Summary1 Herbivores 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 caerulescens A.O.U.) has led to loss of vegetation, and exposure and partial erosion of sediment. 2 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. 3 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. 4 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. 5 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.
SUMMARY(1) The effects of grazing by captive goslings of the lesser snow goose on the vegetation of the La Perouse Bay salt marsh were investigated. On seven separate occasions during the summers of 1986 and 1987 goslings fed on different swards of Puccinelliaphryganodes for up to 150 min. Net above-ground primary production (NAPP) and forage quality (amounts of nitrogen and carbon in tissues) of vegetation were measured in grazed and ungrazed plots.(2) Grazing early in the season resulted in increased NAPP of swards of Puccinellia, as predicted by the herbivore-optimization model. The greatest enhancement of production compared to that in ungrazed swards occurred in plots which received a low to moderate period of grazing (30-90 min). When the period of grazing was 120 or 150 min, NAPP was reduced.(3) The increase in NAPP above that of ungrazed swards was dependent upon the presence of goose faeces. In the absence of faeces NAPP did not increase, indicating the deleterious effects of clipping per se by the geese on the regrowth of swards.(4) A moderate period of grazing resulted in the maintenance of the nitrogen content of shoots (forage quality) throughout the season compared to the corresponding amount in shoots of ungrazed plants. Input of nitrogen from faeces alone was insufficient to account for the net accumulation of nitrogen in above-ground vegetation of swards which regrew following grazing.(5) The ability of swards to recover from the effects of grazing decreased over the course of the summer. With each successive experiment the maximum amount of regrowth decreased, and occurred in plots which received shorter grazing bouts, reflecting the increased foraging efficiency of the goslings.(6) The growth response exhibited by swards of Puccinellia when grazed depends upon the rapid recycling of nutrients via faeces, the intensity of grazing, and the phenology of plant growth. The ability of the herbivore-optimization model to predict the response of vegetation to the effects of grazing is relatively limited unless these and other factors are considered.
The Mid-Continent Population of the lesser snow goose, which breeds in the eastern and central Canadian Arctic and sub-Arctic, and winters in the southern United States and northern Mexico has increased 5-7% annually from the late 1960s to the mid-1990s, largely because of increased survival in response to an agricultural food subsidy. The rise in numbers complements the increased use of nitrogen fertilizers and a corresponding rise in yields of rice, corn, and wheat along the flyways and on the wintering grounds. In sub-Arctic migration areas and at Arctic breeding colonies, foraging by high numbers of birds has led to loss of coastal vegetation, adverse changes in soil properties and the establishment of an alternative stable state of exposed sediment, which can be detected with LANDSAT imagery. At a local scale, gosling growth, size and survival decreased in affected areas and other taxa have been adversely affected. The food subsidy on wintering and migration areas appears insufficient to meet reproductive demands as foraging in spring continues to occur on southern Hudson Bay staging and nesting areas. The recent introduction of liberal hunting regulations may reduce population size in the near term, but the revegetation of these coastal ecosystems will take decades to achieve. The present pattern of vegetation loss in these Arctic coastal systems is likely to continue in the forseeable future.
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