Nine years of change in the flora of Ellerburn Bank, a limestone grassland in the North York Moors

摘要

Limestone grassland is one of the most biodiverse habitats on Earth at a small scale (Wilson er al., 2012). Floristic richness within Europe can reach 80 species per m2 (Butaye er al., 2005) and many plants are found in no other habitat. Limestone grassland also supports important invertebrate communities as well as specialist birds (UK Steering Group, 1998). However, it is also one of Europe's most threatened habitats (WallisDeVries ef al., 2002). In 1998 there was a maximum of 41,000 hectares of lowland calcareous grassland remaining in the UK (UK Steering Group, he. cit.). Loss of habitat continues from forestry, conversion of pastureland to crops and land abandonment (WallisDeVries er al., loc. cit.), leading to reductions in the area and increasesin the isolation of habitat patches (Fisher & Stocklin, 1997). Quality reductions come from the abandonment of traditional agricultural practices leading to land-use intensification such as increased fertilization, herbicides, reseeding and frequent or early mowing (WallisDeVries et al., loc. cit). They may also result from overgrazing, undergrazing leading to the encroachment of scrub, Bracken Pteridium aquilinum and coarse grasses (Bobbink & Willems, 1987), and atmospheric nitrogen deposition leadingto loss of richness (van den Berg et al., 2011). The latter threats mean that their quality may decline even if sites are enclosed in protected areas, due to pervasive forces beyond the control of reserve managers (van den Berg et al., loc. cit.) or as aconsequence of suboptimal management. Indeed, JNCC declared that only 29% of Sites of Special Scientific Interest (SSSI) and 27% of Special Areas of Conservation (SAC) on lowland calcareous grassland in the UK were in favourable condition in 2006, belowthe average for habitats in general (Williams, 2006). Optimal management is difficult to achieve because the effects of different practices can vary depending on local conditions (Klimek et al., 2007) and the taxonomic group under consideration (WallisDeVries et al., loc. cit.).