Chapter 19: Population Ecology

Population A population is a group of organisms that belong to the same species and live in a particular place at the same time. Populations can be measured in terms of size, density, dispersion, growth rate, age structure, and survivorship.

Properties of Populations Population Size A population’s size is the number of individuals that the population contains. Population Density Density is a measure of how crowded the population is. Age Structure A population’s age structure indicates the percentage of individuals at each age.

Properties of Populations, cont. Dispersion Dispersion describes the distribution of individuals within the population and may be random, uniform, or clumped.

Populations can increase, decrease, or stay the same over time Population Dynamics Populations can increase, decrease, or stay the same over time

What do you think might affect a population’s growth and size?

Resources, Space, Predators, Disease, and Habitat Key Point #1: A Limiting Factor is something that keeps a population from growing infinitely Think: What are the 5 major limiting factors? Resources, Space, Predators, Disease, and Habitat

There are 2 different types of limiting factors

1. Density-Independent Factors Key Point #2: Density-independent factors are not affected by the population size Do not depend on the number of individuals present in an area. Examples: unusual weather, natural disasters, and seasonal cycles

2. Density-Dependent Factors Key Point #3: Population-limiting factors, such as competition, are density-dependent because the effect on each individual depends on the number of other individuals present in the same area. Examples: competition, disease, and predation

You Try: Density-Dependent or Independent?

Predation

Weather Patterns

Disease

Competition for Resources

Survivorship Curves Patterns of Mortality Populations show three patterns of mortality or survivorship curves: Type I (low mortality until late in life) Type II (constant mortality throughout life) Type III (high mortality early in life followed by low mortality for the remaining life span).

Survivorship Curves, cont. Type I = large mammals (i.e., humans, elephants) Type II = birds, lizards, rodents Type III = fish, insects

Population Growth Rate Scientists define the growth rate of a population as the amount by which a population’s size changes in a given time. Population Size Birth rate - death rate = growth rate If the birth rate increases, the population increases If the death rate increases, the population decreases

Exponential Growth The exponential model describes perpetual growth at a steady rate in a population. The model assumes constant birth and death rates and no immigration or emigration. Assumes unlimited resources

Logistic Growth In the logistic model, birth rates fall and death rates climb as the population grows. When the carrying capacity is reached, the number of individuals the environment can support is reached and population growth becomes stable.

Logistic Growth, cont. Limiting factors (i.e., food, water, space) will limit a population’s growth, so it cannot exceed carrying capacity

You Try: Exponential or Logistic?

You Try: Exponential or Logistic?

You Try: Exponential or Logistic?