Quantifying dispersal in British noctuid moths

Dispersal is an important process in the ecology and evolution of organisms, affecting species’ population dynamics, gene flow, and range size. Around two thirds of common and widespread British macro-moths have declined in abundance over the last 40 years, and dispersal ability may be important in determining whether or not species persist in this changing environment. However, knowledge of dispersal ability in macro-moths is lacking because dispersal is difficult to measure directly in nocturnal flying insects. This thesis investigated the dispersal abilities of British noctuid moths to examine how dispersal ability is related to adult flight morphology and species’ population trends. Noctuid moths are an important taxon to study because of their role in many ecosystem processes (e.g. as pollinators, pests and prey), hence their focus in this study. I developed a novel tethered flight mill technique to quantify the dispersal ability of a range of British noctuid moths (size range 12 – 27 mm forewing length). I demonstrated that this technique provided measures of flight performance in the lab (measures of flight speed and distance flown overnight) that reflected species’ dispersal abilities reported in the wild. I revealed that adult forewing length was a good predictor of inter-specific differences in flight performance among 32 noctuid moth species. I also found high levels of intra-specific variation in flight performance, and both adult flight morphology and resource-related variables (amount of food consumed by individuals prior to flight, mass loss by adults during flight) contributed to this variation. Analysis of Rothamsted Insect Survey data and National Moth Recording Scheme data of changes in moth abundance and distribution patterns in the UK over the past 4 decades provided some evidence that dispersal ability (measures of adult forewing length) contributed to species’ population trends. The analysis indicated that species with intermediate dispersal ability may be declining more than those of either high or low dispersal ability. I conclude that the new tethered flight technique demonstrated in this thesis provides opportunities for multi-species and cross-taxon comparisons of dispersal ability. The utility of wing length as a proxy for dispersal ability may facilitate the inclusion of dispersal information into analyses for a wide range of insects. Dispersal has some effect on moth distribution and abundance changes but other factors such as habitat availability and responses to changing climate are likely to play a role.