Odonata origins, biogeography, and diversification in an Eastern North American hotspot: multiple pathways to high temperate forest insect diversity
Jeffrey D. Corser,
New York Natural Heritage Program, SUNY College of Environmental Science and Forestry, Albany, NY, USA
Correspondence: Jeffrey D. Corser, New York Natural Heritage Program, SUNY College of Environmental Science and Forestry, 625 Broadway, 5th Floor, Albany, NY 12233, USA. E-mail: email@example.com
We assessed the origins and historical biogeography of a rich regional odonate fauna in New York State (NYS), Northeastern United States.
We computed North American (NA) range centres and NYS range margins and reviewed the taxonomic literature to provide a useful phylogenetic framework for the fauna. We analysed results from a newly completed Odonata atlas using generalised linear anova models to assess the effects of species' origins and zoogeographic affinities on relative frequency and extinction risk metrics.
Phylogenetic reconstruction based on taxonomic nomenclature revealed different patterns of diversification. Zygoptera in NYS is mainly of neotropical origin ˜ 60 Ma displaying a pattern of tropical conservatism, but with a burst recent of Plio–Pleistocene speciation in certain groups. Alternatively, Anisoptera contains crown group endemic taxa and other very old lineages from the Mesozoic era before the breakup of Pangaea, highlighting the evolutionary significance of the Appalachian Mountains as an important global centre of temperate forest freshwater diversity.
These high regional levels of odonate diversity have been brought about by at least three different mechanisms: dependence on forests, predominance of non-ecological speciation mechanisms, and niche conservatism across hundreds of millions of generations.
NYS lies at a crossroads of both ancient and more recent Odonata evolution comprising separate boreal, temperate, and tropical faunas. Those species encountered less frequently and having higher overall extinction risk metrics generally tended to be the boreal species on the rear edge of their range, a widespread phenomenon for the insects of many regions generally attributed to ongoing climate change.
The order Odonata has traditionally been thought of as having a tropical origin (Norling, 1984; Corbet, 1999), but nearly a century ago Odonatologists knew that species richness of some ‘primitive’ families peaked at more northerly latitudes than it did for the more ‘modern’ families (Kennedy, 1928). Overall, this ancient (Carboniferous ~ 325 Ma) insect order does not demonstrate a strong north–south diversity gradient, with middle, rather than more equatorial latitudes, containing higher species richness (Keil et al., 2008; Heiser & Schmitt, 2010; Paulson, 2011). Unlike a great many organisms, certain aquatic macroinvertebrates including odonates (Boyero, 2002; Mayhew, 2007) do not seem to adhere to the hypothesis of tropical conservatism as an explanation for the latitudinal gradient of species richness (Weins & Donoghue, 2004). Rather, the overriding pattern for Odonata is the preponderance of non-ecological speciation mechanisms (i.e. sexual selection) as drivers of diversity (Misof, 2002; McPeek & Gavrilets, 2006; Davis et al., 2011; Svensson, 2012), while there is also some support for the water–energy hypothesis (Keil et al., 2008; Rakosy et al., 2012).
The root of the Anisopteran (dragonflies) phylogenetic tree (Bybee et al., 2008; Davis et al., 2011) is an extant early Mesozoic relict, cryptic cold-adapted lotic taxon with four known species (Anisozygoptera; Epiophlebidae), which are confined to seasonal temperate cool mountain streams in East Asia. Carle (2012) reviewed the extreme phylogenetic isolation and morphological uniqueness of this ‘living fossil’ attributing its > 180 Myr persistence to specialisation in high elevation, groundwater fed, cool mountain streams, and detritus-based (forested) ecosystems that have demonstrated high stability in the face of massive global perturbations. The persistence of this Mesozoic, cold-adapted taxon in highly seasonal environments upsets the notion of tropical conservatism as an overarching explanation for latitudinal diversity gradients because it is premised upon cold tolerance always being manifested as a derived trait within clades (Weins & Donoghue, 2004). Nevertheless, Weins et al. (2006) suspected that tropical conservatism was limited to moderately recent groups, and that older ancestral groups might instead trace their origins to the temperate regions.
