Chapter 14 Population Ecology • MHR 47314.2 Describing the Growth of Populations
In July 1996, Canada’s human population was
about 29 672 000. By July 2000, it was roughly
30 750 000. What processes accounted for this
change in population size? Birthand immigration
(the movement of individuals into a population)
added to the population, while deathand emigration
(the movement of individuals out of a population)
reduced it. These four basic processes cause change
in the size of all populations. In Canada, birth and
immigration rates are higher than death and
emigration rates, so the population is growing.
If the opposite were true, the population would
be declining.
Growth of Populations in
Unlimited Environments
Humans probably move among populations
more than is typical of most species. For many
populations, immigration and emigration rates are
either very low or are roughly equal to each other,
and thus can often be ignored. For this reason, and
for the sake of simplicity, ecologists tend to focus
on only two of the four processes — births and
deaths — when considering how population size
changes. Imagine a population growing under ideal
conditions: with lots of food, space, shelter, and so
forth. You could use the following word equation
to describe how such a population changes in size:
Change in
population
size during a
time interval
Number of
births during
the interval=−Number of
deaths during
the intervalThis word equation can be effectively
summarized in a mathematical model. Using Nto
represent population size, tfor time, and the Greek
letter ∆(delta) for change, we can use ∆Nto
symbolize a change in population size and ∆tto
represent a change in time (that is, a time interval
such as a day or a year). If Bis the number of births
during a time interval and Dis the number of
deaths, the word equation becomes:
∆N
∆t=B−DThe absolute number of births (B) that occurs
during a given time interval depends on the size
of the population at the beginning of the interval
(which determines how many individuals are
present to give birth). The same is true for deaths
(D). A small population will probably experience
fewer deaths during a time interval than a large
one, simply because there are fewer individuals
that can die.
It is useful to have a mathematical model that
describes change in population size for all
populations, regardless of their size. Therefore it
is better to express Band Das rates (the average
number of births or deaths that occur per individual
during a specified time interval). Ecologists use bto
symbolize per capita (which means per individual)
birth rate — the average number of offspring
produced by an individual during a given time
interval. For example, if 50 offspring are born in a
population of 1000 individuals during an interval,
the birth rate bwould be 50/1000=0.05. Knowing
the birth rate, we can then calculate the expected
number of individuals (B) that would be added
during an interval to a population of any size. For
example, if b=0.05and N= 500 , 0.05× 500 = 25EXPECTATIONS
Analyze the components of population growth and explain the factors that
affect the growth of various populations.
Use conceptual and mathematical models to determine the growth of
populations of various species (e.g., use the concepts of exponential and
logistic (sigmoid) growth to describe and predict the future size of various
populations.)
Differentiate exponential and logistic growth curves.
Predict the future size of an exponentially growing population, given its
growth rate and starting size.
Describe how limits on the supply of necessary resources can affect the
growth of populations.