Environmental Engineering Reference
In-Depth Information
and hence are potentially playing with a strategy that has been molded by
the species' evolutionary history.
The survival probabilities (p) in conjunction with age class-specific
birthrates (m) and numbers of individuals in an age or stage class can be used
to calculate the survival and production of each age or stage class. For ex-
ample, suppose two year olds survive to become three year olds with prob-
ability 0.8. Suppose an average two year old produces 1.2 female offspring
(this means that if, say, there are 10 females and collectively they produced
12 female offspring, then on average there are 1.2 offspring per female). Sup-
pose there are six hundred two year olds in the population that survive at a
rate of 0.8.We can then calculate the number surviving to age three as 600
x 0.8 = 480. If the 600 produce 1.2 offspring each, on average, before they
become one year older, then there will be 720 newborns that come from
two year olds that age to become three year olds.These calculations can then
be made for each age class.We can then calculate the number of individu-
als (n) in each age class (x) in a specific time period t (n
x,t
) to understand
population structure in a given time period.We can add up all of the n
x,t
values to calculate total population size in time period t (N
t
).This can be
repeated for each future time period based on data from the previous time
period.Thus, if we wish to calculate population size fifty time steps into the
future based on age structure, age-specific survival, and reproductive values,
and population size in time 1, we simply iterate (i.e., repeat using data from
the previous time step) through each calculation fifty times beginning with
data from time 1.This task is routinely automated using computers that can
calculate future population sizes in a matter of milliseconds.
Sensitivity of Populations to Disturbances
There are a number of things that we can do with this model. For exam-
ple, suppose we were interested in the fate of a threatened species such as
the Giant Panda (
Ailuropoda melanonleuca
). Pandas live predominantly in the
Qinling Mountains in southwestern China (Zou and Pan 1997).They are
specialist feeders on bamboo and their livelihood is threatened by habitat
destruction (Zou and Pan 1975).To obtain key data, Zou and Pan (1997)
studied the population for ten years.They discovered that females, on aver-
age, become sexually mature at age 3.5; males take two years longer on av-
erage maturing at age 5.5. Breeding occurs in March and April. Mothers
give birth in August in a den.The reproductive interval, the time from one
reproductive bout until the next, is about 2.5 years.This prolonged period
