Effective population sizes and migration rates in fragmented populations of an endangered insect (Coenagrion mercuriale: Odonata)


Phill Watts, Marine and Freshwater Biology Research Group, The Biosciences Building, School of Biological Sciences, Liverpool University, Crown Street, Liverpool, L69 7ZB, UK. Tel.: +44(0)151 7954384. Fax: +44(0)151 7954404. E-mail: p.c.watts@liv.ac.uk


  • 1Effective population sizes (Ne) and migration rates (m) are critical evolutionary parameters that impact on population survival and determine the relative influence of selection and genetic drift. While the parameter m is well-studied in animal populations, Ne remains challenging to measure and consequently is only rarely estimated, particularly in insect taxa.
  • 2We used demographic and genetic methods to estimate Ne and m in a fragmented population of the endangered damselfly Coenagrion mercuriale to better understand the contrast between genetic and field estimates of these parameters and also to identify the spatial scale over which populations may become locally adapted.
  • 3We found a contrast between demographic- and genetic-based estimates of these parameters, with the former apparently providing overestimates of Ne, owing to substantial underestimation of the variance in reproductive success, and the latter overestimating m, because spatial genetic structure is weak.
  • 4The overall Ne of sites within the population network at Beaulieu Heath, the largest C. mercuriale site in the UK, was estimated to vary between approximately 60 and 2700.
  • 5While Ne was not correlated with either the total numbers of adults (N) or the area of habitat, this parameter was always less than N, because of substantial variance in reproductive success. The ratio Ne/N varied between 0·006 and 0·42 and was generally larger in smaller populations, possibly representing some ‘genetic compensation’.
  • 6From a simple genetic model and these data on Ne and m, it seems that populations of C. mercuriale have the potential to respond to localized spatial variation in selection and this would need to be considered for future genetic management of this endangered species.