Oct 22 2013
Introduction to Predation & Herbivory
Over half the species on Earth get energy by feeding on other organisms, in a variety of types of
interactions. Some kill-then eat, others graze on living organisms.
All are exploitation—a relationship in which one organism benefits by feeding on, and directly
harming, another. (+/-)
Herbivore: eats tissues of living plants or algae.
Predator: kills and eats other organisms, referred to as prey.
Parasite: lives in or on another organism (its host), feeding on parts of it. Usually doesn't’t kill
the host. Some parasites (pathogens) cause disease. Individual parasites usually feed on only
one or a few host individuals.
Not all organisms fit neatly into these categories. Some predators, such as wolves, also eat
berries, nuts, and leaves. Some herbivores (sheep) eat helpless birds as well as plants.
Parasitoids: insects that lay an egg on or in another insect host. After hatching, larvae
remain in the host, which they eat and often kill. They are unusual parasites (consume
most or all of the host, usually killing it) or unusual predators (over lifetime, feed on only
one host individual)
Predators & Herbivores
Predators and herbivores both tend to feed on multiple individuals of prey or food plants in
their lifetimes (diff from parasites in this way)
Herbivores don’t usually kill the food organisms as predators do. There are exceptions.
Herbivores that eat seeds can be considered predators as each seed contains a unique genetic
individual which you are essentially killing. Additionally, some herbivores can strip entire regions
bare of vegetation which causes many plants to die.
Some predators forage throughout their habitat in search of food (wolves, sharks, hawks)
Others are sit-and-wait predators, remaining in one place and attacking prey that move within
striking distance or enter a trap (spiders web)
These include sessile animals, such as barnacles, and carnivorous plants.
In either case, predators tend to concentrate effort in areas that yield abundant prey. (whether
foraging or sit & wait)
Wolf packs follow seasonal migrations of elk herds. Sit-and-wait predators such as spiders
relocate from areas of scarce prey to
areas where prey are abundant.
(more spider webs where prey is
Most predators eat a broad range of prey
species (in relation to what is most
abundant), but specialist predators show a
preference (e.g., lynx eat more hares than
expected based on hare abundance).
Some predators concentrate on whatever
prey is most abundant. For example, when
researchers provided guppies with two kinds of prey, they ate disproportionate amounts of
whichever prey was most abundant. These predators may form a search image of the most
common prey type and orient toward that prey. Or, learning enables them to become
increasingly efficient at capturing the most common prey. Predators like these tend to switch
from one prey species to another. (in some cases, prey switching is consistent with optimal
Herbivores eat different parts of the plant
(how we group them)
Large herbivores may eat all
aboveground parts, but most specialize
on particular plant parts
Leaves are most commonly eaten,
because they are often the most
nutritious part, except for seeds (high
nitrogen content. However, compared to animals, the nitrogen content in leaves is low) Leaves
are also typically available year round.
Herbivores can reduce the growth, survival, or reproduction of plants. Herbivores that eat seeds
can impact reproductive success. By removing photosynthetic tissues, there is a great reduction
in plant growth (belowground herbivores have impact on growth)
Most herbivores have narrow diets. This is largely a result of insects. Most herbivory insects only
feed on one (or a few) plant species. Some herbivores feed on plant fluids by sucking sap, etc. Other herbivores (e.g., grasshoppers) feed on a wide range of species. Large browsers, such as
deer, often switch from one tree or shrub species to another. In addition, they eat all or most of
the above ground parts of plant species.
Adaptations to Exploitative Interactions
Organisms have evolved a wide range of adaptations that help them obtain food and avoid
Life changed radically with the appearance of macroscopic predators about 530 million years
ago (organisms large enough to be visible to the naked eye)
Within a few million years, after the evolution of the first large predators, prey had evolved
defenses, such as body armor and spines.
Predators exert strong selection pressure on their prey: If prey are not well defended, they die.
