Populations Introduction to Population Dynamics

I. Populations A. What is a population?

•  The world is divided into millions of species.

•  Within a species, a population is a group of individuals that are actually available to each other for mating. ��� •  A species may be spread across the globe, while a���population tends to���cluster together.

B. Examples

C. Population Dynamics

1. Populations grow when birth rate + immigration rate (# of

organisms moving into a population) > death rate + emigration

rate (# of organisms moving out of a population)���

2. Populations decline when birth rate + immigration rate (# of

organisms moving into a population) < death rate + emigration

rate (# of organisms moving out of a population)

A.  Populations and Resources

•  Resources determine how a given population will grow.

•  In general, any population of organisms with plenty of resources and ideal conditions tends to grow fast.

II. Population Growth

•  This type of growth is called exponential growth.

B.  Doubling Time

•  Exponential growth has a fixed doubling time. There is a certain

number of days, or months, or years in which the population

doubles.

C. Exponential Growth

•  At first the population grows slowly, but at some point it takes

off.

1. Resources, matter and energy in short supply limit how large a

population can grow, therefore resources can be ���

limiting factors.

A. No population can continue exponential growth forever

because resources are finite.

III. Limiting Factors

1. Density-dependent limiting factors control population size when a population is large and crowded. a. examples: competition, predation, parasitism and crowding

2. Density-independent limiting factors control population size regardless of whether a population is large or small. a. examples: natural causes such as geothermal activity, extreme weather

B.

IV. Carrying Capacity & Population Growth

A.  Carrying Capacity

•  Limiting factors determine the carrying capacity, or the number

of individuals in a population that can be sustained indefinitely in

a given ecosystem.

•  When a population is small compared to the carrying capacity, it

grows rapidly. What happens next depends on many factors.

B. J Curve

•  The population slows as it approaches the carrying capacity and

levels off, reaching a steady state.

C.  J Curve

• A period of rapid population increase followed by collapse.

•  Population growth disrupts the ecosystem so badly that its

carrying capacity is reduced, and the species suffers a die-off

from which it never fully recovers.

J Curve Example

gThe End