CHAPTER 53 POPULATION ECOLOGY
I. Dynamic processes influence population density, dispersion, and demographics Population affected by: Density
# per unit area
Dispersion
Pattern of spacing within population boundaries
A. Density and dispersion 1. DENSITY How to determine population size Count all
Sample a plot -> extrapolate Count # of blades of grass in a 200 square meter field? # individuals / sample area = density density X total area = total population
Count an indicator
Bobcat tracks
Eagle nest
Black bear scat
Mark-recapture Capture a random sample, tag, release Wait a few weeks Capture another random sample Estimate population size Hectors dolphin, New Zealand
x = number of marked dolphins in the second capture Fig 53.2 n = total number of dolphins in second capture s = number of marked dolphins N = total population size
N = sn/x
Births
Births and immigration add individuals to a population.
Immigration
Deaths
Deaths and emigration remove individuals from a population.
Emigration
2. DISPERSION Clumped dispersion Example: mushrooms group in areas that favor growth
Shitake
Pholiota
(a) Clumped
Uniform dispersion (rare)– can result from direct interactions between individuals such as territoriality
(b) Uniform
Example: creosote bush – competition for water creates uniform dispersion
Mohave desert
Random dispersion – unpredictable because each individual is independent from others
(c) Random
B. Demographics – study of population change over time
Factors that influence population density and dispersion 1. Life table
Follow a cohort = group of same age from birth to death
Imagine a population of 1,000 individuals born at
the same time in the same place. As time progresses, some individuals die, so there are fewer and fewer individuals present each year. But when do most individuals die? Do most individuals live to old age or do many individuals die at young ages?
Belding’s ground squirrel life table
Compare male and female death rates
2. survivorship curve – plot data Number of survivors (log scale)
1,000
Relatively constant death rate but lower survival rate for males
100
Females 10
Males
1 0
2
4 6 Age (years)
8
10
Survivorship curves
Type 1 curve - high survival rate of young, live out most of expected life span and die in old age.
Number of survivors (log scale)
1,000
I
100 II 10 III 1 0
50 Percentage of maximum life span
100
Survivorship curves (cont) Type II curve- relatively constant death rate
throughout their life span. Type III curve - many young, most of which die very early in their life.
3. Reproductive rates (females) Measure reproductive output of cohort
What does this data indicate?
II. The exponential model Exponential growth (geometric)
“unlimited resources” Why cant exponential growth continue forever?
The rate of increase is constant But, much higher number of individuals produced as population increases Example: Bacteria divide every 20’ when resources are unlimited
III. Logistic growth
What is meant by “carrying capacity”?
Maximum population size an environment can
support Limiting factors include: Water, energy, shelter, refuge, food, nesting sites……
Note S-shaped curve
This approximates logistic growth
This population overshot the carrying capacity of the environment
IV. Life history traits are products of natural selection
A. Evolution and life history Traits that effect reproduction and survival
Variables When reproduction begins (age) Frequency of reproduction Number of offspring
1. semelparity Produce once and die Pacific salmon hatches in a stream, migrates to the open ocean, requires 1 – 4 years to matures, returns to the stream to spawn, produces 1000s of eggs, dies.
Agave grows in desert for years, when there is wet year, it sends up a flowering stalk, produces seeds, dies
2. Iteroparity – repeated reproduction produce few, but large, offspring
Artic fox pups
B. Evolutionary trade offs between reproduction
and survival If you care for a large number of offspring, you
yourself may have compromised survival Kestrel study
Transferred chicks around to make defined brood sizes Small 3 – 4 chicks Normal 5 – 6 chicks Large 7 – 8 chicks
Next measure % of male and female parent birds that survive the winter Both parents care for young
Parents surviving the following winter (%)
RESULTS 100
Male Female
80 60 40 20 0
Reduced brood size
Normal brood size
Enlarged brood size
Selective pressures and natural selection if young have high mortality produce large #s young Or
Make an extra parental investment
Brazil nut tree has a few large seeds with high nutrients
Sardines have many offspring
mouse pups
Squirrel monkey has 1 offspring
V. Many factors that regulate population growth are density dependent A. Mechanisms of density dependent regulation 1. competition for resources
Example Soay sheep
2. toxic wastes
Example: yeast
3. predation
Example: trout
4. intrinsic factors
Example: mouse
5. territoriality
Example: cheetah
6. disease
Example: the flu
Toxic wastes – yeast produce ethanol in
fermentation, and ethanol is toxic to yeast.
Predation
Prey population size affects predator population Predators can switch prey when certain prey is plentiful
Intrinsic factors
High population density stress hormonal changes delay sexual maturation decrease in birth rate
Territoriality
Increases resources for each animal survival
(a) Cheetah marking its territory
Territoriality in nesting behavior
(b) Gannets
Disease – increased transmission in dense
populations
B. Population Dynamics
Study of interactions between biotic and abiotic factors that cause variation in population size
1. Stability and Fluctuation Most populations are relatively stable over time, but…. may fluctuate
Soay sheep Population fluctuates widely due to: Harsh wet winters food availability High density more parasites
2,100
Number of sheep
1,900
1,700 1,500
1,300 1,100
900 700
500 0 1955
1965
1975
1985 Year
1995
2005
Predation may cause population density fluctuation
Changes in predation affect population size 2,500
50 Moose
40
2,000
30
1,500
20
1,000
10
500
0 1955
1965
1975
1985 Year
1995
0 2005
Number of moose
Number of wolves
Wolves
The human population is no longer growing exponentially but is still increasing rapidly U.N. Says 7 Billion Now Share the World
6 5
4 3 2 The Plague
1 0 8000 B.C.E.
4000 3000 2000 1000 B.C.E. B.C.E. B.C.E. B.C.E.
0
1000 C.E.
2000 C.E.
Human population (billions)
7
To maintain zero growth either birth rate = death rate
This is demographic balance
Most of the population growth is in developing countries Access to health care, birth control, education for women in developing countries reduces family size
Rapid growth Afghanistan Male Female
10 8
6 4 2 0 2 4 6 Percent of population
Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 10 8
Slow growth United States Male Female
6 4 2 0 2 4 6 Percent of population
Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 8
No growth Italy Male Female
6 4 2 0 2 4 6 8 Percent of population
Age structure – the relative number of people of varying ages
Infant mortality and life expectancy
80
50
Life expectancy (years)
Infant mortality (deaths per 1,000 births)
60
40 30
20
60
40
20
10 0
0 Indus- Less industrialized trialized countries countries
Indus- Less industrialized trialized countries countries
Global carrying capacity for humans Estimate is 10 – 15 billion
ecological footprint - the land and water area
needed to sustain humans Countries vary greatly in footprint size
What factors limit the carrying capacity? Food Water Space Non-renewable resources Waste