Feedback dynamics of grazing lawns: coupling vegetation change with animal growth

Abstract

We studied the effects of grazing by Black Brant (Branta bernicla nigricans) geese (hereafter Brant) on plant community zonation and gosling growth between 1987 and 2000 at a nesting colony in southwestern Alaska. The preferred forage of Brant, Carex subspathacea, is only found as a grazing lawn. An alternate forage species, C. ramenskii, exists primarily as meadow but also forms grazing lawns when heavily grazed. We mowed plots of ungrazed C. ramenskii meadows to create swards that Brant could select and maintain as grazing lawns. Fecal counts were higher on mowed plots than on control plots in the year after plots were mowed. Both nutritional quality and aboveground biomass of C. ramenskii in mowed plots were similar to that of C. subspathacea grazing lawns. The areal extent of grazing lawns depends in part on the population size of Brant. High Brant populations can increase the areal extent of grazing lawns, which favors the growth of goslings. Grazing lawns increased from 3% to 8% of surface area as the areal extent of C. ramenskii meadows declined between 1991 and 1999. Gosling mass was lower early in this time period due to density dependent effects. As the goose population stabilized, and area of grazing lawns increased, gosling mass increased between 1993 and 1999. Because larger goslings have increased survival, higher probability of breeding, and higher fecundity, herbivore-mediated changes in the distribution grazing lawn extent may result in a numerical increase of the population within the next two decades.

Long-Term Effects of Grazing and Global Warming on the Composition and Carrying Capacity of Graminoid Marshes for Moulting Geese in East Greenland

“…Madsen and C. Jaspers, unpubl.). The structuring role of goose grazing has also been demonstrated in studies employing long-term exclosures in grazed marshes (Person et al 2003;Gauthier et al 2004). Owing to its small and low leaves protruding from the moss carpet, C. subspathacea is possibly the most grazing tolerant species amongst the Carex species which may explain its dominance in the grazed part of the marsh.…”

Density-dependent switches in diet: a likely mechanism for negative feedbacks on goose population increase?

“…Person et al (2003) found that the areal extent of grazing lawns increased with an increase in population size in breeding areas of black brant (Branta bernicla nigricans) in southwestern Alaska. This led to improved foraging conditions which resulted in higher growth rates of goslings.…”

“…These increasing numbers have resulted in increased grazing pressure in many traditional breeding areas. Studies from these (sub-) arctic-breeding areas showed contrasting effects of increased grazing pressure on the vegetation, leading either to improved (Cargill and Jefferies 1984;Hik and Jefferies 1990;Person et al 2003) or deteriorated foraging conditions (Zellmer et al 1993;Gadallah and Jefferies 1995b;Jefferies and Rockwell 2002) with potential feedbacks on population growth.…”

“…As most geese demonstrate high fidelity to their breeding grounds and brood rearing areas (Cooch et al 1993;Lindberg and Sedinger 1998), geese often switch to alternative foods. Both in areas where positive (Person et al 2003) and negative feedbacks (Zellmer et al 1993;Gadallah and Jefferies 1995b) were observed, diet switches occurred from depleted preferred to less preferred species. However, the vegetation and abiotic conditions responded differently to these shifts in grazing.…”

“…Additionally, changed abiotic conditions prevented a recovery of the vegetation to its original state (Zellmer et al 1993;Gadallah and Jefferies 1995b). In contrast, in southwestern Alaska, increased goose grazing on alternative food plants (Carex rameskii) led to a vegetation dominated by the preferred food plant (Carex subspathacea) and increased the areal extent of the preferred vegetation type (Person et al 2003).…”