Chapter 52:Population Ecology

Population Ecology Study of the factors

that affect population size and composition.

Population Individuals of a single

species that occupy the same area.

Important Characteristics1. Density

Number of individuals per unit area or volume. Ex:

Diatoms - 5 million/m3

Trees - 5,000/km2

Deer - 4/km2

2. Dispersion Pattern of spacing among individuals. Types:

1. Clumped2. Uniform3. 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

Survivorship Curves 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 Strategies1. "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 Affect is related to N (pop. size). As N increases, mortality increases. Ex: Food, nesting space, disease

2. Density- Independent Factors 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. Know exponential and logistic growth

curves and equations. Know density and density-independent

growth factors.