Population Ecology

1
Chapter 8

• Population Ecology

2
Types of Species in Communities

• 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

3
Indicator Species

• 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

4
Keystone Species

• species whose removal from its community may
  dramatically alter the structure and function of
  the community
• roles
• pollinators
• top predators
• decomposers

5
Foundation Species

• 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

6
Population Distribution

• Three general patterns (see below)
• Most populations live in clumps although other
  patterns occur based on resource distribution.

Figure 8-2
7
Why Clumping?

• Resource availability varies from place to place.
• Living in herds, flocks, or schools provides
  protection from predators and population
  declines.
• Fish, birds, caribou, antelope, zebra
• Predators that live in groups are afforded a
  better chance of catching prey and getting a
  meal.
• Wolves, hunting dogs
• Temporary animal groupings may occur for mating
  and caring for young.
• Dolphin, albatross

8
Changes in Population Size

• Populations increase through births and
  immigration
• Populations decrease through deaths and
  emigration

9
Population Age Structure

• 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.

10
Limits on Population Growth Biotic Potential
vs. Environmental Resistance

• 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.

11
Exponential and Logistic Population Growth
J-Curves and S-Curves

• 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

12
Environmental Resistance
Carrying capacity (K)
Population size (N)
Biotic Potential
Exponential Growth
Time (t)
Fig. 8-3, p. 163
13
Population Growth Curves

• 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 (BI gt DE)
• Stationary phase when then population is in
  dynamic
• equilibrium (BI DE)
• Death phase in which the population declines
• (BI lt DE)

14
Can a Population Exceed Its Carrying Capacity?

• 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.

15
Overshoot
Carrying capacity
Number of sheep (millions)
Year
Fig. 8-4, p. 164
16
Population overshoots carrying capacity
Population Crashes
Number of reindeer
Carrying capacity
Year
Fig. 8-6, p. 165
17
Population Density and Population Change Effects
of Crowding

• Population density the number of individuals in
  a population found in a particular area or
  volume.
• A populations 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

18
Types of Population Change Curves in Nature

• 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

19
Hare
Lynx
Population size (thousands)
Year
Fig. 8-7, p. 166
20
REPRODUCTIVE PATTERNS

• 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.

21
Reproductive PatternsOpportunists and
Competitors

• 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
22
Reproductive Patterns

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

Figure 8-10
23
Survivorship Curves Short to Long Lives

• 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.

Number of individuals
age
24
WORK CITED

• Population Ecology. (1998) Cyber Ed.