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Chapter 52Chapter 52
Population Population EcologyEcology
Population Ecology• Population ecology is the study of the
fluctuations in population size and composition and their ecological causes
• A population is a group of individuals of a single species that simultaneously occupy the same general area, use the same resources, and respond to similar environmental factors
Characteristics of Populations
• Density – the number of individuals per unit area; the mark-recapture method is an estimate of population density– N = (No. marked in 1st catch) X (total no. in
2nd catch) No. of recaptures (marked) in second catch
• Dispersion – pattern of spacing among individuals– Can be clumped (associated with food and
mating), uniform (associated with competition), or random (independent)
Types of Dispersion
Demography• Demography is the study of vital
statistics that affect population size– Life table – an age-specific summary of the
survival pattern of a population
Demography• Survivorship curve – plot of number in
cohort (a group of individuals of the same age) alive at each age– Type I – low mortality of young and mortality
increases with age; few offspring produced with good parental care (humans, mammals)
– Type II – constant death rate (some squirrels, hydra, and some lizard species)
– Type III – high mortality of young; organisms that produce a large number of offspring with little or no parental care (fish and marine invertebrates)
Demography• Survivorship curves
Reproductive Table• A reproductive table is an age specific
summary of reproductive rates in a population
Life History• Life history – the traits that affect an
organism’s schedule of reproduction and survival– Big-bang reproduction (semelparity)–
a single reproductive episode (salmon and agaves)
– Repeated reproduction (iteroparity) – adults produce large numbers of offspring over many years (oaks and lizards)
Limited Resources• Trade offs between investments in
reproduction and survival• Darwinian fitness is measured by how many
offspring survive to reproduce offspring themselves
• Finite resources mean trade offs for reproducing organisms– Reproduce at an early age– Produce many offspring each time they reproduce– Reproduce many times in a lifetime
• It is best to have few, well-cared for offspring
Limited Resources• It is best to have few, well-cared for
offspring
Population Growth• Δ pop. = births – deaths (no emigration or
immigration)
• Let N = population size and let t = time Δ N B = no. of births Δ t D = no. of deaths
• Now, let births = (per capita rate) X (pop.) B = bN
• We can now rewrite the equation as Δ N Δ t
= B – D
= bN – dN
Population Growth• We can now rewrite the equation as
Δ N Δ t
• We then use r to identify the differences in the per capita birth rates and death rates: r = b – d
• If r is positive the pop. is growing; if r is negative the pop. is declining; and if r is 0 there is ZPG
• We can now rewrite the equation asΔ N or dNΔ t dt
= bN – dN
= rN = rN
Exponential Population Growth• Exponential growth – idealized
population in an unlimited environment (no limiting factors)dNdt
• This type of pop. growth produces the J-shaped curve
= rmaxN
Exponential Population Growth• An example of exponential growth in
nature
Population Growth• Exponential growth assumes unlimited
resources which is not very realistic• What are some limiting resources you can
think of?• Ultimately, there is a limit to the number of
individuals that can occupy a habitat• Carrying capacity (K ) is the maximum
population size that a particular environment can support at a particular time
• The logistic population growth model incorporates the effect of population density on the per capita rate of increase
Logistic Population GrowthdNdt
= rmaxN (K-N ) N
Logistic Population Growth
Population Limiting Factors
• Density dependent limiting factors (K – selection) – selection for life history traits that are sensitive to population density– d increases with pop. density; b decreases
with population density– An example of negative feedback
• Density independent limiting factors (r – selection) – selection for life history traits that maximize reproductive success in uncrowded environments– No effect on d or b
Population Limiting Factors
• What are some population-limiting factors?
• What is the relationship between population growth and biotic and abiotic influences?
Population decline of the northern
pintail
Population Limiting Factors
• What are some population-limiting factors?
• What is the relationship between population growth and biotic and abiotic influences?
Human Population Growth• It took all of human time on Earth (3 million years)
for the pop. to reach 1 billion in 1800• It took 130 years to reach the second billion• The third billion was added 30 years later in 1960• The fourth billion was added 15 years later in 1975• In 1987, only 12 years later, the world population
reached 5 billion• It reached 6 billion in 1999• The human population is projected to reach 7.8
billion by the year 2025• The world’s pop. increases by 214,000 people per
day• It takes 12 years to add 1 billion people to our pop.
Human Population Growth
Human Population Growth• A regional human population can exist in one of
two configurations to maintain population stability:– Zero population growth = high birth rates –
high death rates– Zero population growth = low birth rates –
low death rates• The movement from the first toward the second
state is called the demographic transition• The world’s population growth is regional; it is
near equilibrium (0.1%) in developed nations and 1.7% in less developed countries, where 80% of the world’s population lives
Demographic Transition• It took Sweden 150 years for the
transition, and Mexico only half the time
Age Structure
•The age structure of a population influences present and future growth
•A large proportion of individuals of reproductive age or younger results in more rapid growth
•Age structure also predicts future social conditions and needs
Age Structure
Growing at Growing at 2.1% per 2.1% per
yearyear
Growing at Growing at 0.6% 0.6% per yearper year
Zero Zero growthgrowth
Ecological Footprint• The concept of ecological footprint takes
into account the multiple constraints involved in estimating human carrying capacity
• Ecologically productive areas are defined as arable land, pasture, forest, ocean, built-up land, and fossil energy land
• Taking into account ecologically productive areas and converting it into land area per person gives us an estimate of about 1.7 hectares per person.
Ecological Footprint