Environmental Engineering Reference
In-Depth Information
which species would win the competition. Whereas
Grime's (1979) explanation of competitive abilities of
plants is largely based on differences in plant char-
acteristics such as relative growth rate and plant
morphology in the context of vegetation processes,
Tilman (1982) was able to predict competitive hierar-
chies, tested with unicellular organisms as in Gause's
experiments, from their patterns of resource depletion
(related to population growth). In theory, the winner
is the species with the lowest minimum resource
requirement, expressed as the resource concentration
at which growth and mortality are equal. This point
of view seems to contrast with Grime's view, saying
that the species with the highest resource capture
(related to growth rate) will win the competition.
However, the two views are not mutually exclusive:
species A can gain dominance in the early phase of
an experiment, at non-equilibrium transient conditions,
because it has the highest growth rate and captures
a higher amount of the resource, whereas species B
can become the ultimate winner at equilibrium con-
ditions, because it utilizes the resource more effici-
ently; that is, it has a lower minimum requirement.
Eventually, species B may competitively exclude
species A; that is, if it survived as a subordinate in
the transient period, and if an equilibrium can be
achieved anyway (Fig. 5.2). Unicellular organisms of
different species in continuous-flow cultures do have
an opportunity to arrive at one equilibrium state. In
plant communities, succession can become inhibited
before any potentially final equilibrium or climax stage
can develop, exemplified by the dominance of
Molinia
caerulea
in wet heathlands (Berendse & Elberse 1990),
of
Brachypodium pinnatum
in calcareous grasslands
(Bobbink
et al.
1989) and of
Elymus athericus
in
coastal salt marshes (Bakker 1989).
Studies of plant interactions focus on only a few
dominant species in plant communities under more
or less homogeneous environmental conditions. The
increasing interest in biodiversity issues motivated
researchers to wonder how subordinate species can
remain coexisting in competitive plant communities.
This resulted in a renewed interest in environmental
heterogeneity and unpredictability. Huisman and
Weissing (1999) offered a solution to the so-called
plankton paradox, based on the dynamics of the
competition itself, by showing that (i) resource com-
petition models can generate oscillations and chaos
100
10
Species A
Species B
50
5
R
A
*
R
R
B
*
0
0
0
1
2
3
4
Time
Fig. 5.2
Population responses of two species (A and B)
competing for a single limiting resource (
R
), showing
that species A can be dominant in the early phase of
competition because it can utilize the resource rapidly,
whereas species B can take over due to its lower
minimum resource requirement (
R
*). Population size,
resource level and time are given in arbitrary units;
these will vary depending on the organism. After
Tilman (1988). Reproduced by permission of Princeton
University Press.
when species compete for three or more resources
and (ii) these oscillations and chaotic fluctuations
in species abundances allow co-existence of many spe-
cies on a handful of resources. So, whereas classical
competition theory predicts competitive exclusion of
species with similar requirements, recent ideas stress
that species diversity may be explained by a multitude
of processes acting at different scales, and that sim-
ilarities in competitive abilities often may facilitate
coexistence (Bengtsson
et al.
1994). It is still to be
seen to what extent these ideas fit in processes at the
level of terrestrial animal and plant communities.
Competitive abilities in developing communities in
ecological restoration are difficult to predict, because
in such cases the initial conditions and the early-
colonizing species may have a long-term rather than
a transient impact. In recent competition theory it has
been shown that competitive hierarchies or coexistence
among a number of species may very well depend on
the initial species composition and abundance, which
may imply chaos and unpredictability (Huisman &
Weissing 1999).
