THE GENETIC BASIS OF THE ECOLOGICAL AMPLITUDE OF SPARTINA PATENS . I. MORPHOMETRIC AND PHYSIOLOGICAL TRAITS

Abstract

For any species one can delimit a general geographical boundary and a set of ecological tolerances. These limits may fluctuate temporally and spatially in response to the heterogeneity of the biotic and abiotic environment or as a result of changes in the genetic constitution of the species. The margin of a species range may be viewed as a dynamic boundary. A colonizing propagule that has crossed the current species boundary may be able to extend that species range either if it lands in an open ecological niche equivalent to that which it left or, if it adjusts to the new environment, through plastic responses. Alternatively, the establishment of a colonizing individual may depend on its differing genetically from the population of origin in its ecological or niche requirements. The ecological amplitude of a species may thus reflect the evolutionary potential of the species. The limits to the distribution of a species may be viewed as an evolutionary problem closely aligned to the question of what limits a species' ability to respond to natural selection (Antonovics, 1976). To delineate analytically the evolutionary processes which control the distribution of a species, one approach would be a comparative study of the genetic structure of different populations of a species which have successfully exploited a wide range of habitats. Such a study might reveal those factors which have permitted divergence as well as those which may be retarding further expansion at the species boundaries. In this and subsequent papers we propose to address the problem of the evolutionary basis of ecological amplitude and limits to species distributions by examining a population of the salt marsh cord grass Spartina patens (Aiton) Muhl. which shows attenuation in abundance on a microgeographic scale across a series of ecotones from salt marsh to swale to dune to beach. We direct our attention to the following questions: (1) Has genetic divergence occurred among adjacent subpopulations? (2) To what degree are peripheral subpopulations genetically adapted to marginal conditions? (3) What are the characteristics of a species with a wide ecological amplitude? Spartina patens is a highly variable, rhizomatous perennial grass (Fernald, 1950; Mobberly, 1956) which occurs along the Atlantic coast from 50?N in Canada (Fernald, 1926; Rousseau, 1974) to 16?N in Central America and to 13?N in the Caribbean (Urban, 1920). A few inland locations are found in New York, Michigan, and Ontario, some of which are saline or brackish and it has been introduced into the Mediterranean (Hulten, 1958). North of approximately 42?N, S. patens is restricted to the salt marsh community. South of approximately 25?N, S. patens is confined to beaches and low coastal dunes. The Carolinas mark the center of distribution, and here it is a dominant element of the coastal zone. On the Outer Banks of North Carolina it most commonly dominates the high salt marsh, but will also successfully colonize and dominate sand dunes, swale grasslands, sand flats, and scrublands (Au, 1974). Community analyses and correlations of distribution with some 18 environmental vari-