Environmental Engineering Reference
In-Depth Information
soil glycine and ammonium stores.
Furthermore, low-bush cranberry
obtains most of its nitrogen forms
earlier in the growing season and at
a shallower rooting depth than cot-
tongrass. The herb Labrador tea
(
Ledum palustre
) and the shrub dwarf
birch (
Betula nana
) use mainly ammo-
nium but differ in the seasonal timing
of uptake. Each species divides up the
nitrogen pool on the basis of chemi-
cal form, seasonal timing of use, and rooting depth. In other words, they
minimize competitive overlap by dividing up the resource in ways that give
each of them an exclusive spatial or temporal advantage.
Many exploitative competitors also can co-occur when they share a
common resource because they also have an exclusive resource (figure 2.2b).
In such cases, no single competitor species can dominate the other because
each species has a safety net in the form of a resource that is available for
their own exclusive use.Thus, the shared resource does not determine the
outcome of competition. Species diversity is maintained in this system be-
cause the consumer species (e.g., C
1
and C
2
) mutually limit their abundance
through exploitation of their shared resource species (S) and thus do not
reach abundances in which they can overexploit their exclusive resource
species (E
1
and E
2
).
The species that wins in an ex-
ploitative competitive system (fig-
ure 2.1c) is the one that consumes
the resource the quickest or draws
it down to levels where individuals
of the other species can no longer
meet their food requirements and
sustain themselves.
PREDATION: THE KEYSTONE OF ECOLOGICAL STRUCTURE
The examples presented above represent two ways that diversity originates
and is maintained in ecological systems. In a food chain, species diversity is
maintained vertically by consumptive interactions between trophic levels;
in the competitive system, species diversity is maintained horizontally
through fine partitioning of resources. In reality, many ecological systems
are more complex than this because they are driven by combinations of ver-
tical and horizontal diversifying mechanisms.We can begin to explore the
kinds of complexity that arise by combining vertical and horizontal fac-
tors with two more examples that are effectively variations on a theme.
In figure 2.2c, the carnivore species again mediates competitive interac-
tion, but in this case it is an interference competition system. Species di-
versity is maintained in this system because the abundance of a
