D-shaped, gets its name from the stripes, usually around 2 cm, up to 3 cm.
Ch. 5 population regulation part
PopulationRe gulationChapter 5
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)
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
Small-magnitude irregular fluctuations Small random changes in density of one order of magnitude or less
Large-scale irregular fluctuations Large random changes in density of several orders of magnitude
Cycles Regular interval changes in population density
Irruptions Occasional, unpredictable population explosions
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
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
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
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)
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
Stress, Territoriality Stress may regulate density by causing physiological reactions to high densities Territoriality may regulate density by excluding some individuals from reproducing
Genetic Polymorphism Hypothesis, dispersal Genetic composition changes in response to density Saturation dispersal, presaturation dispersal (reduces inbreeding)
Nonequilibriumtheories ofpopulationre gulationAbiotic Extrinsic Regulation,Metapopulations, Chaos theory
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.
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
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)
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.
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.
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
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…
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
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
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?)