The extreme persistence and morphological stasis shown by these thermophilic biphasic predaceous agile insects are also underscored by the ~ 35 Ma (Oligocene) Scudder (1890) fossils from Colorado that are nearly indistinguishable from extant NA Aeshnids. The co-occurring fossilised insects and plants reported by Scudder (1890) indicated the existence of a mild temperate climate at ~ 30–35°N surrounded by a Sequoia forest. Even today in the Western Hemisphere, many of the earliest living Anisopteran families reach their highest diversity in temperate, not tropical zones (Kennedy, 1928; Kalkman et al., 2008). This assemblage predates the long warm period prior to the Eocene when much of the modern biodiversity is thought to have been generated, and after which modern latitude diversity gradients became established as a result of high seasonality outside the lower latitudes (Weins et al., 2006; Archibald et al., 2010).
Because of this very long history, many boreal, temperate, and tropical-centred odonate clades have since diversified, producing paradoxical biogeographical patterns and unlike other regions (e.g. Sternberg, 1998; Heiser & Schmitt, 2010; Juen & DeMarco, 2012), NA surprisingly lacks a thorough biogeographical underpinning. Yet with high richness levels, northeastern states like New York (NYS) offer fertile ground to assess the historical biogeography and possible causes of high diversification in an insect order that has survived here for >100 Myr (Louton, 1982; Carle, 1995), while contributing much to our understanding of key evolutionary and ecological concepts (Cordoba-Aguilar, 2008).
A recent odonate inventory in southwestern NA (comparatively sized to NYS) by Stevens and Bailowitz (2009) detected only about half of the species numbers as NYS, and most eastern U.S. states have much larger species lists than all of Europe combined (Kalkman et al., 2008). Yet even within the eastern NA deciduous forest biome, lower latitudes normally renowned for their high overall biodiversity like the lower Mississippi valley are eclipsed in terms of Odonata species density by the more recently glaciated northeastern states (Bried & Mazzacano, 2010). Despite these generalities, a basic understanding of odonate distribution patterns has largely been lacking in NA, and only recently have the first coarse-scale dot maps been published (Donnelly, 2004a,b,c).
Here, we offer insights into the evolution and diversification of Odonata by examining its origins and historical biogeography in this diversity hotspot as a first approximation until much anticipated complete phylogenies and more refined distribution maps become available (Ballare & Ware, 2011; Dijkstra & Kalkman, 2012). First, we report on some new results (2005–2009) from a statewide odonate atlas; we then synthesise the widely disparate systematics literature into broadly time-calibrated phylogenies for each suborder in NYS. Next, we compute NA range centres and NYS range margins to infer more recent post-Pleistocene zoogeographical origins and historical biogeography of the group within wider Eastern/central NA. Finally, we discuss our results in the context of some leading hypotheses for the causes of high insect diversity (Mayhew, 2007), so that we can better anticipate emerging conservation issues.
NYS comprises 141 300 km2 in a geologically heterogeneous region of northeastern NA. The state stretches from the Atlantic Ocean to the Great Lakes (Fig. 1) and elevations range from sea level to 1629 m. The climate displays marked temperature seasonality and central NYS lies along a tension zone separating a cool continental climate with coniferous and northern hardwood forests from a warmer subtropical climate to the south composed mainly of oak-hickory forests. The state is ~ 63% forested, with substantial regrowth since the early 1900s following widespread agricultural clearing. NYS habitat types are varied, with 264 natural communities identified in 21 subsystems within seven systems (Edinger & Howard, 2008). Annual precipitation is high (75–125 cm) and equally distributed throughout the year.
The NYS dragonfly & damselfly survey
Our survey was organised akin to the now-familiar Breeding Bird Atlas model (e.g. McGowan & Corwin, 2008) except that instead of being assigned to specific grid cells, trained volunteer surveyors were provided with survey locale guidance, while allowing for some site selection flexibility. The aim of our atlas (White et al., 2010) was to gain a clear understanding of the fine-scale distributions of all 193 odonate species ever documented from NYS and to detect any potential new arrivals or losses. A standardised survey protocol was followed by these 270+ volunteers who focused the bulk of their field efforts capturing adults with aerial nets at breeding habitats, as well as collecting exuviae (shed skins), as our survey target was established breeding populations. Nearly 4400 georeferenced surveys were conducted across the state during March–November, 2005–2009. Data were submitted to a centralised coordinator to ensure standardisation, including species identification by specialists. Over 8600 specimens and photographs were verified and vouchers were placed in the NYS Museum.
