Environmental Engineering Reference
In-Depth Information
of environmental stresses (or crossed stresses) and species with different life-history traits and
stages into account will fill the gap in studies of direct species facilitations.
S
PECIES
F
ACILITATION AND
P
LANT
Z
ONATION
IN
C
OASTAL
W
ETLANDS
The arrangement of plant communities in coastal wetlands exhibit zonation gradients
from the seaside inland or towards estuarine marshes. Species competition, abiotic stress and
herbivory are regarded as the regulators of communities across these gradients (Pennings &
Bertness 2001; Crain et al. 2004). Species facilitation is anticipated to extend the ecological
realized niche of species, and thus expand species distribution or provided refuge for rare
species of the community (Bruno et al. 2003). Bertness & Hacker (1994) revealed that
J.
gerardi
facilitated the extension of
I. frutescens
distribution into a more stressful environment
in New England salt marshes. Hence, interspecific facilitation clearly extends the habitable
zones of some species; meanwhile the intraspecific facilitation extends and regulates species
zonation.
S
PECIES
F
ACILITATION AND
D
IVERSITY
P
ATTERNS IN
C
OASTAL
W
ETLANDS
The intermediate disturbance hypothesis proposes that the relationship between species
richness and the stress gradient is a humped-back shape (Sousa 1979) and the stress tolerance
ability of species and degree of competition faced by species are the most important
underlining processes of species richness patterns in plant communities (Shea et al. 2004). On
the other hand, Hacker & Gaines (1997) put forward a conceptual model including species
facilitation to explain species richness patterns. This model proposed that species facilitation
increases species numbers in intermediate, high stress environments and that stress tolerant
species are released from intensive species competition in intermediate stress environments
which increases species richness.
Recently, Michalet et al. (2006) proposed a new concept model to explain species
richness patterns along stress gradients. The common point of the Hacker & Graines (1997)
and Michalet et al. (2006) models is that species facilitation could increase species diversity
when facing harsh environmental stress (i.e., biotic and abiotic stress). This point is supported
by many experiments. In northern New England salt marshes,
Triglochin maritime
surrounded themselves with elevated rings that ameliorated waterlogging stress and harbored
more species than neighboring bare spaces (Fogel et al. 2004). However, Michalet et al.
(2006) pointed out that species facilitation cannot extend species distribution into very high
stress environments, because above a threshold level of stress, the effect of species facilitation
would gradually reduce with increasing stress levels and vanish at very high stress levels.
They also proposed that the role of species facilitation on species richness patterns along
stress gradients depends on the life-history strategy of both the benefactor and beneficiary.
The Michalet et al. (2006) conceptual model, predicted that the impact of species facilitation
