Population EcologyPopulation Ecology

1. Density and 1. Density and DistributionDistribution

2. Growth2. Growtha. Exponentiala. Exponentialb. Logisticb. Logistic

3. Population 3. Population Limiting FactorsLimiting Factors

4. Human 4. Human population growthpopulation growth

Examples of applications

• Invasive species

• Endangered species

• Pest control (e.g., agriculture)

• Human population growth

DensityDensity: the number of organisms in a given area: the number of organisms in a given areaDistributionDistribution: how the organisms are spaced in the area: how the organisms are spaced in the area

Population. Population. Individuals of same species occupying same general Individuals of same species occupying same general area.area.

Changes in population sizeChanges in population size

Growing

Fluctuating

Shrinking

Northern Pintail Duck

Questions

• Why do populations change in size?

• What factors determine rates of population growth or decline?

• How do these differ among species?

2. Population growth

• Two models of population growth:– Exponential– Logistic

2. Population Growth2. Population Growtha. exponential growtha. exponential growth

The change in population size (The change in population size (NN) in an interval of time is) in an interval of time is

number of births – number of deaths, ornumber of births – number of deaths, or

∆∆NN = B - D = B - D ∆∆tt (ignoring immigration and emigration)(ignoring immigration and emigration)

If If bb (birth rate) is the number of offspring produced (birth rate) is the number of offspring produced over a period of time by a population, and over a period of time by a population, and dd (death (death rate) is the average number of deaths for that rate) is the average number of deaths for that population,population,

∆∆NN = bN – dN = bN – dN oror ∆N∆N = (b – d)N = (b – d)N ∆ ∆tt ∆t ∆t

Population GrowthPopulation Growth: exponential growth: exponential growth

The difference between the birth rate and the death The difference between the birth rate and the death rate is the per capita growth raterate is the per capita growth rate

r = b - dr = b - d

The growth equation can be rewritten asThe growth equation can be rewritten as ∆∆NN = rN = rN oror dNdN = rN = rN

∆ ∆tt dt dt

Exponential growth occurs when resources are Exponential growth occurs when resources are unlimited and the population is small (doesn’t unlimited and the population is small (doesn’t happen often). The r is maximal (happen often). The r is maximal (rrmaxmax) and it is called ) and it is called

the the intrinsic rate of increaseintrinsic rate of increase..

r can also be negative (population r can also be negative (population decreasing)decreasing)

if r is zero, the population does not if r is zero, the population does not change in sizechange in size

thus, the rate of increase (or decrease) thus, the rate of increase (or decrease) of a population can change over time.of a population can change over time.

Exponential growth does not happen often:Exponential growth does not happen often:

reindeer slide

Reindeer on the Pribalof Islands, Bering Sea

Or indefinitely:

2.b. Logistic growth2.b. Logistic growthMost populations are limited in growth at some Most populations are limited in growth at some carrying carrying capacity (K) capacity (K) (the maximum population size a habitat can (the maximum population size a habitat can accommodate)accommodate)

Logistic Growth EquationLogistic Growth Equation: incorporates changes in : incorporates changes in growth rate as population size approaches carrying growth rate as population size approaches carrying capacity.capacity.

dNdN = r = rmaxmaxNN

dtdt(K - N)(K - N) KK

Near carrying capacity natural selection will favor traits that Near carrying capacity natural selection will favor traits that maximize reproductive success with few resources (high densities).maximize reproductive success with few resources (high densities).

Density-dependent selection.Density-dependent selection.

K-selectionK-selection

r- and K-selectionr- and K-selection

Below carrying capacity natural selection will favor traits that Below carrying capacity natural selection will favor traits that maximize reproductive success in uncrowded environments (low maximize reproductive success in uncrowded environments (low densities).densities).

Density-independent selection.Density-independent selection.

r-selectionr-selection

Any characteristic that varies according to a Any characteristic that varies according to a change in population density.change in population density.

Density-dependentDensity-dependent

Any characteristic that does not vary as Any characteristic that does not vary as population density changes.population density changes.

Density-independentDensity-independent

food availability, territories, water, nutrients, food availability, territories, water, nutrients, predators/parasites/disease, waste accumulationpredators/parasites/disease, waste accumulation

weather events, salinity, temperature weather events, salinity, temperature

• Density dependent birth and death rates. Many of these reflect – competition for resource (food/energy,

nutrients, space/territories).– predation, parasites, disease– waste accumulation (e.g., ethanol)

3. Factors that limit population growth3. Factors that limit population growth

• Density independent survivorship or mortality– Extreme weather events– Fluctuations in wind and water currents

4. Factors that limit population growth4. Factors that limit population growth

Interactions among population-limiting factorsInteractions among population-limiting factors

The dynamics of a population result from the interaction The dynamics of a population result from the interaction between biotic and abiotic factors, making natural populations between biotic and abiotic factors, making natural populations unstable.unstable.

