Community Interactions and Adaptations

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 offspring, 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 to 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 frequencies 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.

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.

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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Match the title to the paragraph of the text.

Chemical Warfare   Bats and Moths have evolved counteracting strategies   Camouflage conceals both predators and their prey   Importance of community interactions   Some organisms gain protection through mimicry   Plants and Herbivores have many evolutionary adaptations   Predator-Prey interactions shape evolutionary adaptations

 

Pair work. Ask and answer 6 questions to Text 1.

Read the texts using your dictionary.

Retell one of the texts.

Text 2

The Camel

The camelis adapted to survive in a hot, dry and sandy environment. Adaptive physical features are the closable nostrils and long eyelashes, which help keep out wind-blown sand. The feet are broad and splay out under pressure, so reducing the tendency to sink into the sand. The thick fur insulates the body against heat gain in the intense sunlight.

Physiologically, the camel is able to survive without water for 6-8 days. Its stomach has a large water-holding capacity, though it drinks to replace water lost by evaporation rather than in anticipation of water deprivation.

The body temperature of a 'thirsty' camel rises to as much as 40 °C during the day and falls to about 35 °C at night. The elevated daytime temperature reduces the heat gradient between the body and the surroundings, so less heat is absorbed. A camel is able to tolerate water loss equivalent to 25 per cent of its body weight, compared with humans for whom a 12 per cent loss may be fatal. The blood volume and concentration are main­tained by withdrawing water from the body tissues.

The nasal passages are lined with mucus. During exhalation, the dry mucus absorbs water vapour. During inhalation the now moist mucus adds water vapour to the inhaled air. In this way, water is сonserved.

The role of the camel's humps in water conservation is more complex. The humps contain fat and are therefore an important reserve of energy-giving food. However, when the fat is metabolized during respiration, carbon dioxide and water (metabolic water) are produced. The water enters the blood circulation andwould normally be lost by evaporation from the lungs but the water-conserving nasal mucus will trap at least a proportion of it.

 

Text 3

Adaptations to thin air

Prehistoric and contemporary human populations living at altitudes of at least 2, 500 meters above sea level may provide unique insights into human evolution, reports an interdisciplinary group of scientists.

Indigenous highlanders living in the Andean Altiplano in South America and in the Tibetan Plateau in Asia have evolved three distinctly different biological adaptations for surviving in the oxygen-thin air found at high altitude.

The Andean and Tibetan plateaus rise some 4 kilometers above sea level. As prehistoric hunter-gatherers moved into these environments, they encountered desolate landscapes, sparse vegetation, little water, and a cold, arid climate.

In addition, early settlers to the high plateaus likely suffered acute hypoxia, a condition created by a diminished supply of oxygen to body tissues. At high altitudes the air is much thinner than at sea level. As a result, a person inhales fewer oxygen molecules with each breath.

Symptoms of hypoxia, sometimes known as mountain sickness, include headaches, vomiting, sleeplessness, impaired thinking, and an inability to sustain long periods of physical activity. At elevations above 7, 600 meters, hypoxia can kill.

The Andeans adapted to the thin air by developing an ability to carry more oxygen in each red blood cell. That is: They breathe at the same rate as people who live at sea level, but the Andeans have the ability to deliver oxygen throughout their bodies more effectively than people at sea level do.

They have higher hemoglobin concentrations in their blood. Hemoglobin is the protein in red blood cells that ferries oxygen through the blood system. Having more hemoglobin to carry oxygen through the blood system than people at sea level counterbalances the effects of hypoxia.

Tibetans compensate for low oxygen content much differently. They increase their oxygen intake by taking more breaths per minute than people who live at sea level.

In addition, Tibetans may have a second biological adaptation, which expands their blood vessels, allowing them to deliver oxygen throughout their bodies more effectively than sea-level people do.

Tibetans' lungs synthesize larger amounts of a gas called nitric oxide from the air they breathe. One effect of nitric oxide is to increase the diameter of blood vessels, which suggests that Tibetans may offset low oxygen content in their blood with increased blood flow.

 

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