Population Biology

Population

• A group of organisms of the same species living in the same area and interacting with one another.

• Factors affecting growth rate1. Birth Rate

2. Death Rate

3. Rates of Migration (Immigration & Emigration)

Exponential Growth – “J” curve

1. Rapid rate of increase when resources are unlimited and optimal growing conditions are present.

2. The populations double with each generation, such that the larger the population is, the faster it grows.

3. Obviously this rate of growth cannot continue indefinitely.

Number of Species over Time

Logistic Growth “S” Curve

1. This type of growth grows rapidly at first resembling exponential growth, but levels off toward later stages.

2. Related to the fact that resources are limited, and as the population grows larger, it is harder for each individual to get what it needs, so growth slows.

3. Population may slow down either because of decrease in birth rate or an increase in death rate (or both)

4. Populations tend to stabilize around a specific number, although there may be slight fluctuations around that amount.

Carrying Capacity

Carrying capacity – the maximum number of organisms of the same species that can be supported in a specific are indefinitely.

a. Once the carrying capacity is reached, certain factors work to keep population in check (including lack of food, overcrowding, predations, accumulation of waste)

Carrying Capacity, cont.

b. If a natural population overshoots the carrying capacity, three thing can happen:

1. It will die back to the original cc.

2. It will die back, but because of damage to the environment, cc. is lower

3. It will become extinct.

Factors that Restrict the Size of a Population

Limiting Factors of Environmental Resistance

Density-Dependent Limiting Factors

1. The degree of influence depends on the size of the population.

2. Examples: competition, predation, parasitism, stress caused by crowding, disease

3. Seldom totally eliminate a population

4. Some populations are self-controlling in that they have mechanisms to reduce competition – delayed maturation caused by malnutrition, migration, cannibalism.

Density-Independent Limiting Factors

1. The degree of influence is not related to population size.

2. Examples: pollution, habitat destruction, natural disasters, weather

3. Usually just reduce the population below the cc. but they have the ability to totally eliminate a population

Relationships Within a Population

Age Structure

• Consider especially the pre-reproductive compared to the post-reproductive years. – 1. Very young and very old and more

susceptive to disease– 2. If there are huge numbers of young adults,

the population will grow; if there are mostly elderly, the population will decline.

Sex Ratios• In a monogamous species, the ratio of

males to females should be about equal. In deer and pheasants, for example, that is not as important, because one male often fertilizes many females.

Behavior1. Territory- a defended area which insured

the occupants will have enough resources for themselves and their offspring.

2. Social hierarchy – wolves, chickens, social status determines which individuals eat or breed.

3. Natural Cycles – lemmings, snowshoe hare & lynx, ruffed grouse

Relationships Between Populations of Different Species

Relationships Between Populations of Different Species

A. Since two different species cannot indefinitely occupy the same niche, separation of the habitat into microhabitats reduces competition.

B. Theory of Predation – Predators seldom eat all of their prey. As the prey becomes scarce, it is harder to find something to eat.

C. Disease, Commensalism, Parasitism, etc.

Relationships Between Populations of Different Species

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

The End