Environmental Engineering Reference
In-Depth Information
temperatures, the significant effects of
Typha
on soil
oxygen disappear, presumably because of increased
microbial and root respiration and the interaction
between
Myosotis
and
Typha
becomes competitive. In
the field, the overall effect of
Typha
on
Myosotis
was
positive, as
Myosotis
plants growing next to trans-
planted
Typha
were larger and produced more fruit
than those isolated from
Typha
plants.
This phenomenon of so-called third parties affect-
ing the competitive abilities of two other species was
called mediation of competition (Allen & Allen 1990).
Similar to the suggestion of Price
et al.
(1986) in their
review on mediation by parasites, parasites act as a
third party affecting the interference competition or
resource competition between two other species (see
Pennings & Callaway 1996). In the 1990s this was
termed 'apparent competition', but I prefer the more
clear-cut notion of mediation, because it is applic-
able to both negative and positive interactions.
Mediation by parasites is very common in nature and
must be regarded as one of the major types of inter-
action in ecological systems, comparable in import-
ance to direct competition, predation, parasitism or
mutualism (Price
et al.
1986). Experimental and
theoretical studies have provided new insight into
the mechanisms and conditions that could influence
coexistence or exclusion (reviewed by Hudson &
Greenman 1998). Parasite-mediated competition is
particularly significant, given the rise in emerging dis-
eases and the opportunity that pathogens have to reduce
host abundance. Parasite-mediated competition can
act when an invading species introduces a parasite to
vulnerable, resident species. This might have been the
scenario when grey squirrels (
Sciurus carolinensis
) were
introduced to Britain, bringing with them a parapox
virus that reduced the competitive ability of the
native reds (
Sciurus vulgaris
). The reverse can also take
place. If a parasite weakens the competitive ability
of the resident species, invasion can occur. Interac-
tions between the pathogens of rabbits (
Oryctolagus
cuniculus
) and hares (
Lepus europaeus
) provide some
field evidence that pathogens can influence a com-
petitive outcome. Studies in the Netherlands found
a stomach worm (
Graphidium strigosum
) to be more
pathogenic in hares than rabbits. Jefferies (1999)
reviewed interactions between herbivores and micro-
organisms affecting nutrient fluxes in the ecosystem,
and discussed special conditions where increased
dominance from herbivores overrides the regulatory
controls imposed by other organisms, which leads to
trophic cascades and discontinuous vegetation states.
5.5 Concluding remarks
For management purposes, an integrated approach
is often required. Therefore, to link up with the pre-
vious chapter on ecosystems and biotic communities,
and the next one, which deals with populations, one
must pay attention to the plea by WallisDeVries
et al.
(2002) for an integrated approach to conservation
of diversity in biotic communities at the ecosystem
level. They exemplified their views by referring to
calcareous grasslands. In these ecosystems, the bio-
logical diversity is high and includes a variety of
rare species from different taxonomic groups. Among
plant communities, chalk grasslands rank as one of
the richest in plant species, both at a small spatial scale
and on a large scale. They include for instance highly
valued orchids. A potentially high species richness
is found, as well, for butterflies and various other inver-
tebrates. Calcareous grasslands are predominantly
semi-natural communities and thus require some
form of management by grazing or mowing, which
raises conflicts about the best management regime for
particular groups of species. Indeed, managing for plant
diversity requires low nutrient levels, which can be
arrived by grazing, mowing or even mulching. But the
importance of heterogeneity in vegetation structure
is emphasized for the invertebrates, some groups of
which require a less-intensive management by graz-
ing at low stocking rates or by rotational mowing.
Botanically centred management may thus lead to
an impoverished vertebrate fauna. This can only be
avoided by an integrated approach that considers
various taxonomic groups of the biotic community.
What these considerations, with respect to nature
conservation and management, imply in the context
of restoration ecology is hard to say, apart from the
need - to state it in terms of a metaphor - to pay
attention to the forest if one aims at studying or even
preserving a tree population (see Odum 1983).
