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Chapter 5 Populations

Big Idea

Big Idea:

Interdependence in Nature

Key Question:

What factors contribute to changes in populations?

How Populations Grow

Section 5.1

Describing Populations

Ecologists are interested in answering questions about ecosystems

Ecologists study a population’s:

Geographic Range

Density & Distribution

Growth Rate

Age Structure

Geographic Range

The geographic range describes the spaces where it lives

Range can be very small (bacteria) or very large (migratory fish/birds)

Density & Distribution

Population density is the number of individuals of a species per unit of area

This describes how the organisms are spread across their range

The distribution may be random, uniform, or clumped

Growth Rate

A population’s growth rate tell how the population is changing

Positive growth rate leads to an increase in individuals

Negative growth rate lead to a decrease in individuals

Age Structure

Age Structure

The numbers of males and females in different age groups of a population

This is important because in animals only the females between certain ages can reproduce

Population Growth

The size of a population changes based on how many individuals are added to it or removed from it

How are individuals added or removed?

Birthrate and Death Rate

A population may grow if more individuals are born than die in any period of time

If the death rate is higher, populations may shrink

If birthrate and death rate are equal, then a population remains the same in overall number

Immigration and Emigration

Immigration

The movement of individuals into an area occupied by an existing population

Emigration

The movement of individuals out of an area

If immigration is higher than emigration, a population will grow

Exponential Growth

Types of Growth

Imagine a population has and endless supply of food and space, has protection from predators and disease, and all waste products are removed – under these ideal conditions, a population will grow

Organisms that Reproduce Rapidly

A single bacterium can divide to form two cells every 20 minutes

This means that in the span of an hour, a single bacterium can divide to become 8 individual cells

This becomes 64 cells in 2 hours, 512 cells in 3 hours, 4096 cells in 4 hours, and so on and so forth

Exponential Growth

A growth pattern in which the individuals in a population reproduce at a constant rate

Logistic Growth

Populations in the real world don’t grow exponentially for very long

What causes population growth to eventually slow down over time?

Phases of Growth

Phase 1: Exponential Growth

Phase 2: Growth Slows Down

Phase 3: Growth Stops

Logistic Growth Curve

Logistic Growth

A growth pattern in which a population’s growth rate slows or stops, following a period of exponential growth

These growth curves are shaped like the letter S

Carrying Capacity

Carrying Capacity

The largest number of individuals of a population that a given environment can support

This is shown as a broken horizontal line on a logistic growth graph

Population size may increase or decrease just a little, but will mainly stay very near the carrying capacity

Limits to Growth

Section 5.2

Limiting Factors

Why do populations grow as they do?

What factors cause the growth to slow down?

What determines the carrying capacity of an environment for a species?

Limiting Factor

Limiting Factor

A factor that causes the growth of a population to decrease

Could include the number or predators or how much food is available

They determine the carrying capacity of an environment for a species

Density-Dependent Limiting Factors

Density-Dependent Limiting Factor

A limiting factor that depends on population size

Several different kinds

Predators

Herbivores

Parasites and Disease

Competition

Overcrowding

Density-Independent Limiting Factors

Density-Independent Limiting Factor

A limiting factor that affects all populations in similar ways, regardless of population size

Examples include: bad weather events such as hurricanes or drought, natural disasters such as wildfires, or human caused disasters like farming/logging

Human Population Growth

Section 5.3

Historical Overview

Human population grew very slowly for most of human history

Food was hard to find and the death rate was high

Exponential Human Population Growth

The Industrial Revolution led to a boom in human population growth

Goods could be shipped across the world, food supplies became more reliable, healthcare and medicine improved, sanitation and nutrition became important, and the death rate steadily decreased as the birthrate increased

Patterns of Human Population Growth

Demography

The scientific study of human populations

Birthrates, death rates, and the age structure of the population are all studied in order to find reasons why populations are growing or that growth is slowing down

The Demographic Transition

Demographic Transition

A change in a population from high birthrates and death rates to low birthrates and death rates

Stage I: Birthrate and death rate are equally high

Stage II: Death rate begins to fall but birthrate remains high

Stage III: Birthrate falls to meet the death rate

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