Transcript In populations being controlled by density

Population regulation (checks on growth) is the control of
the size of a population. It is the tendency of any
A
population to achieve or return to a size at equilibrium
or
in harmony with the surrounding environment…in other
words, reach its carrying capacity (K).
If a population tends to
B
remain about the same size,
then it is said to be stable.
There are basically two
different types of natural
population regulation classified according to the
types of factors that control
the size of the population
A density-dependent factor is one where the effect of the
factor on the size of the population depends upon the
original density or size of the population.
In general, density-dependent factors are biotic in nature, such as diseases,
parasites, competition for scarce resources, and predation. Some are even
behavioral in nature, such as resorting to cannibalism and killing young when
overcrowded
• A disease is a good example of a density-dependent
factor. If a population is dense and the individuals live
close together, then each individual will have a higher
probability of catching the disease than if the
individuals had been living farther apart. Not only will a
greater number of individuals be affected, but, more
importantly, a greater proportion of the population will
be affected if they are living close together.
Intraspecific: Competition between members of the
same species.
The gypsy moth: a dramatic
• For food
example of how competition
among members of one
• For space
species for a finite resource —
• For mates
in this case, food — caused a
sharp drop in population
Interspecific: Competition between members of different
species…
• For food
• For space
• For sunlight
In populations being controlled by density-dependent
factors, growth rates are usually inversely proportional to
population density. For example, if the population density is
high, the growth rate is low. Conversely, if the density is
low, the growth rate is high.
Density
Growth
• Occur in ecosystems where the communities have MANY
species, ie., where many biological interactions are taking
place, (high biodiversity)
• Occur in ecosystems that are NOT usually stressed
periodically by physical factors (such as periodic flooding
through the area), i.e., ecosystems that are usually more
stable.
A stable population (near K) in a stable
environment is most indicative of what type of
reproductive strategy?
Give 3 examples of biotic density-dependent
factors that can affect a population.
What types of competition can occur for food
resources? Describe.
What types of competition can occur for mates?
Describe.
What types of ecosystems are usually inhabited by
populations controlled by density-dependent
factors?
A density-independent factor is one that is not related in
any way to the size of the population at any given time. The
effect of the factor on the size of the population is
independent of and does NOT depend upon the original
density or size of the population.
• weather is an example of a density-independent factor. Seasonal
temperature changes, or severe storms and floods coming through
an area can just as easily wipe out a large population as a small one.
• habitat destruction: a harmful pollutant put into the environment,
e.g., a stream. The probability of that harmful substance at some
concentration killing an individual would not change depending on
the size of the population. Deforestation is another example.
• food, in short supply, would affect any population…small, or large
• are physical factors, such as weather factors or the
presence of harmful chemicals.
Characteristics of populations controlled primarily by
density-independent factors include:
• much less biological control and the control is
a more haphazard, physical control.
• Unlike the case for density-dependent factors,
in populations being controlled by densityindependent factors, growth rates do not
seem to show any trend at all relative to
population density. (no inverse relationship)
• occur in ecosystems where the communities have FEW
species, i.e., where fewer biological interactions are taking
place, and more physical interactions are taking
place…such as Island communities.
• ecosystems ARE usually stressed
periodically by physical factors (such
as periodic flooding through a flood
plain).
While population regulation factors are classified
into two types, what usually happens in nature is
that a population is actually controlled by a
COMBINATION of density-dependent and densityindependent factors.
Some populations will be primarily controlled by one
type and other populations will be controlled
primarily by the other type. Both types of factors
are external forces on the population
Describe three density-independent factors
that can control a population’s size.
Generally, how do density-independent
factors differ from density-dependent
factors?
How do ecosystems where populations are
controlled by density-independent factors
differ from ecosystems that contain
populations controlled by density-dependent
factors?
Human population control is the practice of artificially
altering the rate of growth of a human population.
Historically, human population control has been
implemented by limiting the population's birth rate, usually
by government mandate, and has been undertaken as a
response to factors including:
• high or increasing levels of poverty
• environmental concerns
• religious reasons
• overpopulation
While population control can involve measures that
improve people's lives by giving them greater control of
their reproduction, some programs have exposed people
to exploitation.
Population control may use one or more of the following
practices although there are other methods as well:
•
•
•
•
•
•
•
•
Generally speaking, if you want
contraception
to increase the population, you
abstinence
can:
• increase birth rates
medical abortion
• decrease death rates
• increase immigration
encouraging emigration If you
want to decrease the
decreasing immigration population size, you can:
• reduce the birth rate
• decrease immigration
sterilization
Increasing the death rate is
euthanasia
considered unethical!
genocide
Many countries have instituted population policies that
regulate births or immigration, with the idea of matching
population size to the country’s resources. The two
categories are: Pronatalist policies and Antinatalist policies.
Encourages births and
large families so that the
population grows.
Discourages births and
encourages small family
sizes so that population
growth either slows, or
stabilizes over time.
In the 1930s, France adopted a policy
that paid families subsidies for raising
children and paid a maternal salary to
women who had additional children
beyond two (the replacement level)
East Germany and Romania had new
home loan policies to encourage young
parents to have children, by forgiving
some portion of the debt with each
new child born. Abortion was also
banned.
Singapore and China instituted policies
raising the legal age of marriage to shorten
a woman’s period of fertility. Low-cost
housing was made less available when
women had more than two children
With over one billion people (1/7 of the world’s population),
and only 7% of the world’s farmable land, there was clearly a
mismatch of population and resources.
In the 1960s, Chinese government instituted the following
policies:
• Family Planning: Birth control pills were made accessible
to all
• Education promoted delaying childbirth until later in life
• Women were pressured to have only one child (One child
policy)
• Many became sterilized or had abortions, which were
now legal
• Opportunities were extended to women in employment
• Social security pensions meant that the elderly did not
necessarily have to rely on their families for a living
• Increase the age of marriage.
• Know and understand the carrying capacity of
the environment. Determine what population
the country’s resources can support, and still
provide a decent standard of living.
• Make decisions regarding desired rate of
growth…match the resources with growth rate.
• Implement family planning, education, and
programs to increase the standard of living,
employment and security, while decreasing birth
rate
• Conserve environmental resources as much as
possible.
For what two reasons might a country
adopt a pronatalist policy?
For what two reasons might a country
adopt an antinatalist policy?
What type of policies did China adopt
to help deal with their overpopulation
problems?