Environmental Engineering Reference
In-Depth Information
Figure 4.2.
(a) Change in per individual birth or mortality rate as a function of increas-
ing population density of a focal species. Birthrate (solid line) declines with population
density because fewer resources are available on a per capita basis to allocate toward off-
spring production. Mortality rate increases with population density because fewer
resources are available to support individual survival. The intersection of the two lines
represents the point at which birth rate balances mortality rate—equilibrium or carry-
ing capacity (K).This is where the population growth levels off after a period of increase
(see figure 4.1b). (b) Competitor species can decrease a focal species' birth rate and
increase death rate leading to a new equilibrium (B
c
) in which the focal species is lim-
ited below its carrying capacity K. Predators can have the same qualitative effect as com-
petitor species if they scare prey and thereby cause prey to spend less time feeding
because they must be vigilant. This in turn reduces resource intake, which can lower
birthrates and increase mortality rates across all prey densities. (c) Predators can increase
prey mortality rate by capturing them (dotted lines) leading to a new equilibrium level
that depends on whether or not the predator is inefficient (P
1
) or highly efficient (P
2
).
emphasis on single population here.The reason that we must be careful to
link the term
carrying capacity
with single population dynamics will become
clear when discussing the effects of predator and competitor species, and
the policy implications of carrying capacity.
If it is true that populations must live within the confines imposed upon
them by the environment, what happens when populations exceed their
