Factors driving larval abundance and adult size of the threatened Lestes macrostigma (Odonata): keys for water management and habitat restoration

Abstract

Freshwater insects have dramatically declined during the last decades. Lestes macrostigma is a damselfly of temporary brackish ponds, and is threatened in Europe. Better understanding its larval ecological requirements is imperative to inform conservation management measures and habitat restoration programs. We studied in a set of 33 temporary ponds the effect of 14 biotic and abiotic variables (including hydroperiod, pond surface area, water salinity, oviposition plant availability and predator relative abundance) on L. macrostigma larval abundance and adult size at emergence. Contrarily to our expectations, salinity level and oviposition plant availability did not drive the species larval abundance. Instead, the later the flooding date of the pond in autumn/winter the higher the larval abundance in the next spring. This effect seemed mediated by the aeshnid dragonfly larvae, as the size and relative abundance of these predators were lower in later flooded ponds. Larval abundance of L. macrostigma also increased with decreasing pond surface area. Lestes macrostigma adults tended to be bigger when they emerged from ponds with higher water levels, likely because deeper waters have lower water temperatures; this larger size may positively affect adult lifespan and fecundity. Our results contribute to explaining the species strong inter-annual variation in population abundance and further illustrate the threat of artificial early flooding of ponds for Mediterranean species.

Implications for insect conservation

To improve L. macrostigma conservation actions, our results advocate to avoid temporary pond flooding before late autumn and to maintain high water levels until adults emerge in the next spring. Further, to increase habitat availability, our data indicate the importance of creating a network of small and deep temporary ponds.