Water temperature,Water temperature,Competition,Competition,Cannibalism.Cannibalism.

Population-Limiting FactorsPopulation-Limiting FactorsSome populations have regular boom-and-bust cycles.Some populations have regular boom-and-bust cycles.

PredationPredationFood shortage in Food shortage in winterwinter

Prey availabilityPrey availability

Population. Population. Individuals same species occupying same general area.Individuals same species occupying same general area.

Have geographic boundaries and population size.Have geographic boundaries and population size.

Key characteristicsKey characteristics Density.Density. Individuals per unit of area or volume. Individuals per unit of area or volume. Distribution:Distribution: uniform, clumped, random. uniform, clumped, random.

Additions (+) : Births and Immigration.Additions (+) : Births and Immigration.Subtractions (-) : Deaths and emigration.Subtractions (-) : Deaths and emigration.

SUMMARYSUMMARY

Demography. Demography. Studies changes in population size.Studies changes in population size.

Life histories. Life histories. Affect reproductive output and survival rate and Affect reproductive output and survival rate and thus population growth.thus population growth. Life history strategies are trade-offs between survival and Life history strategies are trade-offs between survival and reproduction.reproduction.

Population GrowthPopulation Growth Exponential. J-shaped. Idealized, occurs in certain conditions.Exponential. J-shaped. Idealized, occurs in certain conditions. Logistic. S-shaped. A little more realistic. Carrying capacity.Logistic. S-shaped. A little more realistic. Carrying capacity.

K-selection. Density-dependent selection.K-selection. Density-dependent selection. r-selection. Density independent selection.r-selection. Density independent selection.

Population growth is slowed by changes in birth and death Population growth is slowed by changes in birth and death rates with density.rates with density.

Interaction of biotic and abiotic factors often results in unstable Interaction of biotic and abiotic factors often results in unstable population sizes. In some populations they result in regular population sizes. In some populations they result in regular cycles.cycles.

4. Human population growth

Human Population= 6,596,469,500 (earlier today)Human Population= 6,596,469,500 (earlier today)Exponential growth since Industrial Revolution: better nutrition, Exponential growth since Industrial Revolution: better nutrition, medical care and sanitation.medical care and sanitation. Growth rates ( r )Growth rates ( r )1963: 2.2%(0.022), 1990: 1.6%, 2003: 1.3% (200,234/day), 2015: 1%1963: 2.2%(0.022), 1990: 1.6%, 2003: 1.3% (200,234/day), 2015: 1%

Growth will slow Growth will slow down either due to down either due to decreased births or decreased births or increased deaths.increased deaths.

Likely both as Likely both as suggested by age-suggested by age-structure pyramids: structure pyramids: relative number of relative number of individuals in each individuals in each age-class.age-class.

http://www.ibiblio.org/lunarbin/worldpop

When and how will human population growth stop?

• This question is likely to be answered one way or another in your lifetime.

• What is Earth’s carrying capacity for human’s?

• Have we already exceeded K?

• What are consequences of human population growth for other species on this planet?

Human impact

• Depends on – Total human population– Consumption by each individual– Ecological impact of each unit of consumption

• I = PAT (Ehrlich and Ehrlich)– P = population– A = affluence– T = technology

Unknown what Unknown what the carrying the carrying capacity of Earth capacity of Earth for humans is. A for humans is. A useful concept is useful concept is the ecological the ecological footprint: land footprint: land needed to needed to produce produce resources and resources and absorb wastes absorb wastes for a given for a given country.country.

World Wildlife Fund for NatureWorld Wildlife Fund for Nature

Ecological footprints for various countries and the world

Human population has been growing exponentially for a Human population has been growing exponentially for a long time.long time.

A reduction is expected either through lower birth rates or A reduction is expected either through lower birth rates or higher death rates. The age-structure suggest different higher death rates. The age-structure suggest different scenarios for individual countries.scenarios for individual countries.

Humans appear to be above Earth’s carrying capacity.Humans appear to be above Earth’s carrying capacity.

SUMMARYSUMMARY