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Chapter 1, cont'd. Predator - Prey Relationship


The classic predator-prey relationship describes the encounter between a carnivore and another animal, in which the carnivore eats the prey animal, in whole or in part, as a source of energy and nutrients. The major factor controlling the encounter is an issue of diminishing returns. The predator engages the potential prey with a reasonable expectation of gaining energy, but it comes with the metabolic cost of capturing and eating the prey. Only if the energy gained by eating the prey is greater than the metabolic costs, the predator gains positive net energy. As a result, it is to the predator's advantage to seek one of the weaker individuals from among the potential prey individuals, in order to minimize the metabolic cost of capturing the prey. This principle is often illustrated (in Nature videos for the non-science audience) with large predators, such as African cats (a favorite seems to be the Cheetah, Acinonyx jubatus), and herds of potential prey, such as antelopes (another favorite is the Impala, Aepyceros melampus). The better films (actual footage of an encounter in the wild) show the Cheetah “cutting” a young Impala from the herd, and driving it away from its mother before pouncing on it to kill it. Some of the films depicting the cat cutting an old, partially disabled individual appear to be “staged;” [I have watched the filming of a completely staged Puma (Puma concolor) “capturing” a healthy, adult Mule deer (Odocoileus hemionus) in New Mexico, but the deer came from a petting zoo in Santa Fe and the mountain lion (raised from a kitten, feed only hamburger, and star of a circus-like show) came from a commercially operated desert “museum” near Tucson, Arizona. They got 18 seconds of usable footage, which on the TV show appeared to last about a minute and a half, as the lion chased the deer past the same shrub about 5 times.] Most predator-prey encounters are less exciting to watch, which is why there aren't many films depicting them. Watching a songbird capturing insects on the ground, or a bat diving to catch insects as they fly, does not make “good” Nature films.

    An interesting hypothesis concerning predators suggested that the environment is patchy: the prey individuals tend to occur in clumps with minimal prey available between the clumps. When the predator (such as a predatory bird) begins the search for potential prey, the first potential prey individual seen becomes a “search image” which serves as the “goal” for the subsequent search for food. As long as the search image prey can be found, the predator will continnue looking for the same species, while ignoring other potential prey species. If, however, sufficient time passes without find a suitable prey individual, the predator will search for any prey. When another prey individual is found, the predator will change the search image for subsequent searching. It has been reported that when the predator birds are searching for prey individuals they tend to spend more time in those geographic patches which are similar to the patch where the search image was found. This is presumed to maximize the probability that suitable prey will be found. This has been described as “risk-sensitive foraging.” The data (from an entire breeding season) supporting this hypothesis (at a seminar I attended) makes a good case for accepting the hypothesis.

Herbivores as predators:

There have been attempts to analyze herbivore foraging in the same terms as for carnivore foraging. The primary difference in herbivore foraging compared to carnivore foraging is that herbivores can feed with only partial defoliation of the plant individual, as a mechanism to assure that the plants will not just survive, but will recover in a relatively short time, maximizing the food available for later foraging. Although this seems a logical mechanism to provide a long term food supply for the herbivore, many herbivores have been shown to cause full (or complete) defoliation of the plant individual before moving on to another plant. The most spectacular instances of this behavior involve outbreaks of migratory insects: such as the Army worm caterpillar (Pseudaletia unipuncta in Utah, Texas, Oklahoma…) of prairie grasslands, the Forest tent capillar (Malacosoma disstria in Minnesota, Pennsylvania, Ohio,Indiana, Illinois, Iowa, Missouri, Alberta (Canada)…) of Woodlands, and the Migratory locust (in North America: Nebraska, Utah Melanoplus sanguinipes, now extinct; in the rest of the World Locusta migratoria, not extinct) of deserts and semi-arid grasslands.
    Another factor in herbivory involves selection of seeds or fruits as the preferred food. In these strategies, seed predation reduces the future food supply, as there will be fewer seeds to germinate and replace those plant individuals which do not survive to the following year; while fruit consumption maximizes the future population of plants by germination of those seeds which pass through the digestive system of the herbivore, with the added benefit that the seeds are “planted” with fertilizer (the herbivore's fecal material), encouraging better germination and growth of the new plants. The remaining issue in herbivory includes the defensive mechanisms of the plants to discourage herbivory. These mechanisms include mechanical barriers to slow the rate of successful feeding, such as hard shelled seeds, and various spikes and thorns to limit access to the edible parts of the plant; and chemical barriers including mild barriers (foul taste) to strong barriers (poisons which range from declined health of the herbivore to death of the herbivore).


Unlike the herbivores, carnivores generally must kill the entire prey animal to eat even a rather small percent of the animal carcass. The only exceptions I personally am familiar with are cases where the prey species can re-grow missing parts (notably some lizards can regrow their tail after a predator has broken the tail off and eaten it).
    There have been several documented cases of cyclic populatioins sizes driven by coupled predator-prey populations. Perhaps the most famous of these is the Artic Snowy owl (Bobo scandiacus) - Lemming (Synaptomys spp.) system of the Arctic region: the Snowy owls produce chicks at a rate which depends on the number of Lemmings present [more Lemmings eaten, more chicks produced], while the number of Lemmings depends on the number of Snowy owls feeding on them [more Snowy owls eating, fewer Lemmings]. This produces cyclic populations of both predators and prey. As the Owl population goes up, the lemming population goes down; and as the lemming population goes down the Snowy owl population goes down causing the Lemming population to increase so the Owl population increases. This coupled predator-prey population cycle has also been reported for the Coyote (Canis latrans) - Jackrabbit (Lepus spp) system in the short grass prairies of Kansas.


Parasites are a special case of predator. While they damage the host organisms, the hosts normally survive and recover, so the parasite species (different individuals) can feed on the same host individual. It is unusual for the parasite to kill the host but some parasites tend to kill most of their hosts.

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