Environmental Engineering Reference
In-Depth Information
species have on each other.That is, by vying for resources or space with
members of another species, the per capita share of resources again declines.
This decline in resource share causes an overall decline in per capita birth
rates and increase in per capita mortality rates of the focal species (figure
4.2b).The consequence is that the focal species population may reach a new
balance (B
c
) between birth and mortality rates that depends on the density
of the competitor species (the extrinsic factor).The population density at
this new equilibrium will be less than that in the absence of the competi-
tor species (figure 4.2b).That is, the competitor species (external factor) lim-
its the focal species population below its carrying capacity.
Predators affect focal populations differently than competitor species.
Predators can scare prey, which causes prey to spend less time feeding be-
cause they must be vigilant.This in turn reduces resource intake, which can
lower birth rates and increase mortality rates across all prey densities.This
would have the same qualitative net effect on per capita birth and mortal-
ity rates as competition and thus reduce the focal species below its carry-
ing capacity (figure 4.2b). Predators also increase mortality of their prey by
hunting and capturing them. The new equilibrium density of the focal
species will fall below that in the absence of predators.The exact level at
which the new balance is achieved will depend on whether predators are
inefficient (P
1
) or highly efficient (P
2
) at capturing their prey. (See figure
4.2c.)
Weather
Point equlibria and limit cycles all re-
sult from fixed birth and death
processes—called
deterministic
processes
—that lead to order, even
though there can be fluctuating or
oscillatory behavior in the dynamics.
All deterministically oscillatory pop-
ulations cycle about an equilibrium
but they do not reach that equilib-
rium exactly at any instant in time.
Instead, they continually overshoot or
undershoot the equilibrium. When
they overshoot or undershoot, the
density-dependent adjustments in fit-
All deterministically oscillatory pop-
ulations cycle about an equilibrium
but they do not reach that equilibri-
um exactly at any instant in time.
Instead, they continually overshoot
or undershoot the equilibrium.
When they overshoot or under-
shoot, the density-dependent
adjustments in fitness components
(survival and reproduction) cause a
directional reversal in population
density. Thus the populations are
continually drawn back or attracted
to equilibrium.
