Population Population RegulationRegulationChapter 5Chapter 5

Preconditions…Preconditions…

Populations change over timePopulations change over time Populations cannot grow indefinitelyPopulations cannot grow indefinitely Logistic curveLogistic curve Logistic equation represents equilibrium view Logistic equation represents equilibrium view

of population regulation (if perturbed, of population regulation (if perturbed, population returns to equilibrium value, K)population returns to equilibrium value, K)

Other views see population fluctuations as Other views see population fluctuations as random over time, without returning to random over time, without returning to equilibrium (due to disturbance)equilibrium (due to disturbance)

BackgroundBackground

Population regulationPopulation regulation: fluctuations in : fluctuations in abundance with feedback mechanisms to abundance with feedback mechanisms to increase or decrease density toward Kincrease or decrease density toward K

Population controlPopulation control: ecological mechanisms : ecological mechanisms which control upper limit of densitywhich control upper limit of density

Density is a result of combination of factorsDensity is a result of combination of factors In general: In general: ΔΔN = (b + i) – (d + e), where N is N = (b + i) – (d + e), where N is

population size, b is births, d is deaths, i is population size, b is births, d is deaths, i is immigrants, e is emigrantsimmigrants, e is emigrants

Patterns of Patterns of Population Population FluctuationFluctuationSmall-magnitude irregular Small-magnitude irregular fluctuations, Large-scale irregular fluctuations, Large-scale irregular fluctuations, Cycles, Irruptionsfluctuations, Cycles, Irruptions

Small-magnitude irregular Small-magnitude irregular fluctuationsfluctuations

Small random Small random changes in density changes in density of one order of of one order of magnitude or lessmagnitude or less

Large-scale irregular Large-scale irregular fluctuationsfluctuations

Large random Large random changes in changes in density of density of several orders several orders of magnitude of magnitude

CyclesCycles

Regular interval changes in population Regular interval changes in population densitydensity

IrruptionsIrruptions

Occasional, unpredictable population Occasional, unpredictable population explosionsexplosions

Equilibrium TheoriesEquilibrium Theories

Central difference among theories lies in the Central difference among theories lies in the relative importance of relative importance of density-dependentdensity-dependent factors and factors and density-independentdensity-independent factors. factors.

Density-dependent factors have an Density-dependent factors have an increasing effect with increasing densityincreasing effect with increasing density

Density-independent factors have an effect Density-independent factors have an effect that does not vary with densitythat does not vary with density

Extrinsic Biotic SchoolExtrinsic Biotic School

Accepts importance of density-dependent Accepts importance of density-dependent factorsfactors

Emphasizes external biotic factorsEmphasizes external biotic factors

Food supplyFood supply PredationPredation DiseaseDisease

Food supplyFood supply

Evidence shows that food-supply is a strong Evidence shows that food-supply is a strong determinant of density.determinant of density.

Birds frequently die of starvation.Birds frequently die of starvation. Areas with high food supplies tend to have Areas with high food supplies tend to have

high bird densities. (correlation Vs. high bird densities. (correlation Vs. causation)causation)

Artificially supplemented food studiesArtificially supplemented food studies Naturally supplemented food studiesNaturally supplemented food studies

PredationPredation

Difficult to establish (need to know density Difficult to establish (need to know density differences of predators with varying prey differences of predators with varying prey densities)densities)

Studies indicate that predator species Studies indicate that predator species depress prey populationsdepress prey populations

Removal experiments yield ambiguous Removal experiments yield ambiguous resultsresults

““Top-down” or “bottom-up” controversyTop-down” or “bottom-up” controversy

Disease and parasitismDisease and parasitism

Increased densities may increase the rate of Increased densities may increase the rate of transmissiontransmission

Increased density frequently correlates with Increased density frequently correlates with increased disease rateincreased disease rate

However, correlation may not indicate However, correlation may not indicate causation (food supply, red grouse)causation (food supply, red grouse)

The Intrinsic SchoolThe Intrinsic School Based on mechanisms intrinsic to the Based on mechanisms intrinsic to the

populationpopulation Aka the population is self-regulatedAka the population is self-regulated Also relies on density-dependenceAlso relies on density-dependence

Stress, territoriality, genetic polymorphism Stress, territoriality, genetic polymorphism hypothesis, dispersalhypothesis, dispersal

Stress, TerritorialityStress, Territoriality

Stress may regulate density by causing Stress may regulate density by causing physiological reactions to high densitiesphysiological reactions to high densities

Territoriality may regulate density by Territoriality may regulate density by excluding some individuals from reproducingexcluding some individuals from reproducing

Genetic Polymorphism Genetic Polymorphism Hypothesis, dispersalHypothesis, dispersal

Genetic composition changes in response to Genetic composition changes in response to densitydensity

Saturation dispersal, presaturation dispersal Saturation dispersal, presaturation dispersal (reduces inbreeding)(reduces inbreeding)

Nonequilibrium Nonequilibrium theories of theories of population regulationpopulation regulation

Abiotic Extrinsic Regulation, Abiotic Extrinsic Regulation, Metapopulations, Chaos theoryMetapopulations, Chaos theory

Abiotic Extrinsic RegulationAbiotic Extrinsic Regulation

Density-independent, abiotic factorsDensity-independent, abiotic factors Weather, temperature, moisture, sun-Weather, temperature, moisture, sun-

exposure, rainfall, etc…exposure, rainfall, etc… These factors are sufficient to explain density These factors are sufficient to explain density

variations. Populations do not encounter variations. Populations do not encounter ideal conditions long enough for density-ideal conditions long enough for density-dependent factors to be of importance.dependent factors to be of importance.