We built a composite surrogate phylogeny for all NYS species using both morphological and molecular studies from the literature (a full list of > 50 sources is available upon request) for each suborder based on taxonomic rank. The resulting nomenclature was inputted into TreeMaker (Crozier et al., 2005), which converts the systematic nomenclature into an inferred phylogeny. The tree is outputted to a NEXUS file readable by the program FigTree (Rambaut, 2012). For Zygoptera, the taxonomic hierarchy included suborder, family, subfamily, genus, subgenus, and species; while for Anisoptera, we used suborder, superfamily, family, subfamily, tribe, genus, subgenus, and species. Branch lengths for the cladograms were set to unity and fossil and molecular clock estimates were mapped onto the appropriate nodes when dates were given by the authors, and biogeographical origins were inferred from sister taxa relationships from the original sources. In the absence of complete fossil-calibrated phylogenies, these roughly dated approximations based on Linnaean taxonomy have proven robust for biodiversity and conservation applications (Crozier et al., 2005; Davis et al., 2010; Ricotta et al., 2012).
Estimating recent range margins and centres
We determined where NYS occurs in relation to each species' overall NA distribution (centre, northern, southern, eastern, or western range boundary) following the approach of Zuckerberg et al. (2009) to categorise all species as northern, southern, etc. We used our new fine-scale dot maps to pinpoint species whose precise overall NA range margin falls directly within the borders of NYS. We determined the NA latitudinal and longitudinal range centre (to the nearest 0.5°) for each NYS species using Donnelly's (2004a,b,c) maps and following the formula given in Hof et al. (2006) [(northern range boundary + southern range boundary) × 2−1], where the most outlying dot was taken to be the boundary. We then assigned that distributional centre to its corresponding terrestrial ecoregion in Ricketts et al. (1999). Species' NA range centres were assumed to approximate biogeographical refugial areas (i.e. regions of recent origin) during the last glacial maximum (LGM) ~ 12 Ka. Such an analysis was performed by Sternberg (1998) for European odonates and is not intended necessarily to pinpoint the actual glacial refugia, but to elucidate relative latitudinal affinities amongst the fauna of a region.
Current species status
To estimate each species' relative frequency, we calculated the number of individual verified site records in White et al. (2010) divided by total the number of site records for both orders (× 100). We used updated NatureServe S ranks as a proxy for each species' extinction risk; similar to IUCN criteria, S rank is an integrated metric of rarity and conservation concern in NYS (S1 = high imperilment to S5 = least concern) (see White et al., 2010 for further description). We used generalised linear anova models to explore multivariate relationships and to assess the effects of species' recent origins and zoogeographic affinities on these relative frequency and extinction risk metrics. All continuous variables were untransformed; χ2 tests (α = 0.05) were used for testing significant differences among categorical variables and Tukey–Kramer HSD q* tests (α = 0.05) were used for all post hoc comparisons. All analyses were performed in JMP (SAS, 1995).
Evolutionary origins and historical biogeography
The composite dated phylogeny for damselflies is shown in Fig. 2. We highlight three major patterns: (i) a burst of recent endemic speciation in Eastern/central NA coinciding with extreme climate deterioration during repeated Plio–Pleistocene glaciations since ~ 5 Ma; (ii) a major episode of vicariance ~ 30 Ma during the mid-Oligocene, about the time some of the last land bridges between Europe and NA sundered; and (iii) a primarily neotropical biogeographical origin of NYS damselflies beginning around 60 Ma, with repeated colonisations of Eastern/central NA by independent radiations, most recently by Argia, Ischnura.
