V. Values discussion

Answer the following questions. Then discuss your answers with your classmates.

1. When can ecosystem change?

a. As conditions are altered.

b. As conditions are not altered.

с As the relationships in the system are in a dynamic balance, d. other

2. What would be observed as a change in the ecosystem?

a. Any continuing increase in population.

b. A population's remaining constant over a long period of time, с Any continuing decrease in population.

d. other

3. How is recruitment defined?

a. Maintaining or increasing a population.

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b. Making it through the early growth stages to become part of

breeding, reproducing population, с Substantial population increase, d. other

4. What does a relatively low reproductive rate result in?

a. a substantial population increase

b. high recruitment

с low recruitment >*U

d. other

5. What are additional factors that influence population growth and geographic distribution?

a. the ability of animals to mitigate

b. defense mechanisms

с resistance to adverse conditions and disease d. other

6. What does environmental resistance effectively reduce?

a. the early stages of growth

b. adverse conditions с recruitment

d. other

7. When will the population size remain constant?

a. If recruitment is not sufficient to replace losses in the breeding population.

b. If recruitment is at the replacement level.

с If the reproductive rate for a species remains constant, d. other

8. What may environmental resistance effect in certain situations?

a. reproduction с mortality (death)

b. certain pollutants d. other

9. When may the system be said to be balanced?

a. Ifbiotic potential remains constant.

b. As long as decreased populations restore their numbers.

с As long as the balance is maintained within a certain range, d. other 10. Why do populations remain within a certain size range?

a. Because population density increases.

b. Because population density decreases.

с Because most factors of environmental resistance are density-dependent, d. other

11. What human impacts are readily resulted in extinction?

a. ecosystem destruction с exploitation

b. pollution d. other

12. What does the biotic potential of many species depend on?

a. flock of birds с a herd of deer

b. a minimum population base d. other

VI. ROLE-PLAY Choose one of the following situations to act out.

1. An audience (which can be the whole class) is asking a lecturer questions about the relationships in ecosystems (for example, about interbreeding, reproducing groups, reproductive and death rate, problems of ecosystem balance).

1. A specialist who knows how birth rate and death rate are balanced for each species in the ecosystem is giving advices to a group of farmers. The farmers (which can be the whole class) will then ask questions.

1. A reporter is interviewing a scientist about additional factors that influence population growth and geographic distribution of ani­mals and seeds.

1. A scientist from Russia is talking with a scientist from the United States about the result of the interplay between biotic potential and environmental resistance.

1. You are speaking about recruitment. From your point of view, pop­ulation increase may be nil if recruitment is low. Your colleague has an opposite opinion.

1. Two environmentalists are talking about certain pollutants that af­fect reproduction.

1. A lecturer is explaining to a group of students (which can be the whole class) specific mechanisms of population balance. The au­dience will ask questions after a short (5 minutes) talk.

1. A Russian politician is speaking to a group of reporters (which can be the whole class) about human impacts on ecosystems. The re­porters will then ask questions.

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9. A scientist from Russia and a scientist from the United States are comparing human impacts such as ecosystem destruction, habitat alteration, pollution, and exploitation in the two places.

9. A specialist who knows that the problem of ecosystem balance boils down to a problem of how birth rate and death rate are balanced for each species in the ecosystem is arguing with someone who doesn't know this.

VII. FOR DISCUSSION AND COMPOSITION

Choose one of the following to discuss or write about.

1. Are all the relationships in the system in a dynamic balance? Dis­cuss.

1. Describe a change in the ecosystem. What does maintaining or in­creasing a population depend on?

1. Describe additional factors that influence population growth and geographic distribution of animals and seeds. Are all these factors referred to as the biotic potential of the species?

1. What do you know about biotic and abiotic factors? What is re­ferred to as environmental resistance?

1. Is it possible to foresee the result of the interplay between biotic potential and environmental resistance? Discuss.

1. Give examples when environmental resistance may effect repro­duction as well as causing mortality (death) directly.

1. Write about the result of a dynamic balance between biotic poten­tial of a population and environmental resistance.

1. Why do population remain within a certain size range? Discuss.

1. Describe human impacts on ecosystems. Why do they readily re­sult in extinction?

:) VIII. RENDERING IN ENGLISH

Очень важно понять, что нет сил или жестких структур, ко­торые предохраняют экосистемы от изменения. Фактически эко­системы могут изменяться, даже резко, когда изменяются усло­вия. Сохранить данный состав видов в течение долгого времени экосистемам позволяет то, что все взаимодействия (взаимоот­ношения) в них находятся в динамическом равновесии.

