Figure 23.9: Growth of populations according to Malthus’ exponential (or J-curve) growth
model (left) and Verhulst’s logistic (or S-curve) growth model (right) ( 14 )
shown as an “S-shaped curve” inFigure23.9. Why do you think this is? You would be
right if you said because various factors limit the growth of populations. Can you think of
which factors these could be?
Limiting Factors
Limiting factorsthat can lower the population growth rate include reduced food supply
and reduced space. These can have the effect of lowering birth rates, increasing death rates,
or can lead to emigration. This growth model is known as the logistic (S-curve) model, and
looks different than the one for exponential growth (Figure23.9). In this case, the growth
rate begins as proportional to the size of the population, but at higher population levels,
competition for limited resources leads to lower growth rates. Eventually, the growth rate
stops increasing and the population becomes stable.
This plateau in growth is known as thecarrying capacity, or the maximum population
size that can be supported in a particular area without degradation of the habitat. Limiting
factors determine what the carrying capacity is.
In general, a limiting factor is a living or nonliving property of a population’s environ-
ment, which regulates population growth. There are two different types of limiting factors:
density-dependent factors and density-independent factors.