Population size and survival estimates for the rare damselflies, Coenagrion mercuriale and Ischnura pumilio

Coenagrion mercuriale is rare in the UK and is threatened across Europe. Ischnura pumilio is also threatened in the UK. Both species have suffered population declines in recent years and are vulnerable to habitat fragmentation and loss. Yet, reliable population size and survival estimates are scarce in odonate species. This study provides mark-release-recapture estimates of these parameters for UK stronghold populations of both species. Surveys were performed at four locations in southern England between 2001 and 2006. A total of 12071 adult individuals were marked across nine populations. Mark-release-recapture modelling techniques were used to provide survival and recapture probabilities and population size estimates. This study presents the first Horvitz-Thompson estimates of population size in odonates, which are among the highest reported for damselflies. Maximum estimates for a single site were 636624997 for C. mercuriale and 7453382 for I. pumilio. More males than females were captured at all sites, but calculated estimates indicated a female-biased sex ratio in adult I. pumilio at one location. Daily survival probability is among the highest published for damselflies. Male and female survival was equal or very similar in all populations. Further effects of maturity, age, site, and time on survival were identified. Estimated population sizes are much greater than previously thought, suggesting that where habitat is maintained, populations of threatened odonates can be very large. Furthermore, greater proportions of females have been estimated where wider searching techniques were employed. This has implications for future study design if estimates are to be reliably used for conservation management.