NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
Biotic Potential – the innate capacity for increase under ideal conditions.
Example: house fly  lays 120 eggs/generation
About 120 generations / year
1 pair can produce 6 trillion flies in 7 years.
Example: Bobwhite quail
Breed first at one year of age – breed once per year
Average clutch size of 14 eggs.
Year
1
2 (8)(14)
3 (64)(14)
Young
14
112
896
+ Adults
+2
+16
+ 128
Total
16
128
1024
Thus, 1 pair of bobwhite produce 1024 quail in 3 years  Biotic Potential
(doesn't include mortality and assumes all are breeding)
housefly
quail
sparrow
N
rabbit
deer
Time
Differences in biotic potential occur between species.  Rate of increase is
faster for some species than others.
However, not all offspring live to reproduce.
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NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
2
Factors that affect populations : as defined by Aldo Leopold in Game
Management.
1. Decimating Factors – Kills animal directly
 hunting
 diseases / parasites
 predation
 accidents
2. Welfare Factors – Where deficient predispose an animal to death
 food
 water
 cover (escape cover and thermal cover)
 special factors
 grit for birds
 dust baths for rabbits
 tree holes for nests (wood ducks)
 and many others.
Always more efficient to manage welfare factors.
Difficult to manage decimating factors (except hunting, that is only by setting bag
limits)
LIMITING FACTORS
Loss to
predators
Loss because
of inadequate
food supply
N
Death from winter
storms due to lack
of cover
Hatching
of young
in
spring.
Spring
Summer
Fall
Winter
NRES 310: Wildlife Ecology and Management
3
6 October 2008
Population Ecology Lecture
Limiting Factor—factor which is most limiting to growth of population. Usually
occurs in some hierarchy. This means that death from winter is the limiting
factor. So if you manage for predator losses or food supply losses you will still
wind up low because of deaths from winter storms because there's not enough
cover to maintain the population. (i.e. that is all that the habitat can support so
you should manage for the most limiting factor first.).
Density Independent population growth.
Population models for density independent species.
dN / dt = r N
dN / dt = change in numbers over change in time
r = intrinsic rate of increase
N = population size or number of organisms
N
Increases
toward
infinity
TIME
Classic J-shaped curve.
Little intraspecific competition,
Some environmental
perturbations causes the
population to crash
Compensatory
Mortality ?
Surplus – Leopold termed the
"harvestable surplus" – true if
mortality is compensatory
N
Refuge Effect
T
Population doesn't fall
below a given point.
NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
4
Population crashes before reaching K.
Is mortality compensatory or additive?
Is mortality from harvest added to natural mortality?
Additive --(Ducks - redheads)
Or can one type of mortality be substituted for another?
Compensatory -- (upland game birds bobwhite quail)
1) Density Independent – the rate of mortality does not increase with
population size.
or
No Relationship
Mortality
Rate
b=0
MR
Population Size
TIME
POPULATION SIZE
1
100
2
200
3
300
4
400
5
500
J – shaped growth curves.
Population Size
MORTALITY
10
20
30
40
50
Population crashes before being limited by resources.
 Insects – killing frost
RATE
10 % (i.e. the rate
doesn't change
with increasing
pop size.
NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
DENSITY DEPENDENCE
5
Mortality rate increases with population size.
K
N
Inflection
point
Logistic or
S - shaped Curve
T
Exponential growth curve
dN / dt =rN
Modification of exponential growth to include concepts of K and logistic growth
K = carrying capacity – max. no. animals that
the habitat can support
dN / dt = rN (K-N)
K
At K dN / dt = 0 because N = K
Rate of increase declines with increasing population size.
Change in rate of increase operates through nutritional constraints because
intraspecific competition increases with increasing population size.
Intraspecific competition 
Natality
Nutrition
Mortality
Mortality rate increases with population size
Mortality
Rate
Population Size
NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
Mortality most often occurs among the poorest competitors.
1. young
2. old
3. sick
4. infirm
The greater the value of r the more likely that a population is to overshoot K.
Overshoot of K may damage habitat causing an actual reduction in K (meaning
damage to habitat results that is can support fewer animals).
Problems with the logistic
1. cannot handle time lags – produced by delays in age at first breeding.
2. You must know K beforehand (this is very rare)
3. It assumes r is a constant, but mortality changes with population density, r=0
at K.
4. It assume population growth is symetrical.
5. If K/2 is the inflection point, why work equation?
r and K selection. Different evolutionary strategies.
r – from intrinsic rate of increase density independent growth of populations
K – from carrying capacity, density dependence growth of populations
Characteristics of each – handout
I (man)
Log No.
surviving
II (song birds)
III (Oysters)
Age
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NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
7
Survivorship curves.



Iteroparity – organisms with > 1 reproductive attempt during lifetime.
straight line implys equal mortality with respect to age (type II)
semelparity – organisms with one reproductive cycle during its life time.
R and K selection
r – strategists
r-strategy is to make full use of habitats which because of temporary nature,
keep many of the populations at any given moment on the lower, ascending parts
of the growth curve.
Species adapted to life in a short-lived, unpredictable habitat. After timber
harvest or fire
Species succeed best if
1) discover habitat quickly
2) reproduce rapidly
3) disperse in search of new habitat as existing ones become unfavorable
ex. annual plants, some small mammals.
K-strategists
Species live in stable habitats, so its advantageous to confer competitive ability.
Ex. climax old growth forest
K-selection could result in increased specialization or increased territorality
K-strategists are able to maintain the densest populations at equilibrium.
Continium, rather than discrete categories must have two animals to compare.
Bacteria are r-selected and deer are K-selected. Deer are r-selected compared
to bighorn sheep.
NRES 310: Wildlife Ecology and Management
6 October 2008
Population Ecology Lecture
8
Table 5.4 Some of the correlates of r and K selection
Climate
Mortality
Survivorship
Population Size
Intra- and interspecific
competition
Selection factors
Length of life
r selection
Variable and/or
unpredictable; uncertain
Often catastrophic,
nondirected, density
independent
Often Type III
Variable in time,
nonequilibrium; usually
well below carrying
capacity of environment,
unsaturated communities
or portions thereof;
ecologic vacuums;
recolonization each year
Vairable, often lax
K selection
Fairly constant and/or
predictable; more certain
More directed, density
dependent
1. rapid development
2. high maximal rate of
increase, rmax
3. Early reproduction
4. Small body size
5. Single reproduction
6. Many small offspring
1. slower development
2. greater competitive
ability
3. delayed reproduction
4. larger body size
5. repeated reproduction
6. fewer larger progeny
Short, usually less than 1
year
Leads to
Productivity
Stage in succession
Early
Source: After Pianka (1970)
Usually types 1 and 2
Fairly constant in time,
equilibrium, at or near
carrying capacity of the
environment; saturated
communities; no
recolonization necessary
Usually intense
Longer, usually more
than 1 year
Efficiency
Late, climax