The Functional Evaluation on S Alterniflora Beneficial

In its native habitat, S. alterniflora is valued. The species is highly productive, exporting approximately 1300 gm-2 of detritus annually to the estuarine system (Edwards & Mills,2005). Moreover, S. alterniflora is highly regarded for erosion control, as well as fish and wildlife values in its native range (Simenstad & Thom, 1995). In the east coast of America, some waterfowl and wetland mammals eat the roots and shoots of this plant. In addition, stands of S. alterniflora can serve as a nursery area for mangroves, and estuarine fishes and shellfishes.

There are also some economically beneficial uses for S. alterniflora. The species is palatable to livestock, so the plant's continued spread may increase available pasture. Efforts have also been made to use S. alterniflora in paper production (Qin & Chung, 1992; Qin et al., 1997).

As an "ecological engineer", S. alterniflora colonizes on the severe habitat as pioneer species of community succession and transforms open mudflats into densely vegetated marshes (Travis et al., 2002, 2004, 2005). Consequently, coast is stabilized and erosion is controlled. In addition, S. alterniflora plantation could be a countermeasure against disasters of sea level rise with the siltation and salt marsh elevation, as well as the methods to produce lands for the tidelands cold been reclaimed. Furthermore, the function of S. alterniflora have been explored abundantly in China, such as soil amelioration, pollution control, animal fodder, green manure, fish feed, primary producer of detritus food webs, food additive, etc. (Chung, 2006).

4.1.2 Negative Impacts

Firstly, invasion and expansion of S. alterniflora results a series of negative ecological impacts. S. alternifolra may displace native plants, such as Zostera marina (seagrass) at lower elevations, which provide important refuge and food sources for fish, crabs, waterfowl, and other marine life (McMillan et al., 1995), and Salicornia virginica, Triglochin maritimum, Jaumea carnosa, and Fucus distichus at higher elevations (Simenstad and Thom, 1995). As S. alterniflora population expanding, shorebirds and waterfowl will lose potentially important foraging and refuge habitat. In the Willapa National Wildlife Refuge, S. alterniflora has displaced an estimated 16 to 20 percent of critical habitat for wintering and breeding aquatic birds (Foss, 1992). At the same time, bottom-dwelling invertebrate communities of mudflats will be replaced by salt marsh species. Benthic fauna of S. alterniflora meadow is generally depleted in comparison to nearby mudflats, and its invertebrate opulations in the sediments are smaller than populations in intertidal mudflats (He et al., 2007). Juvenile chum and Chinook salmon may lose prey resources and other important attributes of mudflat nurseries (Simenstad & Thom, 1995). In short, mud- and sandflat communities based on bottom-dwelling microalgae will decline, being replaced by food webs driven by the supply of Spartina detritus (Simenstad and Thom, 1995).

Secondly, some of the very traits that make S. alterniflora valued are also the greatest causes for concern. S. alterniflora dense and tall stems and leaves slow tidal water, thus trapping sediment. As a result, S. alterniflora causes tidelands to raise more than they would if they were unvegetated or vegetated by other species (Thompson 1991; Chung et al., 2004). Sedimentation rate was up to 260 mm/year in China (Chung et al., 2004). The sedimentation with such rate alters the topographic and hydrographic characteristics of coast, i.e. before Spartina colonization, foreshores or estuaries consisted primarily of bare, gently-sloping mudflats with shallow tidal channels, Spartina replaces gradually sloping mudflats with badly drained marshes that commonly have steeply sloping seaward edges and deep, steep-sided channels (Gray et al., 1991). In addition, infestations of Spartina may block some navigational channels. The large and dense populations at or in river mouths may cause particular problems by decreasing flow and leading to increased flooding, especially during periods of heavy precipitation and/or above normal tides.

Thirdly, the expansion of S. alterniflora also produces some detrimentally economical effects. The habitats of many native animal species, including some endangered birds and economic mollusks, such as Bullacta exarata, were threatened by S. alterniflora invasions (Luiting et al., 1997). In addition to wildlife impacts, S. alterniflora poses threats to a multimillion dollar shellfish industry in a local economy (Grevstad et al., 2003).

Lastly, changes associated with S. alterniflora also impact recreation. Loss of beach habitat and navigation routes, reduced water access, and other alterations to the estuarine ecosystem may result from the spread of S. alterniflora. Therefore, activities, such as fishing, hunting, boating, bird watching, botanizing, and shellfish harvesting, that are dependent on the extant intertidal ecosystem could be negatively impacted by the continued spread of Spartina (Ebasco Environmental, 1993).

In general, the functions of S. alterniflora should be objectively evaluated according to the local hydrographic and geographic characteristics. Just as to understand the different east/west perspectives on the values of Spartina, it is important to recognize that Atlantic/Gulf coast estuaries are fundamentally different than their Pacific Coast counterparts. The Pacific Coast is macrotidal, while the Atlantic/Gulf coasts are microtidal (Simenstad & Thom, 1995). In addition, the Pacific Coast is more geologically active, and tectonic activity has a much greater influence on coastal marsh systems. Subsidence due to compaction of marsh soils that results from insufficient sediment supply is less important on the Pacific Coast (Thom, 1992). Finally, on the East Coast, the prevailing wisdom is that salt marshes are the key to productivity of estuarine systems because of the contribution of their detritus. However, on the West Coast, secondary productivity from tidal mudflats is more important than detritus exported from salt marshes. The sustainable development of China is severely limited by the inconsistency of supply-demand for land, and the reclamation of tideland can effectively mitigate this condition. The main functions of S. alterniflora are coastal stabilization, erosion control and reclamation. Thus, beneficial function of S. alterniflora is larger than its negative impacts from the view of national macroscopical economics. On the standpoint of ecology and local economic development, the invasion of S. alterniflora is a nuisance rather than a friend.

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