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