Environmental Engineering Reference
In-Depth Information
Table 5.2
Simplified presentation
of different interactions between
two species (A and B), when they
meet (On) or do not meet (Off ).
On
Off
Species A
Species B
Species A
Species B
Competition
−
−
0
0
Allelopathy
0
−
0
0
Parasitism
+
−
−
0
Facilitation
0
+
0
0
Mutualism
+
+
−
−
−
Disadvantage;
+
advantage; 0 indifference.
is difficult to know if the facilitator is unaffected or
suffers in a way, for example by becoming outcompeted.
and/or reproduction of the competing individuals
concerned'. The latter part could be summarized by
stating that the process leads to a reduction in one
or more fitness components.
A large amount of literature is available on inter-
specific resource competition as a phenomenon,
starting in the early 1930s with Gause's famous
experiments with
Paramecium
species feeding on
either the same food resource (bacteria) or on dif-
ferent food resources (bacteria and yeasts). From
these experiments Gause's principle of competitive
exclusion was derived, implying that the number of
species that can coexist cannot exceed the number
of limiting resources. Competition theory was at that
time mathematically related to population growth, in
terms of the Lotka-Volterra logistic growth curves,
which suggest
r
and
K
being important parameters of
success. MacArthur and Wilson (1967) elaborated on
this approach by formulating the concept of
r
- and
K
-selection, resulting in a colonizing strategy and a
competitive or maintenance strategy respectively. In
this concept, competitive ability is assumed to have
been evolved at the expense of colonizing ability
of species; there would be a trade-off between
r
- and
K
-characteristics. Fitness of individuals is considered
a compromise between these contrasting choices,
expressed in terms of the relative importance of dif-
ferent fitness components, as for example generative
versus vegetative reproduction of plants.
Gause noticed that the order of competitive abilit-
ies between two species of
Paramecium
, both feeding
on one and the same resource, did not depend on the
densities at the start of the experiment, which implies
that population growth rate could not easily explain
5.3.2 Competition
Competition between organisms can be direct, for
space or territory, or indirect, for resources. Direct or
interference competition reveals a winner and a loser,
or the two parties remain equal, although in this case
there is a cost of energy anyway which implies a
relative loss as compared to having no interference.
In indirect or exploitation competition there are usu-
ally two losers. If a large plant competes for nitrogen
with a small plant, each of them has a negative
impact on the other. In view of the absolute amount
of nitrogen taken, the exploitation competition can be
asymmetric, but in terms of the relative loss of fitness
of each of the plants the competition can be symmetric.
In animal ecology a distinction is made between con-
test competition and scramble competition. Contest
competition is a form of asymmetric competition and
may equal interference competition, whereas scramble
competition is a form of symmetric competition and
is associated with exploitation competition. Mostly,
loss of fitness between two parties is measured in a
relative way, as compared to the potential fitness of
an organism, and not in terms of the total amount of
resource captured.
Begon
et al.
(1996) provided a useful working
definition of exploitation competition: 'An interac-
tion between individuals, brought about by a shared
requirement for a resource in limited supply, and
leading to a reduction in the survivorship, growth
