CAUSES OF LANDSCAPE-SCALE RARITY IN COBBLE BEACH PLANT COMMUNITIES

Abstract

In most communities, a considerable number of the constituent species will only occupy a small percentage of sites or habitat patches. However, the factors that limit the distribution and abundance of rare species are seldom examined and are poorly understood. I tested several explanations of landscape-scale rarity in the New England cobble beach plant community, an assemblage of halophytic forbs associated with fringing beds of the marine intertidal grass Spartina alterniflora. Spartina reduces flow velocity and stabilizes the cobble substrate, thereby reducing the burial of seeds and seedlings of other species. Frequencies of the presence of forb species behind 387 sampled Spartina beds distributed along 120 km of shoreline in Narragansett Bay, Rhode Island, USA, ranged from 0.005 to 0.382. Both the whole forb assemblage and individual species were strongly nested with respect to patch size and species richness. Species that were rare on a landscape scale were generally only found behind the longest beds, and bed length was positively related to species diversity. Experimental seed additions to 40 beds ranging in length from 8 to 688 m indicated that intermediate-sized beds often occupied by common species are not habitable by three rare forbs. When present behind the longest beds, these rare species are always associated with a microhabitat characterized by a relatively fine-grained and highly stable substrate. A series of substrate manipulations combined with experimental seed additions indicated that the rare annuals Spergularia marina, Salicornia biglovii, and Atriplex arenaria are restricted to the longest Spartina beds because smaller beds lack suitable substrate. These experiments also suggest that seed supply, competitors, and herbivores are not the proximate causes of the absence of these species from smaller beds, although dispersal limitation may prevent the colonization of many larger habitable patches. The microhabitat required by the rare forbs appears to be generated by the deposition of fine-grained particles behind the center of the longest Spartina beds. Consequently, bed length is causally related to the distribution of populations at a landscape scale and to species richness and composition at the scale of a whole habitat patch.