Annals of Page 1 of 13 doi:10.1093/aob/mct195, available online at www.aob.oxfordjournals.org

Capture mechanism in Palaeotropical pitcher (Nepenthaceae) is constrained by climate

Jonathan A. Moran1,*, Laura K. Gray2, Charles Clarke3 and Lijin Chin3 1School of Environment and Sustainability, Royal Roads University, Victoria, British Columbia, V9B 5Y2 Canada, 2Department of Renewable Resources, University of Alberta, Edmonton, Alberta, T6G 2H1 Canada and 3School of Science, Monash University, Bandar Sunway, Selangor, 46150 Malaysia * For correspondence. E-mail [email protected]

Received: 6 May 2013 Returned for revision: 3 June 2013 Accepted: 3 July 2013 Downloaded from

† Background and Aims (Nepenthaceae, approx. 120 species) are carnivorous pitcher plants with a centre of diversity comprising the , Borneo, Sumatra and Sulawesi. Nepenthes pitchers use three main mechan- isms for capturing prey: epicuticular waxes inside the pitcher; a wettable peristome (a collar-shaped structure around the opening); and viscoelastic fluid. Previous studies have provided evidence suggesting that the first mechanism may be more suited to seasonal climates, whereas the latter two might be more suited to perhumid environments. In this http://aob.oxfordjournals.org/ study, this idea was tested using climate envelope modelling. † Methods A total of 94 species, comprising 1978 populations, were grouped by prey capture mechanism (large peri- stome, small peristome, waxy, waxless, viscoelastic, non-viscoelastic, ‘wet’ syndrome and ‘dry’ syndrome). Nineteen bioclimatic variables were used to model habitat suitability at approx. 1 km resolution for each group, using Maxent, a presence-only species distribution modelling program. † Key Results Prey capture groups putatively associated with perhumid conditions (large peristome, waxless, visco- elastic and ‘wet’ syndrome) had more restricted areas of probable habitat suitability than those associated putatively with less humid conditions (small peristome, waxy, non-viscoelastic and‘dry’ syndrome). Overall, the viscoelastic

group showed the most restricted area of modelled suitable habitat. at Royal Roads University Library on August 26, 2013 † Conclusions The current study is the first to demonstrate that the prey capture mechanism in a carnivorous is constrained by climate. Nepenthes species employing peristome-based and viscoelastic fluid-based capture are largely restricted to perhumid regions; in contrast, the wax-based mechanism allows successful capture in both per- humid and more seasonal areas. Possible reasons for the maintenance of peristome-based and viscoelastic fluid-based capture mechanisms in Nepenthes are discussed in relation to the costs and benefits associated with a given prey capture strategy.

Key words: Biogeography, carnivorous plants, climate envelope modelling, Nepenthes, Nepenthaceae, Palaeotropics, , capture mechanism.

INTRODUCTION For example, they are not typically found in the lowland diptero- carp forests that historically covered much of the region. The monotypic family Nepenthaceae comprises approx. 120 It has been suggested that the Nepenthaceae provide an species of carnivorous pitcher plants (McPherson, 2009). example of adaptive radiation, based on pitcher specialization Invertebrate prey is attracted to the pitcher via a combination for nutrient capture (e.g. Chin et al., 2010; Pavlovicˇ, 2012). of , colour patterns and, in some cases, fragrance (e.g. Certainly, there exists a range of specializations. For example, Moran, 1996; Moran et al., 1999, 2012; DiGiusto et al., 2008, derives a significant proportion of its ni- 2010). Nepenthes benefit from prey-derived nutrients, which trogen from interception of falling litter (Moran et al., augment the supply available via the roots (Moran and Moran, 2003; Pavlovicˇ et al., 2011), while four other species 1998; Pavlovicˇ et al., 2007). The centre of diversity of (Nepenthes baramensis, Nepenthes lowii, and Nepenthes comprises the islands of Borneo, Sumatra, Sulawesi Nepenthes macrophylla) sequester nutrients from mammalian and the Philippines; the range extends north to Assam, west to excreta (Clarke et al., 2009; Chin et al., 2010; Grafe et al., Madagascar and south-east to Australia and New Caledonia 2011; Wells et al., 2011). is a (Danser, 1928). Nepenthes pitcher production incurs significant specialist (Moran et al., 2001; Merbach et al., 2002), resource costs (Osunkoya et al., 2007, 2008). Therefore, in and Nepenthes bicalcarata has a mutualistic nutritional associ- common with other carnivorous plants, Nepenthes pitcher ation with the ant Camponotus schmitzi (Clarke and Kitching, plants are restricted to habitats in which the nutritional benefits 1995; Merbach et al., 2007; Bazile et al., 2012; Thornham from carnivory outweigh the costs of producing carnivorous et al., 2012). However, the majority of Nepenthes species organs (Givnish et al., 1984). As a consequence, members of studied thus far capture and digest a range of small arthropods, the genus are somewhat patchily distributed across their range. and many are particularly attractive to ants (e.g. Juniper et al.,

