Document: JAN-3-80-8

Spartina alterniflora: Tracking and predicting the spread of an intertidal invasive in Willapa Bay.

CIVILLE, J.C.*

University of California Davis CA 95616 USA ¹

Abstract:
Spartina alterniflora Loisel., (smooth cordgrass), is a valuable native component of Atlantic and Gulf Coast estuaries. It was likely introduced into Willapa Bay, a Pacific Coast estuary, near the turn of the twentieth century. The initial colonies remained at low levels for decades before gaining the attention of local oyster growers, biologists and wildlife refuge managers in the 1970s; subsequent explosive growth and loss of migratory shorebird feeding grounds has lead to its designation as a noxious plant and environmental emergency by the Washington State legislature. A textbook example of clonal growth, S. alterniflora forms visually striking arrays of circular patches merging into meadows on otherwise open tideflats. Despite the ease with which the human eye detects the progressive spread of the invasion, it is difficult to assess the distribution and spread rate of cordgrass on an ecosystem level. This can largely be attributed to the Bay's size and the nature of the invaded habitat: 360 km2 at mean high tide, with over 190 km² of soft mud tideflats exposed at mean low tide. Incorporating demographic data into Integrated Pest Management (IPM) strategies at ecosystem levels is necessary to effectively direct control and eradication efforts. Low altitude color infra-red photographs (1:6000) were acquired along standardized flight lines in 1994 and 1997. These were digitally scanned, then georeferenced with digital orthophotos and imported into image analysis and a geographic information system for analysis. The 1994 and 1997 data indicate that nearly 13 km² of S. alterniflora is spread over 53 km² of tideflats, increasing by as much as 300 and 400 percent in shallow areas of the Bay. As part of a recent initiative to incorporate biological control into cordgrass IPM, this analysis of spatial distribution and growth trends will be used to determine the most appropriate control methods for different ages and locations of cordgrass populations.

Keywords: Spartina alterniflora, cordgrass, salt marsh, biological invasions, noxious species, invasive species, remote sensing, trend analysis, Willapa Bay, Washington State

This abstract is being presented at: 1:30 PM in session:
Oral Session #65: Wetlands, Estuaries and Salt Marshes.