Exam 4 Study Notes: 52, 53, 54, 55
A
population
is a group of individuals of the same species that occupy the same area at the same time.
A
community
is an assemblage of interacting species.
An
ecosystem
is populations of organisms and their interaction with the physical or abiotic environment,
connected by the movement of energy, nutrients, and organisms.
Population ecology
is the study of how and why the number of individuals in a population changes over
time.
Population size changes in response to changes in birth, death, immigration, and emigration rates.
The study of these factors is
demography
.
Biologists use a variety of
mathematical and analytical tools to study population ecology/demography.
Life tables
summarize the probability that an individual will survive and reproduce in any given
year over its entire lifetime.
They show that individuals may have distinct ways of allocating
energy and resources to activities that promote survival versus activities that promote
reproduction.
This tradeoff between survival and reproduction is the most fundamental aspect of
a species’ life history.
The
growth rate of a population can be calculated from lifetable data or from the direct
observation of changes in population size over time.
Exponential growth occurs when the per
capita growth rate,
r
, does not change over time.
With
discrete generations (no overlap): N
t+1
= R
0
N
t
and N
t
= R
0
t
N
0
where R
0
is the per capita net
reproductive rate (average # of female offspring a female produces every generation.
If R
0
= 1,
then N does not change. If R
0
> 1, then the population is growing and if R
0
< 1, then the
population is declining.
With
continuous generations and no immigration or emigration: r (intrinsic rate of increase) is the
difference between births and deaths per time interval.
ΔN/Δt = dN/dt = (bd)N = rN, so
N
t+1
= e
rt
N
0
.
If r = 0, population replaces itself.
If r < 0, the population is increasing.
If r > 0, the
population is decreasing.
This assumes no seasonality, identical organisms (no age structure),
and unlimited resources.
Eventually, however, growing populations approach the
carrying capacity of their environment—
resulting in logistic growth.
As population density increases,
survivorship
(probability that an
individual will live from time t to time t
1
) and
fecundity
(number of female offspring produced
per female from t to t
1
) may decrease—leading to increased death rates, lowered birth rates, and a
reduction in growth rate.
Agespecific fecundity
is the average number of offspring produced by
a female of a given age class.
Changes in population size
through time may occur as a result of changes in the
age structure of
populations—specifically the number of juveniles in various age classes.
(Few juveniles, many
adults may be declining or stable in size… Many juveniles = likely rapid increase in size)
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Immigration rates and
generation time (the average time between a mother’s first offspring and
her daughter’s) also impact a population’s future.
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 Fall '07
 GILCHRIST
 Ecology, population size

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