One type of migration that you are probably pretty familiar with is the direct, often sea-
sonal, movement of a species that results in a predictable change for that population size.
Maybe you’ve heard that ‘birds fly south for the winter.” Examples of this migration are
the thousands-of-miles migrations that many birds perform in the fall and then again in
the spring when they return to their original habitat (Figure23.8). Another example of a
long-distance migration is the movements of Monarch butterflies from their Mexican winter-
ing grounds to the northern summer habitats (in various regions of the United States) and
back again. These types of migrations move entire populations from one set of location and
environmental conditions to another.
Figure 23.8: A flock of barnacle geese,Branta leucopsis, fly in formation during the autumn
migration in Finland. ( 26 )
Population Growth
Under ideal conditions, given unlimited amounts of food, moisture, and oxygen, and suitable
temperatureandotherenvironmentalfactors,oxygen-consumingorganismsshowexponential
or geometric growth, where as the population grows larger, the growth rate increases. This
is shown as the “J-shaped curve” inFigure23.9. You can see that the population grows
slowly at first, but as time passes, growth occurs more and more rapidly.
These ideal conditions are not often found in nature. They occur sometimes when popu-
lations move into new or unfilled areas. If ideal conditions were found all the time, what
would you expect to happen to populations?
In nature, limits occur. One basic requirement for life is energy; growth, survival and
reproduction all require this. Do you think energy supplies are limited or unlimited?
The answer is they are limited and therefore organisms must use these resources and others
wisely. How do you think this affects the way organisms grow and what do you think the
growth rate would look like?
In nature, under more realistic conditions, at first populations grow exponentially (J-shaped
curve), but as populations increase, rates of growth slow and eventually level off. This is