Каждый вид в экосистеме существует в виде популяции, т.е. воспроизводящей группы. Свойство системы оставаться стабиль­ной (самовыживание) в течение долгого периода времени озна­чает, что популяция каждого вида в экосистеме остается более или менее постоянной по размеру и географическому распро­странению. Любое продолжительное увеличение или уменьше­ние популяции считается изменением в экосистеме. В свою оче­редь, пребывание популяции постоянной в течение долгого времени означает, что воспроизводство уравновешивается смерт­ностью. Таким образом, проблема равновесия экосистемы сво­дится к проблеме, как сбалансированы рождаемость и смертность каждого вида в экосистеме.

Сохранение или увеличение популяции зависит не только от темпа размножения, важным является также пополнение вида. Например, многие рыбы откладывают тысячи и даже миллионы икринок, а растения — тысячи семян. Тем не менее увеличение популяции может равняться нулю, т.е. пополнение очень низкое. Другими словами, большая часть молодых рыбок и растений по­гибает на ранних стадиях роста. И наоборот, даже относительно низкий темп размножения может привести к значительному уве­личению популяции при высоком пополнении.

Дополнительными факторами, которые влияют на рост и географическое распространение, являются: способность жи­вотных мигрировать, а семян — распространяться в подобную среду обитания в других регионах; способность адаптироваться и завоевывать новую среду обитания кроме той, которая была первоначально занята; защитные механизмы и сопротивление неблагоприятным условиям и болезням. Все эти факторы, вме­сте взятые, называются способностью вида к размножению. Не­смотря на различные стратегии относительно способности к размножению, у них есть одно общее — каждый вид имеет вы­сокую воспроизводительную способность и быстро увеличивает свою популяцию при благоприятных факторах для высокого по­полнения. Действительно, каждое новое поколение будет при­умножаться числом женских особей. Например, кролики, даю­щие потомство 20 штук, из которых 10 — женские особи, могут увеличиваться в каждом поколении в 10 раз: 10, 100, 1000 и т.д.

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Это явление в популяциях обычно называется популяционным взрывом.

Но в естественных экосистемах популяционный взрыв про­исходит редко, так как все условия среды не бывают благоприят­ными долгое время. Один или два абиотических и биотических фактора становятся лимитирующими. Сочетание абиотических и биотических факторов, которые могут ограничивать увеличе­ние популяции, называется устойчивостью среды. Увеличивает­ся ли популяция, остается устойчивой или уменьшается, явля­ется результатом динамического равновесия.

В основном, популяции остаются в пределах определенной величины, так как большинство факторов устойчивости среды зависит от плотности. Это означает, что, когда плотность попу­ляции (число особей на единицу площади) увеличивается, со­противление среды становится более интенсивным и вызывает увеличение смертности, так что рост прекращается или снижа­ется. И наоборот, когда плотность популяции уменьшается, со­противление среды обычно ослабевает, позволяя популяции вос­становиться.

С другой стороны, воздействие человека легко приводит к вымиранию, так как оно не зависит от плотности популяции. Такие воздействия человека, как разрушение экосистемы, нару­шение среды обитания, загрязнение и эксплуатация, происхо­дят при любой плотности популяции.

TEXT

Ecosystems: Mechanisms of Population Balance

1. There are some specific mechanisms that provide population balance in nature. It is necessary to focus on one mechanism at a time, but keep in mind that in natural ecosystems all of the mechanisms are working in concert to create the overall balance. Knowledge of these mechanisms will make us aware of how ecosystems may be upset and the consequences that may result.

1. A classic example of population balance is that between the lynx, a member of the cat family, and hares, a member of the rabbit family, as observed in Canada from 1850 to 1930. When the hare pop­ulation is low, each hare can find abundant food and plenty of plac­es, to hide and raise offspring. In other words, the hares' environ­mental resistance is relatively low, and their population increases despite the presence of the lynx predator. As the hare population in­creases, however, each hare has relatively less food and fewer hiding places. More hares provide easier hunting for the lynx so that, with plenty of hares to feed lynx young, the lynx population begins to fall. As the hare population falls, the food and shelter available to each hare again increase. Also, surviving hares are those that are healthi­est and best able to escape from the lynx. Hunting becomes harder for the lynx; many of them starve, and their population begins to fall. These factors sum up to lower environmental resistance for the hares, and their population increases again, repeating the cycle. These events explain the fluctuating but continuing balance found between the hare and lynx populations.

