Download Population size

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

Number of reindeer
2,000
1,500
1,000
500
1910
1920
1930
Year
1940
1950
Fig. 9.6, p. 201
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



Large populations



Less genetically diverse
Less resistant to disease
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
Population Dispersion
Clumped
(elephants)
Uniform
(creosote bush)
Random
(dandelions)
Fig. 9.2, p. 199
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

POPULATION SIZE
Growth Factors
Growth factors
(biotic potential)
Abiotic
Favorable light
Favorable temperature
Favorable chemical environment
(optimal level of critical nutrients)
Biotic
High reproductive rate
Generalized niche
Adequate food supply
Suitable habitat
Ability to compete for resources
Ability to hide from or defend
against predators
Ability to resist diseases and parasites
Ability to migrate and live in other
habitats
Ability to adapt to environmental
change
Decrease factors
(environmental resistance)
Abiotic
Too much or too little light
Temperature too high or too low
Unfavorable chemical environment
(too much or too little of critical
nutrients)
Biotic
Low reproductive rate
Specialized niche
Inadequate food supply
Unsuitable or destroyed habitat
Too many competitors
Insufficient ability to hide from or defend
against predators
Inability to resist diseases and parasites
Inability to migrate and live in other
habitats
Inability to adapt to environmental
change
Fig. 9.3, p. 200
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 Thickness
Doubling Paper Thickness
100000000
90000000
80000000
70000000
Miles
60000000
50000000
Series1
40000000
30000000
20000000
10000000
0
0
10
20
30
Number of Times Doubled
40
50
60
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
K-Selected Species
elephant
saguaro
Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close
to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
Fig. 9.10b, p. 205
Sheep in Tasmania
Number of sheep (millions)
2.0
1.5
1.0
.5
1800
1825
1850
1875
Year
1900
1925
Fig. 9.5, p. 201
r-Selected Species
cockroach
dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching
reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
Fig. 9.10a, p. 205
Snowshoe Hare and Lynx
Population
Population size (thousands)
160
Hare
140
120
100
Lynx
80
60
40
20
0 1845
1855
1865
1875
1885
1895
Year
1905
1915
1925
1935
Fig. 9.8, p. 203