Population Ecology

Native speciesspecies that normally live in a particular

communityNonnative species

also referred to as “invasive” or “alien” speciesspecies that enter a new community either

through immigration or introductionIntroduction may be deliberate or accidentali.e. “killer bees,” Kudzu, zebra mussels, Asian

oysters, domesticated animals

species whose decline or migration indicates a significant change (damage) to a particular community

Serve as “early warning sentinels” of environmental degradation

Examples:AmphibiansTroutBirdsAquatic macroinvertebrates

species whose removal from its community may dramatically alter the structure and function of the community

roles:pollinatorstop predatorsdecomposers

species that alters its habitat in ways that benefit other species behaviors of such species

may influence succession and increase species richness

sometimes identical to the keystone species, while other times serves as a counterbalance

Examples: elephants, kelp, eastern hemlock, mussels

Three general patterns: (see below)Most populations live in clumps although other

patterns occur based on resource distribution.

Figure 8-2Figure 8-2

1. Resource availability varies from place to place.2. Living in herds, flocks, or schools provides

protection from predators and population declines.

Fish, birds, caribou, antelope, zebra

3. Predators that live in groups are afforded a better chance of catching prey and getting a meal.

Wolves, hunting dogs

4. Temporary animal groupings may occur for mating and caring for young.

Dolphin, albatross

Populations increase through births and immigration

Populations decrease through deaths and emigration

How fast a population grows or declines depends on its age structure.Prereproductive age: not mature enough to

reproduce.Reproductive age: those capable of reproduction.Postreproductive age: those too old to reproduce.

Populations with mostly reproductive individuals tend to increase.

Populations with mostly post-reproductive individuals tend to decrease.

Stable populations are equitability distributed among all three categories.

No population can increase its size indefinitely. There are always limits to population growth in nature.

Population change is a balance between:Biotic potential - the intrinsic rate of increase (rr) or the

rate at which a population would grow if it had unlimited resources and…

Environmental resistance – all the factors that act to limit the growth of a population.

Together these determine a populations carrying capacity (KK): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Exponential or geometric growth – starts slowly but accelerates rapidly as population increasesJ-shaped curve plotted on a graph of population

vs. timeLogistic growth – exponential growth followed

by a steady population decrease until the population size levels offS-shaped curveUsually levels off at or near the carrying capacityCarrying capacity is not fixed

Fig. 8-3, p. 163

EnvironmentalResistance

Time (t)

Po

pu

lat i

on

si z

e (N)

Carrying capacity (K)

ExponentialGrowth

BioticPotential

Exhibit four phasesLag – phase characterized by low birth rates, when

the population is adjusting to a new

environment

Growth – phase which shows a dramatic increase in population size (B+I > D+E)

Stationary – phase when then population is in dynamic

equilibrium (B+I = D+E)

Death – phase in which the population declines (B+I < D+E)

Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.

Some populations overshootovershoot their carrying capacity.Reproductive time lagCauses a dieback or a crash

Some populations may increase their carrying capacity by developing adaptive traits (i.e. natural selection)

Some species maintain their carrying capacity by migrating to other areas.

Fig. 8-4, p. 164

Carrying capacity

Year

Nu

mb

er o

f sh

eep

(m

illi

on

s)Overshoot

Fig. 8-6, p. 165

Nu

mb

er o

f re

ind

eer

Populationovershootscarryingcapacity

Carryingcapacity

Year

PopulationCrashes

Population density: the number of individuals in a population found in a particular area or volume.A population’s density can affect how rapidly it

can grow or decline.Density dependent factors Density dependent factors include biotic factors like

disease, competition for resources, predation, and parasitism

Some population control factors are not affected by population density.Density independent factors Density independent factors include abiotic factors like

weather, fire, pollution, and habitat destruction

Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.Stable: fluctuates slightly above and below

carrying capacity.Irruptive: populations explode and then crash to a

more stable level.short-lived, rapidly reproducing species (i.e. algae,

insects)Cyclic: populations fluctuate according to regular

cyclic or boom- and-bust cycles.close predator-prey interactions

Irregular: erratic changes possibly due to chaos or drastic change. populations that inhabit unstable or highly variable

environments

Fig. 8-7, p. 166

Po

pu

lati

on

siz

e (t

ho

usa

nd

s)

Year

LynxHare

Some species reproduce without having sex (asexual).Offspring are exact genetic copies (clones).

Others reproduce by having sex (sexual).Genetic material is mixture of two individuals.Disadvantages: males do not give birth, increase

chance of genetic errors and defects, courtship and mating rituals can be costly (energetically).

Major advantages: promotes genetic diversity, division of labor among the sexes may provide offspring greater protection through critical periods.

r-selected species: Large number of smaller offspring with little parental care

K-selected species: Fewer, larger offspring with higher invested parental care

Figure 8-9Figure 8-9

r-selected species tend to be opportunists while K-selected species tend to be competitors.

Figure 8-10Figure 8-10

The way to represent the age structure of a population is with a survivorship curve.

Late loss population live to an old age.Constant loss population die at all ages.Most members of early loss populations, die at

young ages.

age

Population Ecology. (1998) Cyber Ed.