Environmental Engineering Reference
In-Depth Information
just mentioned, is a frequently cited example; it creates
wetlands and modifi es entire landscapes through its
damming and foraging activities, which are of course
benefi cial to themselves but harmful to other organ-
isms. Often, however, the processes are much more
subtle and nevertheless infl uential, such as the bur-
rowing activities and cast constructions of earthworms
that alter the mineral and organic composition of soil,
accelerate nutrient cycling and facilitate drainage of
soils, ultimately affecting the community composition
of plants, animals and microorganisms. Direct effects
of such biophysical engineering on the part of animals
are usually measured by their indirect consequences,
such as improving the species' own environment (self-
facilitation, as in the case of beavers), or facilitation of
other species (as in the case of earthworms). Examples
of indirect effects of habitat modifi cation are given in
section 6.3.2 .
Allelopathy
- the release of organic compounds from
one plant species, which reduces the germination,
growth or fecundity of other plant species in the com-
munity - has long been considered as a form of unidi-
rectional interference competition between plants.
Here we prefer to consider it as a form of chemical
engineering of the environment. There are many thou-
sands of organic plant compounds released from shoot
materials or exuded by roots, but only a relatively small
number of them have been identifi ed as detrimental
and involved in allelopathy. What's more, even if a sub-
stance exhibits detrimental effects on plants in labora-
tory experiments, it might not truly hamper growth of
other plant species in the fi eld. A good example of
bona
fi de
allelopathy is phenolic substances occurring in
forest soils (e.g. H รค ttenschwiler & Vitousek 2000 , and
references therein). They include, for example, tannins
in the leaves of oak trees (
Quercus
spp.) and bracken
fern (
Pteridium aquilinum
), where they function as
feeding deterrent. Once released from decaying plant
materials into the soil environment, phenolics infl u-
ence plant growth directly by interfering with plant
metabolic processes and by their effects on root symbi-
onts. They also affect ecosystem
nutrient cycling
in
various ways, and interfere with decomposition, min-
eralization and humifi cation (see section 6.3.2).
gate or facultative. Two examples of mutualism that we
will illustrate here, as they should be taken into account
in restoration projects, are (1) plant-mycorrhiza inter-
actions, and (2) plant-pollinator interactions.
Plant - mycorrhiza interactions
can be considered as a
mutualistic symbiosis. For the overwhelming majority
of vascular plants, mutualistic relationships with myc-
orrhizal fungi are of utmost importance. Usually, the
plant provides the associated fungi with carbohydrates
while the mycorrhizae assist their 'host' plants with
taking up water and essential nutrients, especially P,
Cu and Zn, but also N, K, Mg and Ca (see Kuyper & de
Goede 2005 and references therein). However, there
are many different types of mycorrhizal fungi (see
Ozinga
et al
. 1997 and references therein). Approxi-
mately 80% of species of temperate, subtropical and
tropical plant communities are infected by
arbuscular
mycorrhizal fungi
. These fungi are especially effi cient in
the uptake of inorganic P and several other relatively
immobile ions.
Ectomycorrhizal fungi
occur mainly on
woody plants and only occasionally on herbaceous
plants and grasses, and they are especially effi cient in
N - limited ecosystems.
Ericoid mycorrhizal fungi
occur
mainly in the Ericales (heathers and heaths) and are
physiologically comparable with ectomycorrhizae.
Several mycorrhizal fungi have enzymes that break
down organic complexes such as tannin and lignin,
thereby releasing N and P from organic matter found
in the soil, which they then translocate as nutrients in
a form that is more readily available to their host plant.
This is particularly useful to plants in acid soils, where
nutrient uptake by plant roots is often diffi cult.
Plant - pollinator interactions
can be considered as a
nonsymbiotic mutualism. The mutualism is opportun-
istic and fl exible rather than fi xed, neither symmetrical
nor cooperative. The mutual exploitation interest may
be skewed towards a consumer-resource relationship
between the two parties, or even result in antagonism.
In their review on ' endangered mutualisms ' , Kearns
et al
. (1998) pointed out that over 90% of modern
angiosperm species are pollinated by animals of some
kind, including insects, birds, lizards, bats and small
marsupials. Specialist relationships are much more
vulnerable than generalist relationships, but plant-
pollinator interactions are only seldom species specifi c.
Indeed, relatively few plant-pollinator interactions are
absolutely obligate in a strict sense (Johnson & Steiner
2000 ). Many fl owers show specialization in fl oral
traits, yet they are often visited by diverse assemblages
of animals. There is a network of relationships between
Mutualism
Mutualism is a direct interaction between individuals
of different species that results in an increase of fi tness
for both parties. Mutualistic relationships can be obli-
