Population Regulation

1
Population Regulation

• Chapter 5

2
Preconditions

• Populations change over time
• Populations cannot grow indefinitely
• Logistic curve
• Logistic equation represents equilibrium view of
  population regulation (if perturbed, population
  returns to equilibrium value, K)
• Other views see population fluctuations as random
  over time, without returning to equilibrium (due
  to disturbance)

3
Background

• Population regulation fluctuations in abundance
  with feedback mechanisms to increase or decrease
  density toward K
• Population control ecological mechanisms which
  control upper limit of density
• Density is a result of combination of factors
• In general ?N (b i) (d e), where N is
  population size, b is births, d is deaths, i is
  immigrants, e is emigrants

4
Patterns of Population Fluctuation

• Small-magnitude irregular fluctuations,
  Large-scale irregular fluctuations, Cycles,
  Irruptions

5
Small-magnitude irregular fluctuations

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

6
Large-scale irregular fluctuations

• Large random changes in density of several orders
  of magnitude

7
Cycles

• Regular interval changes in population density

8
Irruptions

• Occasional, unpredictable population explosions

9
Equilibrium Theories

• Central difference among theories lies in the
  relative importance of density-dependent factors
  and density-independent factors.
• Density-dependent factors have an increasing
  effect with increasing density
• Density-independent factors have an effect that
  does not vary with density

10
(No Transcript)
11
Extrinsic Biotic School

• Accepts importance of density-dependent factors
• Emphasizes external biotic factors
• Food supply
• Predation
• Disease

12
Food supply

• Evidence shows that food-supply is a strong
  determinant of density.
• Birds frequently die of starvation.
• Areas with high food supplies tend to have high
  bird densities. (correlation Vs. causation)
• Artificially supplemented food studies
• Naturally supplemented food studies

13
Predation

• Difficult to establish (need to know density
  differences of predators with varying prey
  densities)
• Studies indicate that predator species depress
  prey populations
• Removal experiments yield ambiguous results
• Top-down or bottom-up controversy

14
Disease and parasitism

• Increased densities may increase the rate of
  transmission
• Increased density frequently correlates with
  increased disease rate
• However, correlation may not indicate causation
  (food supply, red grouse)

15
The Intrinsic School

• Based on mechanisms intrinsic to the population
• Aka the population is self-regulated
• Also relies on density-dependence
• Stress, territoriality, genetic polymorphism
  hypothesis, dispersal

16
Stress, Territoriality

• Stress may regulate density by causing
  physiological reactions to high densities
• Territoriality may regulate density by excluding
  some individuals from reproducing

17
Genetic Polymorphism Hypothesis, dispersal

• Genetic composition changes in response to
  density
• Saturation dispersal, presaturation dispersal
  (reduces inbreeding)

18
Nonequilibrium theories of population regulation

• Abiotic Extrinsic Regulation, Metapopulations,
  Chaos theory

19
Abiotic Extrinsic Regulation

• Density-independent, abiotic factors
• Weather, temperature, moisture, sun-exposure,
  rainfall, etc
• These factors are sufficient to explain density
  variations. Populations do not encounter ideal
  conditions long enough for density-dependent
  factors to be of importance.

20
Metapopulations

• Population consisting of several patches of
  populations linked by dispersal.
• Patches vary, may go extinct not in equilibrium,
  but overall population survives due to dispersal
  among patches
• Metapopulations are particularly important in
  fragmented habitats

21
(No Transcript)
22
Chaos Theory

• Unpredictable patterns of population growth
• Particularly interesting with r values above 2.69
• Pattern depends on initial conditions
• Not stochastic
• Property of the growth itself (growth equation)

23
(No Transcript)
24
Recapitulating Population Regulation

• There are equilibrium and non-equilibrium
  populations
• Density-dependent and density-independent factors
  affect populations (biotic and abiotic factors)
• It is undeniable that there is no single
  explanation rather, a combination of theories
  applies. To what extent in each case is the
  relative contribution becomes the question.

25
Invasions

• Four stages Transport, Introduction,
  Establishment, Spread
• Invasions follow the logistic curve, usually with
  longer lag phase, followed by exponential growth
• Invasions reach high densities (e.g. zebra
  mussels, Opuntia cactus and cactoblastis moth)
• Escape from density-dependent factors? Probably
  not. Other possibilities.

26
(No Transcript)
27

• Zebra mussel figure

28
Anywhere, everywhere!
29
(No Transcript)
30
Extinction and Risk Analysis

• Extinction is a natural component of populations
  (strongly aggravated by humans)
• Birth rate decreases, mortality increases
• Very low populations suffer the Allee effect
• Anthropogenic habitat loss creates three risk
  factors demographic accidents, habitat
  fragmentation, genetic risk

31
Demographic accidents

• Habitat loss creates population decrease
• With smaller populations, risk of extinction
  increases, due to demographic accidents
• Chance events have a greater impact on small
  populations
• Severe winter, epidemic, predators, etc

32
Habitat fragmentation

• Habitat loss frequently leads to habitat
  fragmentation
• This leads to a metapopulation structure
• Single patches may not be large enough to support
  a breeding population
• Dispersal may not be possible to support
  supplying of extinct patches
• Patches may go extinct simultaneously

33
Genetic risks

• Smaller populations have increased inbreeding and
  genetic drift
• Both lead to increased homozygosity
  (bottlenecking effect leads to loss of alleles)
• Increased homozygosity decreases fitness, and
  thus places population at risk

34
Heath hen on Marthas Vineyard

• Overhunting caused massive population decline
  until 1907
• Population increased moderately thereafter
  (genetic risks?)
• In 1916, fire, storm, cold winter, invasion
  reduced population to 50 pairs (demographic
  accidents-more genetic risk)
• Subsequent years showed sex-ratio skewed toward
  males (demographic accident)
• Extinct by 1932 (any habitat fragmentation?)

35
The end.