Genetic variation and population decline of an endangered hoverfly Blera fallax (Diptera: Syrphidae)

Abstract

Genetic diversity is one of several factors affecting extinction risk in vulnerable populations. In addition to informing conservation management strategies, data on genetic variability can also shed light on the recency and magnitude of historic bottlenecks. The pine hoverfly Blera fallax is one of the rarest invertebrates in the UK, known from just two sites in Scotland. It belongs to an often overlooked, species-rich community that is fundamental to forest function, the saproxylics (that depend on dead wood). To assist current conservation management for B. fallax, including captive breeding and translocations, it is important to know whether genetic factors will limit the success of recovery. Using 12 microsatellite loci, we compared the genetic variation in Scottish and Swedish specimens (Swedish populations are thought to represent a more outbred B. fallax population). As expected, the Scottish population showed significantly lower levels of polymorphism, expected heterozygosity and allelic richness than the Swedish population. Furthermore, significant genetic differentiation was found between the two B. fallax populations (FST = 0.134). We then used an allele frequency-based approach and a Bayesian coalescent-based method to assess genealogical history and detect recent changes in population size. Unexpectedly, data from not only the Scottish but also the Swedish population indicated a strong and relatively recent decline that was more pronounced in Scotland. We discuss the implications of our findings for future conservation management planning, the first undertaking of its kind for saproxylic species in Britain.