Population Ecology
 Native species
 species that normally live in a particular community
 Nonnative species
 also referred to as “invasive” or “alien” species
 species that enter a new community either through
immigration or introduction
 Introduction may be deliberate or accidental
 i.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:
 Amphibians
 Trout
 Birds
 Aquatic macroinvertebrates
 species whose removal from its community may
dramatically alter the structure and function of the
community
 roles:
 pollinators
 top predators
 decomposers
 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 8-2
Resource availability varies from place to place.
2. Living in herds, flocks, or schools provides
protection from predators and population declines.
1.

Fish, birds, caribou, antelope, zebra
Predators that live in groups are afforded a better
chance of catching prey and getting a meal.
3.

Wolves, hunting dogs
Temporary animal groupings may occur for mating
and caring for young.
4.

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 (r) 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 (K):
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 increases
 J-shaped curve plotted on a graph of population vs. time
 Logistic growth – exponential growth followed by a
steady population decrease until the population size
levels off
 S-shaped curve
 Usually levels off at or near the carrying capacity
 Carrying capacity is not fixed
Environmental
Resistance
Carrying capacity (K)
Biotic
Potential
Exponential
Growth
Time (t)
Fig. 8-3, p. 163
 Exhibit four phases
 Lag – 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 overshoot their carrying capacity.
 Reproductive time lag
 Causes 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.
Overshoot
Number of sheep (millions)
Carrying capacity
Year
Fig. 8-4, p. 164
Number of reindeer
Population
overshoots
carrying
capacity
Population
Crashes
Carrying
capacity
Year
Fig. 8-6, p. 165
 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 include biotic factors like disease,
competition for resources, predation, and parasitism
 Some population control factors are not affected by
population density.
 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
Population size (thousands)
Hare
Lynx
Year
Fig. 8-7, p. 166
 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 8-9
 r-selected species tend to be opportunists while K-selected
species tend to be competitors.
Figure 8-10
 The way to represent the age structure of a population is
Number of individuals
with a survivorship curve.
age
 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.
Population Ecology. (1998) Cyber Ed.