Population Dynamics

April 28, 2004

Review

Population (def.) – a group of actively interbreeding individuals Therefore, they’re the same species Individuals are in the same place at the same

time Groups of the same species can be separated

They become separate populations

Factors influencing growth rates of populations Population density Population size Population dispersion Age structure Sex ratio Environmental factors

Population Density

Def. – the number of individuals/area Eg. #prairie dogs/acre

Larger density means that population growth increases But as growth increases, certain factors begin to

limit growth = limiting factors Eventually, population growth decreases

Fig. 9.6, p. 201

2,000

1,500

Nu

mb

er o

f re

ind

eer

1910 1920 1930 1940 1950

Year

1,000

500

Limiting Factors

Fewer resources Parasites and disease Predation Physiological and sociological changes

Population Size

The number of individuals in a population at a given time

Small populations Less genetically diverse Less resistant to disease

Large populations More genetically diverse More resistant to disease

Optimum size

Cheetah

Population Dispersion

The spatial pattern in which the individuals of a population are distributed

3 patterns Clumping Uniformly dispersed Randomly dispersed

Patterns vary in response to limiting factors

Fig. 9.2, p. 199

Clumped(elephants)

Uniform(creosote bush)

Random(dandelions)

Population Dispersion

American Dipper

Spadefoot Toad

Age Structure

The proportion of individuals in each age group in a population

3 groups Prereproductive Reproductive Postreproductive

Each age group has a characteristic birth rate and death rate Birth rate = the number of offspring produced during a certain

amount of time Death rate = The number of individuals who die during a certain

amount of time

Generation Time

The average life span between the birth of individuals and the birth of their offspring

Related to body size A shorter generation time will result in

faster population growth

Sex Ratio

The proportion of individuals of each sex The number of females is usually directly

related to the number of births that can be expected

Environmental Factors Affecting Population Growth

Abiotic Biotic Ability to adapt to environmental change

Natural selection Evolution

Fig. 9.3, p. 200

POPULATION SIZE

Growth factors(biotic potential)

Favorable lightFavorable temperatureFavorable chemical environment(optimal level of critical nutrients)

Abiotic

BioticHigh reproductive rate

Generalized niche

Adequate food supply

Suitable habitat

Ability to compete for resources

Ability to hide from or defendagainst predatorsAbility to resist diseases and parasitesAbility to migrate and live in other habitatsAbility to adapt to environmentalchange

Decrease factors(environmental resistance)

Too much or too little lightTemperature too high or too lowUnfavorable chemical environment(too much or too little of critical nutrients)

Abiotic

BioticLow reproductive rate

Specialized niche

Inadequate food supply

Unsuitable or destroyed habitat

Too many competitorsInsufficient ability to hide from or defendagainst predatorsInability to resist diseases and parasitesInability to migrate and live in other habitatsInability to adapt to environmentalchange

Growth Factors

Growth Curves

Demography = the study of factors that affect the growth and decline of populations

Growth = (birth + immigration) – (death + emigration)

Populations grow exponentially Yields a J-curve

Plotted in terms of doubling time

Doubling Paper ThicknessDoubling Paper Thickness

0

10000000

20000000

30000000

40000000

50000000

60000000

70000000

80000000

90000000

100000000

0 10 20 30 40 50 60

Number of Times Doubled

Mile

s

Series1

Factors Affecting Growth

Resources Fertility Clutch size Frequency of births

Limitations Affecting Growth

Environmental Resistance Determines the carrying capacity (K)

S-curve – population fluctuates around K.

K-selected species

Boom and bust curve – species grow exponentially and use up their resources

R-selected species

Fig. 9.10b, p. 205

Fewer, larger offspringHigh parental care and protection of offspringLater reproductive ageMost offspring survive to reproductive ageLarger adultsAdapted to stable climate and environmental conditionsLower population growth rate (r)Population size fairly stable and usually close to carrying capacity (K)Specialist nicheHigh ability to competeLate successional species

elephant saguaro

K-Selected Species

Fig. 9.5, p. 201

2.0

1.5

1.0

.5

Nu

mb

er o

f sh

eep

(m

illio

ns)

1800 1825 1850 1875 1900 1925Year

Sheep in Tasmania

Fig. 9.10a, p. 205

r-Selected Species

cockroach dandelion

Many small offspringLittle or no parental care and protection of offspringEarly reproductive ageMost offspring die before reaching reproductive ageSmall adultsAdapted to unstable climate and environmental conditionsHigh population growth rate (r)Population size fluctuates wildly above and below carrying capacity (K)Generalist nicheLow ability to competeEarly successional species

Fig. 9.8, p. 203

Po

pu

lati

on

siz

e (t

ho

usa

nd

s)

160

140

120

100

80

60

40

20

0 1845 1855 1865 1875 1885 1895 1905 1915 1925 1935

Year

Hare

Lynx

Snowshoe Hare and Lynx Population