Population Growth

Ch. 4 sec. 3

Factors That Determine Population Growth

• Growth rate: change in a population’s size over time.

• A population’s growth rate is determined by births, deaths, immigration and emigration.

Why are there so many bacteria and so few elephants?

1. Birth and Death Rates

• Mortality: rate of death

• Natality: rate of birth Survivorship Curves:

A graph that shows how the likelihood of death varies with age.

2. Age Structure• Age structure of a population is

related to birth rates, death rates and population growth.

• Young populations (pre and reproductive age) = more births than deaths. – Population increase

• Elderly population = more deaths – Population decrease

• Sex Ratio also influences population size.– Imbalance of males and females = changed

population size

Immigration and Emigration

• Immigration is the arrival of individuals from outside a given area.

• Emigration is the departure of individuals from a given area.

• Migration: temporary movements into and out of an area as a part of a seasonal routine. – Fish, mammals, insects and birds all

migrate

Calculating Population Growth

(birth rate + immigration rate) - (death rate + emigration rate) = net change in population size

Population changes are often expressed as percentages, which we can calculate using the following formula:

1. GROWTH RATE x 100%

How Populations Grow

• Exponential Growth: when a population increases by a fixed percentage each year.

• Logistic Growth: describes how a population’s increased growth is slowed and stopped by limiting factors.

Exponential Growth• Faster and faster growth!

• Only occurs in nature when populations have plenty of food, water, space, and no competition.

• Ex. Mold on bread

Logistic Growth• Growth that slows and stops due

to limiting factors.

• Limiting factors: Any environmental characteristic that can slow population growth and determine carrying capacity• Water

• Space

• Food

• Predators

• Disease

What Limits Population Growth?

1. Carrying capacity

2. Limiting factors

3. Competition

Carrying Capacity• Maximum population an ecosystem can

support.• A population may grow past the carrying

capacity, but cannot stay at that size.

• Example: The state of Pennsylvania can only support a certain number of white tailed deer. There is a limited amount of food, water, and shelter for the deer that live here. If the population grows so much that deer do not have enough of what they need, that population has gone over the carrying capacity and it will shrink.

Limiting Factors• Whatever resource is lacking or

missing for a population• Will ultimately cause a decline in

the population.• Water

• Space

• Food

• Predators

• Disease

Competition• Members within a population use

the same resources and therefore will compete with each other.

• Part of natural selection: those species that can access resources and make best use of those resources will survive.

• Examples: Wolves: territory, food Trees: sunlight

Biotic Potential• Potential that each living thing has to produce offspring

in ideal conditions. • Not every organism is created equally – some can

reproduce quickly and easily – others take quite a long time!

Review Questions1. What is the difference between exponential growth and

logistic growth? Which is more common over long term in nature?

2. You are a population ecologist studying white-tailed deer populations in your state. Populations have been growing exponentially for some time, and food is becoming a limiting factor. Many deer are dying of starvation, and others are in bad health. What do you recommend to state officials? Should people intervene and try to limit deer populations through relocation or hunting? Or should they do nothing and wait for the population to regulate itself? Explain your reasoning.