Environmental Engineering Reference
In-Depth Information
and interact in one way or another? If we can sort
these questions out - biome by biome - then we will
unquestionably be better placed to predict how much
time, energy and capital of all sorts will be required,
or should be allocated, to ecological restoration and
rehabilitation, for reasons 1, 2 and/or 3 given in sec-
tion 16.1. Functional groups have also shown a clear
usefulness in the sticky problem of scaling from
species to vegetation, and higher levels of com-
plexity (Körner 1993, Lavorel & Garnier 2001, Pywell
et al.
2003).
corridors between the patches may be an important
restoration option or programme element, but this
approach may not respond or compensate entirely for
a species characteristic of intermediate or frontier
zones along environmental gradients. Moreover, the
usefulness for species migration among patches
largely depends on the species of plant or animal (see
Chapters 3 and 6).
16.2.4 Ecosystem resilience
Resilience has been defined as the time taken to
return to the steady state of a stable ecosystem after
a temporarily disturbing event (Holling 1973; see also
Fig. 2.3). This is clearly a crucial and very complex
issue as the innate complexity and adaptive nature
of ecosystems must nowadays cope with increasingly
complex multiscalar disturbances all around the globe.
What happens to an ecosystem's developmental trajec-
tory when species disappear locally, and/or new ones
appear and persist (Pahl-Wostl & Ulanowicz 1993, Pahl-
Wostl 1995)? Faced with these or other environ-
mental challenges, does the ecological system show
sufficient adaptive capacity or resilience (Gunderson
2000, Carpenter
et al.
2001, Walker
et al.
2002) for
it to remain recognizable as a system or, rather, does
it somehow mutate, so to speak, or shift to a new
domain of attraction? Can we recognize, design or
choose among alternative ecosystem trajectories and
feedback systems (Odum
et al.
1979, Suding
et al.
2004)? What are the risks of ecological threshold
crossings (Knoop & Walker 1985, Hobbs & Norton
1996)? What are the opportunities that adaptive eco-
systems can exploit? How many alternative states can
a given ecosystem move among in a given configura-
tion, and what is its resistance to the crossing of an
apparently irreversible threshold which pushes it
into a new configuration of alternative states, in a new
basin of attraction? If an ecological system exhibits
resilience, as defined by Holling (1973), what are its
limits and response times to various influences such
as those described in the three previous sections? How
do these features vary from one system to another?
To what extent does the position of a system on a
transformation (or degradation) gradient affect its
resilience and resistance? To what extent can we
identify threshold crossings in the past, or predict them
16.2.3 Ecotones, ecoclines and landscape
boundaries
From this point, we shall leave the subject of suc-
cessional pathways aside and focus instead on the
final or targeted ecosystems intended to be restored,
rehabilitated or designed, and the landscapes in which
they occur. Ecosystems are embedded in landscapes
and they depend largely on processes at work at the
regional scale. In much of Europe, and elsewhere, eco-
logical transition zones have been deeply modified by
human actions (Correll 1991). Human impact on the
functioning of landscape boundaries has been profound
and widespread, and yet at present those bound-
aries appear to be changing quickly as a result of rural
exodus, agricultural abandonment and the rapidly
growing cities and their complex urban/rural inter-
faces, among other things.
Ecotones and landscape boundaries can be diffuse
or sharp (Wiens
et al.
1985, Holland
et al.
1991). Van
der Maarel (1990) distinguished between ecotones
(sharp boundaries with strong environmental fluc-
tuations) and ecoclines (gradual differences in at least
one major environmental factor, fluctuating relatively
little), and stated that ecoclines are the most interest-
ing in terms of species diversity and species rarity.
Can notions of system resilience be applied here,
as some researchers in the new field of biocomplex-
ity maintain? How do the fractal properties of such
zones, and landscapes in general, affect species and
community dynamics (Palmer 1992, Ritchie & Olff
1999b), and how should they influence restoration
efforts? In the case of ecosystems occurring as patches
in a cultivated landscape, where there are boundaries
rather than transition zones, the establishment of
