Ch. 5 population regulation part

1. Population Population
RegulationRegulation
Chapter 5Chapter 5

2. 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)

3. 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

4. Patterns of Patterns of
Population Population
FluctuationFluctuation
Small-magnitude irregular Small-magnitude irregular
fluctuations, Large-scale irregular fluctuations, Large-scale irregular
fluctuations, Cycles, Irruptionsfluctuations, Cycles, Irruptions

5. 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

6. 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

7. CyclesCycles
Regular interval changes in population Regular interval changes in population
densitydensity

8. IrruptionsIrruptions
Occasional, unpredictable population Occasional, unpredictable population
explosionsexplosions

9. 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

11. 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

12. 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

13. 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

14. 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)

15. 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

16. 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

17. 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)

18. Nonequilibrium Nonequilibrium
theories of theories of
population population
regulationregulation
Abiotic Extrinsic Regulation, Abiotic Extrinsic Regulation,
Metapopulations, Chaos theoryMetapopulations, Chaos theory

19. 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.

20. 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

22. 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)

24. 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.

25. 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.

27. Zebra mussel figureZebra mussel figure

28. Anywhere, everywhere!

30. 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

31. 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…

32. 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

33. 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

34. 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?)

35. The end.The end.