NRES 310: Wildlife Ecology and Management

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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 à 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 / Young / + Adults / Total
1 / 14 / +2 / 16
2 (8)(14) / 112 / +16 / 128
3 (64)(14) / 896 / + 128 / 1024

Thus, 1 pair of bobwhite produce 1024 quail in 3 years à Biotic Potential

(doesn't include mortality and assumes all are breeding)

Differences in biotic potential occur between species. à Rate of increase is faster for some species than others.

However, not all offspring live to reproduce.

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

Spring Summer Fall Winter

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

Classic J-shaped curve.

Little intraspecific competition,

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

TIME / POPULATION SIZE / MORTALITY
1 / 100 / 10
2 / 200 / 20
3 / 300 / 30
4 / 400 / 40
5 / 500 / 50

J – shaped growth curves.

Population crashes before being limited by resources.

·  Insects – killing frost

DENSITY DEPENDENCE

Mortality rate increases with population size.

Exponential growth curve dN / dt =rN

Modification of exponential growth to include concepts of K and logistic growth

dN / dt = rN (K-N) K = carrying capacity – max. no. animals that

K the habitat can support

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 à Natality

Nutrition

Mortality

Mortality rate increases with population size

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

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.

Table 5.4 Some of the correlates of r and K selection

r selection / K selection
Climate / Variable and/or unpredictable; uncertain / Fairly constant and/or predictable; more certain
Mortality / Often catastrophic, nondirected, density independent / More directed, density dependent
Survivorship / Often Type III / Usually types 1 and 2
Population Size / Variable in time, nonequilibrium; usually well below carrying capacity of environment, unsaturated communities or portions thereof; ecologic vacuums; recolonization each year / Fairly constant in time, equilibrium, at or near carrying capacity of the environment; saturated communities; no recolonization necessary
Intra- and interspecific competition / Vairable, often lax / Usually intense
Selection factors / 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
Length of life / Short, usually less than 1 year / Longer, usually more than 1 year
Leads to / Productivity / Efficiency
Stage in succession / Early / Late, climax

Source: After Pianka (1970)