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.

2. 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.

3. 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-

Unit Eight

Ecosystems: Mechanisms of Population Balance

 

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.

5. 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.

8. 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.

Unit Eight

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

EXERCISES

I. READING COMPREHENSION

1. What is the main idea of the article?

a. In natural ecosystems all of the mechanisms are working in
concert.

b. It is necessary to focus on one mechanism at a time.

с There are some specific mechanisms that provide population balance in nature.

Ecosystems: Mechanisms of Population Balance

2. What is a classic example of population balance?

a. between the cat family and the rabbit family

b. between the lynx predator and the hare population
с between the lynx and hares

3. According to paragraph 2, surviving hares are those that

a. are best able to escape from the lynx

b. provide harder hunting for the lynx
с are healthiest

4. In paragraph 3, parasites may infect

a. microbes b. all species of plants с animals

5. How do parasite organisms act in terms of population balance (para­
graph 3)?

a. as large predators b. as viruses с as vectors

6. In paragraph 4, parasites may

a. weaken their host с not kill their host

b. make it more vulnerable

7. What are balances between an organism and several natural ene­
mies (paragraph 4)?

a. unstable с less prone to wide fluctuations

b. much more stable

8. What is extremely important to recognize in all balances (para­
graph 5)?

a. a high degree of adaptation of the natural enemy

b. a low degree of adaptation of both the prey or host

с a high degree of adaptation of both the prey or host and the natural enemy

9. According to paragraph 5, when a species from one region is intro­
duced into another there may occur

a. automatic balances с lack of balances

b. ecological disasters

10. What is often involved in the control of carnivore and some herbi­
vore populations, according to paragraph 6?

a. the excess carnivore population с territoriality

b. some herbivore population

11. In paragraph 6, the territory defended is

a. small b. large с wide

12. Which of the following is not true?

102________ _____ __________________________ Unit Eight

Ecosystems: Mechanisms of Population Balance

 

a. Territories are larger in good times.

b. Territories are smaller in lean times.

с The size of the territory varies with resources available.

13. Put the following sentences in the correct order (paragraphs 7, 8).

a. Territoriality is an instinctive behavioral trait in many species.

b. Those individuals unable to claim a territory generally meet an
unhappy end.

с Territoriality does not change the basic principle of population being a dynamic balance between biotic potential and environ­mental resistance.

14. According to paragraph 8, throughout history, virtually all wars be­
tween nations have involved

a. territorial encroachment

b. territorial disputes

с the most sophisticated weaponry available

15. What do almost all of us aspire to owning (according to para­
graph 8)?

a. a fence b. a piece of land с a neighbor

II. VOCABULARY

From the list of words and expressions below, choose the correct word or expression foreach blank space. Use each word or expression only once.

 

in the same way	in terms of	be upset	escape
in the face of	ecological disasters	lag time	infect
damping effect	parasites	diminish	fence
behavioral trait	abundant	assure	hare
rear a brood	food web	aspire	lynx

1. Knowledge of these mechanisms will make us aware of how eco­systems may and the consequences that may result.

2. When the_____ population is low, each hare can find____ food

and plenty of places.

3. Also, surviving hares are those that are healthiest and best able to
from the_____ .

4._____ population balance, parasitic organisms act_____ as large

predators.

5. Almost all of us____ to owning a piece of land that we can put a

_____ around and call ours, and the bigger the piece of land, the

better.

6. All species of plants, animals, and even microbes may be__ by

7. In a_____ , a population of any given organism is affected by a

number of predators and parasites simultaneously.

8. Thus, the_____ between increase in the prey population and that

of the natural enemy is____ .

9. These factors have a great_____ on the rise and fall of the prey

population.

10. Such introductions may lead to what are commonly called _____ as balances fail.

11. The territory defended is large enough to_____ the "owners" of

being able to gather enough food to successfully____ .

12._____ limited resources, however, territoriality creates a mecha­
nism of selecting the strongest.
13. Territoriality is an instinctive____ in many species.

III.PREPOSITIONS

Fill in the correct preposition for each blank space.

Much more abundant and ecologically important_____ popula­
tion control are a huge diversity____ parasitic organisms. These or­
ganisms range_____ tapeworms, which may be a foot or more____

length_____ microscopic disease-causing protozoans, fungi, bacte­
ria, and viruses. All species____ plants, animals, and even microbes

may be infected____ parasites.__ terms_________ population bal­
ance, parasitic organisms act____ the same way__ large preda­
tors. As the population density___ the host organism increases,

parasites and their vectors (agents that carry the parasites_____ one

host_____ another), such_____ disease-carrying insects, have little

trouble finding new hosts, and infection rates increase, causing die-
back. Conversely, when the population density____ the host is low,

transfer_____ infection is impeded, and there is a great reduction

_____ levels_____ infection, a condition that allows the population

to recover.

104________________Unit Eight