The composite phylogeny for dragonflies is shown in Fig. 3. Repeated patterns are more intricate than for damselflies, but we can discern that: (i) dragonflies have occupied Eastern/central NA much longer than damselflies, with the basal representatives of many major families apparently originating in what is now Eastern/central NA when it was still part of the supercontinent Pangaea during the Mesozoic era > 180 Ma; (ii) consequently, NA figures prominently in the diversification of dragonflies, with ~ 3/4 of the NYS fauna being derived from biogeographical regions involving NA; (iii) the colonisation of temperate Eastern/central NA by tropical libellulids began about the same time as the damselflies ~ 60 Ma and has proceeded in waves, most recently by the genus Libellula; (iv) there has been an explosive recent endemic radiation of the lotic gomphids in Eastern/central NA; and (v) groups originating across NA/Eurasia tend to contain certain members that today are confined to the boreal regions, yet the basal representatives of those clades inhabit temperate Eastern/central NA (i.e. Aeshna, Leucorrhinia, Somatochlora, Sympetrum).
Recent origins: current range margins and centres
We added five new species to the overall NYS list, but we were unable to confirm the presence of 18 of the least common 188 species depicted in Donnelly (2004a,b,c). There was no significant phylogenetic effect (P > 0.13) on species' relative frequency or extinction risk metrics, and families tended to contain both rare and more frequently encountered species. Species near their southern and western range margins in NYS were significantly more rare (P = 0.0005) and were encountered less frequently (P = 0.055; q* = 2.75) than species on their northern and eastern range margins. As a result of our field efforts, we determined that 45 species have a range margin falling directly within the borders of NYS and we detected apparent leading edge movements into NYS from all directions, and range retractions away from the state to the north and the south.
Ecoregional differences in range centres between Anisoptera and Zygoptera were not significant (χ2 > 0.28), and although dragonflies had a 1° overall more northerly distribution of their latitudinal range centres compared to damselflies, this difference was non-significant (P = 0.17). There was, however, a strong genus-level effect (P < 0.001), and the range centres of the major groups spanned almost 15° latitude; tropical genera had more southerly latitudinal centres, while those with northern origins had centres well into heavily glaciated regions (Figs 1 and 4). On the basis of the location of these distributional centres, a small subset of currently diversifying groups have NA range centres very close to the LGM in NYS (~ 42°N latitude), while others had significantly (P < 0.0001; q* = 4.1) more poleward range centres (~ 50°N).
Two thirds of the odonates in NYS were either near their range centre, or else along their northern range boundary, with the remaining third split amongst all other directions. This pattern also had a significant (P < 0.003) family-level effect, as most of the Libellulids (skimmers) and Coenagrionids (pond damsels) were near their northern margins, whereas 60% of the Gomphids (clubtails) were near the centre of their overall range. Aeshnids (darners), Corduliids (emeralds), and Lestids (spreadwings) were more evenly split amongst all directions. Likewise, of the 45 species whose overall NA range margin fell directly within the borders of NYS, over half were on their northern margin, but a substantial number (~ 30%) were on their southern margin; Corduliids and Coenagrionids made up about three fourth of those species. Overall, the species making up the current NYS odonate fauna trace their recent origins to five distinct regions: (i) endemic northeastern NA (9%); (ii) the central and southern Appalachian Mountains (41%); (iii) the Great Lakes (26%); (iv) boreal Canada (17%); and (v) neotropics (7%).
Although we found 10% fewer species overall compared to over a century of historical records assembled by Donnelly (2004a,b,c), every species we did not encounter was already very rare or a suspected vagrant. This amounted to roughly equal proportions within each suborder and these apparent range retractions were species specific and not phylogenetically constrained. We discovered five new species and there is increasing evidence pointing to ongoing marked distributional flux of odonate populations owing to climate change (Hassall & Thompson, 2008; Corser, 2010; Ott, 2010), and it is clear that extant groups of these mobile insects have traversed huge spans of time and space across the earth over the past few 100 Myr (Coope, 2004; Grimaldi & Engel, 2005). Below, we explore the nature of this northeastern NA Odonata hotspot in the context of potential causes of high levels of insect diversity (Mayhew, 2007).
