Populations can’t increase forever…
Population Regulation
• Factors affecting population size, growth
“A realistic basis for… dealing with the changes of numbers in
populations would include the following propositions:
– Density dependent factors
– Density independent factors
(a) All populations are constantly changing in size;
(b) The environments of all organisms are constantly changing;
These processes involve factors that have the ability to
either Limit or Regulate animal populations
(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.”
Limitation ≠ Regulation
Factors may Limit population growth, but not
necessarily Regulate growth
–P.R. Ehrlich and L.C. Birch
Populations can’t increase forever…
Limiting and Regulating Factors
• Limiting factors = density independent
• Not all factors will contribute to regulation
– Natural or anthropogenic process affecting rate of
population change
• Weather
• Predation
– Effects of regulatory (density-dependent)
factors may not be greater than nonregulatory (density-independent) factors
• Rainfall in S Texas, snow in northern areas
• 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
– Density dependence requires change in
demographic variables with density for
regulation to occur
• Reproduction
• Survival
Hypothetical recruitment and mortality for a
naturally regulated population
Limitation, Regulation in Reality
• Stochasticity!
Carrying capacity
Demographic rates
1.2
1
Population growing Population declining
Recruitment
– Fancy word for randomness, usually
environmental variation
• Seasonal changes
• Weather (rainfall, snow, etc.)
0.8
0.6
– Consequences of randomness:
0.4
Mortality
0.2
0
Population density
• Equilibrium or CC is not a constant
• Population size is not constant
• Mix of density-dependent, independent effects
from the same factors
1
Limitation, Regulation in Reality
Limitation, Regulation in Reality
• Limitation vs. Regulation?
– 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
Limitation, Regulation in Reality
Limitation vs. Regulation?
• Food
• Weather
• Predation
• Disease
1986
Year
1996
Limitation vs. Regulation?
• 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
Food
Limitation vs. Regulation?
• Weather
– Limitation likely
• Affects all regardless of density
• Snow
• Rain
– Effects worse when near K
• Animals already in poor condition
Weather
Limitation vs. Regulation?
• Predation
– Limitation likely
• 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
Limitation vs. Regulation?
Disease
• Disease
– Limitation likely, 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
Limitation vs. Regulation?
Limitation, Regulation in Reality
Effects of changes in management
• Cover
– Limitation likely
• Specialists
– Bighorn sheep, mountain goats
• High density may destroy cover (indirect)
– Lack of access to cover = mortality from
predation or exposure
– Heavily grazed vs. ungrazed areas
Limitation, Regulation in Reality
Limitation, Regulation in Reality
Effects May be Sex- or Age-Specific
Effects May be Sex- or Age-Specific
Response to regulation is imperfect
Limitation, Regulation in Reality
• 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
1986
Year
1996
4
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
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