Environmental Engineering Reference
In-Depth Information
species were introduced through human activity. To date, many hypotheses have been
proposed to explain the invasion success of exotic species. Some of the species invasion
hypotheses include: the natural enemy hypothesis, the evolution of increased competitive
ability hypothesis and the novel weapon hypothesis. These hypotheses emphasize the effect
of the absence of natural enemies; increased competitive ability in the absence of natural
predators due the ability to allocate more resources to growth and/or fecundity and
biochemical interactions between native and invasive species, respectively (Callaway &
Ridenour 2004). Species facilitation has also been reported as an explanation for successful
invasion of exotic species. Maron & Connors (1996) demonstrated facilitated invasion of
exotic weedy plants into disturbed Californian coastal prairies through soil enrichment by the
nitrogen-fixing shrub Lupinus arboreus . Cavieres et al. (2008) found nurse cushion species
( Azorella monantha ), which is native to the high Andes of central Chile, also exhibited a
facilitative effect specific to the invasive species, Taraxacum officinale . Tecco et al. (2006)
revealed that the invasive species, Pyracantha angustifolia facilitated recruitment of both the
native species, Condalia montana and the exotic species, Ligustrum lucidum.
In coastal wetlands, species invasions have been linked with human disturbances and
eutrophication. Few studies have linked species facilitation with invasion. Battagia et al.
(2009) reported that the native, Morella cerifera facilitated seed spread and seedling
establishment of the invasive, Triadica sebifera in a floating marsh , but impeded its further
growth. Cushman et al. (2011) found native plant species protected the exotic plant, Ehrharta
calycina from herbivory by black-tailed jackrabbits in a coastal foredune system of northern
California. Thus, it is important to further our understanding of the positive and negative
interactions between invasive species and native species in coastal wetland plant communities
to improve the management of coastal wetlands.
S PECIES F ACILITATION AND E COSYSTEM F UNCTION
IN C OASTAL W ETLAND
Productivity of plant communities is an important ecosystem function of coastal wetland
which indicates ecosystem health. Here we have focused on predictions of how species
facilitation may influence productivity in coastal wetland plant communities. However, to the
best of our knowledge, no studies linking species facilitation with productivity in coastal
wetland plant communities exist. Thus, we propose a conceptual model to explain the
relationship between species facilitation and coastal wetland community productivity based
on studies of other ecosystems (Figure 2). Our model proposes that direct interspecies
facilitation can increase productivity of the plant community through diversity functions ( e.g .,
species diversity or phylogenetic diversity; Valient-Banuet et al. 2006; Maestre et al. 2010).
These functions include niche complementarity (maximizing resource usage among species
with different functional traits), sampling effects (increasing the probability of including
dominant species with greater productivity in more diverse communities) or species
facilitation (increasing survival of species and thus increasing numbers of individual plants)
under a particular environmental stress.
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