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Population
Ecology
Density dependence,
regulation and the Allee effect
ESRM 450
Wildlife Ecology and
Conservation
Wildlife Populations
Groups of animals, all of the same
species, that live together in a
particular area and reproduce
Gotelli, N.J. (1998) A Primer of Ecology
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Population Growth
•  Populations increase in proportion to their size
•  e.g., at a 10% annual rate if increase
–  a population of 100 adds 10 individuals in one year
–  A population of 1000 adds 100 individuals in one year
•  Allowed to grow unchecked, populations growing at a
constant rate will rapidly approach infinity…
Exponential Growth
Nt = N0ert
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Exponential Growth Model – A
Cornerstone of Population Ecology
•  All populations have potential for exponential growth
–  Exponential models valuable because they recognize the
multiplicative nature of population growth (positive feedback yields
accelerating growth)
•  Realistically describes growth of many populations in the
short term
–  i.e., resources are often temporarily unlimited (pest outbreaks,
weed invasions, humans)
Wildlife Populations Have Great
Capacity for Growth
•  e.g., population growth of the ring-necked pheasant
(Phasianus colchicus)
–  8 individuals introduced to Protection Island, Washington, in
1937, increased to 1,325 adults in 5 years:
•  166-fold increase!
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Wildlife Populations Have Great
Capacity for Growth
•  e.g., population growth of the ring-necked pheasant
(Phasianus colchicus)
–  8 individuals introduced to Protection Island, Washington, in
1937, increased to 1,325 adults in 5 years:
•  166-fold increase!
•  Aside - Unforgiven
Film set in 1881; includes scene with pheasants
being shot from a train
Pheasants not imported from China until 1890s
Yet, Infinite Population Sizes Not
Observed
•  Despite potential for exponential increase, most
populations remain at relatively stable levels or fluctuate
around equilibria
–  Paradox noted by Malthus and Darwin
–  for population growth to be checked, a decrease in the birth rate
or an increase in the death rate (or both) must occur as overall
population size gets large
–  What causes these changes in birth and death rates?
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Crowding
•  As populations grow, crowding…
–  Reduces access to food (other resources) for individuals and
their offspring
–  Aggravates social strife
–  Promotes the spread of disease
–  Attracts the attention of predators
•  As a result, population growth slows and eventually halts
•  Process known as DENSITY DEPENDENCE
–  decreasing per capita growth rate* with increasing population
size
* Individual contribution to population growth (survival and reproduction)
The Logistic Growth Model
•  Populations with density dependence (decreasing per
capita growth rate with increasing population size)
modeled using the logistic growth equation
•  Logistic model incorporates idea that populations tend
to level off at carrying capacity (K)
•  K: population size at which no more individuals can be
supported over long time periods
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The Logistic Growth Model
Density dependence
K/2
No density dependence
Logistic Growth Curve
•  Graph of population size (N) versus time (t) for logistic
growth features S-shaped curve (sigmoid growth)
–  Populations below K increase
–  Populations above K decrease
–  Populations at K remain constant
•  Inflection point at K/2 separates accelerating and
decelerating phases of growth
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Not All Factors Cause Populations
to Level Off at K
•  Those that do are called Density-dependent factors
–  Effects increase in intensity with crowding
–  Prevent populations from growing larger in process called REGULATION
–  e.g., food supply and places to live
–  Sometimes also: effects of predators, parasites, and diseases
Owls in a tree cavity
Not All Factors Cause Populations
to Level Off at K
•  Density-independent factors may remove individuals
from populations, thereby depressing numbers, but
cannot regulate them
–  Also call limiting factors
–  Effects do not intensify with crowding; cannot check growth
–  e.g., temperature, precipitation, catastrophic events
(sometimes predation and parasitism)
–  In some cases, can keep populations small if severe & common
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Evidence for DensityDependence
•  Density-dependence difficult
to demonstrate in field
–  Requires manipulation
•  Alter density, measure vital
rates (fecundity and survival)
–  or Natural Experimentation
•  Take advantage of natural
fluctuations in density (problem
– exogenous factors)
•  Best evidence comes mainly
from lab experiments
–  e.g., fruit flies: fecundity and life
span decline with increasing
density in laboratory populations
Pearl (1927) Q Rev Biol
Field Evidence
•  Island warbler populations
underwent bottleneck mid-20th
century, then recovered
•  Thus, researchers were able to
measure population size and
individual vital rates during
recovery
Seychelles Warbler (Acrocephalus sechellensis)
–  Island setting provided for
replicates (multiple islands)…
–  …and minimized influence of
exogenous factors like
predators (i.e., islands acted
as outdoor laboratories)
Brouwer et al. (2009) Ecology
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Overall
Aride
Breeding season
Cousine
Individual reproduction declined with increasing
population size on 4 islands
Populations leveled off in accord with projections
based on density-dependent reproduction
Field Evidence
Bighorn sheep (Ovis canadensis)
Portier et al. (1998) J Zool
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Offspring (lamb)
survival plummeted
with increasing density,
especially during winter
Portier et al. (1998) J Zool
The Allee Effect
•  Under the density-dependent (logistic) growth model
–  Individuals in small populations should thrive; why?
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The Allee Effect
•  Under the density-dependent (logistic) growth model
–  Individuals in small populations should thrive; why?
–  No crowding; plenty of resources
•  Thus, we expect high population growth rate for small
populations
The Allee Effect
•  Under the density-dependent (logistic) growth model
–  Individuals in small populations should thrive; why?
–  No crowding; plenty of resources
•  Thus, we expect high population growth rate for small
populations
•  Sometimes, the opposite is observed
-  Low per capita growth rate at low population density
-  Allee effect
-  also called inverse density dependence or ‘depensatory’ growth
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The Allee Effect
•  Causes? There are 6 that are widely recognized
–  (1) Difficulty finding mates (e.g., plants, broadcast spawners, also
maybe some great whales)
–  (2) Cooperative defense
–  (3) Predator satiation (predation most effective at low prey density)
–  (4) Cooperative feeding
–  (5) Dispersal (high emigration at low density; animals more likely or
better able to colonize occupied sites)
–  (6) Habitat alteration (presence of conspecifics increases
recruitment to a population by altering biotic or abiotic conditions in
a way that increases fitness of recruits)
Kramer et al. (2009) Pop Ecol
Evidence for Allee Effects
Once widespread
across Vancouver Island (31000km2)
Now, only roughly 32 in wild
(critically endangered)
170 captive in 4 breeding facilities
Vancouver Island marmot (Marmota vancouverensis)
Brashares et al. (2010 ) J Anim Ecol
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Mechanisms?
•  VIM now lead solitary lives
-
-
historically, as much as 90%
of time spent within 20m of a
conspecific
Now, none
•  Also, VIM now range widely
•  Consequently
–  High energy expenditure
(movement)
–  Higher predation mortality
(greater exposure and fewer
alarm calls)
–  Reduced foraging efficiency
(more time devoted to
vigilance)
Brashares et al. (2010 ) J Anim Ecol
Population Ecology and
Conservation
•  An understanding of population growth, density
dependence and the Allee effect has great applied value
–  Identify high and low quality habitat (e.g., comparison of growth
rates)
–  Identify populations in trouble (sharp population declines); project
future trajectories
–  Set appropriate harvest rates (crowded populations can be
harvested sustainably due to compensatory growth)
–  Explain why small populations are not growing as expected (Allee
effects)
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Five Minute Paper
Questions & Insights
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