1

Population ecology

Definitions

• Habitat

• Population

• Community

• Ecosystem

• Biotic Factors

• Abiotic Factors

Characteristics of Populations

• Size

• Age Structure

• Density

• Distribution

45.1

Age Structure Diagrams

Shows age distribution of a population

Rapid Growth

Slow Growth

Zero Growth

Negative Growth

• Different species occupying the same

area compete…

– These interactions influence the density and dispersion of individuals and

populations.

Distribution

45.1

Distribution

clumped • Clumped populations:

– Suitable physical, chemical, and biological conditions are patchy, not uniform.

– Social groups

– Offspring not mobile

45.1

2

Distribution

• Rare in nature

• Result of fierce

competition for very

limited resources

uniform

45.1

Distribution

• Uniform environmental

conditions

• Members are neither

attracting nor repelling each other

random

45.1

Determining population size

• Direct counts are most accurate but

seldom feasible

• Sample an area, then extrapolate

• Capture-recapture method is used for mobile species

45.2

Capture-Recapture Method

Total captured in sampling 2

Marked individuals in sampling at time 2

Total population size

Marked individuals in sampling at time 1

45.2

Survivorship Curves

• represent age-specific patterns of death for a

given pop in a given environment

• Each species has a characteristic curve.

• Three types of curves are common in nature.

45.5Fig. 45-10a, p.805

3

Fig. 45-10b, p.805 Fig. 45-10c, p.805

Life History Patterns

• Patterns of timing of reproduction and

survivorship

• Vary among species

• Summarized in survivorship curves and

life tables

Reproductive Strategies

• Different environments and population densities can favor different reproductive strategies

• r-selection favors traits that maximize number of offspring

• K-selection favors traits that improve offspring quality

Life History and Reproductive Strategies--

Certain combinations of traits tend to occur together:

Trait r-selected K-selected

Lifespan Short Long

Growth rate Fast Slow

1st Reproduction Early Late

Reproductive effort High Low

Parental care Less More

Offspring Many, small Few, large

Reproductive

episodes

all at once more than once

Kind of environment Variable Stable

r- or K- selected?

• Mouse

• Mushroom

• Oak tree

• Human

• Elephant

• Dandelion

4

Population Growth

45.3

r

• Net reproduction per individual per

unit time

• Can be used to calculate rate of

growth of a population

45.3

r = b - d

Exponential Growth

45.3

Population growth per unit time

(G)

Per capita growth rate

(r)

Total population

(N)

x=

G=r x N

Exponential Growth: Any quantity that is growing at a rate proportional to its size.

TIME

PO

PU

LATI

ON

SIZ

E

45.3

Biotic Potential

• Biotic potential: Maximum rate of increase

per individual (r) under ideal conditions

• Varies between species

• In nature, biotic potential is rarely reached

45.3

Limits on Growth of

Populations

Limiting factor: any essential

resource that is in short supply

5

Actual rate of increase is influenced by

environmental conditions:

• nutrient supply

• competition for space

• Pollution

• floods

• etc.

45.4

Carrying Capacity (K)

• Maximum number of individuals that

can be sustained in a particular

habitat

• Logistic growth occurs when

population size is limited by carrying

capacity45.4

Logistic Growth

45.4

Logistic Growth

Population

growth per unit time

(G)

Net

reproduction per indiv. per

unit time

(r)

#

Individuals

(N)

x=Proportion

of resources not yet used

x

45.4

TIME

PO

PU

LATI

ON

SIZ

E

Fastest growth of pop.

Births > Deaths

Resources

abundant

Resources

become limiting Births = Deaths

Carrying capacity

45.4

Overshooting Capacity

• Population may

temporarily increase

above carrying

capacity

• Overshoot is usually

followed by a crash;

dramatic increase in

deaths

Reindeer on St. Matthew’s Island

Fig. 45-9, p.803

6

Density-Dependent Factors

• Logistic growth equation

deals with density-

dependent controls

• Limiting factors become

more intense as

population size increases

45.4

• Factors unaffected by population

density.

• Natural disasters, climate changes, etc.

Density-Independent factors

Human Populations World population density

Human Population Problems

• World pop reached 6 billion in

1999

current population

• About 2 billion live in poverty

• Most resources are consumed by

the relatively few people in

developed countries

Side-Stepping Growth Controls

• Expanded into new habitats

• Agriculture increased carrying

capacity; use of fossil fuels aided

increase

• Hygiene and medicine lessened

effects of density-dependent controls

7

Human Population Growth Future Growth

• Exponential growth cannot continue

forever

• Breakthroughs in technology may

further increase carrying capacity

• Eventually, density-dependent factors

will slow growth

Fertility Rates

• Worldwide, average annual rate of

increase is 2.6%

• Total fertility rate (TFR) is average

number of children born

• Highest in developing countries, lowest

in developed countries

Fig. 45-17b, p.815