Tropical and temperate conservatism
We found evidence that older groups demonstrate temperate, as opposed to tropical conservatism as Weins et al. (2006) predicted, and furthermore regions that they have inhabited the longest – in this case the freshwaters of the Appalachian Mountain forests in Eastern/central NA – contain the highest richness levels while serving as refugia during episodes of deteriorating climate. As outlined by Carle and Cook (1984) and Carle (1995) in the case of the basal dragonfly families (Fig. 3), we found that temperate Eastern/central NA has been a major evolutionary repository of global Anisopteran diversity since at least Mesozoic times > 180 Ma. In fact, Dillon and Robinson (2009) concluded from > 25 years of careful study that certain living freshwater faunal elements of the Appalachians might even be relicts of Palaeozoic era watersheds.
On the other hand, Zygoptera in NYS demonstrate the more typical post-Eocene tropical conservatism pattern (Weins & Donoghue, 2004; Weins et al., 2006) and these odonates are relative newcomers to Eastern/central NA compared to many of the much older endemic dragonflies (Figs 2 and 3). This alternate subordinal pattern might not occur in Eurasia, however (Heiser & Schmitt, 2013), where the signature of historical biogeographical patterning appeared to be weak at broad regional (100 000s km2) scales (Rakosy et al., 2012). Despite their tiny size, certain damselflies' remarkable colonisation abilities (e.g. Sherratt & Beatty, 2005) seem to obscure the fact that although they are primarily tropical-originated insects, certain groups readily colonise and rapidly radiate in temperate and even boreal regions (Pritchard, 2008). Nevertheless, many of these younger lineages might well be ‘evolutionary dead ends’ as it is the pruning of these (often more lentic) extinction-prone clades that yields monotypics having larger range sizes and favouring long-term persistence (Jansson & Dynesius, 2002; Hadly et al., 2009).
Recent post-glacial recolonisation, cryptic northern refugia, and sexual selection
We found the same recent origins as Beatty and Beatty (1968) and NYS has clearly been repeatedly recolonised along pre-Pleistocene invasion routes (Carle, 1995), primarily from refugia in unglaciated nearby forested realms just to the south. A broad north(east)ward recolonisation pattern along either side of the Appalachian uplands is evident (Fig. 1) being parallel to the modern prevailing wind direction – yet these are also well-known pathways for many organisms in Eastern/central NA (Soltis et al., 2006). This strong Pleistocene imprint is also found across Eurasia (Sternberg, 1998; Kosterin, 2005) and post-glacial mixing of separate temperate, tropical, and boreal faunas has also been identified in other Odonata hotspots (Heiser & Schmitt, 2010; Morrone, 2010). At the same time, species' responses to repeated glacial cycles have been idiosyncratic, resulting in the highly complex distribution patterns seen today (Soltis et al., 2006; Stewart et al., 2010).
Our fine-scale distribution maps in White et al. (2010) depict this same intra-specific pattern. For example, many populations of the same putative species have recolonised NYS since the LGM from separate glacial refugia primarily along two different pathways: an eastern coastal route up the Hudson River valley (Corser, 2010) continuing northward into New England and eastern Canada, and another from the southwest, up the Ohio River valley into southwestern Ontario and NYS and around the Great Lakes (Fig. 1). This has created a suture zone where hybridisation and hidden cryptic diversity would be expected (Hewitt, 2004; Swenson & Howard, 2005), and indeed ongoing odonate hybridisation is well-documented in the vicinity of this broad contact zone (Donnelly, 2003, 2004c).
This landscape has thus played a pivotal role in serving as both a conduit, as well as generator of diversity, where odonate communities were continuously disassembled and reassembled into new collections of glacial races (McPeek & Gavrilets, 2006). McPeek and colleagues' extensive study on Enallagma and Lestes damselflies in northeastern NA pointed to relatively recent (100 000 years) explosive radiations, incipient species formation with numerous examples of morphologically distinct subspecies, hybrid zones, and secondary contact among differentiated forms showing demographic signatures of major bottlenecks, range fragmentation, and range expansions caused by repeated Pleistocene glacial cycles (Turgeon et al., 2005).
