1. Population
Re gulation
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: ΔN = (b + i) – (d + e), where N is
population size, b is births, d is deaths, i is
immigrants, e is emigrants

4. Patter ns 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

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
re gulation
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

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)

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.

27.  Zebra mussel figure

28. Anywhere, everywhere!

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 Martha’s
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. T he end.