Population Ecology

Population: A group of organisms that belong to the same species that live in the same place at the same time.

Properties of a Population I

Boundaries: May be imposed by an environmental

factor May be arbitrarily chosen to simplify a

population study. Different than properties of an individual:

Individuals are born, reproduce, and die. A population study focuses on HOW MANY individuals are born, how many die, etc…

Properties of a Population II: Population Size

The number of individuals it contains Important, but can be hard to measure directly

If the population is small and the organisms are immobile, then the organisms can be counted.

Usually individuals are too abundant, too widespread, or too mobile to be counted and the number must be estimated using the random sampling method.

Suppose we want to know the number of sunflowers in a field. The ecologist counts the number in a 10 km squared area and

finds in that small patch there are 1,000 sunflowers. Assuming the field in 100 km squared, it is reasonable to

assume that there are 10,000 sunflowers overall. (1,000 sunflowers x 10)

Properties of a Population III: Population Density

Measures how crowded a population is. Expressed as the number of individuals per unit of area or

volume. (example: 30 people per square kilometer)

Properties of a Population IV: Dispersion

The spatial distribution of individuals within a population. Clumped: individuals are clustered

together Often occur when resources are

clumped Also occurs due to an individual’s

social behavior (herds, flocks) Even: individuals are separated by a

fairly constant distance. Due to social interactions-

organisms want to get as far away from each other as possible.

Random: each individual’s location is independent of the other individual’s locations.

Usually due to wind dispersal.

Population Dynamics I

Change in size and composition over time. Birth rate: number of births in a given time Death, or mortality, rate: number of deaths

in a given period of time Life expectancy: how long on average an

individual is expected to live.

Population Dynamics II: Patterns of Mortality

Tend to conform to one of three patterns.

Curves are call survivorship curves: show the likelihood of survival at different ages throughout the lifetime of an organism.

Population Growth Rate

The amount by which a population size changes over time. Birth Death Immigration Emigration

For simplicity, can assume immigration = emigration

Birth rate – death rate = growth rate

Exponential Model

Population that increases rapidly after only a few generations: the larger the population gets, the faster it grows.

J-shaped curve- grows slowly when the population is small, speed up as more individuals are added.

Limitations- populations grow exponentially under rare conditions and for short periods of time.

In reality- limiting factors restrain the growth of a population.

Logistic Model

Accounts for the influence of limiting factors

Carrying capacity (K): the number of individuals the environment can support over a long period of time.

Assumes that carrying capacity is constant

Population Regulation

Two kinds of limiting factors Density-independent (weather, floods,

fires): reduce the population by the same proportion regardless of a population’s size.

Density-dependent (include resource limitations): shortages of food, triggered by increasing population density.

All populations fluctuate in size

Questions: Answer the following questions in on a separate sheet of paper. Be sure to write out the question and answer the question in complete sentences.

1. Explain how two populations can be the same size but have different densities. 2. Explain two difficulties and ecologist might have in counting a population of migratory

birds. Develop and explain a method for estimating the size of such a population. 3. According to the exponential model, how do birth and death rate change with

population size? 4. Describe two differences between the exponential model and the logistic model. 5. Write a brief paragraph supporting the assumption that immigration and emigration

are insignificant when studying global human population.