Population regulation (checks on growth) is the control of the size of a population. It is the tendency of any population to achieve or return to a size.

Population regulation (checks on growth) is the control of the size of a population. It is the tendency of any 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 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

B

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.

• 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.

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

Intraspecific: Competition between members of the same species.

• For food• For space• For mates

Interspecific: Competition between members of different species…• For food• For space• For sunlight

The gypsy moth: a dramatic example of how competition among members of one species for a finite resource — in this case, food — caused a sharp drop in population

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 density-independent 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 density-independent 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:

• contraception• abstinence• medical abortion• encouraging emigration• decreasing immigration• sterilization• euthanasia• genocide

Generally speaking, if you want to increase the population, you can:

• increase birth rates• decrease death rates• increase immigrationIf you want to decrease the population size, you can:• reduce the birth rate• decrease immigrationIncreasing the death rate is considered unethical!

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.

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.

Discourages births and encourages small family sizes so that population growth either slows, or stabilizes over time.

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?