Science 10
Outcomes
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Sustainability of Ecosystems
Investigation 8 – Populations
During the 1990’s, the number of sea otters living near the Aleutian Islands off the
coast of Alaska fell sharply, by about 90 percent. Looking for an explanation, scientists
discovered that killer whales had started eating sea otters. Killer whales usually eat
seals and sea lions, not the much smaller otters. Study the Figure 1 to find out why the
whales changed their feeding habits, and what the effects were.
Figure 1 Changes in the feeding habits of killer whales in the Pacific Ocean
The story of the sea otters shows how changes in a population in one part of a food web
affect populations in other parts of the web. A linked series of events extended all the
way from the fish to the seals, to the killer whales, to the otters, to the kelp forest. In a
food web, each organism survives by gaining food energy from the trophic level below it.
The organisms, in turn, may be used as food by species in the trophic level above it.
The population size of each species is continually adjusted by the species’ interactions
with both its food supply and the consumers that use it as food. In addition, each
species may compete for food resources with others in its own trophic level.
Carrying Capacity
Can any population of organisms keep growing forever? What might limit its numbers?
The largest population of a species that an environment can support is called the
carrying capacity of the environment for this species. You have already discovered
how food supply, competition, and predators affect population size. Here is a summary
of the four main factors that determine carrying capacity.
o
Materials and Energy All populations of organisms are ultimately limited by the
amount of usable energy from the Sun, as well as the supply of water, carbon,
and other essential materials.
o
Food Chains The population sizes at any trophic level are limited by the
population sizes (or the biomass) in all the levels below it. In other words,
populations are limited by their food supply. Populations are also limited by
organisms in the levels above them, which use them as food. Thus animal
populations are limited by their predators and plant populations are limited by
herbivores.
o
Competition Each organism has the same needs as other organisms of its
species. The demand for resources (such as food, water, mates, and space)
results in competition among individuals for these resources. For example, each
wolf in an area may use deer as its main source of food. If the deer population is
low and the wolf population is high, competition for food among wolves
increases.
Competition among members of the same species is called
intraspecific competition. The wolves may also compete with other predators,
such as pumas and coyotes, that live in the area and also feed on deer.
Competition between species is called interspecific competition.
Both
intraspecific and interspecific competition can limit population growth.
o
Density Depending on their size, environment, and way of life, different species
have different needs for space. The need for space can determine an organism’s
population density: how many individuals can live in an area at one time.
Think of animals that live close together and animals that live far apart. How
might you explain this difference between them?
If population density increases beyond a suitable level for a particular species, it
produces conditions that tend to limit further growth in numbers. For example,
overcrowding may increase stress and promote the spread of diseases or parasites.
Among some animals, overcrowding leads to increased aggression and neglect of
offspring. These factors increase death rate and decrease birth rate, which together
reduce the population size. Factors that increase in significance as a population grows
are called density-dependent factors.
Other factors can limit a population, regardless of its size. For example, a forest fire
may kill most of the snakes in an area of forest, whether there are ten or ten thousand
of them. Such factors are called density-independent factors because their effect on
population size does not depend on how many individuals there are in the population.
What other density-independent factors might limit population size?
Questions
1.
2.
Give one example of each of the following types of limiting factors.
a)
Density-dependent
b)
Density-independent
What is meant by the term “carrying capacity”?
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3.
Give an example of
a)
Interspecific competition
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b)
Intraspecific competition
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4.
How might overcrowding lead to a reduction in population size?
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5.
How might an increase in the population of plants in an area lead to an increase
in the population of hawks?
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6.
How might an increase in the population of hawks in an area lead to an increase
in the population of plants?
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