Ecology Lecture No.12: Predation & Herbivory
Thursday October 18 , 2012
-Over half of the species on Earth engage in exploitation, a relationship in which one organism benefits
by feeding on, and directly harming, another. Herbivores eat tissues of living plants or algae. Predators
kill and eat other organisms, referred to as prey. Parasites live in or on another organism (its host),
feeding on parts of it. Usually doesn’t kill the host. Some parasites (pathogens) cause disease.
-Not all organisms fit neatly into these categories. Some predators, such as wolves, also eat berries,
nuts, and leaves. Parasitoids are insects that lay an egg on or in another insect host. After hatching, the
larvae remain in the host, which they eat and often kill. Although predators and herbivores both tend to
feed on multiple individuals of prey or food plants in their lifetimes, herbivores don’t usually kill the food
organisms as predators do.
-While some predators forage throughout their habitat in search of food, others are sit-and-wait
predators that remain in one place and attack prey that move within striking distance. These include
sessile animals, such as barnacles, and carnivorous plants. Predators tend to concentrate their efforts in
areas that yield abundant prey. For example, wolf packs follow seasonal migrations of elk herds. Sit-and-
wait predators will often move from areas of scarce prey to areas where prey is abundant.
-Predators have a wide range of feeding tactics: Most predators feed on whatever prey is abundant,
while others will specialize in hunting certain preferential species or concentrate on whatever prey is
most abundant. Such predators orient towards a specific prey type by either forming a search image of
the most common prey type or learning and enabling increasingly efficient methods at capturing the
most common prey. In some cases, prey switching is consistent with optimal foraging theory.
- Most herbivores feed on a narrow range of plants (only one or a few plant species) and will eat
different parts of that given plant. Large herbivores may eat all aboveground parts, but most specialize
on particular plant parts. Large browsers, such as deer, often switch from one tree or shrub species to
another, while other herbivores like grasshoppers feed on a wide range of species. Belowground
herbivores can also have an impact.
-Plant leaves are most commonly eaten by herbivores because they are often the most nutritious part,
excluding the seeds. In this way herbivores can reduce the growth, survival, or reproduction of plants
(especially by eating seeds). Some herbivores feed on plant fluids by sucking sap. Adaptations To Exploitative Interactions:
-Organisms have evolved a wide range of adaptations that help them obtain food and avoid being eaten.
Within a few million years of macroscopic predators first appearing 530 million years ago, prey had
evolved defenses, such as body armor and spines. This is because predators exert a strong selection
pressure on their prey: If the prey species are not well defended, they die. Herbivores exert a similar
selection pressure on plants.
Animal Defenses To Escape Being Eaten:
-Physical defenses of prey include: Large body size in elephants, rapid movement in gazelles and body
armour in snails and sea urchins. Warning or aposematic coloration is used by prey as predators quickly
learn not to eat organisms that have toxins. Crypsis is defense mechanism by which the prey is
camouflaged, or resembles its environment. Mimicry is where the prey resembles another organism that
is toxic or very fierce. Certain behavioural defenses of prey include: Not foraging in open areas, keeping
lookouts and forming defensive circles (in musk oxen).
- There can be trade-offs present between behavioral and physical defenses. For example, snails with
the thickest shells are the last to take refuge.
Plant Defenses To Escape Being Eaten:
- Some plants engage in masting and produce huge numbers of seeds in some years and hardly any in
other years. These plants will temporarily hide from seed-eating herbivores, and then overwhelm them
by sheer numbers later on. Compensation, or the removal of plant tissue in order to stimulate new
growth, aids plants in tolerating the effects of herbivory. Under full compensation there is no net loss of
plant tissue. The removal of leaves can decrease self-shading, also resulting in increased plant growth.
Removal of apical buds may allow lower buds to open and grow.
- In field gentians, 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.
Structural & Chemical Defenses Of Plants:
-Tough leaves, spines and thorns, saw-like edges, and pernicious (nearly invisible) hairs that can pierce
the skin are all examples of structural defenses of plants. Some are induced defenses that are produced
in response to a herbivore attack. For instance, some cacti increase spine production after being grazed.
Secondary compounds are toxic chemicals used by plants to reduce herbivory. Other compounds attract
predators or parasitoids that will attack the herbivores. Some of these chemical defenses are produced
all the time, while others are induced.
-Most predators have evolved physical features that allow for the quick capture of prey. This is evident
in the unattached skull bones of snakes, which enable th