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Биология Community Interactions and Adaptations
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A

An ecological community consists of all the interacting populations within the ecosystem. Community interactions such as predation, parasitism and competition help limit the size of populations. The interacting web of life that forms a community tends to maintain a balance between resources and the number of individuals consuming them. When populations interact with one another, influencing each other`s ability to survive and reproduce, they serve as agents of natural selection. For example in killing prey that are easiest to catch, predators leave behind those individuals with better defenses against predation. These individuals leave the most off-spring, and over time their inherited characteristics increase within the prey population. Thus, as community interactions limit population size, they simultaneously shape the bodies and behaviors of the interacting populations. The process by which two interacting species act as agents of natural selection one another over evolutionary time is called coevolution.

The most important community interactions are competition, predation, parasitism and mutualism. Their importance is illustrated by the adaptations that have evolved under the environmental pressures exerted by these interactions over evolutionary time.

B

To survive, predators must feed and prey must avoid becoming food. Therefore, predator and prey populations exert intense environmental pressure on one another , resulting in coevolution. As prey become more difficult to catch, predators must become more adept at hunting. Environmental pressure endowed the cheetah with speed and camouflage spots, and its zebra prey with speed and camouflage stripes. It has produced the keen eyesight of the hawk and the warning call of the ground squirrel, the stealth of the jumping spider and mimicry of the fly it stalks.

C

Most bats are nighttime hunters that navigate and locate prey by echolocation. They emit extremely high-frequency and high-intensity pulses of sound and, by analyzing the returning echoes, create an image of their surroundings and nearby objects.

Under environmental pressure from this specialized prey-location system, certain moths have evolved simple ears that are particularly sensitive to the frequences used by echolocating bats. When they hear a bat, these moths take evasive action, flying erratically or dropping to the ground. Some moths have evolved a way to jam the bats` echolocation mechanism by producing their own high-frequency clicks. In response, when hunting a clicking moth, a bat may turn off its own sound temporarily and zero in on the moth by following the moth`s clicks. These interactions illustrate the complexity of coevolution adaptations.

D

Both predators and prey have evolved colors, patterns and shapes that resemble their surroundings. Such disguises render animals inconspicuous even when they are in the plain sight.

Some animals closely resemble specific objects such as leaves, twigs, bark, thorns, or even bird droppings. Camouflaged animals tend to remain motionless rather then to flee their predators.

Predators who ambush are also aided by camouflage. For example, a spotted cheetah becomes inconspicuous in the grass it watches for grazing mammals. The frog-fish closely resembles the algae –covered rocks and algae on which it sits motionless, dangling a small lure from its upper lip. Small fish notice only the lure and are swallowed as they approach it.

E

Mimicry refers to a situation in which a species evolves to resemble something else. For example, once warning coloration evolved, there rose a selective advantage for tasty, harmless animals to resemble poisonous ones. The deadly coral snake has brilliant warning coloration and the harmless mountain king snake avoids predation by resembling it.

F

Both predators and prey have evolved a variety of toxic chemicals for attack and defense. The venom of spiders and poisonous snakes serves both to paralize prey and to deter its predators. Many plants produced defensive toxines. For example lupins produce alkaloids which deter attack by the blue butterfly, whose larvae feed on the lupin`s buds.

Certain mollusks, including squid and octopus emit clouds of ink when attacked. These "smoke screens" confuse their predators and mask their own escape.

An interesting example of chemical defense is seen in the bombardier beetle. In response to the bite of an ant, the beetle releases secretions from special glands into an abdominal chamber. There enzymes catalyze an explosive chemical reaction that shoots a toxic, boiling hot spray onto the attacking ant.

G

Plants have evolved a variety of chemical adaptations that deter their herbivorous "predators." Many, such as the milkweed, synthesize toxic and distasteful chemicals. Animals rapidly learn not to eat foods that make them sick, and so milkweeds and other toxic plants suffer lit­tle nibbling. Consequently, such plants are often very abundant; any animal immune to the plant poisons enjoys a bountiful food supply. As plants evolved toxic chemicals for defense, certain insects evolved increasingly efficient ways to detoxify or even make use of the chemicals. The result is that nearly every toxic plant is eaten by at least one type of insect. For example, monarch butterflies lay their eggs on milkweed; when their larvae hatch, they consume the toxic plant. The caterpillars not only tolerate the milkweed poison but also store it in their tissues as a defense against their own predators. The stored toxin is even retained in the metamorphosed monarch butterfly.

Grasses have evolved tough silicon (glassy) substances in their blades, discouraging all herbivorous predators except those with strong, grinding teeth and powerful jaws. Thus, grazing ani­mals have come under environmental pressure for longer, harder teeth. An example is the coevolution of horses and the grasses they eat. On an evolutionary time scale, grasses evolved tougher blades that reduce predation, and horses evolved longer teeth with thicker enam­el coatings that resist wear.

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