The ecology and conservation of endangered saproxylic hoverflies (Diptera, Syrphidae) in Scotland

Abstract

Hoverflies are important for their roles in ecological and environmental services, and are also charismatic species of conservation interest in their own right. Almost half of all hoverflies are saprophages, which are organisms that feed on dead or decaying organic matter, and these include saproxylic species that depend on deadwood. Deadwood and its associated community are a rich source of forest biodiversity and are fundamental to forest function, but due to poor management, many saproxylics are threatened or endangered, and techniques for conserving saproxylic species are poorly developed. In this thesis I study the ecology and conservation management of an endangered UK saproxylic fly, the Pine hoverfly, Blera fallax (Linnaeus) (Diptera, Syrphidae) and the dispersal ability of the similarly endangered Aspen hoverfly, Hammerschmidtia ferruginea (Fallén) (Diptera, Syrphidae). My main goals were to clarify methods to support their recovery in active programmes of species conservation in Scotland, UK. For B. fallax, this included experimenting with habitat creation techniques, investigating the best conditions for larval growth and assessing competition effects. In addition, I evaluated the genetic variability of the remaining population in Scotland by comparing it with one in Europe to determine whether genetic constraints may limit recovery. For H. ferruginea, I determined dispersal ability with field experiments involving mark and recapture techniques.

By cutting holes at the surface of stumps of Pinus sylvestris, breeding habitat was created artificially for B. fallax at the remaining known locality for this species in the UK. Over 4 years, 81 % of holes were colonized by B. fallax, and by up to six other saproxylic syrphid species. The most successful holes were those cut into the heartwood, seeded with pine chips and sawdust and partially covered, as indicated by a combination of field occupancy monitoring and lab growth experiments. Observations of larval morphology and behaviour within rot holes revealed specializations that largely segregate the species in both time and space, and may mitigate interspecific competition between B. fallax and three more common syrphid species. I further demonstrated that B. fallax has a life history that features facultative semivoltine development, which may be a bet-hedging strategy to cope with fluctuating levels of larval food. Fifty B. fallax larvae were successfully reared and bred in captivity and from these, 430 descendent laboratory reared larvae and adults were released across three relocation sites. After initial success at the first re-location site when a new generation of larvae appeared in holes in 2010, a population crash at all sites occurred in the following year, possibly caused by adverse weather conditions. This disappointing result highlights the vulnerability of small populations to stochastic events, and means that survival of B. fallax may now depend on those larvae that are semivoltine, supplemented by animals currently being reared in captivity. My genetic analyses revealed similarly troubling information that highlights the precarious existence of B. fallax in Scotland: compared with a population in Sweden, Scottish B. fallax had significant less neutral genetic variation, and showed signs of a recent and severe bottleneck that reduced the effective population size to just 12 (CI: 0 - 266) individuals at some point in the last 200 years. Mindful of these challenges, I exploit my new data on the ecology and life history of B. fallax and combine it with techniques for captive rearing and for monitoring the genetic health of B. fallax into specific protocols and general prescriptions for the on-going recovery and management of this species.

In order to assess the dispersal ability of H. ferruginea (and therefore its potential for recolonizing newly created habitat), in May to July over two years, adults were marked and released from a central point and subsequently monitored at the breeding site, decaying aspen wood Populus tremula, where adults tend to assemble for mating and oviposition. Adults were resighted visiting logs of decaying aspen set out at 1 km intervals along transects up to 7 km away. Up to 10 % of released individuals were resighted up to 5 km from the central release point. Most dispersing individuals (68 %) were resighted at 1 km, which I propose as the optimal distance for managing aspen for this species.

Both of these hoverflies are case studies of techniques for recovering endangered saproxylic flies. Overall, my findings greatly increase fundamental knowledge of the ecology and natural history of these flies, and clarify some of the practical approaches that will be required in their conservation.