Effects of introduced species on delta functions

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Control of introduced species (e.g. Spartina, Typha, Lythria) is feasible and necessary to prevent loss of critical habitat services like rearing for estuarine dependent fish.

Process based delta restoration typically avoids funding for seasonal introduced species control, which is often based on limited assessment of the scope of the issue, and doesn’t define a clear end point or goal for management. The problem of new species invasion is commonly described as the extent of invasion, and is rarely linked to the loss of a specific service. It was anticipated that Spartina invasion will, if not managed, convert a substantial portion of tidal flats to marsh, resulting in a significant change in the nearshore landscape. Perennial pepperweed has increased its distribution in the Puyallup Estuary. Other freshwater species are widely distributed, with Reed canarygrass a common understory of even relatively undisturbed freshwater wetland vegetation. Delta specific targets for introduced species control would ideally be based on a reasoned analysis of what changes are acceptable, and what changes are unacceptable, while considering also the level of persistence of resources necessary to affect the trajectory of new species invasion.

Delta Strategy Analysis

ESRPDeltaStrategy.PNG

The uncertainties in how this topic affects delta restoration has resulted in its inclusion in the ESRP River Delta Adaptive Management Strategy. This three criteria analysis should build off the analysis above, and supports development of learning projects.

Importance Viability Policy Relevance

The potential future change in vegetation by introduces species has not been predicted. The effects of localized or landscape spread of these species has not been studied. Modification of vegetation may have some effect on biota.

Defining a tolerable level of change from new species introductions will be difficult, and it is unclear that some quantification of harm or lack thereof is the limiting factor in decision making.

Management of introduced species could become a significant consumer of the limited resources available for ecosystem management, leading to the frequent use of toxins or introduction of additional species to control problem species. Developing a body of evidence to clarify the costs and benefits of introduces species control is useful for defining long term restoration targets.