Environmental Engineering Reference
In-Depth Information
6.1
INTRODUCTION
vores and predators that perch higher in the food
chain. A seminal paper was that of Lindeman (1942),
who studied fl uxes of energy (derived from the biomass
of different feeding or trophic levels) through the
aquatic food web of Cedar Bog Lake in Minnesota. His
example was followed by a whole generation of ecosys-
tem ecologists, inspired by Eugene P. Odum, among
others (see Golley 1996). Their work highlighted the
structuring effects of energy fl ow in ecosystems, limit-
ing productivity of successive
trophic levels
as progres-
sively less energy is available when moving from
primary producers (e.g. plants and algae) via herbiv-
ores to predators and, fi nally, the top predators. As
every trophic level beyond the fi rst one respires and
discards (as urine and faeces) part of its consumption,
less energy is available for higher trophic levels. As a
consequence, food chains and food webs often have a
pyramidal shape when biomass or energy is expressed
per trophic level.
Ecosystem ecologists soon found that ecosystem pro-
ductivity is determined not only by fl uxes of energy, but
also by nutrient fl ows (see e.g. DeAngelis 1992). All
organisms require and take up a certain proportion of
macronutrients (e.g. nitrogen and phosphorus, or sili-
cate, in the case of diatoms), as well as numerous
minor or oligo-elements. These nutrients are necessary
for organisms to build new tissue and to compensate
for tissue turnover. In many ecosystems, energy and
carbon (a major element of carbohydrates) are not the
limiting factor to the growth of organisms; rather, it is
the limited availability of nutrients for part or all of the
year. A key difference here is that availability of nutri-
ents is not so much determined by input and output
from the ecosystem but rather depends much more
on local sources such as mineralization of nitrogen
through the decomposition of dead plant material. As
a consequence, the pathways followed by these nutri-
ents moving through the ecosystem often appear to be
cyclic, rather than linear, as is generally the case for
energy and carbon.
At the heart of the aforementioned approaches to
ecosystem ecology lies the notion that most natural
ecosystems, most of the time, are 'in balance', thanks
to
resistance
and
resilience
that develop as emerg-
ing attributes of ecological systems over evolutionary
time. In other words, except when an ecosystem is
heavily disturbed by humans or by catastrophic disrup-
tions such as earthquakes or volcanic eruptions, the
system stays in some sort of equilibrium. This implies
that the inputs and outputs of an ecosystem, including
The focus of restoration ecology, and the closely allied
practice of ecological restoration, is the structure, com-
position and functioning of ecosystems in a given land-
scape. Recall that an
ecosystem
encompasses the
interactions between species of a
biotic community
,
and between each of them and the abiotic environ-
ment in which they live. Recall also that a
landscape
is made of an assemblage of interacting systems,
including ecosystems, each with its own community.
In the present chapter, we start by giving a brief history
of successive scientists' views on the subject of ecosys-
tems as an ecological unit. Thereafter we present a
number of examples of direct and indirect interactions
between and among species within a biotic commu-
nity, which in turn affect community structure and, at
a higher level still, whole-ecosystem functioning. Then,
we consider complexity in species interactions as a
variable to be considered for those studying or attempt-
ing to carry out ecosystem restoration in a world of
environmental change. At the end of each section, we
indicate the impact that our exposé may or should have
on the further development of the science of
restora-
tion ecology
. We conclude by refl ecting on the desired
'attributes of restored ecosystems' as given by
SER
Primer on Ecological Restoration
(SER 2004), and on the
interface between theory and practice in general.
6.2
ECOSYSTEMS
The views that ecologists have of ecosystems have
developed and evolved enormously since the term was
fi rst introduced, 75 years ago, by A.G. Tansley (1935).
Among other things, it changed as a result of the way
that successive generations of ecologists, anthropolo-
gists, and archaeologists considered so - called ' natural '
systems. Until recently, a 'balance of nature' paradigm
prevailed, so let's begin our survey there.
6.2.1 Ecosystems in a supposedly
'balanced' world
The fi eld of ecosystem ecology traditionally studies
fl uxes of energy and matter through and between eco-
systems. The fi rst studies on the functioning of ecosys-
tems focused on the transfer of energy, and how energy
fl uxes determine productivity of plants, and the herbi-
