Slow response of grassland specialists to habitat fragmentation in well-connected calcareous grasslands

Abstract

Due to the transition from traditional land use to modern agriculture throughout Europe, semi-natural grasslands are subject to severe environmental changes. Both agricultural intensification and abandonment have caused fragmentation of semi-natural grasslands in Europe with adverse effects on biodiversity. However, species extinction can possibly occur with a substantial delay to habitat deterioration. Meanwhile, there is increasing evidence that such time-delayed extinctions represent an essential but still less-considered challenge in the conservation of grassland biodiversity. In this study, we analyzed the effects of past and present landscape conditions on vascular plant, Orthoptera and butterfly specialists in calcareous grasslands in one of the German biodiversity hotspots and provide implications for conservation.

Patch area as well as patch connectivity significantly decreased from 1970 to 2015. However, this decrease was most dramatic between 1970 and 1990; since then further habitat fragmentation is proceeding at a much lower level as a result of intensified conservation measures. Due to regular management, the studied patches were generally characterized by a high habitat quality and were still large and well-connected compared to most other regions in Central Europe. The number of specialist vascular plant and butterfly species was positively related to both past and present fragment size. However, models including past patch area performed better in both species groups. In contrast, Orthoptera specialist species richness was in neither case affected by patch area. Additionally, the number of specialist species was better explained by past than present habitat connectivity across all species groups.

Our results clearly suggest that habitat specialists can persist in calcareous grasslands for several decades, despite increasing pressure of habitat fragmentation. In general, past landscape conditions were more strongly correlated to today's specialist species richness indicating the existence of an extinction debt across all species groups. Contrary to our expectations, this was also true for butterflies which are generally known to be relatively mobile and short-lived. However, due to regular management and still sufficient area and connectivity of the patches, even organisms that are more sensitive to fragmentation may persist in such well-connected landscapes. However, current patch area and connectivity were still important for the number of butterfly and plant specialists. In contrast, the current number of Orthoptera specialists within a patch seems to depend mainly on other factors such as habitat quality. Consequently, conservationists should focus on the maintenance of habitat quality in calcareous grasslands, as well as the restoration of former patches to counteract future biodiversity loss.