Population Ecology

Study of the factors that affect population size and composition.

Population

Individuals of a single species that occupy the same area.

Important Characteristics

1. Density

2. Dispersion

Density

Number of individuals per unit area or volume.

Ex: Diatoms - 5 million/m3

Trees - 5,000/km2

Deer - 4/km2

Dispersion

Pattern of spacing among individuals.

Types:1. Clumped

2. Uniform

3. Random

Clumped Dispersion May result form a patchy

environment. May increase chances for

survival. Ex:

Schooling behavior Flocks of birds

Uniform Dispersion

Often the result of antagonistic interactions between individuals.

Ex: Territories Spacing between desert plants

Random Dispersion

Often the result of the absence of strong attractions or repulsions between individuals.

Not a common pattern.

Demography

The study of the vital statistics that affect population size.

Ex: Birth and Death rates

Demographic Factors

Age structure of the population.

Birth and Death rates. Generation time. Sex ratio and reproductive

behavior.

Life Tables

Mortality summary for a cohort of individuals.

First developed from life insurance studies.

Life Tables Show

Mortality rate per year. Life span of the organism. Fecundity (birth rate).

Survivorship Curve

Plot of the numbers of a cohort still alive over time.

Curve Types: Type I Type II Type III

Type I

Low early deaths. High late deaths. Ex:

Humans Other large mammals

Type II

Constant death rate. Ex:

Annual plants Many invertebrates

Type III

High early deaths. Low late deaths. Ex:

Trees Oysters

Comment

Curve type may change between young and adults.

Ex: Nestlings - Type III Adult Birds- Type II

Life History Strategies

1. "r" or Opportunistic species

2. "k" or Equilibrial species

"r" Species

Increase fitness by producing as many offspring as possible.

Do this by: Early maturation Many reproductive events Many offspring

Result

Maximize reproduction so that at least a few offspring survive to the next generation.

Most offspring die (Type III curve).

"k" Species

Increase fitness by having most offspring survive.

Do this by: High parental care Late maturation Few reproduction events Few offspring.

Result

Maximize survivorship of each offspring.

Few offspring, but most survive (Type I curve).

What is the strategy For a weed? For an endangered species? For Garden Pests?

Population Growth

N/t = b - d Where:

N= population size t = time b = birth rate d = death rate

Rate of Increase

r = difference between birth rate and death rate.

r = b - d

Equation:

N/t = rN N = population size t = time r = rate of increase

From Calculus

The equation N/t = rN becomes:

dN/dt = rmax N

rmax = intrinsic rate of increase

Exponential Growth

dN/dt = rmax N Characteristic of "r" species. Produces a “J-shaped”

growth curve. Only holds for ideal

conditions and unlimited resources.

Logistic Growth

dN/dt = rmax N K-N K

K = carrying capacity

Result

“S-shaped” growth curve. Characteristic of “k" species. Common when resources are

limited.

Comment

K is not a constant value. Populations often oscillate

around “K” as the environment changes.

Additional Comments

Populations often overshoot “K”, then drop back to or below “K”.

AP Exam rarely asks you to work the equations, but you should be able to give them.

Regulation of Population Size

1. Density- Dependent Factors

2. Density- Independent Factors

Density-Dependent

Affect is related to N. As N increases, mortality

increases. Ex: Food, nesting space,

disease

Density-Independent

Affect is not related to N. Mortality not related to

population size. Ex: Weather and climate

Population Cycles

Cyclic changes in N over time.

Often seen in predator/prey cycles.

Ex: Snowshoe Hare - Lynx

Causes

Density dependent factors. Chemical cycles. Saturation strategy to

confuse predators.

Age Structure Diagrams

Show the percent of a population in different age categories .

Method to get data similar to a Life Table, but at one point in time.

Importances

Can be used to predict future population growth trends, especially for long lived species.

Exponential Growth

Produces age structures that are a triangle or pyramid shape.

Logistic Growth

Produces age structures that have even sizes between most age categories.

Declining Populations

Produce age structures with a narrow base and wider middles.

Summary

Know density and dispersion patterns.

Know Life Tables and survivorship curves.

Be able to contrast and compare “r” and “k” strategies.

Summary

Know exponential and logistic growth curves and equations.

Know density and density-independent growth factors.