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1989; Moran, 1996; Moran et al., 1999; Di Giusto et al., 2008; Nepenthes species, suggesting disruptive selective pressure to Bauer et al., 2009). Although Nepenthes pitchers appear to be adopt one of two capture mechanisms. Similarly, Bonhomme simple gravity-fed pitfall traps, there are three distinct mechan- et al. (2011) established an inverse relationship between the isms by which they effect capture of their invertebrate prey. viscoelasticity of pitcher fluid and the amount of wax produced The first two mechanisms function to trap the prey; the third to in 23 Nepenthes species, once more suggesting disruptive selec- retain it (for a recent review, see Benz et al., 2012). tion. A potential driver of such selection is climate: Bauer et al. (2012a) raised the possibility that a perhumid climate might (1) A wettable peristome (function: trapping). The peristome is favour the peristome-based strategy, whereas drier or more sea- the collar-shaped structure that surrounds the pitcher sonal climates might favour a wax-based strategy (see points 1 opening, and is the location of the major extrafloral nectaries and 2, above). Bonhomme et al. (2011) suggested that the pro- in many species (Juniper et al., 1989). The surface of the duction of viscoelastic fluid (as opposed to waxes) favours the peristome is covered in microscopic radial ridges, arrayed capture of volant prey, which are relatively more common than anisotropically to aid traction in the direction of the pitcher crawling prey such as ants, in montane habitats. Of greater rele-

mouth, but to deny traction in the opposite direction, for vance to the current study, they also demonstrated that the visco- Downloaded from some prey taxa (Bohn and Federle, 2004). Capillary action elastic Nepenthes fluid retains its retentive properties, even when between the ridges also renders the peristome highly wet- diluted by the heavy rainfall typical of such environments. table, and it has been shown that a wetted peristome provides Therefore, we decided to test the idea that the employment of very little traction for insect feet, effectively causing the prey the various capture mechanisms described above might be driven to ‘aquaplane’ and slip into the pitcher. Conversely, many by climatic variables. Specifically, we explored the possibility insect visitors have little trouble safely traversing the peri- that peristome-based and viscoelastic capture strategies are http://aob.oxfordjournals.org/ stome when it is dry (Bohn and Federle, 2004; Bauer et al., restricted to perhumid environments, whereas the wax-based 2008, 2009, 2011, 2012a, b). The width of the peristome strategy is favoured in drier or more seasonal environments. (and thus its presumed effectiveness as a conveying We took a climate envelope modelling approach, using Maxent, surface) varies considerably among species. a widely used species distribution modelling program (Phillips (2) Epicuticular waxes (function: trapping). In many Nepenthes et al., 2004, 2006; Elith et al., 2011; http://www.cs.princeton. species, the upper part of the inner pitcher wall is covered in edu/~schapire/maxent/). one or more layers of epicuticular waxes (Riedel et al., 2003,