MetapopulationsMetapopulations

Population consisting of several patches of Population consisting of several patches of populations linked by dispersal.populations linked by dispersal.

Patches vary, may go extinct; not in Patches vary, may go extinct; not in equilibrium, but overall population survives equilibrium, but overall population survives due to dispersal among patchesdue to dispersal among patches

Metapopulations are particularly important in Metapopulations are particularly important in fragmented habitatsfragmented habitats

Chaos TheoryChaos Theory

Unpredictable patterns of population growthUnpredictable patterns of population growth Particularly interesting with r values above Particularly interesting with r values above

2.692.69 Pattern depends on initial conditionsPattern depends on initial conditions Not stochasticNot stochastic Property of the growth itself (growth Property of the growth itself (growth

equation)equation)

Recapitulating Population Recapitulating Population RegulationRegulation

There are equilibrium and non-equilibrium There are equilibrium and non-equilibrium populationspopulations

Density-dependent and density-independent Density-dependent and density-independent factors affect populations (biotic and abiotic factors affect populations (biotic and abiotic factors)factors)

It is undeniable that there is no single It is undeniable that there is no single explanation: rather, a combination of theories explanation: rather, a combination of theories applies. To what extent in each case is the applies. To what extent in each case is the relative contribution becomes the question.relative contribution becomes the question.

InvasionsInvasions

Four stages: Four stages: Transport, Introduction, Transport, Introduction, Establishment, SpreadEstablishment, Spread

Invasions follow the logistic curve, usually Invasions follow the logistic curve, usually with longer lag phase, followed by with longer lag phase, followed by exponential growthexponential growth

Invasions reach high densities (e.g. zebra Invasions reach high densities (e.g. zebra mussels, Opuntia cactus and cactoblastis mussels, Opuntia cactus and cactoblastis moth)moth)

Escape from density-dependent factors? Escape from density-dependent factors? Probably not. Other possibilities.Probably not. Other possibilities.

Zebra mussel figureZebra mussel figure

Anywhere, everywhere!

Extinction and Risk AnalysisExtinction and Risk Analysis

Extinction is a natural component of Extinction is a natural component of populations (strongly aggravated by humans)populations (strongly aggravated by humans)

Birth rate decreases, mortality increasesBirth rate decreases, mortality increases Very low populations suffer the Allee effectVery low populations suffer the Allee effect Anthropogenic habitat loss creates three risk Anthropogenic habitat loss creates three risk

factors: demographic accidents, habitat factors: demographic accidents, habitat fragmentation, genetic riskfragmentation, genetic risk

Demographic accidentsDemographic accidents

Habitat loss creates population decreaseHabitat loss creates population decrease With smaller populations, risk of extinction With smaller populations, risk of extinction

increases, due to demographic accidentsincreases, due to demographic accidents Chance events have a greater impact on Chance events have a greater impact on

small populationssmall populations Severe winter, epidemic, predators, etc…Severe winter, epidemic, predators, etc…

Habitat fragmentationHabitat fragmentation

Habitat loss frequently leads to habitat Habitat loss frequently leads to habitat fragmentationfragmentation

This leads to a metapopulation structureThis leads to a metapopulation structure Single patches may not be large enough to Single patches may not be large enough to

support a breeding populationsupport a breeding population Dispersal may not be possible to support Dispersal may not be possible to support

supplying of extinct patchessupplying of extinct patches Patches may go extinct simultaneouslyPatches may go extinct simultaneously

Genetic risksGenetic risks

Smaller populations have increased Smaller populations have increased inbreeding and genetic driftinbreeding and genetic drift

Both lead to increased homozygosity Both lead to increased homozygosity (bottlenecking effect leads to loss of alleles)(bottlenecking effect leads to loss of alleles)

Increased homozygosity decreases fitness, Increased homozygosity decreases fitness, and thus places population at riskand thus places population at risk

Heath hen on Martha’s Heath hen on Martha’s VineyardVineyard

Overhunting caused massive population Overhunting caused massive population decline until 1907decline until 1907

Population increased moderately thereafter Population increased moderately thereafter (genetic risks?)(genetic risks?)

In 1916, fire, storm, cold winter, invasion In 1916, fire, storm, cold winter, invasion reduced population to 50 pairs (demographic reduced population to 50 pairs (demographic accidents-more genetic risk)accidents-more genetic risk)

Subsequent years showed sex-ratio skewed Subsequent years showed sex-ratio skewed toward males (demographic accident)toward males (demographic accident)

Extinct by 1932 (any habitat fragmentation?)Extinct by 1932 (any habitat fragmentation?)

The end.The end.