Changes in methods of agricultural production over the last century have involved the rationalization of land use and the widespread intensification of approaches to animal husbandry in Europe. Abandonment of traditional grassland management practices (mowing and grazing) has led to a massive reduction in the area of semi-natural grasslands (Bernes 1994; Eriksson et al. 2002; Poschlod & WallisDeVries 2002), and the distributions of many grassland plants have become increasingly fragmented and disjunct (e.g. Fischer & Stöcklin 1997). Semi-natural grasslands are characterized by high levels of plant species diversity, and the current interest in the sustainable management of biological diversity has focused attention on the status of the remaining grassland fragments. At the same time, fragmented grassland systems within the agricultural landscape provide a valuable arena for the study of the mechanisms – dispersal, migration, gene flow and adaptation – that determine levels of species diversity and gene diversity in natural populations.
There have been a number of recent studies of genetic variation in fragmented populations of grassland species (e.g. Young et al. 1999; Hooftman et al. 2004; Pluess & Stöcklin 2004; Van Rossum et al. 2004). The majority of these studies, in common with studies of fragmented plant populations in general, have focused mainly on stochastic processes and the roles of population size, density and isolation as determinants of levels of within-population gene diversity or the degree of interpopulation differentiation (e.g. van Treuren et al. 1991; Weidema et al. 2000; Hooftman et al. 2004; Vekemans & Hardy 2004). An increasing number of studies have also attempted to interpret the structure of genetic variation within and between fragmented plant populations in terms of metapopulation processes, and the dynamics of colonization and extinction in spatially discrete habitat patches (Cruzan 2001; Tero et al. 2003; Hilfiker et al. 2004). In contrast, the role of local adaptation and selection as determinants of genetic structure in fragmented population systems has received less attention (cf. Lönn et al. 1996; Kalisz et al. 2001; Volis et al. 2004).
A few studies (e.g. Giles & Goudet 1997; Cronberg 2002; Jacquemyn et al. 2004) have explored the possibility that, especially in long-lived perennial plant species, the structure of genetic variation may be associated with the historical properties of the landscape and with past population size rather than with present population size and isolation. The coastal archipelago along the northern Baltic coast, where a progression of new islands is being generated by ongoing isostatic uplift, has provided an unique system where genetic variation can be analysed in the context of the history of habitat availability (Giles & Goudet 1997; Cronberg 2002). However, for agricultural landscapes, it is more difficult to obtain temporal data on the distribution of habitats. Although aerial photographs provide data on habitats and land use back to the first half of the 20th century, detailed maps that cover earlier historical periods are relatively rare. Studies of genetic variation have been carried out on fragmented populations of a woodland species, Primula elatior L., in Belgium where data on the distribution of forests in the agricultural landscape can be extracted from topographic maps that date back to the end of the 18th century (Jacquemyn et al. 2004). We have investigated genetic variation in a grassland species (Briza media L.) in a Swedish agricultural landscape that is covered by a series of aerial photographs and cadastral maps that extends back to the early 18th century.
Most studies of gene diversity in fragmented grassland plants have focused on the relict populations that remain after the severe reduction of previously wider distributions (e.g. van Treuren et al. 1991; Young et al. 1999; Van Rossum et al. 2004). Briza media is not, however, restricted to remnant fragments of grassland that have had a long historical continuity, but is also present in younger grasslands. This perennial grass is widespread and relatively common (Dixon 2002) and belongs to a suite of species that that are regarded as indicators of species-rich, unimproved, semi-natural grasslands in Sweden (e.g. Ekstam & Forshed 1992). In contrast to B. media, many of the other indicator species are known to be slow colonizers of new grassland sites and their occurrence is thought to be linked to a long history of grassland continuity (e.g. Eriksson 1998).
The study was carried out within a local agricultural landscape in which semi-natural grassland fragments are embedded in a mosaic of forest habitats and arable fields. All the present-day grassland fragments within the study area have been characterized in terms of their historical continuity (over the last 265 years) and their previous land use, as well as in terms of their present size and surrounding matrix of habitats. Within the study area, B. media is both widespread in grasslands that have had a long historical continuity and able to colonize grazed grasslands on abandoned arable fields after a few decades. Indeed, it has its highest frequencies in young grasslands on previously arable sites (M.T. Sykes, H.C. Prentice and O. Jonsson, unpublished data).
We investigate the extent to which gene diversity and the present structure of within-deme genetic variation in B. media can be explained by the historical continuity and past land use of grassland sites, and by properties of the present-day grasslands and their surrounding landscape matrix.