a discussion of whether biotic factors are equally important
as physical factors in generating evolutionary patterns in
the fossil record (Gould and Eldredge, 1993). On the one
hand, predation has been thought to influence the rates of
species diversification and extinction (Conway Morris,
1998; Jablonski, 2000). Two processes in particular are
thought to play an important role in these patterns of
macro-evolution: coevolution, or the reciprocal evolutionary change between prey and predator (Futuyma and
Slatkin, 1983), and escalation, or adaptation without
reciprocal change (Vermeij, 1994). Conversely, others
think that random physical occurrences are a better
explanatory mechanism for the evolution of species over
long periods of time (Gould, 1990). Thus, mass extinctions
caused by random physical events completely overwhelm
the evolutionary history generated from natural selection
among organisms. See also: Coevolution; Diversity of Life
through Time
Recent work using fossil data has led to preliminary and
exciting tests of the different perspectives. Accumulating
evidence indicates predation and the process of escalation
also played a key role in Phanerozoic ecosystems and
influenced biological diversification (Kelley and Kowalski
Hansen, 2003; Huntley and Kowalewski, 2007). Bivalves,
gastropods, trilobites and brachiopods with their extensive
fossil records have been very informative and this data
demonstrate that escalation can explain some fossil patterns. Other taxa with less complete fossil records such as
cephlapods, bryozoans and dinosaurs are providing additional support for the important role predator–prey
interactions play in species diversification (reviewed in
Kelley and Kowalski Hansen, 2003). This is an exciting
area in predation research with much left to be discovered.
Future opportunities on understanding the role of predation in generating patterns in the fossil record are
numerous and will provide profound insight into the history of life on earth. See also: Coexistence; Mesozoic
Marine Revolution
Summary and Conclusions
The interactions between predators and prey are so varied
and complex that they almost defy summary. Predators are
the primary movers of energy through communities and
are an important factor in the ecology of populations. The
amount and frequency of predation determines the mortality of prey and birth of new predators. Mathematical
models and empirical research suggests that a specialized
predator and its prey should oscillate: predators increase
when prey are abundant, prey are driven to low numbers by
predation, the predators decline and the prey recover.
6
Some simple systems do cycle, but in most systems, alternative prey and complex trophic interactions probably
dampen simple predator–prey cycles. Predator–prey systems are potentially unstable at the population level. At
least three factors are likely to promote stability and
coexistence: refuges for prey, alternative prey for predators
and predator–predator interactions. At the community
level, predation has often been shown to have strong direct,
indirect and cascading effects. Predator–prey interactions
have economical implications for agriculture and land
management.
In response to complex predator–prey interactions,
traits that decrease the likelihood of being predated and
traits that increase the efficacy of predators are under
strong selection. Prey species evolve complex physical and
behavioural defences that reduce their vulnerability to
predators. Predators evolve other adaptations to counter
those defences. These changes are observed both in extant
species and also in the fossil record. This process has
resulted in a vast array of prey defences and predator
counter-defences. Ancient predation, just like the predator–prey interactions we observe today, played a central
role in determining ancient ecosystem structure. Thus
predation is important to understanding the history of life
on earth. Much future research is needed to unravel the
complexities of predator–prey interactions and its effects;
however, all in all, predation is a critical process
for maintaining species diversity in both ecological and
evolutionary time.
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Future Reading
Barbosa P and Castellanos I (eds) (2005) The Ecology of Predator
Prey Interactions. Oxford: Oxford University Press.
Ruxton GE, Sherratt TN and Speed MP (2004) Avoiding Attack:
The Evolutionary Ecology of Crypsis, Warning Signals and
Mimicry. Oxford: Oxford University Press.
Turchin P (2003) Complex Population Dynamics. Princeton:
Princeton University Press.
Vermeij GV (1993) Evolution and Escalation: An Ecological History of Life. Princeton: Princeton University Press.
ENCYCLOPEDIA OF LIFE SCIENCES & 2009, John Wiley & Sons, Ltd. www.els.net
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