Population regulation results from a combination of density-dependent and density-independent factors that influence birth and death rates. Density-dependent factors like food supply, predation, and disease have effects that increase with population density, while density-independent factors like weather fluctuate independently of density. Various theories propose population equilibrium driven by these factors or non-equilibrium dynamics involving metapopulations and chaos. Small habitat loss can increase extinction risk through reduced population size and genetic problems or fragmentation leaving populations vulnerable to accidents.

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.