Environmental Engineering Reference
In-Depth Information
Gomulkiewicz 1997 ; Bridle & Vines 2007 ). In situa-
tions with too little gene fl ow, small increases can play
an important role in both rescuing populations from
extinction, and introducing novel genes that may be
adaptive in changing environments (Sexton
et al
.
2009). While experimental evidence on the role of
gene fl ow on the success of restorations is lacking,
there are examples in natural systems that suggest
that gene fl ow can play a role in the ability of popula-
tions to evolve in response to novel pressures (Kawecki
2008). Population history may play a role in determin-
ing the optimal level of gene fl ow to maintain viable
and adaptable restored populations. Species with his-
torically large, interconnected ranges that have
recently been fragmented might respond positively to
increased gene fl ow, but caution may be required
when manipulating gene fl ow among populations that
have a history of isolation and disjunct distributions
(Edmands 2007 ).
a matrix of biotically degraded but abiotically similar
habitats (Prevedello & Vieira 2010). For instance, Mat-
thews
et al
. (2009b) found that native species diversity
decreased in restored wetlands closer to urbanization,
which they suggested was due to increased seed disper-
sal of invasive species from the urbanized land to the
restorations. In these situations, invasive propagule
pressure (seed arrival) can overwhelm local interac-
tions and make long-term success of the restoration
project diffi cult to achieve (DiVittorio
et al
. 2007 ; Rein-
hardt & Galatowitsch 2008). The latter authors found
that wetland restoration was successful when both the
propagule pressure of the invasive is minimized and
native species seeds are added - neither management
technique on their own was successful in establishing
a native wetland community. Controlling incoming
seeds from invasive species is diffi cult if the restoration
area is surrounded by invaded habitat. In these cases,
the scope of the restoration project would need to be
enlarged to address the surrounding matrix or a long-
term sustained commitment to invasive species control
would be required.
21.4.2
Dispersal
In highly
fragmented
landscapes (see Chapter 5),
suitable habitats for native species are often far apart
from each other and small in size. This again presents
the case where connectivity is a double-edged sword: it
is good for
native species
persistence but increases
invasions
from the surrounding matrix. Species abun-
dance is often found to be limited by the amount of
seeds that arrive in an area; if more seeds of that
species were to arrive, more individuals would recruit
and the species would be more abundant (Clark
et al
.
2007). In addition, the degree to which abundances
are depressed due to lack of seeds often relates to the
extent of landscape dispersal barriers (Seabloom
et al
.
2003). Seed addition in restoration projects can
address these barriers, although it often differs from
natural dispersal in that it occurs only in the initial
stages of restoration. Continued seed input year after
year is advantageous because recruitment events often
depend on specifi c weather conditions; interannual
variation in recruitment can enhance both genetic and
species diversity (Wright
et al
. 2005). In addition, new
species or genotype arrival via seed addition may be
necessary for the system to track environmental
change (Honnay
et al
. 2002 ).
The type of matrix surrounding habitat patches may
also be important to restoration trajectories, particu-
larly in cases where the restoration project is located in
21.5
MAINTAINING BIODIVERSITY
Levels of biodiversity (genetic, species and functional
diversity) within an ecosystem will undoubtedly be
important in infl uencing the system's sensitivity to
change. Diversity of species and functional groups has
been shown to infl uence a range of biogeochemical
processes, trophic interactions and resistance to bio-
logical invasions (Hillebrand & Matthiessen 2009).
Plant genotypic diversity has also been found to have
similar effects (Hughes
et al
. 2008 ). While empirical
results are somewhat mixed, species diversity may
regulate temporal variability
ecosystem processes
(e.g. productivity and nutrient cycling; Baez & Collins
2008 ; Isbell
et al
. 2009), making the system more
stable. Often increased stability at the ecosystem level
due to species diversity is accompanied with increased
variability at the population level (e.g. particular
species density and biomass), termed 'compensatory
dynamics ' (Gonzalez & Loreau 2009 ). Thus, restora-
tion projects with ecosystem service goals may benefi t
from increased species diversity to keep ecosystem
function stable, while restoration projects with more
species - specifi c goals may benefi t from high genetic
diversity of the target species to maintain desired
species richness.