2007; Poppinga et al., 2010). These waxes can interfere with at Royal Roads University Library on August 26, 2013 an insect’s foothold in a number of ways. First, wax crystals MATERIALS AND METHODS can detach from the top layer to clog the claws, thus denying Nepenthes species population localities and pitcher characteristics traction. Secondly, thewaxysurface is unwettable, denying a foothold to insects that rely on capillary forces for traction Geographical co-ordinates were recorded for populations of 94 (Juniper and Burras, 1962; Gaume et al., 2002, 2004; Gorb Nepenthes species (Table 1). This represents approx. 80 % of et al., 2004; Gorb and Gorb, 2006; Wang et al., 2009; the species currently described (McPherson, 2009). A total of Scholz et al., 2010). Finally, the microscopic roughness of 1978 populations were recorded from Thailand, Laos, Vietnam, the wax crystal layer itself may reduce the effective area Cambodia, Peninsular Malaysia, Sumatra, Java, Borneo, the available for traction by the prey’s adhesive pads (Scholz Philippine Archipelago and Sulawesi. The populations of 69 et al., 2010). In many Nepenthes species, epicuticular species were observed directly in the wild by one of us (C.C.); waxes work in conjunction with semi-lunate cells. These data on the remaining species were derived from examinations are modified stomatal guard cells that line the upper part of of material, cultivated plants and taxonomic descrip- the inner pitcher wall. They present an anisotropic surface tions (e.g. Danser, 1928). For many Nepenthes species with that denies traction to the claws of insects attempting to limited geographical distributions, the observations represent escape (Pant and Bhatnagar, 1977; Owen and Lennon, the entire currently known geographical range. Members of the 1999; Gaume et al., 2002; Thornhill et al., 2008; Wang Nepenthaceae are remarkable for high degrees of endemism: et al., 2009; Gorb and Gorb, 2011). In contrast to the many are montane with restricted geographical ranges, often a action of the peristome, the waxy zone appears to operate single mountain or a small number of adjacent mountains (e.g. equally well under both wet and dry conditions. N. macrophylla, N. clipeata, N. aristolochioides; Clarke, 1997, (3) Viscoelastic pitcher fluid (function: retention). A number of 2001). Consequently, for 76 of our study species (approx. Nepenthes species produce pitcher fluid with viscoelastic 80 %), only a small number of populations (,10) are currently properties. Insects falling into this type of fluid find it impos- known to exist. In contrast, a small number of species have sible to extricate themselves, and quickly drown (Gaume and very large geographical ranges, and a consequently larger Forterre, 2007; Gaume and Di Giusto, 2009; Bonhomme number of identified populations. For example, 460 populations et al., 2011). of N. gracilis were identified, spanning Thailand, Peninsular Malaysia, Sumatra, Borneo and Sulawesi; similarly, there were There is considerable variation in the degree to which a given 342 populations of N. ampullaria identified, encompassing Nepenthes species employs each mechanism. Bauer et al. Thailand, Peninsular Malaysia, Sumatra and Borneo (Table 1). (2012a) divided the capture strategy of Nepenthes species into Initially, we considered randomly sub-sampling from the set of two broad groups: peristome based and wax based. In addition, populations for thesewide-ranging species, to limit any potential Benz et al. (2012) demonstrated an inverse relationship taxonomic bias in our results as a result of one or two species nu- between peristome width and extent of the waxy zone in eight merically dominating a given prey capture mechanism group Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants Page 3 of 13

TABLE 1. Summary of Nepenthes species used in the study, including pitcher characteristics