New species that came into being during these glacial cycles colonised newly opened, well-watered terrain, chasing the retreating glacier north (Siepielski et al., 2010), some all the way to Eurasia. In eastern NA, the transition from a temperate to a boreal forest-dominated odonate assemblage lies near the terminus of the Appalachian Mountains in the Canadian Maritimes (Larson & Colbo, 1983). NYS lies along the rear edge of this assemblage, with these boreal species mainly occurring in regions of the state with the shortest growing seasons. Likewise, most of the truly boreal NYS odonates on the rear edge of their often large ranges well north into Canada (i.e. many Somatochlora) are often ranked highly for conservation concern in northeastern NA and elsewhere (Bried & Mazzacano, 2010; DeKnijf et al., 2011).
Surprisingly, Beringia appears not to have not played a significant role as a LGM odonate refugia; Cannings and Cannings (1994) fingered just one species (Somatochlora sahlbergi) as a Beringian resident during the LGM finding instead that the boreal elements dominating the fauna were mostly recent arrivals from the southeast. This implies that the boreal assemblage might be derived from the temperate component (Belyshev & Kharitonov, 1978; Cannings & Cannings, 1994), and repeated isolation of temperate species in cryptic northern refugia during repeated glacial cycles can play an important role in generating certain boreal species over a couple of million generations (Barraclough & Vogler, 2002; Weir & Schluter, 2004; Carstens & Knowles, 2007). Our data support this view because there is a preponderance of NA range centres for several groups that clusters just south of the approximate latitude of the LGM in NYS (Fig. 4).
McPeek et al. (2010) provide an overview of how sexual selection on male genitalia has promoted the recent diversification of Enallagma damselflies in northeastern NA, and Svensson (2012) used a damselfly example to highlight the predominance of non-ecological speciation mechanisms, such as sexual selection and thermal adaptation, suggesting that reproductive isolation often precedes ecological (habitat) differentiation in these insects. Our dated phylogenetic approach (Fig. 2) also uncovered a role for Tertiary vicariance in the deeper diversification of Zygoptera in NA. For example, the opening of the North Atlantic ocean ~ 30 Ma facilitated generic-level diversity in many insects (Noonan, 1988), including the establishment of the Coenagrioninae in Eastern/central NA (Fig. 2; Guan et al., 2013).
Although much understudied compared to Zygoptera, incipient species formation arising from sexual selection in the Anisoptera is also rampant (Misof, 2002). For most dragonflies, however, diversification patterns are much older than Plio–Pleistocene because even the most derived groups like Leucorrhinia and Libellula show complex histories of repeated episodes of dispersal, vicariance, and radiation across the Holarctic, back and forth between NA and Eurasia over the past 10 to 100 Ma (Fig. 3; Artiss, 2004; Kosterin, 2005). Even more ancient supercontinental patterns of vicariance and dispersal define the biogeographical patterns for these old Holarctic groups, involving land bridges, widespread climate change, and repeated patterns of dispersal, radiation, and subsequent extinction of lineages (Sanmartin et al., 2001). Nevertheless, certain groups such as the Sympetrum dragonflies demonstrate many of the same more recent post-glacial fragmentation patterns in the vicinity of NYS as do some of the damselflies (e.g. Donnelly, 2004b; Paulson, 2011).
Evolutionary role of temperate forested ecosystems and conservation implications
Continental-scale distributions and dragonfly richness patterns are known to conform to the water–energy hypothesis (Keil et al., 2008), highlighting the role of forested ecosystems in promoting high levels of both temperate and tropical odonate diversity (Paulson, 2006). Forested ecosystems provide the landscape matrix within which both lentic and lotic aquatic oviposition sites are embedded, and the keystone role that forests play in the maintenance of healthy aquatic systems where significant forest regrowth has occurred like NYS is plain (Huntington et al., 2009). Intact mature forests are vital to the viability of odonate populations because they serve as thermoregulatory refugia where pre-reproductive adults feed and attain sexual maturity, greatly enhancing their survival rates (Corbet, 2006). Rith-Najarian (1998) empirically demonstrated how intact older forests foster both larval and adult life histories resulting in greater dragonfly species abundance and diversity in the landscape manifesting as regional patterns of species density.
