Transcript Population Ecology - CarrollEnvironmentalScience

• In looking at nature…never
forget that every single organic
being around us may be said
to be striving to increase its
numbers. – Charles Darwin,
1859
Major Characteristics of a
Population
• Population dynamics is a study of how
populations change in:
• size (total # of individuals)
• density (# of individuals in a certain
space) and
• age distribution (the proportion of
individuals in each age in a population) in
response to changes in environmental
conditions.
• Dispersion patterns in nature
Density
Clumped
• Why clumping? 4 reasons:
• 1. The resources a species needs vary greatly
in availability from place to place.
• 2. Living in herds, flocks, and schools can
provide better protection from predators.
• 3. Living in packs gives some predators a better
chance of gaining a meal (hunting advantage).
• 4. Some animal species form temporary groups
for mating and caring for their young.
Dispersion Pattern: Uniform
• Some species maintain a fairly constant
distance between individuals.
• Creosote bushes in the desert have this
pattern so that there is better access to
scarce water resources.
Uniform
Dispersion Pattern: Random
• Organisms with a random distribution
(dandelions) are fairly rare. The world is
mostly clumpy.
Entrances and Exits on the
Global Stage
• Populations increase through births and
immigration and decrease through deaths
and emigration.
• Population change= (births +
immigration) – (deaths + emigration)
• These variables depend on changes in
resource availability and other
environmental changes.
Age Structure
• A population’s age structure can have a strong
effect on how rapidly its size increases or
decreases.
• Usually described in terms of organisms that
are:
– not mature enough to reproduce (the
prereproductive stage)
– and those that are capable of reproduction (the
reproductive stage),
– and those that are too old to reproduce (the
postreproductive age)
Survivorship Curve
• The size of a population that includes a large
proportion of young organisms is likely to
increase.
• In contrast, the size of a population dominated
by individuals past the reproductive stage
(older) is likely to decrease.
• If there is a fairly even distribution between
these stages, the population will remain stable
(births = deaths).
3 types of survivorship curves:
• Early loss, constant loss, late loss
Intrinsic rate of increase (r)
• The rate at which a population would grow if it
had unlimited resources.
• Individuals in populations with a high rate of
growth typically reproduce early in life, have
short generation times, reproduce many times,
and have many offspring each time they
reproduce.
• Ex: houseflies. Descendents of a single female
housefly could total 5.6 trillion within 13 months.
• This is exponential growth.
– That would suck.
Environmental Resistance
• Fortunately, no population can grow indefinitely.
There are always limits to growth.
• Decrease factors: too much or too little light,
temperature too high or low, unfavorable
chemical environment, low reproductive rate,
specialized niche, inadequate food supply,
poor habitat, too many competitors, inadequate
defense from predators, diseases and
parasites, inability to migrate or adapt to change.
Carrying Capacity (K)
• Together, biotic potential and environmental
resistance determine the carrying capacity.
• This is the maximum # of individuals of a
given species that can be sustained
indefinitely in a given amount of space.
• The growth rate of a population decreases as it
nears the carrying capacity of its environment
because resources (food and water) begin to
dwindle.
Carrying Capacity
Our man Edward O. Wilson (and
Robert H. MacArthur)
• These two suggested that species could
be classified into 2 fundamental
reproductive patterns:
• r-selected species
• And
• K-selected species
r- selected species
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Cockroach
&
dandelion
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
• Early successional species
K- selected species
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Elephant
&
Saguaro cactus
Fewer, large 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
• Late successional species
Review:
• Population dynamics is a study of how
populations change in:
• Dispersion patterns:
• 3 ways that age structure is broken down:
• Some limiting factors:
• What is carrying capacity?
• Explain r- selected species vs. K- selected
species.
Homework:
• Find 2 graphs on-line:
• A J- curve graph and an S-curve graph
• Explain in a paragraph what is going on
ecologically and why
• Monday we will discuss graphing and
practice it!