Environmental Engineering Reference
In-Depth Information
Fundamentally, this says that species 1 and 2 will coexist stably if their fitness ratio
lies between
r
and 1/
r
. Thus, the less the niche overlap, the greater the difference in
fitness that is tolerated compatible with species coexistence. These conditions
involve two kinds of differences between species that affect species coexistence
in opposite ways. First are fitness differences, which are measured in terms of the
deviation of the fitness ratios from 1. This ratio fundamentally measures the overall
relative degrees of adaptedness of the species to their common environment.
A species with a larger value of
k
can be thought of as a better performer, and
therefore inequality in this respect understandably favors exclusion.
A complication arises due to the fact that when different focal species are present,
the resource species or predator species maintained in the food web may be different
[
17
]. When we ask if species 1 can exclude species 2, those resources and predators
present when species 1 is present alone are used to calculate the fitnesses and niche
overlap for both species 1 and 2. When we ask if species 2 can exclude species 1,
a different set of resources and predators might be present. This means that in some
cases, the
r
and
k
ratio for each end of the inequality in (
Eq. 13.6
) will be different,
and will have to be calculated based on which focal species is present. While, this
complication does not alter the fundamentals as to whether a given species can
exclude another species from a community, it is important to keep in mind that
r
and
the
k
's may change with the circumstances [
26
,
27
].
What do these critical quantities measure? Although the
k
's and
r
both involve
aspects of resource consumption and predation, they measure independent aspects.
In particular, the quantity
r
is independent of how well the species are adapted to the
environment. Instead it compares species in terms of which trophic links (links to
resources and predators) are most important to them, and how important they are, as
illustrated in
Fig. 13.2
. Importance is measured in terms of the ability of that link to
generate density-dependent feedback [
17
]. This comparison shows how much the
species interact with each other through their resources and natural enemies and so
how much conflict there is between them: the larger
r
is, the larger the conflict.
Naturally, two individual organisms have greater similarity and therefore greater
conflict through trophic relationships within species than between species (not
counting the effects of overall fitness differences), and
r
is a relative measure
comparing between-species interactions through trophic links to within-species
interactions. In other words, it compares the strength of interspecific feedback
loops with intraspecific feedback loops, in essence adjusted to equal overall fitness.
Thus, the ratio
k
1
/
k
2
and niche overlap
r
represent two different kinds of comparison
between species.
Exclusion Principles
Recognition of these two separate kinds of ecological comparison between species
resolves a conundrum about the competitive exclusion principle [
28
], which is often
stated as “no two species can occupy the same ecological niche.” It has been