Arid subtropical climates, even at significantly lower latitudes in NA (Stevens & Bailowitz, 2009) support an order of magnitude lower odonate diversity than NYS. Similarly, over the past few million years as the tropical forests of Africa experienced increasing aridification, the African odonate fauna became impoverished, favouring the evolution of widespread lentic generalists (Damm et al., 2010). On the other hand, in neotropical forests, site- and landscape-scale diversity of Odonata can reach astonishingly high levels (Paulson, 2006; Gonzalez Soriano et al., 2011). At broader scales though, two of tropical Mexico's best surveyed and highest diversity states – Chiapas and Veracruz – that together are a bit larger than NYS, contain nearly identical concentrations of species richness as NYS (Gonzalez Soriano & Novelo Gutierrez, 2007).
Does the evolution of both Odonata and temperate forested ecosystems bear this out? All the modern families of Odonata have been in existence since the Mesozoic era (Grimaldi & Engel, 2005), and the basal family members of our estimated tree are all much older than 100 Ma (Fig. 3). Carle and Cook (1984) and Carle (1995) hypothesised that they had originated in cool groundwater-fed seeps and streams in the Gymnosperm forests of the mid-latitudes of eastern NA. Pangaea was breaking apart at this time and the NA land mass had just broken away from EurAfrica, and NYS would have been at a slightly lower latitude (~ 35°N) than today (Noonan, 1988; Sanmartin et al., 2001) with fluctuating temperate palaeoclimates (Dera et al., 2011).
This predates the rise of Angiosperms during the Cretaceous period (< 100 Ma) and the deciduousness that is the hallmark of these forests is a reflection of the increasing seasonality of the climate (Wolf, 1987). Their pre-adapted cold-hardy aquatic larval stage must have buffered certain odonates from ever more extreme temperature fluctuations post-Eocene (Archibald et al., 2010), and these detritus-based groundwater-fed ecosystems are known to offer an unparalleled buffering effect even during extreme environmental deterioration leading to mass extinctions (Robertson et al., 2013). Although fossil preservation is low in the humid erosional forests of Eastern/central NA (Dillon & Robinson, 2009), there are very few stem group Odonata (extinct lineages) in NA compared to other parts of the globe and the oldest Odonata fossil evidence from NA that we could find dates to around the Palaeocene ~ 60 Ma (Wighton & Wilson, 1986). Thus, we cannot rule out relatively lower extinction rates as a contributing factor to the high levels of forested freshwater diversity maintained here for aeons.
Our 5-year atlas detected range margin shifts for a number of odonates, but range contractions leading to species' losses from a region (extirpations) are notoriously difficult to document. Where sufficient fine-scale distribution data are available, the trend in Europe is clearly towards range expansion of warm-adapted tropical odonates and retraction of the more cold-adapted boreal species as the climate warms (Hassall & Thompson, 2008; Ott, 2010). This pattern has also been well-documented for other European insects, but comparable evidence in NA had until recently been lacking. New trend data for 100 butterfly species gathered by citizen scientists in neighbouring Massachusetts between 1992 and 2010 have found this same process of large-scale climate-driven insect species reshuffling in northeastern NA (Breed et al., 2013); we anticipate seeing similar results emerge in the near future.
We must acknowledge the dedicated field efforts of all the citizen scientists who helped us to assemble this database. Funding was provided through NYS Wildlife Grant T-2-1 in cooperation with the U.S. Fish and Wildlife Service division of Wildlife and Sport Fish Restoration. The Nature Conservancy of New York provided funding for manuscript preparation. We thank Timothy Howard for providing critical feedback and advice on earlier versions that greatly improved the manuscript as did the incisive comments of two anonymous reviewers. Paul Novak and Thomas ‘Nick’ Donnelly offered expertise, encouragement, and support in numerous ways. Special thanks go to Bernd Dieter Bruch for translating Sternberg (1998) to English, and NYS DEC librarians Deborah Ferguson, Emily Wager, and Connie Allesse who located many references for us.