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
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
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)
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.
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.
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.
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?)