Species No. of populations Peristome size* Wax Viscoelastic fluid†

Nepenthes adnata 3 Small Present Absent Nepenthes alata 17 Small Present Absent Nepenthes alba 10 Small Present Absent Nepenthes albomarginata 27 Small Present Absent Nepenthes ampullaria 342 Large Absent Absent Nepenthes andamana 5 Small Present Absent Nepenthes argentii 2 Medium Absent Absent Nepenthes aristolochioides 2 Medium Absent Present Nepenthes baramensis 12 Medium Present Absent Nepenthes bellii 5 Medium Absent NA‡ Nepenthes benstonei 9 Small Present Absent Nepenthes bicalcarata 16 Large Absent Absent Nepenthes bokorensis 3 Small Present Absent Downloaded from Nepenthes bongso 13 Large Reduced Present Nepenthes boschiana 3 Large Present Absent Nepenthes burbidgeae 4 Large Absent Present Nepenthes burkei 1 Large Reduced Absent Nepenthes chang 2 Small Present Absent Nepenthes chaniana 5 Medium Present Present http://aob.oxfordjournals.org/ 1 Small Present NA Nepenthes copelandii 1 Medium Reduced Present Nepenthes densiflora 4 Medium Reduced Present Nepenthes diatas 3 Medium Present Present Nepenthes dubia 3 Small Absent Present Nepenthes edwardsiana 2 Medium Present Absent Nepenthes ephippiata 3 Medium Reduced Absent Nepenthes eustachya 14 Small Present Absent Nepenthes eymae 5 Medium Reduced Present Nepenthes faizaliana 3 Large Present Absent

Nepenthes flava 1 Large Absent Present at Royal Roads University Library on August 26, 2013 Nepenthes fusca 47 Medium Reduced Present Nepenthes glabrata 8 Small Present Present Nepenthes glandulifera 1 Small Reduced Present 460 Small Present Absent Nepenthes gracillima 7 Medium Present Absent 86 Small Present Absent Nepenthes hamata 3 Medium Present Absent Nepenthes hirsuta 19 Small Reduced Absent Nepenthes hispida 5 Small Present Absent Nepenthes holdenii 1 Small Present Absent Nepenthes inermis 9 Small Reduced Present Nepenthes izumiae 3 Medium Present Absent Nepenthes jacquelineae 2 Large Absent Present Nepenthes jamban 1 Medium Absent Present Nepenthes kampotiana 3 Small Present Absent Nepenthes kerrii 2 Small Present Absent Nepenthes kongkandana 2 Small Present Absent Nepenthes lavicola 1 Medium Present Absent Nepenthes lingulata 1 Medium Present Absent Nepenthes longifolia 5 Medium Present Present Nepenthes lowii 19 Medium Reduced Absent Nepenthes macfarlanei 15 Medium Present Present Nepenthes macrophylla 3 Medium Present Present Nepenthes macrovulgaris 4 Small Reduced Absent Nepenthes mapuluensis 3 Large Present NA Nepenthes maxima 10 Medium Present Present Nepenthes merrilliana 9 Medium Absent Absent Nepenthes mikei 3 Small Present Absent Nepenthes mira 1 Medium Absent NA Nepenthes mirabilis 148 Small Present Absent Nepenthes muluensis 6 Small Present Absent Nepenthes murudensis 4 Small Present Absent Nepenthes northiana 5 Large Reduced Present Nepenthes ovata 4 Large Reduced Present Nepenthes petiolata 2 Medium Present Present Nepenthes philippinensis 4 Small Present Absent Continued Page 4 of 13 Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants

TABLE 1. Continued

Species No. of populations Peristome size* Wax Viscoelastic fluid†

Nepenthes pilosa 1 Small Reduced Present Nepenthes rafflesiana 238 Medium Reduced Present Nepenthes rajah 5 Large Absent Absent Nepenthes ramispina 7 Small Present Absent Nepenthes reinwardtiana 39 Small Present Absent Nepenthes rhombicaulis 3 Medium Present Absent 68 Small Present Absent 2 Medium Absent Absent Nepenthes singalana 8 Medium Present Absent Nepenthes smilesii 19 Small Present Absent 9 Medium Present Present Nepenthes spectabilis 7 Medium Present Present Nepenthes stenophylla 22 Small Present Present Downloaded from Nepenthes sumatrana 6 Medium Reduced Present Nepenthes suratensis 3 Small Present Absent Nepenthes talangensis 3 Small Absent Present Nepenthes tenuis 1 Medium Absent Present Nepenthes tentaculata 56 Small Present Absent Nepenthes thai 1 Small Present Absent http://aob.oxfordjournals.org/ Nepenthes thorelli 3 Small Present Absent Nepenthes tobaica 13 Small Present Present Nepenthes tomoriana 3 Small Present NA 6 Large Reduced Absent 13 Large Absent Absent 2 Large Absent Absent Nepenthes villosa 5 Medium Present Absent Nepenthes vogelli 7 Medium Absent Present Population total 1978 Species total 94 at Royal Roads University Library on August 26, 2013 * Categories correspond to those presented in fig. 8 of Bauer et al. (2012a) as follows: small ¼ –20 to –10; medium ¼ –5 to +5; large ¼ + 10 to +25 (values refer to the magnitude of the residuals from the peristome width/pitcher length regression relationship). † Field evaluations of pitcher fluid viscoelasticity were based on direct observations, not rheological measurements. This is a potential source of error. ‡ NA ¼ data unavailable. Data sources: Bonhomme et al. (2011); Bauer et al. (2012a); Benz et al. (2012); personal field measurements/observations (C.C.).

(see below for details regarding groups). However, to have down- width/pitcher length regression relationship: positive sampled would have created false absences, which would have values denote larger than average peristomes; negative reduced the accuracy of the models used. It is important to values, smaller than average (Table 1). We did not apply a stress that for the purposes of this study, we were focused correction for phylogeny (see Cheverud et al., 1985) to the not on individual species, but on the capture mechanism. peristome size measurements, since Bauer et al. (2012a) Therefore, if a population of plants using a given mechanism found no significant difference between phylogenetically was known and located, it was included in the analysis. For our corrected and uncorrected morphological data for the 65 purposes, the specific status of a plant was less important than species that they examined in their study (i.e. no detectable the fact that, for instance, a population of Nepenthes conforming phylogenetic signal with regards to morphological traits). to a given capture mechanism group or syndrome (see below) Since their 65 species account for 70 % of the total was present at a given location. For each species, the following number of species used in the current study, we believe it pitcher attributes were quantified. highly unlikely that the other species we used would depart from this pattern in any significant way. (1) Peristome size. We adopted the approach of Bauer et al. (2) Presence/absence of wax. We used species data describing (2012a) in assigning peristomewidth classes based on the re- wax production from Bonhomme et al. (2011), Bauer et al. lationship of peristome width to pitcher length. The relative (2012a) and Benz et al. (2012), as well as direct observation peristome size of 65 species was derived from fig. 8 of Bauer of specimens in the field by C.C. (Table 1). For the reasons et al. (2012a); values for the remaining 29 species in the outlined above, we did not apply a phylogenetic correction current study were derived from field measurements by to the wax data-set. C.C. Species were then assigned to one of three peristome (3) Presence/absence of viscoelastic pitcher fluid. For each size categories (small, medium and large). These correspond species, the presence or absence of viscoelastic pitcher to the following categories of peristome width in fig. 8 of fluid was noted. Data for 23 species were taken from the Bauer et al. (2012a): small ¼ –20 to –10; medium ¼ –5 data set presented by Bonhomme et al. (2011); the remaining to +5; large ¼ + 10 to +25. These numerical values refer data were from direct field observations by C.C. A potential to the magnitude of the residuals from the peristome source of error in our approach is that assessment of Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants Page 5 of 13

