A biotic agent promotes large‐scale catastrophic change in the coastal marshes of Hudson Bay

Abstract

SummaryHerbivores may initiate small changes to plant–soil systems that trigger positive feedbacks leading to rapid catastrophic shifts in vegetative states, including irreversible changes in soil properties. In the coastal marshes of Hudson and James bays, foraging by increasing numbers of lesser snow geese (Chen caerulescens caerulescensA.O.U.) has led to loss of vegetation, and exposure and partial erosion of sediment.Multi‐temporal analysis of LANDSAT data has been carried out to detect vegetation change from 1973 to 1999 or later at nine sites in the coastal marshes of these bays where staging and/or breeding geese are present annually.Images were co‐registered, and for each image NDVI (Normalized Differential Vegetation Index) channels were generated. For each location, pairwise normalized differences were calculated between these NDVI images for each successive period defined by the imagery acquisition dates. The resulting secondary NDVI difference images expressed changes in NDVI values for each time interval and yielded three well‐defined classes: water, vegetation decline and no detectable change in vegetation.At the nine widely separated study sites, the intertidal saltmarsh (an ecological sere) has been lost (to a total of 35 000 ha) and an alternative stable state (exposed sediment) established. Similar changes have occurred elsewhere along the 2000‐km coastline where the geese breed or stage.Re‐vegetation of these coastal marshes will take decades because of near‐irreversible changes in soil properties that require erosion and re‐deposition of unconsolidated sediment before large‐scale plant colonization can occur, and because large numbers of geese continue to forage annually producing this dramatic top‐down effect.