Environmental Engineering Reference
In-Depth Information
21.3.2 Thresholds and more
dramatic shifts
effects by their choice of what species to establish
fi rst, the preparation of the soil or the introduction
of soil mycorrhizae or other organisms (MacDougall
et al
. 2008). It is important to consider how restora-
tion actions may infl uence the future dynamics of
the system, either by their success in ameliorating
unwanted past effects or by introducing new effects in
the restoration process. For instance, inoculation with
a single generalist mycorrhizae taxon may help initial
establishment of the plant component in a restoration,
but may not function well in times of stress and/or may
disproportionately benefi t one or two species in the res-
toration mix (Hoeksema
et al
. 2010 ). Alternatively,
Kulmatiski
et al
. (2006b) found that
exotic species
were associated with strong soil history effects in aban-
doned agricultural fi elds in the United States, with evi-
dence indicating that they were able to facilitate their
own growth by maintaining benefi cial microbial
populations and nutrient-cycling rates in these soils.
Thus, in cases such as these, restoration addressing
exotic species removal also needs to address soil micro-
bial constraints (i.e. through topsoil amendments or
removal).
Disturbances or shifts in environmental conditions can
also cause sudden and dramatic shifts in the internal
organization of ecosystems that may cause long-term
divergence from an intended restoration trajectory
and, potentially, a transition to an
alternative state
(Folke
et al
. 2004; Chapter 6). It is important to recog-
nize that trajectories can be nonlinear and sometimes
exhibit
thresholds
, where a small change in the envi-
ronment can cause a large change in structure or func-
tion. In a system that has crossed a threshold, the
factors that are important for its internal dynamics (i.e.
species interactions, abiotic limitations and connectiv-
ity) shift and may fundamentally change. Following
the shift, restoration that proceeds to re-establish base-
line disturbance regimes or composition without
taking into account these shifts in internal dynamics
may prove largely unsuccessful (Suding
et al
. 2004 ).
While it has proven diffi cult to verify alternative states
in natural systems, particularly in the degraded
systems most focused on in restoration, these concepts
have proven useful heuristically (Suding & Hobbs
2009). For instance, Firn
et al
. (2010) studied a system
in Australia where increased ungulate grazing facili-
tated the invasion of a problematic grass species. They
found that applying management strategies that take
into consideration the current dynamics of the novel
system, in this case maintaining grazing and increas-
ing the palatability of the invader via fertilization, were
more successful than attempting to re-establish the
historic baseline conditions (reduced grazing, reduced
nitrogen and removal of the invasive).
The presence of positive
feedbacks
- when changes
are amplifi ed by ecological or environmental processes
- is one mechanism that causes divergent trajectories
in a system (Suding
et al
. 2004). This type of feedback
is often described as founder, priority or legacy effects.
These effects could stem from interactions with other
species (e.g. being the fi rst to colonize an area), interac-
tions with mutualists or pathogens (e.g. supporting
benefi cial or harmful soil organisms) or interactions
with the abiotic environment (e.g. ameliorating harsh
environmental conditions) (Corbin
et al
. 2004 ). Resto-
ration projects often have to address existing legacies
in a degraded site - be it high propagule pressure of
exotic species, changed soil processes or changes in the
availability of mutualists, such as mycorrhizal fungi.
Restoration efforts can create new founder or legacy
21.3.3
Transient dynamics
It is additionally important to realize the importance of
transient dynamics in restoration - when disturbances
shift dynamics away from steady state, and result in
population dynamics that are either amplifi ed or atten-
uated in the short term (Stott
et al
. 2010 ). Legacy and
priority effects can also be transient, and not necessar-
ily result in long-term divergence to different end states
(Collinge & Ray 2009). While population sizes during
these periods may be used to determine restoration
success or further management actions, they do not
necessarily indicate the long-term status of the popula-
tion or assemblage (Wiedenmann
et al
. 2009 ). For
instance, van Katwijk
et al
. (2010) demonstrated the
importance of considering transient dynamics in the
management of eelgrass (
Zostera marina
) populations.
While the site was highly eutrophied for several
decades, comparisons in the early 1990s showed
strong population growth - nearly a doubling in popu-
lation size. Over the next decade, however, population
abundance declined until extinction in 2004. Further
studies indicated that microalgae cover was associated
with eelgrass mortality prior to seedset. Thus, while
the eutrophied location had maximal germination and
