Semi-natural grasslands: landscape, history and plant species diversity

Abstract: This thesis explores the impact of past and present landscape and habitat structure on present levels of vascular plant species diversity within fragmented semi-natural grasslands. Within the Jordtorp-area, a mosaic agricultural landscape on the island of Öland (Sweden), historical land-use changes and the development of semi-natural grasslands was characterized, over a time-period of almost 300 years, using historical maps, aerial photographs and a high-resolution imagery. The study also examines the relative importance of management conditions and habitat history (in terms of continuity, previous land-use), on present-day vascular plant species diversity within the semi-natural grasslands. Present-day grasslands represent 18% of the grassland area found at the beginning of the 18th century. The most dramatic loss of grassland habitat took place between late 19th century and the beginning of the 20th century, resulting in a progressive grassland fragmentation during the 20th century. Results from the Jordtorp-area showed that present levels of small-scale (<1m2) species diversity reflects both present grassland connectivity and grassland connectivity of past landscapes – suggesting that the losses and fragmentation of the semi-natural grasslands within the landscape has had an effect on the plant species communities. However, as some part of the diversity was associated with the grassland connectivity of past landscapes, further diversity declines are to be expected within the grasslands. Present levels of plant species diversity are not only a reflection of the present and past grassland structure, the variation in diversity also reflects the characteristics of the surrounding landscape and land-use legacies related to the historical villages, suggesting that focusing solely on habitat structure provides a limited perspective on the driving forces behind the loss of diversity. Habitat continuity and previous land-use of present grassland fragments had a significant influence on present-day species diversity. The total richness of both generalist and specialist species, on large-scale, increased with habitat age, while the richness of species on small-scale showed no relationship with habitat age. However, the proportion of specialist and generalist species on smallscales was highly dependent on previous land-use. Both present management intensity and the long-term continuity of management were significantly associated with present-day species diversity. A unimodal association between species diversity and distance from nearest settlement suggest that diversity is promoted by a history of moderate grazing intensity. An overall conclusion from the Jordtorp-area is that both habitat quality (environment and management) and habitat availability (connectivity) have a significant impact on species diversity. The results from the thesis emphasize the importance of a landscape perspective in both time and space – embracing the spatial structure of the grassland habitat, the wider landscape as well as the land use history of the area.