A Century of Evolution in Spartina anglica

Abstract

This review describes the origin and spread of the salt marsh grass Spartina anglica. the first specimen of which was collected from Lymington. Hampshire in 1892. and presents new material relating to its ecology and evolution. Using electrophoretically detectable variation. we confirm the strong circumstantial evidence that S. anglica originated by chromosome doubling of the sterile hybrid between the Old World species Spartina maritima and the North American Spartina alterniflora. In addition, extensive analysis of isoenzyme and seed protein phenotypes indicates that S. anglica is almost totally lacking in genetic variation. This may result from a narrow genetic base following a single origin or from a multiple origin from the uniform parents. It is likely to be maintained by the extensive clonal spread of most populations from a few, often deliberately introduced, founders and by preferential pairing at meiosis between identical homologous chromosomes preventing recombination between the component genomes. The implications of these findings are manifold. For example, the genetic uniformity of the species may help to explain why it has a relatively narrow ecological amplitude. A simple multiple-regression model incorporating largely physical, tide-related variables indicates that the distributional limits of Spartina in metres above Ordnance Datum are predicted remarkably well by tidal range, with variation in fetch, estuary area and position on the estuarine gradient significantly improving the prediction (generating equations explaining more than 90% of the variation in both the upper and lower limits). That the niche of the species can be so well defined is most probably due to its recent evolution and its lack of genetic differentiation as well as the predominance of physical, as opposed to biological, factors limiting its downshore spread. The species' genetic uniformity, coupled with its frequent occurrence as dense, monospecific stands, may also account for the recent rapid spread in several populations of the ergot fungus, Claviceps purpurea. For example, in Poole Harbour, Dorset, the average level of infected inflorescences rose from 36% in 1983 to more than 85% in 1988. Spartina anglica is, unusually among temperate grasses, a C4 species (one of only eight such species in Britain), and the implications of this method of carbon fixation in a species whose biomass production and possible northward spread may be limited by early spring and summer temperatures is considered, particularly in relation to projected climatic warming. The short-term causes and consequences of die-back in the southern parts of the plant's range are described, including the changes in low water tidal channels accompanying the invasion and subsequent decline of Spartina in a south coast harbour. In a section describing the species' interaction with other species we discuss the effects of Spartina's invasion on aerial and benthic invertebrates and on overwintering wading birds, particularly the dunlin, Calidris alpina, the decline in numbers of which correlates with the spread of the grass in British estuaries. Soil conditions, grazing and temperature are important factors affecting the competitive interaction of S. anglica with Puccinellia maritima, the latter invading S. anglica swards more rapidly in sandier, grazed marshes in more northern latitudes. The consequences of the genetic bottleneck that occurred during the speciation process and the relative youth of S. anglica in evolutionary terms are emphasized throughout by comparing the species with P. maritima. The greater variation, population differentiation and niche breadth of the latter species are particularly evident. Finally, we look forward to the changes which may occur with global warming and rising sea levels and how they may affect the second century of the species' evolution. © 1991 Academic Press Limited