Ecology II: Communities and Populations

Population: the total number of a certain species in a given area

3 Characteristics of Populations

1. Geographic Distribution

2. Population Density

3. Growth Rate

I. Population Growth

Affected by 3 Factors:

1. Number of Births

2. Number of Deaths

3. Number of Individuals entering or leaving

Population Dynamics

Wolves in Yellowstone

Size, Distribution, Density

Size

- Fluctuates constantly- Influenced by many

factors-Growth =- (b-d) (i-e)

Distribution

- Provides clues about environmental/social factors that affect populations

3 Types:1. Clumped2. Uniform3. Random (rare)

Density

- Number of individuals per unit area

- May indicate resource availability

II. Population Demographics

- The study of how population factors change over time

A. Density Dependent Factors

- Affects populations as density varies

Ex.-disease-predation-Limiting Resources1. Food (nutrients)2. Water3. Space4. Habitat

B. Density Independent Factors

- Often abiotic- Natural disasters- Non-communicable

disease

Population Growth Curves

Exponential Growth: populations reproduce at a constant rate

Occurs when resources are unlimited

Logistic Growth: slowing or stopping of population growth after exponential growth

Occurs when resources become scarce

Carrying Capacity: the largest number of individuals that an environment can support

Decline Phase: the population crash due to several causes

Causes for Decline:1. Stress2. Disease susceptibility3. Predation4. Competition5. Parasitism

Sampling: estimating size and densityForms:1. Quadrat: small section

of land, count individuals, extrapolate

2. Transect: a line across different areas

3. Mark-and-recapture:Catch as many as possible, mark them, repeat, marked/new total

III. Human Population Growth

History:

-population grew slowly for thousands of years-disease kept the population low-most of history – 50% of children died-500 years ago human population began growing

-invention of agriculture-increased food supply-improved sanitation, health care-exponential growth

Reasons for growth:

1. Invention of Agriculture

2. Increased food supply

3. Improved sanitation and healthcare