Flight of the dragons: a global review of migration in Odonata.

Insects are the most abundant and ecologically important animal migrants. Yet, we know relatively little about the patterns and processes underlying insect migration. Dragonflies (Anisoptera) and damselflies (Zygoptera) comprise the ancient insect order Odonata, whose ancestors were the first organisms to fly on Earth. Several members of Odonata are known to migrate long distances and have considerable impacts on ecosystems through biomass and nutrient transfer, pest control and species interactions. However, most aspects of odonate migration remain unknown and available data have not been fully reviewed from a global perspective in over two decades. This lack of consensus has repercussions on species monitoring, specialised conservation efforts and scientific progress. Here, we review odonate migration ecology, addressing: (i) what odonate migration is; (ii) why odonates migrate; and (iii) which odonate species migrate. We define two types of odonate migration: multi-generational migration, where back-and-forth migratory journeys are completed over several generations, and single-generational migration, where the same individuals leave and return to the original reproductive habitat. We conclude that within single-generational migration, altitudinal migration is currently the only known strategy, and we present the first complete list of species observed to perform this type of migration, where refuge is temporarily sought at high-altitude sites away from the reproductive habitat. In addition, we generate an exhaustive global list of 85 dragonfly and 15 damselfly species for which migration has been confirmed (total = 100 species) and a list of 85 possible migrants (22 damselfly species; 63 dragonfly species). Consideration of phylogeny suggests that migration has evolved multiple times within Odonata, and is present in four extant dragonfly families and two extant damselfly families. Approximately 1.5% of all odonate species are migratory, with the proportion rising to approximately 2.9% if species deemed to be possible migrants are also included. Thus, overall, migration is a relatively uncommon strategy in Odonata. Among dragonflies, the vast majority (73%) of migratory species occur in the Libellulidae with 62 confirmed migratory species, equating to 5.9% of all libellulids, whereas in damselflies, migrants are divided almost equally between Coenagrionidae (N = 8) and Lestidae (N = 7), with the genus Ischnura in Coenagrionidae having the most migrants of all damselfly genera (N = 5). Biogeographically, the proportion of migratory species is highest in the Palearctic (10.8%), followed by the Nearctic (6.8%), results that may reflect research bias or indicate that migration is an adaptation favoured at high latitudes. Interestingly, most odonate migrants appear to be species of 'Least Concern' according to the IUCN Red List and are potentially resilient to environmental change because of adaptations associated with their migratory strategy, such as opportunism, generalism and long-distance mobility.