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# Ch. 5 population regulation part

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### Ch. 5 population regulation part

1. 1. PopulationRe gulationChapter 5
2. 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. 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. 4. Patter ns ofPopulationFluctuationSmall-magnitude irregularfluctuations, Large-scale irregularfluctuations, Cycles, Irruptions
5. 5. Small-magnitude irregular fluctuations Small random changes in density of one order of magnitude or less
6. 6. Large-scale irregular fluctuations Large random changes in density of several orders of magnitude
7. 7. Cycles  Regular interval changes in population density
8. 8. Irruptions  Occasional, unpredictable population explosions
9. 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. 10. Extrinsic Biotic School Accepts importance of density-dependent factors Emphasizes external biotic factors Food supply Predation Disease
11. 11. 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
12. 12. 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
13. 13. 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)
14. 14. 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
15. 15. Stress, Territoriality Stress may regulate density by causing physiological reactions to high densities Territoriality may regulate density by excluding some individuals from reproducing
16. 16. Genetic Polymorphism Hypothesis, dispersal Genetic composition changes in response to density Saturation dispersal, presaturation dispersal (reduces inbreeding)
17. 17. Nonequilibriumtheories ofpopulationre gulationAbiotic Extrinsic Regulation,Metapopulations, Chaos theory
18. 18. 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.
19. 19. 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
20. 20. 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)
21. 21. 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.
22. 22. 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.
23. 23.  Zebra mussel figure
24. 24. Anywhere, everywhere!
25. 25. 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
26. 26. 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…
27. 27. 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
28. 28. 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
29. 29. 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?)
30. 30. T he end.