Population Growth Notes

Population growth depends on two things

1. Biotic potential - this is the number of offspring that an organism can produce

2. Environmental Resistance - this is the combination of factors which limit the growth of a population.

Biotic Potential                                                         Environmental Resistance

- reproduction rate                                                      - lack of food

- ability to migrate                                                      - lack of water

- ability to invade new habitats                                   - no habitat

- defense mechanisms                                                 - harsh weather

- ability to cope with harsh conditions                       - predators

                                                                                    - disease

                                                                                    - temperature extremes

                                                                                    - parasites

                                                                                    - competition

These two concepts tend to remain balanced in natural ecosystems. If they change, the population grows or shrinks.

When the biotic potential increases too much or the environmental resistance decreases too much, a population explosion results.

Humans have mostly overcome environmental resistance and so we are in a population explosion.

Population growth can be calculated. It depends on four factors

1. births - the number of individuals born in a population

2. deaths - the number of individuals which die in a population

3. immigration - the number of individuals which enter an area

4. emigration - the number of individuals which leave an area

population growth = (births + immigration) - (deaths + emigration) x 100%

                                                initial # of organisms

e.g., Imagine there are 200 birds at a nesting site. One year there are 55 chicks born and 40 birds die. Two birds join the population and three leave. Find the growth rate.

e.g., There are 182 moose in a particular national park. In one year 47 calves are born and 68 animals die. Four moose move into the park and 2 leave. Find the growth rate.

Five factors tend to keep populations balanced

1. Predator-prey relationships - if predators eat too many prey, the prey population will decrease and then so will the predator population. When the predator population decreases, fewer prey are eaten and the prey population increases again. This increase allows the predator population to increase.

2. Plant-herbivore relationship - same as predator-prey

3. Host-parasite relationship - same as predator-prey

4. Carrying capacity - is the largest population the ecosystem can support. If the population becomes too large, the ecosystem cannot support it and it decreases again. When the ecosystem recovers, the population can grow again.

5. Territoriality - some species defend a territory of a certain size. This limits the number of individuals which can live in a given area.

Demographics

Demographics is the study of changes in a population (growth rate, age, gender, etc.)

Why do we study population growth?

1. Governments need to be able to set taxes, insure food supplies, plan for retirement funds, be able to pay for health care, and other government programs.

2. Companies want to know how populations change for advertising.

3. Wildlife biologists study populations to know if humans are interfering with animal populations.