19th century woodland structure controls stand-scale epiphyte diversity in present-day Scotland

Axioms developed from island biogeography theory (i.e. species-area relationships, effects of fragmentation and isolation) are central to the development of conservation strategy. Within this context, the 'extinction debt' hypothesis brings into question an often assumed relationship between species richness and present-day spatial habitat structure (i.e. extent, fragmentation), suggesting instead that the richness and composition of biological communities may lag behind spatial changes in habitat. We examined evidence for an extinction debt among epiphytic lichens, a highly diverse biological group of significant conservation concern. Using sites in Scotland, we compared epiphyte species richness in smaller-scale habitat units (aspen stands) to larger-scale woodland structure (extent and fragmentation) measured at two spatial scales (1 km(2) and 4 km(2)) and for two timeframes, modern (1990s to 2000s) and historic (1860s to 1880s). Species richness was positively related to woodland extent and negatively related to woodland fragmentation; however, richness was explained better by historic woodland structure at a 1-km(2) scale, than by modern woodland structure. The results indicate: (1) a coupling of stand-scale epiphyte assembly and dynamics of the wider woodland ecosystem, and (2) a significant lag in the response of epiphyte species richness to habitat spatial structure. However, the effect of spatial habitat structure is different between species groups with contrasting traits. The effect of decreasing woodland extent on epiphyte richness is generally more severe for microlichens (comprising a greater number of rare and specialist species) than the more generalist macrolichens.