1. Much more abundant and ecologically important in population control are a huge diversity of parasitic organisms. These organisms range from tapeworms, which may be a foot or more in length to mi-

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croscopic disease-causing protozoans, fungi, bacteria, and viruses. All species of plants, animals, and even microbes may be infected by par­asites. In terms of population balance, parasitic organisms act in the same way as large predators. As the population density of the host or­ganism increases, parasites and their vectors (agents that carry the par­asites from one host to another), such as disease-carrying insects, have little trouble finding new hosts, and infection rates increase, causing dieback. Conversely, when the population density of the host is low, transfer of infection is impeded, and there is a great reduction in levels of infection, a condition that allows the population to recover.

4. Parasites may not kill their host, but they generally weaken it and make it more vulnerable to adverse conditions and to attack by larger predators. It is commonly observed that the animals killed by large predators are infected with parasites, whereas animals killed by hunters are generally healthy. In a food web, a population of any given organism is affected by a number of predators and parasites simulta­neously. Consequently, the balance can be thought as a balance be­tween the population of an organism and its natural enemies. The wide swings in populations noted in the hare-lynx case are generally typical of very simple ecosystems involving relatively few species. Balances between an organism and several natural enemies are generally more stable and less prone to wide fluctuations because different natural enemies come into play at different population densities. Also, when the preferred prey is at a low density, the population of the natural enemy may be supported by its feeding on something else. Thus, the lag time between increase of the prey population and that of the natu­ral enemy is diminished. These factors have a great damping effect on the rise and fall of the prey population.

4. In all such balances, however, whether simple or complex, it is extremely important to recognize that a high degree of adaptation is involved on the part of both the prey or host and the natural enemy. This adaptation is such that a given natural enemy is incapable of com­pletely eliminating its prey or host but yet is capable of limiting the prey or host population to a certain density. Putting any predator and prey or host and parasite together does not lead to an automatic bal­ance. Such lack of balance is shown all too clearly by what may occur when a species from one region is introduced into another. Such in-

traductions may lead to what are commonly called ecological disas­ters as balances fail.

6. In discussing predator-prey balances, it was said that in lean times the excess carnivore population — the lynx, for instance, — simply starved. Actually, another factor is often involved in the control of carnivore and some herbivore populations: territoriality, which refers to individuals or groups claiming a territory and defending it against others of the same species. For example, the males of many species of songbirds stake out a territory at the time of nesting. Their song has the function of warning other males to keep away. Male wolves and other carnivores, including dogs, stake out a territory by spotting it with urine, the smell of which warnes other males to stay away. The territory defended is large enough to assure the "owners" of being able to gather enough food to successfully rear a brood. The size of the territory defended varies with resources avail­able. In lean times territories are larger; in good times they are smaller.

7. The obvious advantage of territoriality is that individuals that are able to successfully claim and defend a territory will have enough re­sources to rear a well-fed, healthy next generation. Those individuals unable to claim a territory generally meet an unhappy end. Continually chased out of one territory after another, they fall victim to any of the factors of environmental resistance, or at the very least they are unable to breed and raise young. Territoriality does not change the basic princi­ple of population being a dynamic balance between biotic potential and environmental resistance. In the face of limited resources, however, ter­ritoriality creates a mechanism of selecting the strongest and fittest to survive and breed, while eliminating the genes of the weaker individuals.

7. Territoriality is an instinctive behavioral trait in many species. By keeping populations in check, territoriality helps to maintain the bal­ance of the ecosystem and thus to ensure survival of the species. Many people have observed that humans are also a territorial animal, even to an extent that greatly exceeds any other species. Almost all of us aspire to owning a piece of land that we can put a fence around and call ours, and the bigger the piece of land, the better. Throughout history, virtu­ally all wars between nations have involved territorial disputes, and nations continue to arm themselves to the teeth with the most sophis­ticated weaponry available - always, they say, to defend themselves against the threat of territorial encroachment by their neighbors.

7. 

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USEFUL WORDS AND EXPRESSIONS			Ц:
paragraph 1	transfer	herbivore	0}
keep in mind	impede	males	;a
in concert	level	stake out	■Л
overall balance	parasitic organisms	nesting	>j
be upset	disease-causing	keep away	*;K
paragraph 2 lynx	in the same way paragraph 4	stay away assure rear a brood	Щ Ж
hares	vulnerable		fo
abundant	food web	paragraph 7	to
escape	swings	chase out	•-..-/
starve	prone	victim	Ј>5
paragraph 3 huge diversity	come into play prey lag time	in the face of paragraph 8
tapeworms	diminish	behavioral trait
protozoans	damping effect	in check	hi
fungi infect	paragraph 5	aspire fence	it
parasites	introduce	territorial disputes	'■CI
in terms of	ecological disasters	to arm to the teeth	Я
host organism disease-carrying insects P В Р infection rates lean time		weaponry > j territorial encroachmeiij neighbors
dieback	carnivore