Environmental Engineering Reference
In-Depth Information
into its two fundamental components: per individual survival (or rather its
opposite, mortality) and birth.When population size increases, per individ-
ual birthrates will decline because there are increasingly fewer resources
available per individual to allocate to reproduction. Per individual mortal-
ity rates will rise with increasing population size, again due to declining
abundance of resources that would normally be allocated to survival. (See
figure 4.2a.) Whenever birth or mortality rates change with or depend on
population size within a location, we say that the population is undergo-
ing
density-dependent
growth. (Population size within a geographic location
is called population
density
.) By contrast, fitness of individuals in popula-
tions undergoing the unbounded exponential growth described above is
not influenced by population density. Such populations are said to undergo
density-independent
growth.
Ecological Balance and Carrying Capacity
In the face of intensifying competition with rising population size, per in-
dividual net fitness eventually will be reduced to the point where the per
individual birthrate is exactly offset by per individual death rate (figure 4.2a).
At this point, individuals are merely replacing themselves over their lifetime
and the population will neither grow nor shrink in size. Instead it will re-
main at a steady state.This steady state is called
equilibrium
.The reader may
be more acquainted with the vernacular term for equilibrium,“balance of
nature.”The problem with the term
balance of nature
, however, is that it gives
the impression that there is a single natural balance.Yet, several different pop-
ulations could exist at their own unique balance in different geographic lo-
cations. Hence, the idea that life on earth is in a single balance of nature is
a popular but unfortunate misconception.
We have now explained how environments limit species populations.The
equilibrium population size is effectively the fixed maximum population
size that can be sustained or “carried” by the limiting supplies of resources
or space in a geographic location. In
ecology, this is formally known as
car-
rying capacity
, usually labeled K. In fig-
ure 4.1b, K for the Tasmanian
population of sheep is on the order of
1,670,000 individuals. Carrying ca-
pacity is synonymous with equilib-
rium
for a single population
. Note the
At this point, individuals are merely
replacing themselves over their life-
time and the population will neither
grow nor shrink in size. Instead it
will remain at a steady state. This
steady state is called
equilibrium
.