Herbivores exert similar selection pressure on plants (although death is typically not result)
Adaptations to Escape Being Eaten
Large size (elephants).
Rapid movement (gazelles).
Body armor (snails, sea urchins).
Poison: Species use poisons to defend themselves against predators. Species that contain toxins
are often brightly colored (aposematic mechanism)
Warning (aposematic) coloration: Species that use poisons to defend themselves use warning
coloration as a first line of defense against predators. Predators learn not to eat organisms that
are brightly colored as they may have toxins.
Mimicry: Mimicry has two forms. Organisms can use it to resemble less palatable organisms or
physical features of the environment (less desirable prey) Crypsis is another form of mimicry which is a form of camoflauge (resembling background) Finally, the prey can use it to resemble
another organism that is toxic or very fierce (harmless flies that resemble yellow jacket wasps)
Not foraging in open areas when predators are abundant (where prey is most
Defensive circles (Muskoxen)
There can be trade-offs between behavioral and physical defenses. Snails with
thickest shells are the last to take refuge. Snails with shells that break easily are
quickest to take refuge.
Plants also have defenses.
Some produce huge numbers of seeds in some years and hardly any in other years
(masting). Masting allows plants to hide (in time) from seed-eating herbivores, then
overwhelm them by sheer numbers.
Plants can also produce their leaves at times of year when herbivores are scarce
Some plants have growth responses that allow them to tolerate effects of herbivory.
Compensation: Removal of plant tissue stimulates new growth. Compensation can be
full or partial
Full compensation—no net loss of plant tissue.
Compensation may occur when the removal of leaves can decrease self-shading,
resulting in increased plant growth. Removal of apical buds (those at ends of
branches) may allow lower buds to open and grow.
In field gentians (type of plant), early herbivory results in compensation, but later in the
season it does not. If too much tissue is removed, or there aren’t enough resources for
growth, compensation cannot occur. The below image shows that clipped plants grew
more branches, and produced more flowers than unclipped plants. Plants clipped on July 12 produced the most fruits. Plants clipped on July 28 did not have time to
compensate fully for their loss of the tissues
Plants have many structural defenses:
Tough leaves, spines and thorns, saw-like edges, pernicious (nearly invisible) hairs that
can pierce the skin. Some are induced defenses which are produced in response to
herbivore attack. (some cacti increase spine production after being grazed)
Plants also have chemical defenses:
Secondary compounds: function to reduce herbivory. Some are toxic. Other compounds
attract predators or parasitoids that will attack the herbivores.
Some are produced all the time; others are induced defense.
Predators/herbivores exert strong selective pressures on their prey and food plants, which can
lead to improvements in the defenses of those organisms. This in turn imposes a strong
selective pressure on predators and herbivores.
For any prey defense mechanism, there is usually a predator with a countervailing offense.
Cryptic prey could be detected by smell or touch instead of sight.
Physical features for prey capture
Snakes can swallow prey that are larger than their heads. The skull bones aren’t rigidly
attached to one another, which allows the snake to open its jaws massively.
Body form of a cheetah enables large bursts of speed to catch its prey
Some predators subdue prey with poisons (e.g., venomous spiders).
Some use mimicry, blending into their environment so that prey are unaware of their presence.
Some have inducible traits (e.g., a ciliate that adjusts its size to match the size of the available
prey). If the ciliate is small and its prey is large it will increase in size (vice versa)
Some predators can detoxify or tolerate chemicals made by prey organisms.
Garter snakes are able to eat the rough-skinned newt, which has large amounts of
tetrodotoxin (TTX), a potent neurotoxin. Some garter snake populations are resistant to
TTX, but there are costs. Resistant garter snakes can’t move as fast as nonresistant
snakes. In addition, once the snakes swallow a poisonous newt, they are immobilized for up to 7 hours. During this time it is vulnerable to predation and may suffer heat
Plant defenses can also be overcome by herbivores.