165
See also: Distribution of species over space and time 162–163 ■ Metapopulations
186–187 ■ Metacommunities 190–193 ■ Overpopulation 250–251ORGANISMS IN A CHANGING ENVIRONMENT
flies kept in a bottle. Initially, their
fertility rate increased. However,
as the population density grew,
competition for resources increased,
and eventually reached a bottleneck.
After this, the flies’ fertility rate
dropped; their numbers continued
to increase but slowly, and generally
the population level stabilized.
Similarly, Pearl found that
when the number of hens in a pen
increased, the birds struggled to
find enough food. As the space
between them reduced, the henslaid fewer eggs and, as their
fertility rate declined, the rate of
population growth slowly stabilized.Variable strategies
The two key variables in Verhulst’s
equation are the maximum capacity
of a species to reproduce (r), and the
carrying capacity of the area (K).
Organisms are either r-strategists
or K-strategists. R-strategists, such
as bacteria, mice, and small birds,
reproduce rapidly, mature early,
and have a relatively short life.
K-strategists, such as humans,
elephants, and giant redwood
trees, have a slower reproduction
rate, take longer to mature, and
tend to live longer. Ecologists
study r-strategists, which are often
found in unstable environments,
to assess risks to their necessary
high reproduction levels, and study
K-strategists in more predictable
environments to ensure long-term
species survival. ■Thomas Malthus
Malthus was born in Surrey,
UK, in 1766, the seventh child
of a prosperous family. After
studying languages and
mathematics at the University
of Cambridge, he took a post
as curate of a rural church. In
1798, he published an essay
arguing that the rate of
increase in human populations
outstrips much steadier rises
in food production, leading to
inevitable starvation. Malthus
went on to publish six further
editions of the essay, and he
made a number of visits to
Europe to gather population
data. In 1805, he was appointed
Professor of History and
Political Economy at the East
India Company College in
Hertfordshire. He became
increasingly involved in
debate about economic policy,
and criticized the Poor Laws
for causing inflation and
failing to improve life for the
poor. Malthus died in 1834.Key works1798 An Essay on the Principle
of Population
1820 Principles of Political
Economy
1827 Definitions in Political
EconomyFruit flies are small, common flies that
are attracted to ripe fruit and vegetables.
They are popular for laboratory studies
because they reproduce so quickly and
are easy to cultivate.Biologists are at the
present time in no
way likely to suffer
ostracism if they
venture to study
human problems.
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