Predation & herbivory (article) | Ecology | Khan Academy
Herbivory is a form of predation in which the prey organism is a plant. In fact, this was the only television channel I was allowed to watch as a kid—I thought it was is seen in the cycling of the lynx, a predator, and the snowshoe hare, its prey. become too abundant, due to competition—may also be a factor in the cycle. Snowshoe hares (Lepus americanus) are a critically important prey species for Berg, for their support; and my wife, Deborah Berg, and our children Tyler, Kerry Murphy for sparking my curiosity in Canada lynx and snowshoe hares in .. ) and are an important food source for many mammalian and avian predators. The aptly named snowshoe hare has particularly large feet and a winter-white coat. In the summer though, their fur turns brown, taking up to ten weeks to.
After all, many of us have watched bears catching salmon, lions eating zebras, or octopuses capturing prey on the nature channel. In fact, this was the only television channel I was allowed to watch as a kid—I thought it was amazing!
Nature shows on television highlight the drama of one animal killing another, but predation can also take less obvious forms. For instance, when a mosquito sucks a tiny bit of your blood, that can be viewed as a form of predation.
So can herbivory, in which an animal—say, a cow or a bug—consumes part of a plant. What counts as predation?
Predators and their prey
A predator is an organism that consumes all or part of the body of another—living or recently killed—organism, which is its prey. In the broad definition, however, the zebra is too! Nor does a predator necessarily have to kill its prey.
Instead, as in a grazing cow or a bloodsucking mosquito, it may simply take a portion of the prey's body and leave it alive. Population dynamics of predators and prey Populations of predators and prey in a community are not always constant over time. Instead, in many cases, they vary in cycles that appear to be related.
The most frequently cited example of predator-prey dynamics is seen in the cycling of the lynx, a predator, and the snowshoe hare, its prey. Strikingly, this cycling can be seen in nearly year-old data based on the number of animal pelts recovered by trappers in North American forests.
The number of hares fluctuates between 10, at the low points and 75, toat the high points. There are typically fewer lynxes than hares, but the trend in number of lynxes follows the number of hares. The classic explanation is this: As hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, it kills so many hares that the hare population begins to decline.
This is followed by a decline in the lynx population due to scarcity of food. When the lynx population is low, the hare population begins to increase—due, at least in part, to low predation pressure—starting the cycle anew. Today, ecologists no longer think that the cycling of the two populations is entirely controlled by predation.
For instance, it appears that availability of plant foods eaten by the hares—which decreases when hares become too abundant, due to competition—may also be a factor in the cycle.
Defense mechanisms against predation When we study a community, we must consider the evolutionary forces that have acted—and continue to act! Species are not static but, rather, change over generations and can adapt to their environment through natural selection.
Predator and prey species both have adaptations—beneficial features arising by natural selection—that help them perform better in their role. For instance, prey species have defense adaptations that help them escape predation. These defenses may be mechanical, chemical, physical, or behavioral. Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey.
Chemical defenses are produced by many animals as well as plants, such as the foxglove, which is extremely toxic when eaten. The millipede in the lower panel below has both chemical and mechanical defenses: The prey population eventually recovers, starting a new cycle.
T Paramecium, which also proved useful in test-tube studies of competition, was placed in culture with a predaceous protozoan.
Predators and prey
These laboratory studies found that cycles were short-lived, and the system soon collapsed. However, if one added more paramecium every few days, the expected cycle was observed. These results suggested that the predator-prey system was inherently self-annihilating without some outside immigration.
The question then arose: Observing that frequent additions of paramecium produced predator-prey cycles in a test-tube led to the idea that in a physically heterogeneous world, there would always be some pockets of prey that predators happened not to find and eliminate.
Perhaps when the predator population declined, having largely run out of prey, these remaining few could set off a prey rebound. Spatial heterogeneity in the environment might have a stabilizing effect. A laboratory experiment using a complex laboratory system supports this explanation. A predaceous mite feeds on an herbivorous mite, which feeds on oranges. A complex laboratory system completed four classic cycles, before collapsing.
Observations of prickly pear cactus and the cactus moth in Australia support this lab experiment. This South American cactus became a widespread nuisance in Australia, making large areas of farmland unusable. When the moth, which feeds on this cactus, was introduced, it rapidly brought the cactus under control. Some years later both moth and cactus were rare, and it is unlikely that the casual observer would ever think that the moth had accomplished this. Once the cactus became sufficiently rare, the moths were also rare, and unable to find and eliminate every last plant.
Inadequate dispersal is perhaps the only factor that keeps the cactus moth from completely exterminating its principal food source, the prickly pear cactus.
Prey defenses can be a stabilizing factor in predator-prey interactions. Predation can be a strong agent of natural selection. Easily captured prey are eliminated, and prey with effective defenses that are inherited rapidly dominate the population.
Examples include camouflage in the peppered moth, and prey that are nocturnal to escape detection. Bats capture moths in flight, using sonar to detect them; some moths are able to detect incoming sonar, and take evasive action. Perhaps seriously unbalanced system simply disappear, and those that persist are ones in which the predator is not "too effective", likely because the prey has adaptations to reduce its vulnerability.
The availability of a second prey type -- an alternate prey -- can be stabilizing or destabilizing. Often a predator eats more than one prey. If a predator switches between prey A and B on the basis of their frequency, it will eat A when B is rare and B when A is rare.
The prey should exhibit mild oscillations, and the predator should fluctuate little. This would stabilize prey abundances. However, if one prey species is abundant and the predator is unable to reduce its numbers, the result might be the maintenance of a continuously high predator density. Such an abundant predator might then eliminate a second prey species.
This is a destabilizing effect of an alternative prey. The hare-caribou-lynx relationship in Newfoundland is a complex example of such a destabilizing effect.Predator prey relationships
Complex Interactions in Ecological Communities Predation can have far-reaching effects on biological communities. A starfish is the top predator upon a community of invertebrates inhabiting tidally inundated rock faces in the Pacific Northwest. The rest of the community included mollusks, barnacles and other invertebrates, for a total of 12 species not counting microscopic taxa. The investigator removed the starfish by hand, which of course reduced the number of species to Soon, an acorn barnacle and a mussel began to occupy virtually all available space, out competing other species.