Transcript population ecology 2010

Population Ecology
Chapter Overview Questions
• What are the major characteristics of
populations?
• How do populations respond to changes in
environmental conditions?
• How do species differ in their reproductive
patterns?
What are Population Dynamics?
• Population dynamics are changes that occur in a
population in response to environmental stress or
changes in environmental conditions
• They change in:
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–
–
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Size
Density
Dispersion (spacial patterns)
Age distribution
What Limits Population Growth?
• Four variables:
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Births
Deaths
Immigration (individuals that move in)
Emigration (individuals that move out)
• Variables depend on changes in resource
availability or on other environmental
changes
Population Growth
Pop = Pop0 + (b + i) - (d + e)
ZPG (zero pop growth)
(b + i) = (d + e)
Age Structure: Young Populations
Can Grow Fast
• 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.
Biotic Potential
• Ability of populations of a given species to increase in
size. Intrinsic rate of increase (r) is the rate at which a
population would grow if it had unlimited resources.
– Abiotic Contributing Factors:
• Favorable light
• Favorable Temperatures
• Favorable chemical environment - nutrients
– Biotic Contributing Factors:
• Reproductive rate
• Generalized niche
• Ability to migrate or disperse
• Adequate defense mechanisms
• Ability to cope with adverse conditions
Environmental Resistance
• Consists of all the factors acting jointly to
limit the growth of a population
• Population size of species in a given place
and time is determined by interplay between
biotic potential and environmental
resistance.
• Affects the young more than the elderly in a
population, thereby affecting recruitment
(survival to reproductive age)
Limits on Population Growth:
Biotic Potential vs. Environmental
Resistance
• No population can increase its size
indefinitely.
– The intrinsic rate of increase (r) is the rate at
which a population would grow if it had
unlimited resources.
– Carrying capacity (K): the maximum
population of a given species that a particular
habitat can sustain indefinitely without
degrading the habitat.
Population Growth
• Populations show two types of growth
– Exponential
• J-shaped curve
• Growth is independent of population density
– Logistic
• S-shaped curve
• Growth is dependent on population density
Exponential Growth
• As early as Darwin, scientists have realized that
populations have the ability to grow
exponentially
• All populations have this ability, although not all
populations realized this type of growth
• Darwin pondered the question of exponential
growth. He knew that all species had the
potential to grow exponentially
• He used elephants as an example because
elephants are one of the slowest breeders on the
planet
Exponential Growth
• One female will produce 6 young over
her 100 year life span. In a population,
this amounts to a growth rate of 2%
• Darwin wondered, how many
elephants could result from one male
and one female in 750 years?
19,000,000 elephants!!!
Exponential Growth Graph
Animation: Exponential Growth
PLAY
ANIMATION
Logistic Growth
• Because of environmental resistance,
population growth decreases as density
reaches carrying capacity
• Graph of individuals vs. time yields a
sigmoid or S-curved growth curve
• Reproductive time lag causes
population overshoot
• Population will not be steady curve due
to resources (prey) and predators
Video: Logistic Growth
PLAY
VIDEO
Population Dynamics and Carrying
Capacity
• Basic Concept: Over a long period
of time, populations of species in
an ecosystem are usually in a state
of equilibrium (balance between
births and deaths)
– There is a dynamic balance between
biotic potential and environmental
resistance
Carrying Capacity (K)
• Exponential curve is not realistic
due to carrying capacity of area
• Carrying capacity is maximum
number of individuals a habitat can
support over a given period of time
due to environmental resistance
(sustainability)
Exponential and Logistic Population
Growth: J-Curves and S-Curves
• Populations
grow rapidly
with ample
resources, but
as resources
become limited,
its growth rate
slows and
levels off.
Figure 8-4
Exponential and Logistic Population
Growth: J-Curves and S-Curves
• As a population
levels off, it
often fluctuates
slightly above
and below the
carrying
capacity.
Figure 8-4
What Happens if Population Size
Exceeds Carrying Capacity?
• It will overshoot or exceed the carrying
capacity. This occurs because of a
reproductive time lag, the period needed for
birth rate to fall and death rate to rise in
response to resource overconsumption.
• Population suffers a dieback, or crash
Exceeding Carrying Capacity: Move,
Switch Habits, or Decline in Size
• Members of
populations which
exceed their resources
will die unless they
adapt or move to an
area with more
resources.
Figure 8-6
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 population’s density can affect how rapidly it can
grow or decline.
• e.g. biotic factors like disease
– Some population control factors are not affected by
population density.
• e.g. abiotic factors like weather
How Does Population Density
Affect Population Growth?
• Density-independent population controls affect a
population’s size regardless of density.
– Floods, fires, hurricanes, unseasonable weather, habitat
destruction, pesticide spraying
• Density-dependent population controls are
limiting factors that have a greater effect as
population density increases. Tend to reduce
population size by decreasing natality or
increasing mortality as population increases.
– Competition for resources, predation, parasitism,
disease.
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.
– Cyclic: populations fluctuate and regular cyclic or boomand-bust cycles.
– Irregular: erratic changes possibly due to chaos or
drastic change.
Role of Predation in Controlling
Population Size
• Some species that interact as predator or prey undergo
cyclic changes in their numbers, with sharp increase in
their numbers followed by periodic crashes.
• Bottom-up control occur when species consume food
faster than it can be replenished, and have a decrease in
quantity and quality of food. Can occur without presence
of predators.
• Examples of top-down control of predators on prey
populations include
– Wolves controlling deer populations and moose populations
– Large predatory fish controlling other fish populations in lakes
– Sharks and alligators controlling some fish populations
Types of Population Change
Curves in Nature
• Population sizes often vary in regular cycles when
the predator and prey populations are controlled
by the scarcity of resources.
Figure 8-7
Animation: Capture-Recapture
Method
PLAY
ANIMATION
Reproductive Strategies
• Goal of every species is to produce as many offspring as
possible
• Each individual has a limited amount of energy to put
towards life and reproduction
• This leads to a trade-off of long life or high reproductive
rate
• Natural Selection has lead to two strategies for species: r strategists and K – strategists
• Availability of suitable habitat for individuals of a
population in a particular area is what determines its
ultimate population size.
r - Strategists
• Are opportunist
species
• Spend most of their
time in exponential
growth
• Maximize
reproductive life
• Minimum life
• Can gain a foothold
in changed
environmental
conditions
K
r - Strategists
•
•
•
•
•
•
•
•
•
•
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate – (r)
Population size fluctuates wildly above and below
carrying capacity – (K)
Generalist niche
Low ability to compete
K - Strategists
• Competitor species
• Maintain population
at carrying capacity
(K)
• Maximize lifespan
• Thrive best in
ecosystems with fairly
constant
environmental
conditions
K
K- Strategist
•
•
•
•
•
•
•
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•
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Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close to
carrying capacity (K)
Specialist niche
High ability to compete
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 population, die at young
ages.
Survivorship Curves:
Short to Long Lives
• The populations of
different species
vary in how long
individual members
typically live.
Figure 8-11
Animation: Life History Patterns
PLAY
ANIMATION
Video: Kelp Forest (Channel
Islands)
PLAY
VIDEO
Video: Salmons Swimming
Upstream
PLAY
VIDEO