Environmental Engineering Reference
In-Depth Information
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Figure 21.3
A hierarchical response framework suggested
by Smith
et al
. (2009) to illustrate how different processes
can infl uence the response of an ecosystem over time to
directional environmental change. They distinguish time
frames of (a) individual-level change, (b) reordering of
resident species and (c) species immigration. In addition, the
ecosystem response could be (d) relatively slow in
communities with very long-lived species with slow turnover
rates and (e) relatively rapid in communities easily invaded
by exotic species.
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(c)
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Disturbances
can move the composition of the com-
munity forwards or backwards along this continuum
(Parker 1997). While succession models do encompass
dynamics that do not assume a predictable temporal
trajectory, such as arrested succession (Lichter 2000;
Acacio
et al
. 2007), evaluation of restoration trajecto-
ries is often based on the assumption of smooth turn-
over over time followed by an eventual arrival at a
stable 'climax' level that is characteristic of a natural
reference ecosystem
(Matthews
et al
. 2009a ). This
assumption has been criticized based on empirical evi-
dence (Zedler & Callaway 1999), but it is still widely
utilized as a way to gauge success in restoration. In a
synthesis of wetland restoration projects, Matthews
et al
. (2009a) found little support for the assumption
of simple predictable restoration
trajectories
: differ-
ent indicators of restoration progress showed different
trajectories over time; not all
indicators
of restoration
progress showed an increasing trajectory; and in some
cases recovery took much longer than the time frame
on which
mitigation wetlands
are typically moni-
tored (Figure 21.4). The authors argue for the need to
compare restored sites to a naturally variable set of
reference sites in order to take into account that mul-
tiple restoration trajectories are possible. They also
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Time since wetland restoration (year)
Figure 21.4
Restoration trajectories in Illinois, United
States, wetlands, illustrating how different indicators can
indicate different restoration dynamics as well as the
importance of reference sites. Values are expressed relative
to reference wetlands. Solid lines and symbols denote one set
of reference sites, dotted lines another set of reference sites.
Squares represent values in herbaceous wetlands, and
circles represent values in forested wetlands. C (coeffi cient
of conservatism) is an index based on conservation
rankings of the native species where a higher value
indicates more conservation-valuable native species. From
Matthews
et al
. (2009a) .
point to the mismatch between technocratic standards
of mitigation and ecological dynamics due to the unre-
alistic policy assumption of simple, rapid and predict-
able restoration trajectories.
