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An Assessment of the Geographic Distribution and Status of a Rare Dragonfly, Rhionaeschna mutata, at the Northwestern Edge of its Range
Emily Gaenzle Schilling, Ron Lawrenz, and Holly Kundel

Northeastern Naturalist, Volume 26, Issue 3 (2019): 523–536

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Northeastern Naturalist Vol. 26, No. 3 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 523 2019 NORTHEASTERN NATURALIST 26(3):523–536 An Assessment of the Geographic Distribution and Status of a Rare Dragonfly, Rhionaeschna mutata, at the Northwestern Edge of its Range Emily Gaenzle Schilling1,*, Ron Lawrenz2, and Holly Kundel1 Abstract - Rhionaeschna mutata (Hagen) (Spatterdock Darner) is a rare North American dragonfly, most widely distributed in the eastern US. In 2009, a reproductive population was found in 2 ponds in eastern Minnesota, establishing a substantial northwestern range expansion. We assessed the geographic distribution of the Spatterdock Darner in the region to inform conservation planning for this species. Using previously defined habitat criteria, we identified potential reproductive ponds in the ecoregion with GIS. In 2015 and 2016, we used multiple methods to survey 25 ponds for Spatterdock Darner nymphs, adults, and exuviae. We found no Spatterdock Darners in the region, despite intensive survey efforts targeted at ponds that met the habitat criteria. The Spatterdock Darner may be present in this water-rich region, but was undetected by our efforts, or a local extirpation may have occurred, possibly linked to recent fish colonization in one of the original reproductive ponds. Introduction Rhionaeschna mutata (Spatterdock Darner) is a rare and ecologically restricted North American dragonfly species (Schilling et al. 2019). This species, formerly named Aeshna mutata, was reclassified in 2003 to Rhionaeshna, a primarily tropical genus comprised mostly of species residing in South America (von Ellenreider 2003). Spatterdock Darner is one of only 5 North American species in its genus and the only one with an eastern North American range (Fig. 1). Its taxonomic distinctiveness in eastern North America makes this species a priority for conservation (White et al. 2015). The Spatterdock Darner is considered a species of concern or threatened throughout its range (NatureServe 2017). Although the cause for this species’ rarity is unknown, its habitat specificity along with declining habitat availability and quality—caused by anthropogenic environmental change—are possible contributing factors (Schilling et al. 2019). A narrow set of habitat conditions describes the Spatterdock Darner’s reproductive habitat: small, heavily vegetated, semi-permanent, fish-free ponds with a wooded riparian edge and with sphagnum present (Schilling et al. 2019). Our study focused on a Spatterdock Darner population recently discovered in Minnesota, which established the farthest northwestern record of this species (DuBois et al. 2015). This population was first observed in 2009 in 2 adjacent small (1.7-ha [4.2-ac] and 1.2-ha [3-ac]), fish-free kettle ponds (45°10'44"N, 1Biology Department, Augsburg University, Minneapolis, MN 55454. 2Science Museum of Minnesota’s Warner Nature Center, Marine on St Croix, MN 55047. *Corresponding author - schillin@augsburg.edu. Manuscript Editor: Christopher M. Heckscher Northeastern Naturalist 524 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 Vol. 26, No. 3 92°50'21"W) in a forested region of the St. Croix River valley (St. Paul–Baldwin Plains and Moraines ecoregion; MN-DNR 2017). The closest known population is ~360 km southeast in the Southeastern Wisconsin Till Plains ecoregion (Fig. 1; Omernik et al. 2000). When first observed in Minnesota, a reproductive population was detected in the 2 kettle ponds with the collection of 21 aquatic nymphs (Dubois et al. 2015). Several adults were also observed flying at the 2 ponds that same year (DuBois et al. 2015). Adults, nymphs, and exuviae were captured and observed at these sites over the following 5 years, indicating a viable breeding population for several seasons (DuBois et al. 2015). In 2014, the year preceding our study, depressed numbers of the Spatterdock Darner were collected from these sites. Thus, we initiated this study with the goal of informing state-level conservation planning for this rare and imperiled species. The objectives of our study were to: (1) identify potential Spatterdock Darner reproductive ponds in the St. Paul–Baldwin Plains and Moraines ecoregion in Minnesota, and (2) assess the distribution of this species in Minnesota by surveying a subset of ponds for the presence of Spatterdock Darner nymphs, adults, and evidence of emergence. Materials and Methods Field-study region and study-pond selection Our study region is water-rich, with aquatic habitats comprising more than 11.5% of the landcover within the ecoregion (Fig. 2; MN-DNR 2006). In particular, this region of Minnesota is densely populated with kettle ponds confined in basins Figure 1. Current known distribution of Rhionaeschna mutata (Spatterdock Darner) in the northeast based on records (gray circles) submitted to Odonata Central database (Abbott 2018). Map indicates the location of our study population () and the closest known reproductive population (▲) 360 km southeast (map from www .odonatacentral.org). Northeastern Naturalist Vol. 26, No. 3 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 525 created by receding glaciers 10,000 y ago. Topographically low in comparison to other areas in the state, the St. Paul–Baldwin Plains and Moraines ecoregion is dominated by a large moraine and areas of outwash from the Superior lobe (MNDNR 2017). As a result, the drainage network is poorly developed throughout most of the region, which has prevented fish colonization of kettle ponds until recently, as humans have increasingly disrupted natural barriers to fish movement (Rahel 2007, Schilling et al. 2009). The 2 ponds where the Spatterdock Darner was first documented in Minnesota fit this paradigm. We used previously identified reproductive habitat criteria as a guide for selecting potential breeding ponds in our study region as study sites (Schilling et al. 2019). We aimed to select small, heavily vegetated, semi-permanent, fish-free ponds with a wooded riparian edge and sphagnum present. We remotely assessed some of these criteria (Table 1) with a geographic information system (ArcGIS for Desktop version 10.3; ESRI Inc., Redlands, CA). We used the National Wetland Inventory (NWI) polygon coverage for east-central Minnesota (Macleod et al. 2013) overlaid with the Ecological Classification System in Minnesota coverage (MNDNR 2017) to locate ponds in the same ecoregion (St. Paul–Baldwin Plains and Moraines) as the original reproductive sites. We selected all waterbodies within this ecoregion that are hydrogeomorphically categorized as terrene ponds (palustrine unconsolidated bottom or aquatic bed wetlands completely surrounded by uplands). Figure 2. Map of ponds surveyed in Minnesota for Rhionaeschna mutata (Spatterdock Darner) in 2015/2016 (figure also shows surrounding waterbodies) . Northeastern Naturalist 526 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 Vol. 26, No. 3 The reason for narrowing our selection to terrene ponds is that they lack inflowing and outflowing streams and are thus isolated from the surface hydrology network (Macleod, et al. 2013). Hydrological isolation is useful as a proxy for fish-free, as hydrologically isolated ponds lack natural routes of colonization by fish (Magnuson et al. 1998, Nolby et al. 2015, Schilling et al. 2008). All waterbodies within this hydrogeomorphic category are small <10.12 ha (25 ac) and semi-permanent. The National Wetlands Inventory describes these ponds as, “Code G: Water covers the substrate throughout the year except in years of extreme drought, or Code F: Surface water persists throughout the growing season in most years. When surface water is absent, the water table is usually at or very near the land surface.” (US Fish and Wildlife Service 2018). To identify ponds with a wooded riparian edge, we created a 100-m buffer around all selected ponds and overlaid the Minnesota Landcover Classification System coverage (MN-DNR 2015). We then selected waterbodies with >50% of their buffer classified as “forest”. We visited a subset of the remotely assessed ponds (n = 25; Fig. 2) within a 3-km radius of the original reproductive sites to ground-truth for Spatterdock Darner reproductive habitat (Table 1). We assessed fish presence/absence using minnowseines (0.91 m x 6.10 m [3 ft x 20 ft] net with 0.32-cm [1/8-in] mesh; Fig. 3) and baited minnow traps set overnight in each study pond. We determined the maximum depth of ponds with a distinct littoral zone by boat using a hand-held depth meter. We visually assessed “boggy” conditions by searching the shoreline perimeter for the presence of sphagnum and measuring pH (1 date/pond with a handheld YSI Professional Plus multiparameter water-quality meter; Hach, Loveland, CO). We also visually assessed aquatic vegetation cover and type in each pond. We surveyed for Spatterdock Darner presence at all ponds that met reproductive habitat criteria, as described below. Dragonfly surveys We conducted 2 years of field surveys (2015 and 2016) to assess the presence of Spatterdock Darner adults and nymphs in our study ponds. The Spatterdock Darner is the earliest emerging darner species in northeastern North America, with a flight period of late May through July; other darners have a mid-late summer flight period Table 1. Number (and percentage) of study ponds in the St. Croix River Valley, MN exhibiting characteristics of breeding habitat for Rhionaeschna mutata (Spatterdock Darner). An asterisk (*) denotes characteristics that were remotely assessed and ground-truthed. Number (and percentage) of study ponds ( n = 25) Habitat characteristics exhibiting characteristic Nuphar spp. (yellow waterlilies) 13 (52%) Heavily vegetated 25 (100%) Boggy (sphagnum/peat) 10 (40%) Wooded riparian edge* 25 (100%) Fishless* 14 (56%) Shallow/small* 25 (100%) Semi-permanent* 25 (100%) Nymphaea spp. (white waterlilies) 11 (44%) Northeastern Naturalist Vol. 26, No. 3 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 527 (Beatty and Beatty 1969, New York Natural Heritage Program 2017, Oldham 2007). Therefore, we began our collection period in mid-May and continued through June to target the F-0 and F-1 (nearly mature) nymphs and emerging tenerals. We used the methods described below to survey dragonflies at least once per study pond during this period. We selected a subset of 9 ponds (including the 2 original reproductive ponds) to be sampled more frequently (2–8 times/pond) in both spring and late fall (to target large overwintering nymphs) over the 2-y study period. We employed various sampling methods to target all dragonfly life stages, with an emphasis on collecting nymphs and exuviae. Nymph and exuviae collections are essential for identifying successful dragonfly reproductive habitat. Adults are vagile and their presence near a waterbody may not indicate viable reproductive behavior (Raebel et al. 2010). Exuviae collections are particularly effective for verifying successful development within a certain waterbody, as source–sink dynamics may affect reproductive success (Foster and Soluk 2004, Raebel et al. 2010, Sigutova et al. 2015). We sampled nymphs in the littoral zone of each pond using 3 types of equipment chosen to increase sampling surface area to maximize catchper- unit effort: large dipnets (40.64 cm x 33.02 cm [16 in x 13 in] net with 0.48-cm [3/16-in] mesh [www.frabill.com #3529]), nylon minnow seines (0.91 m x 6.10 m [3ft x 20 ft] net with 0.32-cm [1/8-in] mesh), and sweep frames (0.91 m x 0.91 m [3 ft x 3 ft] wood frame, 0.32-cm [1/8-in] mesh hardware cloth]). On each sampling Figure 3. Our minnow-seine sweep sampling technique in one of the 2 small, fishless, kettle ponds where a breeding population of Rhionaeschna mutata (Spatterdock Darner) was first detected in Minnesota. Northeastern Naturalist 528 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 Vol. 26, No. 3 date, we conducted 5 composite (3 passes each) dipnet sweeps, 2– 3 minnow-seine sweeps, and ~10 frame sweeps per pond. We collected exuviae by visually searching shoreline vegetation for ~1 observer h/sampling date/pond and preserved for later identification representative specimens of all collected dragonfly species. We were particularly interested in locating ponds that supported Aeshna tuberculifera (Walker) (Black-tipped Darner), because this species has reproductive habitat characteristics similar to those of the Spatterdock Darner (low pH, vegetated ponds with boggy fringe in forested regions; Nikula et al. 2003, Paulson 2011, Pollard and Berrill 1992) and others have documented the co-occurrence of these 2 species (Mochon 2015, Schiffer and White 2014). We used binoculars to observe dragonfly adults on the wing. Its brilliant blue eyes make the Spatterdock Darner distinguishable from other darners in the region, except for Rhionaeschna multicolor (Hagen) (Blue-eyed Darner), which is the only congeneric found in Minnesota (Fig. 4; Mead 2017). The flight period of Blue-eyed Darner and Spatterdock Darner do not closely overlap, so the 2 are unlikely to be confused (DuBois 2018). Results and Discussion Based on remotely assessable criteria (Table 1), we identified 562 ponds as candidates for supporting reproductive Spatterdock Darner populations in the St. Figure 4. The brilliant blue eyes of Rhionaeschna mutata (Spatterdock Darner) are a key identifying characteristic, making it distinguishable from other darners in the study region. Northeastern Naturalist Vol. 26, No. 3 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 529 Paul–Baldwin Plains and Moraines ecological subsection of Minnesota. The 25 ponds that we selected for ground-truthing exhibited multiple Spatterdock Darner reproductive habitat criteria (Table 1). Study pond size varied from 0.16 ha to 6.68 ha and pH spanned from 5.07 to 7.97 (average = 6.40); the majority (72%) of ponds had a pH < 7.00, and thus were slightly acidic. Maximum depth ranged from 0.76 m to 2.74 m. Fish surveys revealed that 8 ponds supported fish populations, although all of these waterbodies were likely historically fish-free, because they lacked natural routes of fish colonization. We collected no Spatterdock Darners in our study ponds during our 2-y sampling period, including the 2 original reproductive ponds, despite employing intensive collection methods targeting all life-stages at ponds that demonstrated suitable habitat characteristics for Spatterdock Darner. First observed in 2009, the last observation of this species in Minnesota occurred in 2014, the summer prior to initiation of the current study (Dubois et al 2015). Other darner species that we collected in our study ponds include: Aeshna canadensis (Walker) (Canada Darner; n = 12 ponds), Black-tipped Darner (n = 11 ponds), and Anax junius (Drury) (Green Darner; n = 9 ponds). We propose 2 hypotheses for why we collected no Spatterdock Darners during our study period: (1) the appearance/disappearance of the species in the region was a localized immigration/extinction event, explained by source–sink dynamics, and (2) Spatterdock Darners are still present in the region and have gone undetected by our survey efforts. These 2 hypotheses are not mutually exclusive. Localized immigration/extinction event The disappearance of Spatterdock Darners from the original breeding ponds in Minnesota is indicative of this species’ transience, a characteristic which has been documented throughout its range (Schilling et al. 2019). The transience of this species aligns with the findings of Van Allen et al. (2017) that dragonfly communities in fish-free ponds show higher levels of species turnover than in ponds where fish are top predators (Van Allen et al. 2017). The abundance of Dragonfly species in fish-free ponds can fluctuate an order of magnitude from year to year (Schiffer and White 2014; E. Schilling and R. Lawrenz, pers. observ.). It is possible that if this population cycle is particularly extreme in a given year, numbers can become so depleted that the species in question goes locally extinct, unless it is supported by populations occupying nearby ponds across the landscape. While few studies have documented persistent Spatterdock Darner populations, Shiffer and White (2014) observed this species as a regular resident species over a 50-y study of a semi-permanent, “typically” fishless pond in Pennsylvania. Even with this resident population, fluctuations in abundance varied greatly with water levels, periodically requiring recolonization from other source populations. It is possible that the Spatterdock Darner population originally discovered in Minnesota may have been moving through the region, using the ponds to stage dispersal into new habitats. After persisting in the ponds for a few years, the population then experienced a “bad” year, and without nearby source populations for recolonization, went extinct Northeastern Naturalist 530 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 Vol. 26, No. 3 in the region. Our hypothesis that the appearance/disappearance of this species in our study ponds was a stochastic and localized event is supported by the fact that Spatterdock Darners were not collected in any of our study ponds, all of which were located within close proximity to, and provided similar habitat characteristics to, the original reproductive ponds. The apparent rapid local extirpation of Spatterdock Darners from the original reproductive sites in Minnesota was possibly influenced by the recent introduction of fish into 1 of the ponds. The 2 ponds where Spatterdock Darners were initially discovered in Minnesota are isolated kettle ponds, both of which were fish-free at the time of the initial dragonfly surveys in 2009. We also conducted a paleolimnological investigation to assess historical fish absence in these ponds. Using the subfossil remains of Chaoborus americanus (Johannsen), a dipteran with a larval stage that typcially resides in the pelagic zone in fishlesss lakes, in pond sediments as an indicator of historical fish absence (Schilling et al. 2008), we ascertained that both of these waterbodies likely were historically fish-free (E.G. Schilling, unpubl. data). In 2012, however, 3 years after Spatterdock Darners were first discovered in the ponds, R. Lawrenz observed that Culaea inconstans (Kirtland) (Brook Stickleback) fish from an unknown origin had colonized 1 of the 2 ponds. A rusted minnowtrap was discovered on the shoreline of the pond, indicating that the fish may have been introduced illegally (the pond is located on property protected by the Manitou Fund). It is not uncommon for bait-bucket releases of these fish into waters where they are not native (US Geological Survey 2018). Although Brook Stickleback are native to Minnesota watersheds, it is unlikely that these fish naturally colonized this hydrologically isolated kettle pond. Prior to 2012, this pond exhibited all of the identified Spatterdock Darner reproductive habitat criteria except presence of sphagnum, indicating that it was a suitable reproductive site. The riparian edge of the second reproductive pond is less wooded (less than 50% of its perimeter is forested), making it potentially less suitable. The decline in numbers (DuBois et al. 2015) and eventual absence of Spatterdock Darners in both ponds in 2015 and 2016 indicates that a local extirpation occurred, which may be linked to the arrival of fish. Others have demonstrated that fish ponds act as ecological traps for dragonflies (Sigutova et al. 2015), and it is well documented that fish predation directly impacts larval odonates, with some species more vulnerable than others (Crowder and Cooper 1982, Johansson and Brodin 2003, Johnson and Crowley 1980, McPeek 1998, Morin 1984a, Pierce et al. 1985, Schilling et al. 2009, Torben et al. 2010). In addition to direct effects of predation on dragonfly nymphs, fish may compete with Spatterdock Darners for common prey, thus altering food-web dynamics (Morin 1984b). Specifically, Brook Stickleback have been demonstrated to reduce predaceous invertebrate biomass by both direct competition for similar prey items and also by preying on early life-stages of predaceous invertebrates (Hornung and Foote 2006, Wieker et al. 2016). The population decline in the more suitable reproductive pond may have resulted in a corresponding loss in the adjacent pond due to source–sink dynamics (Pulliam 1988). Dragonflies are more likely to experience local extinctions in low-quality Northeastern Naturalist Vol. 26, No. 3 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 531 habitats than in those of higher quality (Suhonen et al. 2010). Habitat specialists, such as the Spatterdock Darner, are more sensitive to changes in environmental conditions, e.g., changes in water quality or community structure through fish stocking, and thus, are more prone to local extinction (Korkeamäki and Suhonen 2002). In our study system, the shift in ecological condition due to the colonization of fish may have led to a cascading effect for the neighboring pond, ultimately resulting in a local extirpation of this species. Imperfect detection It is possible that Spatterdock Darners are still present in this water-rich region but have gone undetected by our survey efforts. Our ability to detect Spatterdock Darners in our study ponds may have been impeded by under-sampling, both in terms of the number of ponds surveyed and our within-pond sampling effort. We surveyed a small number of the hundreds of potential reproductive sites identified by our GIS analysis. Expanded survey efforts may indeed reveal other ponds in the region that support the species. Our study was designed to maximize detection of Spatterdock Darner (e.g., using multiple collection methods targeting all life-stages and timing our nymph collections to maximize the likelihood of detecting the largest nymph stadia). However, within ponds, patchy distribution of nymphs may result in under-detection if the “right” location in an individual pond is not sampled. Similarly, it is possible that Spatterdock Darner exuviae were undetected by our shoreline visual searches. Several studies have documented Odonata emergence several meters off the ground on trees (Bennett and Mill 1992, Corbet 1999, Tennessen 1979, Worthen 2010). The preference of Spatterdock Darners to breed in ponds with wooded uplands may indicate a preference for woody emergence-supports (Fig. 5). We examined downed woody debris for exuviae, but did not search above our sight-line; therefore, they may have gone undetected by our survey efforts. Finally, our method for detecting “suitable ponds” in our study region may have been too coarse to accurately select ponds that are likely to support reproductive Spatterdock Darner populations. The habitat criteria used for detecting potential breeding ponds were developed by Schilling et al. (2019) using the best available information on this species. Due to the paucity of records on breeding populations, these criteria were developed, in part, using adult records, and thus were not restricted to reproductive records alone. The inclusion of adult records when characterizing Odonata breeding habitat criteria may prevent discernment of more idiosyncratic requirements (Patten et al. 2015). This data gap highlights the need for future research focusing specifically on emergence to detect successful breeding populations and associated habitat conditions (Raebel et al. 2010). Implications for conservation Many Odonata species that are considered threatened or rare have limited dispersal and colonization abilities due to either morphological constraints on their movement and/or their reliance on specific habitat conditions (Thompson et al. 2003). Darner dragonflies, such as the Spatterdock Darner, are large-bodied and strong flyers; therefore, their dispersal is unlikely to be morphologically constrained. Northeastern Naturalist 532 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 Vol. 26, No. 3 In addition to our study population, several other new records at the northern edge of this species’ range (collected at similar latitudes to our study population) have been documented over the past 2 decades (Schilling et al. 2019). These new records may be an indicator of the dispersal abilities of this species, and also that it may be shifting the northern edge of its range in response to climate warming. Studies have demonstrated northward range-shifts amongst dragonflies as an adaptive response to climate change (Hickling et al. 2005, Ott 2001). Dragonflies, in general, have demonstrated that they may be well-suited to climate warming, as indicated by their long history in the fossil record relative to other orders and their proven ability to disperse into temperate and subarctic habitats from their tropical origins (Hassall 2008). However, the recent colonization and local extirpation of the Spatterdock Darner in our study ponds indicates that dispersal ability of rare Odonata alone may not ensure their persistence in the face of climate change. Ultimately, the availability of suitable reproductive habitat will dictate whether the Spatterdock Darner will persist in ponds in more northern regions. Our GIS analysis indicates that the St. Paul–Baldwin Plains and Moraines ecoregion is not only water-rich, but also includes a potentially high number of waterbodies that could support this species. The presence of the Black-tipped Darner, a species with similar habitat requirements, in many of our study ponds suggests that these habitats may indeed be suitable. However, local extirpation of Spatterdock Darner following the introduction of fish into Figure 5. Rhionaeschna mutata (Spatterdock Darner) exuvia found on the underside of a downed tree branch, when the species was first detected in Minne sota in 2009. Northeastern Naturalist Vol. 26, No. 3 E.G. Schilling, R. Lawrenz, and H. Kundel 2019 533 1 of the original reproductive ponds in Minnesota is a case in point that this species’ habitat is vulnerable to anthropogenic stressors beyond warming climate. Effective conservation of this species will require consideration of multiple anthropogenic activities that threaten its ability to persist. The rarity of the Spatterdock Darner in our study region in Minnesota supports the conclusions of White et al. 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