Environmental Engineering Reference
In-Depth Information
explicitly to plant-herbivore-predator systems, they
would also apply to any multi-link food-web inter-
action. The central question would be: why are
community cascades apparently absent or rare in
terrestrial habitats? According to Polis
et al.
(2000),
all the cascades that Smith
et al.
(2000) referred to
in terrestrial systems measured interactions within
subsets of a community. Many of the impediments
to community-level cascades would arise from the
complexity of natural systems, and support even for
species-level cascades in terrestrial systems would be
limited. However, a study of butterflies on calcareous
grasslands (Steffan-Dewenter & Tscharntke 2002)
indicated that the specialists of higher trophic levels
(monophagous and strongly oligophagous butterflies)
are more sensitive to habitat fragmentation than are
species of lower trophic levels, in the plant com-
munity. In Chapter 5 in this volume I will, once again
refer to the problem of trophic cascades, then in the
context of metapopulation dynamics, which also
cannot be considered without keeping an eye on the
complexity of the communities they are part of.
ation results from selection and evolution, a process
that is continuous.
4.4.1 Plant-mycorrhiza interactions
The majority, say 80%, of species of temperate, sub-
tropical and tropical plant communities are infected
by vesicular-arbuscular mycorrhizal fungi (VAM
fungi, nowadays called AM fungi). There is every
indication that in such communities a vigorous semi-
permanent population of fungal symbionts with low
'host' specificity is involved in an infection process
which effectively integrates compatible species into
extensive mycelial networks (Francis & Read 1994);
the number of these fungal species is only about 200.
Ectomycorrhizal fungi (ECM) occur mainly on
woody plants and only occasionally on herbaceous
and graminaceous plants and involve more than 5400
fungal species. Ericoid mycorrhizae occur mainly in
the Ericales and are physiologically comparable with
ECM. Non-mycorrhizal plants occur mainly in very wet
or saline ecosystems and in ecosystems with a high
nutrient availability and/or with recently disturbed
soil (see references in Ozinga
et al.
1997). Orchid-
mycorrhiza relations are a special case (see below; Dijk
et al.
1997).
AM fungi are presumably especially efficient in the
uptake of inorganic P (and other relatively immobile
ions such as Cu
2+
, Zn
2+
and NH
4
+
) and are capable of
increasing the P uptake more in nutrient-rich patches
than in soils with a uniform P distribution (Cui &
Caldwell 1996a, 1996b), whereas ECM and Ericoid
mycorrhizae are more efficient in N-limited eco-
systems. In contrast to AM fungi, some ECM and
Ericoid mycorrhizae fungi have abilities to take up N
from organic matter and translocate these nutrients
to their host plant (Read 1991). Enzymatic degrada-
tion by ECM and Ericoid mycorrhizae fungi has been
shown for proteins, cellulose, chitin and lignin.
Changes in the proportion of nutrients in inorganic
or organic form may create changes in the competit-
ive abilities provided by different mycorrhiza types.
The presence of mycorrhizae has been shown to
change the outcome of plant competition in many
cases, both for AM plants and for ECM plants, and is
thus a determinant of plant community structure
(Ozinga
et al.
1997, van der Heijden
et al.
1998). Moora
4.4 Mutualism
We define mutualism as a bidirectional facilitation,
as an interaction between individuals of different
species that lead to an increase of fitness of both
parties, often based on mutual assistance in resource
supply. The benefits usually exceed the costs for both
partners (Bronstein 1994). Mutualistic relationships can
be facultative; leguminous plants for instance can live
with or without
Rhizobium
. They can also be obligate,
a condition for survival: lichens are a symbiosis
between a fungal and an algal component. For
vascular plants in general, mutualistic relationships
with mycorrhizal fungi are of utmost importance.
Many experimental investigations have shown that both
plant and fungal symbionts benefit from the recip-
rocal exchange of mineral and organic resources, but
sometimes the relationship may turn into parasitism.
The case of plant-pollinator relationships is similarly
complicated, because these interactions may ideally be
mutualistic, but in many cases there is a bias in the
balance in favour of one or the other component.
Mutualism may thus be accompanied with mechanisms
indicating a balanced antagonism. The current situ-
