Environmental Engineering Reference
In-Depth Information
There are also examples of mutually negative inter-
actions between
ecosystem engineers
, for example
in the salt marsh-tidal fl at interface, where common
cordgrass (
Spartina anglica
) and the lugworm (
Arenicola
marina
) were found to affect each other negatively by
changing their shared biophysical environment in
opposite ways (van Wesenbeeck
et al
. 2007 ). While
seedlings of
S. anglica
stabilize the sediment with their
roots, preventing
A. marina
from feeding,
A. marina
strongly bioturbates the sediment, preventing
S. anglica
seedlings from establishing. All these examples show
the importance of gaining knowledge about basic
mechanisms in order to understand long-term cascad-
ing effects in communities. Otherwise any attempts at
restoration - inspired
intervention
are likely to produce
unexpected and possibly undesired results.
Allelopathic species (chemical engineers), excreting
organic compounds that can reduce the germination,
establishment, growth, survival and/or fecundity of
other species, can also have a long-term impact on
plant communities, inhibiting succession. In boreal
forests, for example, the ground-layer vegetation in late
post - fi re successions is frequently dominated by dense
clones of the dwarf shrub species
Empetrum hermaph-
roditum
, one of the most widespread plants in the Euro-
pean arctic and boreal biomes. This plant produces
large quantities of phenolics and is conspicuously
avoided by herbivores. Indeed, it is thought to be
responsible for the strong negative effects on tree-
seedling establishment and growth of Scots pine (
Pinus
sylvestris
), among other plant species, and on micro-
bial activity and plant-litter decomposition rates,
thereby contributing to humus accumulation and
reduced nitrogen availability (Nilsson
et al
. 1998 ). In
general, phenolics are highly allelopathic under acidic,
nutrient-poor soil conditions. In calcareous soils, by
contrast, most phenolic compounds are rapidly metab-
olized by microbial activity and adsorption is high.
There are indications that invasive
alien species
may
inhibit the germination of native species by chemical
interference (Ens
et al
. 2009 ; see also Chapter 20 ).
ences therein). Some mycorrhizal species with a
compact structure increase resistance of plants against
drought, pathogens, heavy metals and polyphenolic
substances (Ozinga
et al
. 1997). In spite of a huge
amount of knowledge about the importance of plant-
mycorrhiza interactions, including in ecological resto-
ration interventions, actual applications are still largely
unexplored (Harris 2009).
Effective restoration of pollinators providing pollina-
tor services in ecological restoration projects has also
received too little attention and experimentation, even
though much basic information is available (Dixon
2009 ).
Habitat fragmentation
and other effects of
land use - such as agriculture, grazing, herbicide and
pesticide use - and the introduction of non-native
species have a signifi cant impact on plant-parasite and
plant-pollinator interactions (Kwak
et al
. 1998 , Taki &
Kevan 2007 ; see also Chapter 7 ).
6.3.3
Implications for restoration ecology
In our review thus far, we have touched upon a number
of potential applications of fundamental ecological
insights to ecological restoration projects, but we
readily acknowledge that taking all the potential inter-
actions, and all their indirect effects, into account is
certainly impossible. Our message is twofold. First, res-
toration ecologists should realize that
ecosystem
functioning
is complex and not easy to initiate, mimic
and/or manage. They must, wherever possible, try to
integrate the knowledge presented in their efforts to
make ecological restoration a success, rather than
ignoring the complexity and making a choice for simple
solutions. Secondly, and related to this, we welcome
and applaud initiatives to scientifi cally cope with com-
plexity. Searching for general rules and laws, rather
than presenting a huge amount of detailed informa-
tion, continues to be a challenge for scientists. Though
this 'coping with complexity' may seem to lead us still
farther away from applicability, we consider it useful to
inform the reader about promising initiatives.
Effects of m utualism
The presence of mycorrhizae has been shown to
change the outcome of plant competition in many
cases, both for plants associated with arbuscular myc-
orrhiza and those associated with ectomycorrhiza;
they are thus a determinant of plant community struc-
ture (e.g. Kiers
et al
. 2000 ; Stein
et al
. 2009 , and refer-
6.4 COMPLEXITY IN BIOTIC
COMMUNITIES
The dynamics of communities is obviously not only
determined by interactions between pairs of species:
recall the food webs and pollination networks we
