Environmental Engineering Reference
In-Depth Information
in coastal wetland studies. Additionally, many studies on species facilitation are too short
term to achieve equilibrium in species interactions, thus longer observations are necessary.
T
ESTS OF THE
S
TRESS
G
RADIENT
H
YPOTHESIS
IN
C
OASTAL
W
ETLANDS
The stress gradient hypothesis (SGH) was put forward by Bertness & Callaway (1994) at
a time when ecologists did not appreciate the role of facilitation in communities. The SGH
was based on the species competition model of Grime (1979), which proposes that the
importance of species competition decreases along the stress gradient as productivity
decreases or abiotic stress increases. Bertness & Callaway's (1994) SGH expands Grime's
model to incorporate species facilitation. The SGH predicts that the frequency of facilitative
and competitive interactions between species will vary inversely to each other across abiotic
or biotic stress gradients and that species facilitation is more likely to occur in high stress
environments. Nevertheless, many of the experiments testing the SGH have examined the
strength, but not the frequency of species competition and facilitation along abiotic stress
gradients (Maestre et al. 2009). Maestre et al. (2005) employed a meta-analysis to investigate
tests of the SGH in plant communities of arid environments and found that the SGH was not a
general rule in arid plant communities, although Lortie & Callaway (2006) disapproved this
argument and re-analyzed the data and supported the SGH. Thus, for some specific studies,
the interactions may depend on the types of abiotic stress gradients and measurements of
plant performance. Hence, the predictions of the SGH were refined according to life history
traits (i.e., competitive species or stress-tolerant species; He et al. 2012) and stages (i.e.,
sapling, seedling, juvenile or adult) of interacting species, types of abiotic stress gradients
(i.e., resource or non-resource) and biotic stresses (Maestre et al. 2009; Smit et al. 2009).
Their updates improved the SGH, and may provide a way to explain variations in results
across various ecosystems.
For coastal wetlands, support or not for the SGH depends on spatial scale. Studies on a
local or landscape scale commonly support the SGH, but studies across a geographic scale do
not support the SGH. Alberti et al. (2008) found that crab herbivory stress regulated species
interactions between plants in Argentinean marshes. Competitive interactions dominated
when crab herbivory stress was low in the spring and summer, but facilitative interactions
dominated when crab herbivory stress was high in the fall. On a landscape-scale, Crain's
(2008) work on species interactions in the presence of consumers in oligohaline, brackish and
salt marshes also provided evidence supporting the SGH. On a geographic scale, 3 studies
that investigated the same species, but in high latitude, low salinity Maine marshlands
(Ewanchuk & Bertness 2004), intermediate latitude and salinity Rhode Island marshlands
(Bertness & Ewanchuk 2002) or low latitude, high salinity Georgia and Alabama marshlands
(Pennings et al. 2003) found conflicting results. Species facilitation by neighboring plants
was rare in the Maine, Georgia and Alabama salt marshes, but common in the Rhode Island
marshlands. However, many of the experiments testing the SGH in coastal wetlands only take
a few dominant species into account; few experiments in any ecosystem have investigated the
SGH with whole community-level dynamic data (Goldberg et al. 1999; Rajaniemi et al.
2009). Thus, testing the SGH at the community-level in coastal wetlands while taking types
