If the apple doesn't fall far from the tree: Dispersal limitation and climatic niche conservatism shape dragonfly assemblages across Europe

Adaptations to habitats with low temporal and spatial stability (such as the depicted area in the Scottish Highlands, where air temperatures can drop below minus 10°C until Summer) allowed lentic dragonfly species to recolonize areas north of the 0°C isotherm at the Last Glacial Maximum faster than lotic species after the permafrost and glaciers started to retreat 21 thousand years ago. Until today we see the strong imprint of Europe’s climatic history that underlies the current distribution of dragonfly species and the composition of dragonfly assemblages. Photo by Stefan Pinkert.

By Pinkert, Stefan; Dijkstra, Klaas-Douwe; Zeuss, Dirk; Reudenbach, Christoph; Brandl, Roland; Hof, Christian

It was recognized early on that broad-scale patterns of species richness generally correlate well with gradients of current temperature and with available water and energy. Statistical correlations inferred from contemporary macroecological patterns support the hypothesis that current environmental factors are major determinants of the spatial variation of biodiversity. However, studies focusing on species richness alone do not account for different evolutionary histories of regions and lineages, even though doing so helps understanding the major factors that have shaped the current distribution of biodiversity, such as dispersal and diversification.

In this paper, we used European dragonfly assemblages as a study system to address the hypotheses that phylogenetic conservatism of thermal adaptations as well as differences in post-glacial recolonization of previously glaciated areas between species adapted to running waters (lotic) and standing waters (lentic) determine patterns of endemism and phylogenetic diversity.

Dragonflies are poikilothermic organisms that require energy from their abiotic environment for mobility and for fundamental physiological processes (e.g. development time). The present-day distribution of dragonfly diversity is considered to be the result of tropical niche conservatism (through adaptations such as colour lightness, see Pinkert et al. 2016 in Ecography) that allowed only a few cold-adapted and, hence, younger lineages to colonize and persist in temperate climates, generating a gradient of decreasing phylogenetic diversity with decreasing temperature. However, although already mentioned by Tillyard (1916), a century ago, this long-standing hypothesis of tropical niche conservatism in dragonflies has never been studied empirically. Moreover, recent assessments of dragonfly species richness of North America and Europe reveal rather complex biogeographical patterns, with most species being found within the mid-latitudinal domains of both continents.

In addition, the ecological differences between lentic and lotic species are hypothesized to play an important role in shaping assemblages of freshwater taxa, including dragonflies. Previous studies have shown that lentic species have more northern ranges and greater dispersal propensity than lotic species. Some lentic species are also able to avoid inhabitable conditions through long-distance migration or a diapause state. The proportion of lentic to lotic species thus provides a proxy for the dispersal/colonization abilities of species. Owing to these adaptations, lentic species should have an advantage over lotic species in colonizing and persisting in ditches and seasonal ponds, but also in areas with pronounced seasonal temperature variation. Given a phylogenetic signal in these adaptations, we expected that assemblages in previously glaciated areas are composed of a closely related subset of lentic species, whereas the climatic stability of unglaciated areas in the Mediterranean basin should have supported the accumulation of both lentic and lotic lineages. Given the limited time available since the LGM, we predicted that incomplete recolonization results in a marked decrease in the diversity of dragonfly assemblages at a certain latitudinal breakpoint, for instance at the southern border of the zone of discontinuous permafrost at the LGM along the historical 0°C isotherm.

To address these research questions, we constructed a phylogenetic super-tree of 122 European dragonfly species - the most comprehensive phylogeny of European dragonflies that is currently available – and combined it with their distributions and habitat preferences as well as environmental variables of contemporary climate and the climate at the LGM. This dataset allowed us to conduct the first analysis of the geographical pattern of phylogenetic diversity for a complete group of freshwater insects across an entire continent.

We demonstrated that dragonfly species richness peaked in central Europe, whereas endemism and phylogenetic diversity decreased both gradually from warm areas in the south-west to cold areas in the north-east and with increasing proportion of lentic species. Endemism and phylogenetic diversity were consistently higher in formerly unglaciated areas south of the 0 °C isotherm during the LGM than in formerly glaciated areas. These findings support our hypothesis that the combination of phylogenetic conservatism in thermal adaptations and probably the varying dispersal abilities of species with different habitat preferences have left a strong imprint on the current diversity patterns of European dragonflies. With that we provide the first empirical support for the long-standing assumption that temperature is a major determinate for the geographical distribution of dragonfly lineages. Even more important, our results show that although dragonflies are generally considered to be a group with high dispersal ability and although the temperatures started to rise already 21 thousand years ago, the marked change in the relationship between the proportions of lentic to lotic species with latitude close to the historical 0°C isotherm indicates that lotic species might not be in equilibrium with current climate owing to limited post-glacial colonization and dispersal abilities. Indeed, according to our data, 33 of 42 lotic species are confined to roughly the southern half of Europe, among them 15 of the 18 total European endemic. By contrast, assemblages in previously glaciated areas are markedly more homogenous and are composed of predominantly lentic, widespread and closely-related species. This finding is important to understand community assembly of a broad range of freshwater taxa in previously glaciated areas, but it also implies that the ranges of rare species, including many European endemics, will probably contract with future climatic changes.

The complex diversity patterns of European dragonflies (see e.g. the pronounced contrasts between patterns of phylogenetic diversity and species richness) highlight that species richness alone cannot take into account the evolutionary history of regions and lineages and that only the joint consideration of different components of species diversity, their evolutionary history, and their ecological characteristics in concert with the contemporary and historical variation of climatic conditions allows a comprehensive understanding of the processes shaping the geographical variation of biological diversity.

References:

Pinkert, S. et al. 2016. Colour lightness of dragonfly assemblages across North America and Europe. – Ecography DOI: 10.1111/ecog.02578.

Tillyard, R.J. 1916. The biology of dragonflies Odonata or Paraneuroptera. – Cambridge University Press, Cambridge, UK.

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