Process-based long-term evaluation of an ecological network of calcareous grasslands connected by sheep herding

This paper evaluates the long-term effect of an ecological network of calcareous grasslands, a habitat type that experienced dramatic habitat loss and fragmentation during the 20th century, on species richness of habitat specialist plants. Calcareous grasslands are of special conservation concern as the habitat type with the highest diversity in plant and invertebrate species in central Europe. A baseline survey in 1989 established complete vascular plant species lists for all 62 previously abandoned calcareous grassland patches in the study area and assessed the presence of 48 habitat specialist plant species. An ecological network was initiated in 1989 to reconnect these patches with existing grazed pastures (core areas) through large flock sheep herding where feasible, as sheep are thought to be the primary dispersal vectors for calcareous grassland plants. An evaluation survey in 2009 showed significant increase in species richness of habitat specialist plants in patches reconnected by sheep herding, indicating successful colonizations by habitat specialist plants, while ungrazed patches showed no significant change. Observed increase in species richness between 1989 and 2009 was related to connectivity by sheep herding and the presence of a diversity of structural elements providing microsites for establishment. Baseline species richness of the patches, which had been abandoned since at least 1960, was associated with patch area, supporting the effect of ecological drift, and with vegetation type, which suggests that delays in extinction may be related to site factors governing the strength of competition with later seral species. The implementation of this ecological network represents a long-term natural experiment' with baseline data, manipulation, and evaluation of hypothesized effects on a clearly defined target variable. It thus provides much needed empirical evidence that species loss in fragmented calcareous grassland communities can be counteracted by restoring functional connectivity among remnant patches.