Figure 6.1 Fluctuations around the carrying capacityJ-Curve
A J-curve on a graph records the situation in which, in a new environment, the
population density of an organism increases rapidly in an exponential or
logarithmic form, but then stops abruptly as environmental resistance (e.g.,
seasonality) or some other factor (e.g., the end of the breeding phase) suddenly
becomes effective. The actual rate of population change depends on the biotic
potential and the population size. Population numbers typically show great
fluctuation, such as the characteristic “boom and bust” cycles of some insects or
the cycles seen in algal blooms. This type of population growth is termed
“density independent” as the regulation of growth rate is not tied to the
population density until the final crash.
S-Curve
An S-shaped curve of growth (also known as logistical growth) occurs when, in
a new environment, the population density of an organism increases slowly
initially, in a positive acceleration phase, then increases rapidly, approaching an
exponential growth rate as in the J-shaped curve, but then declines in a negative
acceleration phase until reaching zero growth when the population stabilizes.
This slowing of the rate of growth reflects the increasing environmental
resistance which becomes proportionately more important at higher population
densities. This type of population growth is termed “density dependent” since
the growth rate depends on the numbers present in the population. The point of
stabilization, or zero growth rate, is the carrying capacity of the environment for
that organism. K represents the point at which the upward curve begins to level,
produced when changing population numbers are plotted over time.