Environmental Engineering Reference
In-Depth Information
feedback loops. Niche overlap,
r
, is fundamentally about feedback loops. Low
niche overlap means feedback loops between species are weak relative to those
within species. As emphasized above, this is essential for coexistence to be
stabilized. Thus, mechanisms that bring about low niche overlap are called
stabilizing mechanisms. Stabilizing mechanisms vary from the obvious to
the subtle. Specialization of the members of a guild on different resources is the
stabilizing mechanism termed resource partitioning [
30
], which quite directly leads
to low contributions to
r
(
Fig. 13.2a
,
d
, and
e
). Likewise, if the natural enemies of
the guild members are specialists, feedback loops through predation are separated
(
Fig. 13.2a
and
c
). This is natural enemy partitioning [
17
]. As we shall see below,
these direct and obvious stabilizing mechanisms are far from the only ones. Likely
involved in these scenarios are trade-offs that provide advantages to specialization.
For instance, consumers well equipped to exploit a particular resource, or predators
well equipped to attack a particular species, may not be so well equipped to exploit
a different resource or attack a different species because the very equipment that
works well in a specific situation does not work so well in another [
11
,
31
].
Equalizing mechanisms do not have to involve the feedback loops at all. In the
Lotka-Volterra model considered here, the fitnesses are measured at low density,
and so density feedback has no direct role [
17
]. One can ask what mechanisms
might lead species to be similar in average fitness in a given environment. It is clear
that the laws of physics come in at the ultimate level and constrain performance
differences between species. But there are still numerous ways in which species
might differ in efficiency at a given task. Many trade-offs might be seen as
equalizing mechanisms in that doing well in one respect might mean doing less
well in another respect [
29
]. For instance, defense against predation or harsh
physical environmental conditions might lead to lower growth rates of individual
organisms and perhaps slower rates of reproduction. Thus, one species might have
higher survival rates but suffer in reproduction relative to another species, thus
limiting the fitness differences between species that are possible.
There is as yet no general theory of equalizing mechanisms, but one is likely to
emerge from general principles of community assembly and natural selection.
Natural selection drives species to the limits of what is possible: for instance, not
being defended against harsh conditions, while not growing fast, are certainly
possible in an organism, but if it is also possible through a genetic change to
grow faster if expenditures on defense are low, or to be defended if growth is
slow, then that character is likely to evolve. The constraint on what is possible is
approached, which enforces the trade-off [
4
]. This process happens within species,
as it involves natural selection at the individual level. The process of relevance
between species is community assembly. More efficient species arriving in
a particular locality are likely to displace others there, and that process will continue
until constraints on efficiency are approached,
trade-offs apply, and fitness
differences are minimized.
It should be recognized that many constraints in nature may have both stabilizing
and equalizing aspects to them. Trade-offs associated with resource partitioning
may lead to stabilization as well as equalization, provided similar profit is derived
