Population Regulation. Limiting and Regulating Factors. Limitation, Regulation in Reality. Populations can t increase forever. Limitation Regulation

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1 Population Regulation A realistic basis for dealing with the changes of numbers in populations would include the following propositions: (a) All populations are constantly changing in size; (b) The environments of all organisms are constantly changing; (c) Local populations must be recognized and investigated if changes in population size are to be understood; (d) The influence on population size of various components of the environment varies with population density, among species, among local populations, and through time. P.R. Ehrlich and L.C. Birch Populations can t increase forever Factors affecting population size, growth Density dependent factors Density independent factors These processes involve factors that have the ability to either Limit or Regulate animal populations Limitation Regulation Factors may Limit population growth, but not necessarily Regulate growth Populations can t increase forever Limiting factors = density independent Natural or anthropogenic process affecting rate of population change Regulating factors = density dependent Per capita rate of growth declines with increasing population size, individual fitness declines Increased risk of dying from starvation Decreased reproduction Limiting and Regulating Factors Not all factors will contribute to regulation Effects of regulatory (density-dependent) factors may not be greater than nonregulatory (density-independent) factors Rainfall in S Texas, snow in northern areas Density dependence requires change in demographic variables with density for regulation to occur Reproduction Survival Demographic rates Hypothetical recruitment and mortality for a naturally regulated population Recruitment Population density Mortality Carrying capacity Population growing Population declining Stochasticity! Fancy word for randomness, usually environmental variation Seasonal changes (rainfall, snow, etc.) Consequences of randomness: Equilibrium or CC is not a constant Population size is not constant Mix of density-dependent, independent effects from the same factors 1

2 Hard to quantify effects of factors Rarely know population density Factors interact Random environmental events = random Detecting density-dependence is difficult Long-term data Can not be incorporated into management until long after growth has occurred Food Disease Year Cover Food Food Regulation likely Evidence hard to come by Affected by weather and other factors Food quality and quantity Influence birth, death rates Low energy intake during estrus affects ovulation Low protein during late gestation results in small offspring, high mortality 2

3 Food Affects all regardless of density Snow Rain Effects worse when near K Animals already in poor condition Weather No evidence for regulation Animals in poor condition may be more vulnerable Abundant prey may be targeted Predator removals have short-term effects Predator-prey dynamics may be complex Disease Disease, but hard to quantify No evidence for regulation Animals in poor condition may be more vulnerable May have greater prevalence at high density Eradication of disease has resulted in population increase Rinderpest in Africa Screw worm in Texas 3

4 Cover Specialists Bighorn sheep, mountain goats Effects of changes in management High density may destroy cover (indirect) Lack of access to cover = mortality from predation or exposure Heavily grazed vs. ungrazed areas Effects May be Sex- or Age-Specific Effects May be Sex- or Age-Specific Response to regulation is imperfect Density dependence can have a large effect on population growth Strong effect may lead to overcompensation Population decreases, increases too quickly Erratic, cyclic, chaotic population dynamics Strong feedbacks DO NOT imply stability Year 4

5 Population Regulation Population Regulation Response to regulation is imperfect Density-dependence: negative feedback loop Effects not immediate Growth slows, but does not stop Delayed effects on juvenile growth, recruitment 5