Environmental Engineering Reference
In-Depth Information
(1996), who performed experiments in a semi-arid
region of Australia to rehabilitate degraded landscapes
on bare slopes. Their study revealed that the creation
of a patchy construction of 0.5 m high piles of acacia
branches resulted in a 10-fold increase in water infi l-
tration, with a huge impact on soil quality and plant
production (see also Chapter 4). In other words, they
improved the recovery potential of the site undergoing
restoration by mimicking a natural spatial pattern, and
thereby assisting the natural self-re-organization of the
ecosystem.
ing of the consequences. Our focus will be on these
indirect effects, dealt with thereafter, in view of their
impact on ecosystem structure and functioning, which
is particularly relevant in the practice of ecological
restoration.
Consumption
All organisms require resources to survive, grow and
reproduce. At the level of individuals, the benefi t is
one-sided: a plant needs nutrients and a predator needs
prey, but not the other way around. Among animal
consumers, there are (monophagous) specialists that
consume only one prey species, and (polyphagous)
generalists that consume a wide range of prey. Con-
sumption may result in the death of a complete organ-
ism, as is the case with many predator - prey interactions,
but consumption can also be partial, as for instance
when herbivores consume only part of their food
plants, or plants absorb small quantities of nutrient
pools. Great variations in the relevant temporal scale
also occur across the range of interactions where con-
sumption occurs in ecosystems.
The impact of consumption on community struc-
ture depends very much on the growth rate or growth
potential of the prey species, and the consequences for
the food chain or the food web. Imagine a spatial gradi-
ent with increasing plant productivity, be it due to
increasing annual precipitation in a subtropical region
or to increasing nutrient availability in the temperate
zones of the Earth (see Figure 6.2). At low productivity,
vegetation is too sparse to support herbivores. An
increase of primary productivity, however, should
6.3
BIOTIC COMMUNITIES
Biotic communities are not only shaped by the interac-
tions with the abiotic environment, but also to a large
extent by all sorts of interactions between different
species. Organisms can affect each other by eating, by
direct aggressive interference, by changing the envi-
ronment, and by giving rewards for provisions such
as nectar for pollination. In section 6.3.1, we start
by describing a number of such direct interactions
between species, such as consumption and mutualism.
Then, in section 6.3.2, we consider a number of indi-
rect effects that result from these direct physical inter-
actions, such as competition and facilitation. We notice
that in many scientifi c articles, direct interactions
are measured by their indirect effects. We consider it
essential to explicitly distinguish between the two (i.e.
between mechanisms and consequences). While recog-
nizing that the latter are more relevant in ecological
restoration projects, how can we manipulate the con-
sequences without knowing the mechanisms? Finally,
in section 6.3.3, we refl ect on implications of the pre-
sented information for restoration ecology.
Plants
6.3.1
Direct interactions
As mentioned, we distinguish between direct and indi-
rect interactions between species. We briefl y present
basic knowledge of the following direct interactions:
(1) consumption by plants and animals, with particu-
lar attention to interactions between trophic levels,
(2) parasitism, a form of consumer-resource interac-
tions, (3) ecosystem engineering of plant and animal
species, and (4) mutualisms, especially plant-
mycorrhiza and plant - pollinator interactions. This
brief overview provides the basis for our understand-
Herbivores
Carnivores
Primary productivity
Figure 6.2 The equilibrium densities of plants, herbivores
and carnivores as a function of primary productivity. The
relationships are based on the model of Oksanen et al .
(1981) . (Modifi ed from van de Koppel 1997.)
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