# Population Growth and Regulation

## What's Ecology

• It's not conservation, preservation or recycling!

• It is:

• The scientific study of the relationships between organisms and their environment.

• An attempt to understand the distribution and abundance of organisms.

## What's a Population

• It varies, ecologists use the word "population" in several ways; typically a "somewhat isolated collection of individuals."

• Properties of Populations:

1. Size or Density.

Ecologists characterize the abundance of organisms by measuring their density; individuals per square meter (or per meter cubed).

Density can be measured by counting, by transect or quadrat sampling, and by mark-release recapture.

2. Distribution (or range).

The geographical limits of the population.

3. Dispersion.

The clumpedness of the population; a population may over-dispersed (regularly spaced) or under-dispersed (more clumped than expected).

## Understanding the dynamics of population size.

There are two "basic models" of population growth. Exponential growth, and logistic growth.

• In exponential growth, the population's growth rate increases with population size.

1. The models indicate growth to infinity if r>0, and decline to zero if r< 0.

2. r is called the "intrinsic rate of growth", and is a basic parameter of many ecological models and processes. What are the units of r?

• In logistic growth the population growth is "checked" by resource limitations. As the the population approaches its "carrying capacity" K its growth rate slows down.

In logistic growth there are "density-dependent" changes in growth rate that REGULATE population growth.

Ecologists see "density dependent" regulation as a general feature of population that stabilizes the natural world.

• BUT, for some species density-independent processes are important too.

## Life Tables: accounting for differences in age.

• The exponential and logistic models treat all individuals in a population the same; but we know that baby's don't reproduce but adults do!

• Ecologists take account of this by constructing "life tables" for populations that show

1. the survivorship of different ages &

2. the fecundity of different ages

Life tables show that species vary enormously in their "life history" characteristics.

Some show lots of mortality early in life, but little when they are older. Others show the reverse pattern.

Some reproduce throughout their lives; other reproduce in one enormous bout and die.

## r and K selected life histories.

• Many ecologists believe that these difference in life history reflect the action of natural selection, &

they believe that organisms that frequently find themselves in the "r" dominated part of the logistic growth curve have different life histories than those that find themselves in the K dominated part of the growth curve.

• r-selected organisms live in ephermal (weedy) habitats, they are constant exploitly new habitats, and their population are typically growing fast. This emphasizes "reproduction now". So r-selected species are:

2. Mature and reproduce early

3. Prone to non-repeated reproduction.

4. Produce large numbers of offspring.

Classical r-selected species are weeds, shrimp in ephermal ponds, etc.

• K-selected organisms live in stable persistent habitats, their populations are typical very stable. This emphasizes delayed reproduction, and adult competitiveness. So K-selected species are: