TY - JOUR
AU - Butler, Marguerite, A
AB - Abstract Habitat associations provide clues to the resources that influence the life of animals. Food distribution, structural microhabitat or degree of insolation can determine species’ strategies for energy acquisition, locomotor strategy or thermoregulation. A growing body of research suggests that insolation may be important not for heat, but rather for visual performance in communication, crypsis or prey capture. Odonates (damselflies and dragonflies) are famous for their heliothermic habitat associations. Megalagrion nigrohamatum nigrolineatum is a forest-dwelling damselfly endemic to the island of O’ahu and part of an ecologically diverse adaptive radiation with spectacular body coloration. Although many Megalagrion exploit full sun, nigrolineatum can be curiously found in deep shade raising the possibility that it is shade-seeking. Here, we show that nigrolineatum selects perches based on light, and not perch type or temperature. Surprisingly, they did not select the brightest locations available (as might be expected if they are extending their visual function in a challenging habitat), but chose darker perches in a fairly dark habitat. This strategy opens up niche space that is abundantly available in forests, yet little-occupied by other odonates. We discuss implications of shade-seeking for communication, evolutionary diversification and preserving future evolutionary potential. habitat selection, light intensity, Megalagrion, microhabitat, shade-seeking INTRODUCTION Nearly every aspect of an animal’s fitness is affected by where it chooses to live, whether it be obtaining food, mates or thermal benefits, or avoiding predators (MacArthur & Pianka, 1966; Morris, 2003; Dugatkin, 2014). The distribution of food resources can, in turn, also determine social structure and body size variation (Jarman, 1974) and even sexual dimorphism (Dayan & Simberloff, 1994). The selection of locations for broadcasting social displays can increase conspicuousness to potential mates (Marchetti, 1993; Endler & Thery, 1996) or reduce detection by predators (Endler, 1991). The structural microhabitat can also dictate which locomotor strategies are most effective for predator escape, maintaining territories or capturing prey (Losos, 2009). Habitat selection can also facilitate thermoregulation for optimal physiological performance (Huey et al., 1989; Adolph, 1990; Dubois et al., 2009; Kosheleff & Anderson, 2009; Villen-Perez, Carrascal & Gordo, 2014). Indeed, patterns of microhabitat use can provide powerful insight into the importance of various ecological drivers for focal species, but relatively few studies explore habitat use with respect to multiple ecological factors and attempt to rank their influence (Petit & Petit, 1996; Downes & Shine, 1998; Valentine, Roberts & Schwarzkopf, 2007; Greenville & Dickman, 2009). Ecologists have long recognized that species may have tolerance ranges for physical variables of the environment (Grinnell, 1917), and that systematic measurement of ecological variables can reveal a quantitative description of the ‘niche’ (Hutchinson, 1957). Animals may be generalists or specialists with regard to any ecological resource, which can be readily assessed by quantifying the habitat or microhabitat use of animals in comparison to the distribution that is available in the environment. All else being equal, specialization to use only a portion of the resource distribution indicates a greater influence of that ecological axis on the fitness of a species than generalized use (Ravigne, Dieckmann & Oliveri, 2009). Odonates (dragonflies and damselflies) are famous for strong habitat associations generally with streams and open waterways. Important ecological factors include the availability of thermal resources (especially heliothermy, e.g., May, 1976; Corbet, 1980; Nilsson-Örtman et al., 2012), ambient light quality [for social signalling, e.g., Schultz, Anderson & Symes, 2008; Schultz & Fincke, 2013; or exposure to ultraviolet (UV) Cooper, 2010; Cooper, Brown & Getty, 2015], forest structure and the availability of shade (Shelly, 1982; Paulson, 2006) and access to oviposition sites (Rehfeldt, 1990; Fincke, 1992; Switzer, 2002). As all odonates are visual predators, a sun-loving lifestyle would facilitate social signalling, prey capture and optimal flight performance. However, in complex communities, a shade-seeking strategy would open up a large niche space for odonates to exploit. A few studies suggest that this may be the case (Shelly, 1982; Loiola & De Marco, 2011) but active shade-seeking has rarely been demonstrated for any odonate species, and the ecological implications of selecting darker microhabitats have yet to be explored. Megalagrion nigrohamatum nigrolineatum is a tropical damselfly species which is found in forested streams of Hawaiʻi, and it presents an interesting case because it could be a shade-seeking specialist. It is part of a diverse clade of damselflies that form an endemic adaptive radiation (Polhemus & Asquith, 1996; Jordan, Simon & Polhemus, 2003) that together exploit a variety of habitats ranging from pool and stream habitats to waterfalls and non-stream environments. Indeed, the level of ecological diversity found among these species is unparalleled within any other clade of damselflies. Megalagrion nigrohamatum nigrolineatum is an endemic species to the island of Oʻahu, found within the many stream valleys at small pools on the margins of streams and in slow-flowing tributaries and even at puddles remaining from ephemeral streams. Oʻahu’s stream valleys are carved by erosion, creating many high-walled stream valleys that are deeply shaded. Megalagrion nigrohamatum nigrolineatum is sometimes found in sympatry with its congeners; however, it can often be found alone in relatively dark habitats. Even when in sympatry, they are found in peripheral microhabitats away from the main stream. It typically spends much of its time perched, facilitating microhabitat use measurements with respect to many biotic and abiotic factors. Its unusual shade tolerance provides an interesting test case for whether microhabitat use is associated with perch type, height, temperature or light regime in this species, and how our findings compare with more typical odonate behavioural ecologies. MATERIAL AND METHODS Megalagrion nigrohamatum nigrolineatum is a pool-breeding species of damselfly and aside from the occasional mention of M. n. nigrolineatum being a weak flier that frequently perches along streams, its ecology or behaviour has received little study (Polhemus & Asquith, 1996). It is a territorial species with bright colour patches and can be found along streams that are often cluttered and heavily shaded, living in a structurally heterogeneous environment. In 2012, it was added to the US Endangered Species List based on a landscape-level approach, citing invasive predators, particularly poeciliid fish, and habitat loss as the major threats to this species (Englund, 1999; Polhemus, 2007; 77 FR 57647 57862). Field work Field work was conducted in the Fall (between 20 September and 8 November 2012) and Spring (30 April to 9 May 2013), under permits authorized by the Hawaiʻi Department of Land and Natural Resources (FHM13-300). Our study site was located along ‘Aihualama Stream, in the south Koʻolau mountains, Honolulu, Hawaiʻi on the island of Oʻahu (c. 21°20.123′N, 157°48.167′W, elevation 150 m), which is home to a stable population of M. n. nigrolineatum. The site receives an average of 383-cm annual rainfall (Frazier et al., 2016) and frequently experiences significant cloud cover, and temperature does not differ by season. Megalagrion nigrohamatum nigrolineatum is the only Megalagrion species at this site, found along sections of the stream that are heavily shaded, often perching among its lush vegetation, rocks or stream litter. This species remains active during light rain, which commonly occurs at the study site (Elizabeth Henry, Marguerite Butler, in prep.). Data were not collected on days with heavy rain. Microhabitat use data were collected via focal observation. All focal observations were made by the same researcher (E.R.H.) and occurred between the hours of 9:00 and 17:00. The investigator walked slowly and unidirectionally along the stream until a damselfly was spotted. Observations were made from a distance of 1.5–3 m, recording each perch the damselfly used until the damselfly flew out of sight or the 10-min mark, with typical observations lasting 1.5 min. For half the study, prior to the species being federally protected, we were able to mark and release individuals at the close of observation. Furthermore, field work was spread out over many months, and with an average life expectancy of 1 week, it is unlikely that a significant number of damselflies entered into the data set twice. Care was taken to visit the site at different times of day to balance sampling throughout its activity period and to include only mature (not teneral) damselflies behaving normally. Only undisturbed damselflies that used at least one perch were included in the data set. Tandem pairs were evaluated as a separate data set as their perch use needs may differ from the general population, with each tandem pair treated as a single data point. We were unable to evaluate perch preference for females as they were only observed in tandem. Male–male interactions were noted (perched male flying towards another male can include chase in which other male flies away from focal male, mid-air circling and mid-air grappling). Perch data were supplemented by incidental observations. Microhabitat variables We characterized the microhabitat used by damselflies in terms of perch type and height, temperature and degree of insolation, parameters known to be important in other odonates (May, 1976; Pezalla, 1979; Clement & Meyer, 1980; Corbet, 1999; Remsburg & Turner, 2009; Samejima & Tsubaki, 2010). Perch height (from the ground or stream) was measured in centimetres with measuring tape. Substrate type can be important for oviposition sites, as damselflies are known to oviposit on rocks along margins of streams, in the stream litter or in the tissues of the streamside vegetation itself (Corbet, 1999) and was classified as plant, rock (stream boulders) or stream litter (dead vegetation floating on the water). Many species of odonates are known to be heliothermic (May, 1976) and sometimes select perches for thermal transfer from conduction or access to solar radiation (i.e. basking; Corbet, 1980; Samejima & Tsubaki, 2010). However, Endler (1993) noted that the human eye is not always a reliable indicator of absolute light levels in forest shade, as sun flecks are often darker than they appear as a result of the remarkable adaptability of the human eye to low light levels. Therefore, we recorded three variables to distinguish among light and thermal resources: (1) whether perch sites were in sun flecks (yes/no) as ascertained by the human eye; (2) downwelling irradiance, a quantification of the amount of light energy impinging on a flat surface collected from all directions within a 180° hemisphere (Leal & Fleishman, 2002). Irradiance was collected using an Ocean Optics PS1000 portable spectroradiometer with a 180° acceptance angle cosine- corrected probe (Ocean Optics CC-3-UV) attached to the end of the input fibre-optic that was pointed upwards to measure downwelling light; and (3) substrate temperature in Celsius with an infrared temperature gun thermometer (Raytek ST PropPlus). The range of microhabitat conditions available to damselflies was characterized by taking the same set of environmental data at randomized locations throughout the damselfly study site. A random number generator indicated positions along the length of a stream, with a second random number indicating the location along the width of the stream including the adjacent banks. Analyses Microhabitat preferences were tested by comparing the frequency of environmental conditions used by perching damselflies with the distribution of conditions available throughout the study site by means of the Pearson’s chi-square test (Sokal & Rohlf, 1981). To avoid pseudoreplication, mean values were first computed for individuals with multiple perches so that each individual entered only once into the analysis. Downwelling irradiance data (at perch locations) were analysed by first calculating total intensity in μmol photons/s/m2 for each reading by taking the integral between the wavelengths of 300–720 nm. We used a Wilcoxon signed-rank test to evaluate differences in medians between the distribution of intensities collected at perch and random sites (Sokal & Rohlf, 1981). We plotted the distributions of these data after calculating probability densities using kernel density estimation in the package GenKern 2.23 (Lucy & Aykroyd, 2013). We performed a two-sample Kolmogorov–Smirnov test for differences between the probability distributions of light intensity at perch vs. random locations (Hollander & Wolfe, 1973). All statistical analyses were performed using R statistical computing environment (R Core Team, 2017). RESULTS In total, 202 focal observations were collected on single males and 15 on tandem pairs during 41.5 h of field work. Females were only seen in tandem with a male; therefore, we observed a total of 219 males to 15 females. Damselflies were present at the stream site from 0900 to 1700 h. Damselflies were ‘active’ in both Spring and Fall, and there was no difference in perch use by season or time of day (results not shown). Damselfly activity consisted primarily of perching, with occasional flights to move perches, to sally for prey (all Megalagrion catch insect prey in flight) or engage in social interactions. We observed more damselflies along the upper portion of the stream, which contained numerous small tributaries. Downstream of the study site, where no damselflies are found, the stream is wide and deep with an open canopy. All sections of the stream contain many introduced species which are potential predators including crayfish, fish, frog tadpoles and dragonfly naiads. The stream is covered by a dense canopy composed of many alien tree species, notably Ficus spp. and Falcantaria moluccense. Damselflies chose deep understory habitat along the stream, which was dominated by shade broken by sun flecks (generally very small gaps in the canopy where rays of filtered sun penetrated) as opposed to adjacent open habitat. They would perch near stream sections that were relatively shallow with slow-moving water. The distribution of males along ʻAihualama stream was somewhat patchy, with multiple males competing for apparently favoured locations such as stream litter or particular rocks or perches. Light preferences The availability of sun flecks in sections of the stream where damselflies are found was low, with only 21% of randomly selected potential perch sites in visible sun flecks (‘Random’, Fig. 1). For most activities, damselflies showed no preference for sun flecks. Neither perching male damselflies (n = 283; χ2 = 0.036; d.f. = 1; P = 0.849; Fig. 1) nor tandem pairs (whether ovipositing or not; n = 10, χ2 = 0.0205, d.f = 1. P = 0.886) showed any difference in perch selection with respect to sun flecks. However, male–male interactions occurred more frequently in sun flecks than expected at random (n = 14; χ2 = 11.11; d.f. = 1; P = 0.00086; Fig. 1), involving half of all male–male interactions. Only 14% of males perched in the sun exhibited any territorial behaviour. Figure 1. Open in new tabDownload slide Perched males and tandem pairs show no preference for sun flecks (perched males vs. random locations: n = 283, χ2 = 0.036, d.f. = 1, P = 0.849; perched tandem pairs vs. random locations: n = 10, χ2 = 0.0205, d.f. = 1, P = 0.886). However, male–male interactions preferentially occurred in sun flecks (male–male vs. random: n = 14, χ2 = 11.11, d.f. = 1. P = 0.8.6e−4). Proportion of perches in sun flecks (yellow) vs. shade (grey), as ascertained qualitatively. Asterisk represents significant difference from random and error bars represent 95% confidence limits. Figure 1. Open in new tabDownload slide Perched males and tandem pairs show no preference for sun flecks (perched males vs. random locations: n = 283, χ2 = 0.036, d.f. = 1, P = 0.849; perched tandem pairs vs. random locations: n = 10, χ2 = 0.0205, d.f. = 1, P = 0.886). However, male–male interactions preferentially occurred in sun flecks (male–male vs. random: n = 14, χ2 = 11.11, d.f. = 1. P = 0.8.6e−4). Proportion of perches in sun flecks (yellow) vs. shade (grey), as ascertained qualitatively. Asterisk represents significant difference from random and error bars represent 95% confidence limits. Overall, the mean intensity of light within the M. n. nigrolineatum habitat is low. This is true whether in comparison to downwelling irradiance at other habitats where congeners live (Julio Rivera, Marguerite Butler, in prep.) or in comparison to measured odonate light habitats worldwide (Table 1). Within this relatively dark habitat, perch sites selected by damselflies were significantly different in intensity from randomized locations (differences in distributions of perch vs. random intensities are highly significant: Kolmogorov–Smirnov test D = 0.36, P = 0.002; Fig. 2). However, the selected sites were not brighter. In fact, they were significantly darker, indicating that M. n. nigrolineatum damselflies showed a preference for the lowest light intensities available (median intensity at perches: 16.20 μmol photons/s/m2 vs. at random locations: 20.99 μmol photons/s/m2 are significantly different: Wilcox W = 1036, P = 0.05; Fig. 2). Figure 2. Open in new tabDownload slide Damselflies prefer darker perches. Probability density of downwelling irradiance at perch sites chosen by M. n. nigrolineatum (n = 41) in red and available locations (n = 65) in black. Damselflies show a significant preference for the darker locations within the habitat (Wilcox W = 1036, P = 0.05; Kolmogorov–Smirnov test D = 0.36, P = 0.002). Preference did not change between days. Figure 2. Open in new tabDownload slide Damselflies prefer darker perches. Probability density of downwelling irradiance at perch sites chosen by M. n. nigrolineatum (n = 41) in red and available locations (n = 65) in black. Damselflies show a significant preference for the darker locations within the habitat (Wilcox W = 1036, P = 0.05; Kolmogorov–Smirnov test D = 0.36, P = 0.002). Preference did not change between days. There was no difference in temperature between perch sites. Perch sites in sun flecks were on average 23.3 °C ± 0.21 SE and perch sites in shade were 23 °C ± 0.09 SE (one-sided t-test, T = 1.09, d.f. = 38.9, P = 0.14). Temperature was also uniform across substrate types (ANOVA; F = 1.5543, d.f. = 2, P = 0.2155) and was uncorrelated with perch height (R2 = 0.006, P = 0.1837). Perch type Damselflies showed no preference for perch type. Male damselflies were found on plant, rock and stream litter according to their availability (Fig. 3). Perch height varied for plant, rock and stream litter, with means of 72.6 cm ± 5.8 SE, 29.2 cm ± 3.7 SE and 3.89 cm ± 1.5 SE, respectively, with an overall pattern of using perches low to the ground. Fifty per cent of all perches were less than 35 cm above the ground or stream surface (mean 48 cm ± 3.8 SE). Figure 3. Open in new tabDownload slide Damselflies showed no preference for perch type. Percentage of perches used by substrate type (plant, rock and stream litter; n = 239 individuals), with white bars indicating availability of each type in the habitat based on random sampling (n = 77). Damselfly perch type matched their availability in the habitat (χ2 = 371.82, d.f. = 462, P = 0.9992). Error bars represent SE. Figure 3. Open in new tabDownload slide Damselflies showed no preference for perch type. Percentage of perches used by substrate type (plant, rock and stream litter; n = 239 individuals), with white bars indicating availability of each type in the habitat based on random sampling (n = 77). Damselfly perch type matched their availability in the habitat (χ2 = 371.82, d.f. = 462, P = 0.9992). Error bars represent SE. DISCUSSION We found evidence for active shade-seeking in M. n. nigrolineatum, a finding that is uncommon among odonates (Corbet, 1980, 1999; Cordoba-Aguilar, 2008). To the best of our knowledge, this is the first quantitative demonstration of shade-seeking in this group. We found preferred light levels that are substantially lower than other odonates by up to two orders of magnitude (Table 1), similar to only one other species which is described as forest-dwelling and shade-seeking (Shelly, 1982). This finding is particularly striking because a preference for well-lit sites as explained by ‘heliothermy’ is a dominant theme in describing odonate natural history (e.g. Lutz & Pittman, 1970; Pezalla, 1979; Waringer, 1982; Hamilton & Montgomerie, 1989; Corbet, 1999; De Marco & Resende, 2002; Remsburg, Olson & Samways, 2008; Dixon & Gennard, 2010; Goforth, 2010). Odonates provide many text-book examples of baskers, with thermal gain thought to provide an advantage for flight performance (e.g. Samejima & Tsubaki, 2010). However, M. n. nigrolineatum’s habitat is thermally uniform, thereby eliminating opportunities for conductive heat gain as there are no warmer surfaces to be found. In addition, there is little evidence for radiative heat gain as the measured light intensities are very low, probably due to filtering from the typically heavy cloud cover and forest canopy. Although no study has yet established the minimum irradiance level required to raise damselfly body temperature, Shelly (1982) conducted a correlative study of a basking and shade-seeking damselfly and found that the body temperature of the basking species did not begin to rise until the irradiance level exceeded 35000 lux, an intensity more than an order of magnitude higher than our measured values. Therefore, this habitat may not allow sufficiently intense solar radiation for substantial radiative heat gain (this would also help explain the thermal uniformity of substrate temperatures; see also Hertz, Fleishman & Armsby, 1994). A thermoconforming strategy is known in some Ischnura and Protoneura damselflies and small-bodied dragonflies (Corbet & May, 2008; Carvalho et al., 2013). Thermoconforming species provide a unique opportunity to explore the importance of light for vision independent of any thermal benefit. Table 1. Intensity of habitat light for odonate species including Megalagrion nigrohamatum nigrolineatum Species Location Habitat Mean photon flux (µmol/s/m2) Min photon flux (µmol/s/m2) Reference Damselflies  Megalagrion nigrohamatum nigrolineatum HI, USA Forest 21.03 4.78  Heteragrion erythrogastrum Panama Forest 10.8 Shelly (1982)  Argia difficilis Panama Forest 917.9 Shelly (1982)  Ischnura elegans UK Pond 1250.96 Dixon (2010)  Coenagrion puella UK and France Pond 1154.4 125.4 Dixon (2010); Angelibert & Giani (2003)  Lestes sponsa UK Pond 1294.9 Dixon (2010)  Coenagrion scitulum France Pond 125.4 Angelibert & Giani (2003) Dragonflies  Libellula pulchella MN, USA Pond 1791 Pezalla (1979)  Libellula depressa France Pond 125.4 Angelibert & Giani (2003)  Sympetrum infuscatu Japan Paddy field 1900 Watanabe & Kato (2012)  Cordulegaster bidentata Italy Forest 15.2 Manenti, Siesa & Ficetola (2013) Species Location Habitat Mean photon flux (µmol/s/m2) Min photon flux (µmol/s/m2) Reference Damselflies  Megalagrion nigrohamatum nigrolineatum HI, USA Forest 21.03 4.78  Heteragrion erythrogastrum Panama Forest 10.8 Shelly (1982)  Argia difficilis Panama Forest 917.9 Shelly (1982)  Ischnura elegans UK Pond 1250.96 Dixon (2010)  Coenagrion puella UK and France Pond 1154.4 125.4 Dixon (2010); Angelibert & Giani (2003)  Lestes sponsa UK Pond 1294.9 Dixon (2010)  Coenagrion scitulum France Pond 125.4 Angelibert & Giani (2003) Dragonflies  Libellula pulchella MN, USA Pond 1791 Pezalla (1979)  Libellula depressa France Pond 125.4 Angelibert & Giani (2003)  Sympetrum infuscatu Japan Paddy field 1900 Watanabe & Kato (2012)  Cordulegaster bidentata Italy Forest 15.2 Manenti, Siesa & Ficetola (2013) Open in new tab Table 1. Intensity of habitat light for odonate species including Megalagrion nigrohamatum nigrolineatum Species Location Habitat Mean photon flux (µmol/s/m2) Min photon flux (µmol/s/m2) Reference Damselflies  Megalagrion nigrohamatum nigrolineatum HI, USA Forest 21.03 4.78  Heteragrion erythrogastrum Panama Forest 10.8 Shelly (1982)  Argia difficilis Panama Forest 917.9 Shelly (1982)  Ischnura elegans UK Pond 1250.96 Dixon (2010)  Coenagrion puella UK and France Pond 1154.4 125.4 Dixon (2010); Angelibert & Giani (2003)  Lestes sponsa UK Pond 1294.9 Dixon (2010)  Coenagrion scitulum France Pond 125.4 Angelibert & Giani (2003) Dragonflies  Libellula pulchella MN, USA Pond 1791 Pezalla (1979)  Libellula depressa France Pond 125.4 Angelibert & Giani (2003)  Sympetrum infuscatu Japan Paddy field 1900 Watanabe & Kato (2012)  Cordulegaster bidentata Italy Forest 15.2 Manenti, Siesa & Ficetola (2013) Species Location Habitat Mean photon flux (µmol/s/m2) Min photon flux (µmol/s/m2) Reference Damselflies  Megalagrion nigrohamatum nigrolineatum HI, USA Forest 21.03 4.78  Heteragrion erythrogastrum Panama Forest 10.8 Shelly (1982)  Argia difficilis Panama Forest 917.9 Shelly (1982)  Ischnura elegans UK Pond 1250.96 Dixon (2010)  Coenagrion puella UK and France Pond 1154.4 125.4 Dixon (2010); Angelibert & Giani (2003)  Lestes sponsa UK Pond 1294.9 Dixon (2010)  Coenagrion scitulum France Pond 125.4 Angelibert & Giani (2003) Dragonflies  Libellula pulchella MN, USA Pond 1791 Pezalla (1979)  Libellula depressa France Pond 125.4 Angelibert & Giani (2003)  Sympetrum infuscatu Japan Paddy field 1900 Watanabe & Kato (2012)  Cordulegaster bidentata Italy Forest 15.2 Manenti, Siesa & Ficetola (2013) Open in new tab Alternatively, light preference may be the result of optimization for visual capabilities. This is an especially appealing hypothesis for odonates, as they use vision as a primary sensory modality for both foraging and mating behaviours. A small but growing body of research is indicating that for many odonate species, light intensity and availability of shade may be the best predictors of abundance and community assembly and may be even more important than ambient temperature or vegetation structure (Pezalla, 1979; Anholt, 1992; Paulson, 2006; Remsburg et al., 2008; Saito et al., 2016; this study). Our finding is particularly interesting in demonstrating that visually oriented species can also specialize to low light levels, an environmental condition that is more challenging for vision. We note that our demonstration of dim light preference is probably conservative, as our measurements were made within the vicinity where damselflies are found. In particular, we did not sample the brightest light available nearby—the open meadows and stream sections without canopy that are adjacent to the study site (and damselflies are never found in there). Therefore, at the habitat level, M. n. nigrolineatum selects areas that are deeply shaded. Within the shaded habitat, M. n. nigrolineatum shows no preference with regard to sun flecks (Fig. 1); however, there are varying degrees of light intensity within forest shade and light gaps (Endler, 1993) and M. n. nigrolineatum selects perches at the lowest light levels (Fig. 2). Furthermore, M. n. nigrolineatum may be averse to sunny days as it was not found at the site on ‘sunny’ days (mean: 73.60 μmol photons/s/m2), whereas it was active on cloudy days (mean: 24.33 μmol photons/s/m2; Wilcoxon sign-rank test: W = 658; P = 1.87e−5). These findings present several questions for future study, including the following: (1) For species which rely on visual communication and visual cues in hunting, what is the advantage for preferring darker habitats? (2) Are there compensatory mechanisms for specialization to this visually more challenging habitat? (3) What ecological or evolutionary opportunities are available with this preference? Implications for vision It is no surprise that many members of the adaptive radiation of Megalagrion damselflies can be found in full sun (Polhemus & Asquith, 1996; Cooper, 2010; Cooper et al., 2015; Scales & Butler, 2015; Julio Rivera, Marguerite Butler, in prep.). Many Megalagrion can be found in complex multi-species communities, often with similarly coloured congeners, and bright light should aid in social signalling and discrimination. Perching odonates are also known to take off after prey that are first sighted from their perch locations (Olberg, Worthington & Venator, 2000; Olberg et al., 2005, 2007), a decision which involves discrimination of size and distance to the object, both of which are assessed visually. Although visual discrimination is inherently more difficult at dim light levels (Land & Nilsson, 2002), M. n. nigrolineatum similarly chases after prey or conspecifics from their perch locations. We know that M. n. nigrolineatum has relatively large eyes and facet diameters which should confer greater sensitivity (Scales & Butler, 2015). Whether they have additional adaptations or behaviours to facilitate visual performance under low light is unknown. Although male damselflies showed no preference for perch type, they did choose low-lying perches (mode 35 cm vs. average height 48 cm ± 3.8 SE). The visual acuity of this species can be modelled from eye morphology. Based on an interommatidial angle of 0.658° in its acute zone (Scales & Butler, 2015), the distance at which a 3.5-mm damselfly head can be resolved is c. 29 cm. Therefore, the selected perch heights are consistent with the visual acuity of this species for identifying animals approaching from the water. We note, however, the possibility of observational bias as we did not search for damselflies significantly above human height. Ambient light can also facilitate territorial advertisement and social signalling (Endler, 1993). One mechanism that forest-dwelling animals exploit is the use of high-contrast signals. For example, Phylloscopus birds living in dark habitats have evolved bright body signals and use them strategically with selective display (Marchetti, 1993). These species control their level of conspicuousness by revealing their bright patches only when displaying, displaying in locations that offer high contrast and hiding their signals otherwise (Marchetti, 1993). Some Enallagma damselflies are only active during times of the day when their body coloration has the highest level of contrast with the visual background (Schultz et al., 2008), whereas Megaloprepus caerulatus damselflies use structural colours on wing bands to create flashing signals even in dark habitats (Schultz & Fincke, 2009). Selection of light microhabitats and exposing colour signals to augment conspicuousness when displaying and promoting crypsis at other times is well documented for many forest-dwelling species, particularly birds (Endler, 1986, 1991; Endler & Thery, 1996). Our finding of damselfly male–male interactions occurring more frequently in sun flecks supports the notion that this strategy may be of more general importance. Interestingly, M. n. nigrolineatum has brightly coloured eyes that can be quite conspicuous when viewed face-on, especially against the dark background, as well as a dark upper thorax that can render the damselfly nearly impossible to be seen from above (Christina Linkem, Elizabeth Henry, Marguerite Butler, in prep.). In nearly every context, M. n. nigrolineatum damselflies chose darker perch sites, making single males and tandem pairs harder to be seen, consistent with a strategy for avoiding harassment and predation (Plaistow & Siva-Jothy, 1996; Larison, 2007). There are potential predators on adults at the study site including bulbuls (Pycnonotus cafer), dragonflies (Orthemus ferruginea and Anax strenuus), spiders, chameleons (Trioceros jacksonii) and amphibians (Rhinella marina); however, we are unaware of any documented interaction between these species and Megalagrion, aside from anecdotal observations involving T. jacksonii and A. strenuus. We did not observe any instance of predation, even on damselflies perched in the sun flecks. Therefore, we are not able to speculate on the impact of predation on this species. In contrast, territorial males showed a preference for sun flecks. One possible explanation is that shaded perches provide a refuge for non-territorial males, with territorial males intensely competing for sun flecks, such that there is a segregation of microhabitat use by social status as is commonly found in the damselfly family Calopterygidae (Kirkton & Schultz, 2001; Cordoba-Aguilar & Cordero-Rivera, 2005). However, in our study, a substantial fraction (50%) of male–male interactions occurred in shaded locations, and only a minority of males in sun flecks (14%) engaged in territorial defence. Therefore, our results contradict the notion that territorial males monopolize sun flecks. Territorial males may indeed enjoy an advantage of being more conspicuous in sun flecks; however, the specific site to defend may be more strongly determined by characteristics optimal for oviposition sites, such as leaf litter among still waters where we frequently observed tandem pairs engaging in oviposition. We note that tandem pairs showed no preference for sun flecks. Territorial males may, therefore, prefer perch sites near optimal oviposition sites, ideally with high visibility. The ecological and behavioural findings here correspond well to our previous findings that M. n. nigrolineatum has an eye morphology that seems to be optimized for maximal light capture (among the largest facet diameters in forward-facing region of the eye), and that eye morphology in this genus is evolving in concert with microhabitat use (Scales & Butler, 2015). These lines of evidence are consistent with adaptation to life in a dim habitat. Evolutionary and conservation implications There is a growing concern for the future evolutionary potential for Megalagrion species, given the extensive habitat degradation in the Hawaiian islands. Vulnerability to introduced predators is often cited as a threat to Megalagrion populations; yet, it is worth noting that the ‘Aihualama stream and surrounding forest are highly degraded and still support a stable population of M. n. nigrolineatum. ʻAihualama stream contains introduced crayfish (Procambarus clarkii), shrimp (Palaemonetes), fish (loricarids, poeciliids), dragonfly (Crocothemis servilia) and frogs (Glandirana rugosa and R. marina), several of which have been proposed as predators on damselfly larvae (e.g. Williams, 1936; Englund, 1999; Englund & Polhemus, 2001). Englund (1999) provided distributional patterns suggesting that predators exclude damselflies, but these correlations may have other explanations. Furthermore, laboratory predation experiments may result in highly artificial situations that increase larval susceptibility to predators, whereas stream sites are highly complex with many opportunities for crypsis and avoidance. We found no evidence for high densities of introduced predators excluding damselflies in this natural population. Despite the conservation concern (several species have recently been federally listed, including M. n. nigrolineatum), there has been very little direct ecological study of most Megalagrion species, with the majority of research focused on taxonomy and distribution of the genus (Williams, 1936; Polhemus, 1993; Mlot, 1995; Polhemus & Asquith, 1996; Englund, 1999; Polhemus, 2007) or phylogenetics, phylogeography and genetic structure (Jordan et al., 2005; Jordan, Barruet & Olaf, 2007; Jones, Bogdanowicz & Jordan, 2009). We actually know very little about the ecological requirements of most species, but what we have learned is quite surprising and strongly implicates the importance of the light environment. Recently, Cooper (2010) and Cooper et al. (2015) have proposed that red coloration evolves in Megalagrion as an adaptive response to prevent UV damage at high elevations, balanced by sometimes competing pressures from sexual selection. With the light environment as a primary ecological resource axis, greater attention should be paid to the introduced flora, especially introduced tree species, which can change the structure of the forest itself. At ‘Aihualama, the forest is composed of mostly non-native plants, particularly Ficus planted almost 100 years ago, and invasive F. moluccana trees. The latter species provides extensive shade from their dominant 30-m-wide canopies (Hughes, Johnson & Uowolo, 2011). Its tolerance of deep shade may be the reason why M. n. nigrolineatum is the only damselfly present at this site. Of all Megalagrion species, M. n. nigrolineatum can be found in the narrowest mountain valleys of O’ahu, which as a result of their high walls are deeply shaded for most of the day. Indeed, they are tolerant of very dark valleys with only very small, ephemeral puddles. We were initially quite surprised to find M. n. nigrolineatum in places that were so uncharacteristic of the more typical requirements of odonates. Yet, these small, forested pools of water are a common habitat on Oʻahu, found along the many high-walled stream valleys and tiny tributaries of the Koʻolau mountains of Oʻahu carved over its 3 Myr history via erosion. This habitat preference provides ample ecological opportunity for M. n. nigrolineatum to populate a virtually unoccupied odonate ‘niche’ in a topologically complex landscape and may pave the way for genetic differentiation and diversification. Genetic studies are needed to assess its population structure in the context of its evolutionary history and geography. It is critical therefore that additional ecological studies be undertaken to understand the fascinating evolutionary history of this genus and to be better positioned for future conservation efforts (Preston & Englund, 2007). More generally, light environment may be a significant resource axis for many more species than currently recognized. Although thermal resources are clearly important to many species, light also provides opportunities for diversification in visual strategies. Forests provide among the most complex visual habitat in the terrestrial environment. As in our case, species which can exploit the darkest habitats will have ready access to conditions which will not be suitable for sun-loving species. Shade-seeking behaviour may, therefore, be a tactic which expands ecological opportunity. ACKNOWLEDGEMENTS The authors thank the National Science Foundation for generous support of this project (IOS-0958509 to M.A.B.). We thank R. Sears, J. Laughlin and the staff of the Lyon Arboretum for providing invaluable information about and access to study sites. We also thank D. Stelling for assistance with field work and data collection, and C. King and Hawaiʻi DLNR for invertebrate collecting permits. We thank the Dr Charles and Sandra Guest-Van Riper Endowment Student Travel Award for help with dissemination at conferences. Lastly, we thank four anonymous reviewers for their helpful comments. REFERENCES Adolph SC . 1990 . Influence of behavioral thermoregulation on microhabitat use by two Sceloporus lizards . Ecology 71 : 315 – 327 . Google Scholar Crossref Search ADS WorldCat Angelibert S , Giani N . 2003 . Dispersal characteristics of three odonate species in a patchy habitat . Ecography 26 : 13 – 20 . Google Scholar Crossref Search ADS WorldCat Anholt BR . 1992 . 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TI - Damselflies that prefer dark habitats illustrate the importance of light as an ecological resource
JO - Biological Journal of the Linnean Society
DO - 10.1093/biolinnean/blx122
DA - 2018-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/damselflies-that-prefer-dark-habitats-illustrate-the-importance-of-iTuMt1ae8O
SP - 144
VL - 123
IS - 1
DP - DeepDyve
ER -