Environmental Engineering Reference
In-Depth Information
This paper raised a number of important research
topics, and pushes us to go beyond simply demon-
strating that biodiversity can affect ecosystem func-
tioning. Specifically, it suggests that - in some cases
- biodiversity may be insufficient as a reason to jus-
tify the expense of restoration, at least from a purely
engineering or socio-economic point of view. But, after
having recognized this point of view, we need to com-
ment on it. Firstly, productivity, frequently used as an
estimate of ecosystem functioning, may be considered
as an agricultural or silvicultural rather than an eco-
logical parameter. Secondly, as we emphasized in
the previous section, there are other very important,
compelling reasons to conserve and, where possible,
to restore biodiversity. Indeed, current biodiversity
represents the evolutionary potential of the future, and
in this way it at least affects all ecosystems' long-term
functioning, except perhaps those that are completely
designed and engineered. We therefore suggest focus-
ing on the question of how restoration of ecosystem
functioning can contribute to the restoration of bio-
diversity rather than putting it the other way around
(see Fig. 5.1 in this volume; van Andel 1998a).
Scientific challenges in this area abound and much
remains to be elucidated through careful research.
erally embrace a flux-of-nature paradigm and accept
the reality of many possible steady states, basins of
attractions and above all multiple trajectories that
ecosystems can experience or exhibit in the course
of their development (see Suding
et al.
2004). In that
context, the model of adaptive cycles (Holling 1973)
is a useful alternative metaphor for how ecosystems
develop (see below). Nevertheless, this model or
metaphor of ecosystem development should by no
means be taken by the reader as being definitive
or all-inclusive. In the field it is often hard to see its
application and faced with the myriad forms of living
systems that exist we need an abundance of modelling
approaches (e.g. Ludwig & Tongway 1995, Tongway
& Ludwig 2002). These models of course represent just
one conceptual approach, but they have considerable
power for generating new hypotheses and applications
to conservation, management and restoration. Despite
loss of biodiversity at genetic and species levels, and
despite rampant homogenization of cultures, and the
erasure of former landscape boundaries and transition
zones, the diversity of ecological systems may actu-
ally be increasing at present, rather than decreasing.
But the result may not be entirely to our satisfaction,
or to that of future human generations.
The topic of assembly rules calls for renewed popu-
lation and, especially, community ecology research to
deepen understanding of the genetic, population and
community dynamics within different types of eco-
system: plants, animals and all the rest. A recent topic
(Temperton
et al.
2004) and a conceptual paper
(Belyea & Lancaster 1999) should help the reader
gain an overview of the field. The issue to be con-
sidered here is, to what extent assembly rules that
may have shaped communities in the past can still
be usefully applied to predict the development of
future, newly emerging communities. One key issue
to mention here is that of so-called functional groups
of organisms and the extent to which they can be
used in predictive or retroactive analysis and piloting
(or reconstruction) of communities and ecosystems
regarded as damaged, disturbed or destroyed by
human activities (McIntyre & Lavorel 2001, Gondard
et al.
2003, Pausas
et al.
2003). How does succession
take place, after all, and what are the adaptive cycles,
if any, and the feedback systems, assembly rules and
other inherent functional, evolutionary or simply
dynamic mechanisms that make ecosystems develop
16.2.2 Assembly rules and the
structuring of ecosystems
Restoration of ecosystems is, as was indicated in
Chapter 2 (see Fig. 2.2) a pathway towards some desired
ecosystem, rather than the immediate re-establishment
of such a system. Rehabilitation has much more the
character of a discovery, a guided designer process that
can work out even without a target set. The notion
of assembly rules (see Chapter 5) suggests that the struc-
turing of developing ecosystems may be subjected
to certain general laws of species interactions and
succession. This notion is obviously relevant to the
design
of ecosystems as much as to their restoration
or rehabilitation. The important point, for ecologists,
is to determine to what extent pre-existing eco-
systems are - or can be - taken as models or refer-
ences for the reparative work or redesigning work at
hand. More discussion on this will be provided below.
Contrary to the Clementsian model of succession
leading to some sort of 'climax', ecologists now gen-