viscoelasticity was made based on direct observation of Release 3 (Hijmans et al., 2005; http://www.worldclim.org/ pitcher fluid, rather than rheological measurement. bioclim). This data set comprises gridded precipitation and tem- Nepenthes undergo ontogenic change in pitcher morphology. perature data based on monthly measurements from globally dis- Young plants typically produce terrestrial pitchers that are ovoid tributed weather stations. The data are averaged from the years in shape; as the plant grows, aerial pitchers are produced. These 1950–2000 to generate a single 50 year ‘climate surface’ for are usually more elongate in form. In a number of species, this each month. The resulting surfaces are then utilized to generate ontogenic shift in gross morphology is accompanied by bioclimatic variables (e.g. mean temperature of the wettest changes in wax production, fluid viscoelasticity and/or relative quarter), which are considered more meaningful biologically width of the peristome. In such cases, we took the approach than monthly measurements for defining the environmental tol- that if, for example, viscoelastic fluid was produced by either erances of a species (Graham and Hijmans, 2006; Murienne et al. pitcher form of a given species, then that species was classed , 2009). These variables have been widely used to predict as employing viscoelastic fluid (Table 1). The same approach plant species distributions in other habitat modelling studies et al. et al. a b was taken regarding ontogenic changes in wax production and (e.g. Kumar , 2006; Guisan , 2007 , ; Murienne

et al., 2009; Kumar and Stohlgren, 2009). We did not use Downloaded from peristome size. monthly meteorological data due to differences in seasonal weather patterns over the extensive geographical area covered Nepenthes capture mechanism groupings in the study. For instance, in western Java, the period of highest rainfall occurs between December and February; in contrast, Based on the three attributes outlined above, we grouped the the highest rainfall in northern Thailand occurs during September species into eight putative capture mechanism groups (hereafter (source: Meteorological Service of Singapore; http://www.weather. http://aob.oxfordjournals.org/ called groups), summarized in Table 2. Six of the groups focus on gov.sg). To have used monthly data might have prevented the a single pitcher characteristic (peristome size, viscoelasticity of model from associating particular climate variables with a highly fluid and waxiness). The final two groups represent combinations correlated capture group, thus reducing model accuracy. of these characteristics: the ‘wet’ syndrome comprises species that possess large peristomes, and produce no wax; the ‘dry’ syn- drome comprises species with small peristomes, wax and non- Climate envelope model development and evaluation viscoelastic fluid. It might have been considered optimal to Using the bioclimatic variables discussed above, we have incorporated only species that produce viscoelastic fluid employed Maxent V. 3.3.3k (Phillips et al., 2004) to model at Royal Roads University Library on August 26, 2013 into the ‘wet’ syndrome, based on the observations of Bonhomme group distributions in our geographical area of interest. This et al. (2011). However, onlyseven species possessed pitchers with encompassed Thailand, Laos, Vietnam, Cambodia, Peninsular large peristomes, viscoelastic fluid and no wax, and the total Malaysia, Sumatra, Java, Borneo, the Philippines and Sulawesi. number of populations for this combination of traits was only Maxent is a maximum entropy-based machine learning program 18, far below the threshold of 80 required for meaningful model- that estimates the probability distribution for the occurrence of a ling using Maxent (Elith et al.,2011). The large and medium peri- species based on constraints of environmental variables on stomesize classes were combinedinthe large peristomegroup and species distribution (Phillips, 2006; Phillips et al., 2006; Elith ‘wet’ syndrome. This is because there appears to be little differ- et al., 2011). The current study modelled distributions of capture ence between the two peristome size categories from a functional mechanisms (Table 2) rather than individual species, and gener- viewpoint. For example, both N. bicalcarata (large peristome cat- ated an estimate of the probability of the presence of a given egory) and N. rafflesiana (medium peristome category) employ group that varied from 0 to 1, where 0 was the lowest and 1 the peristome aquaplaning as a prey-trapping mechanism (Bohn and highest probability. The advantage of this modelling approach Federle, 2004; Bauer et al., 2008). is that it only requires presence data and environmental variable layers (continuous or categorical) for the study area (Phillips and Dudı´k, 2008). Moreover, Maxent has been found to produce the Climate variables most accurate models among many different species distribution We considered 19 bioclimatic variables as potential predictors modelling methods (Elith et al., 2006; Hijmans and Graham, of Nepenthes group distributions (summarized in Table 3). 2006; Townsend Peterson et al., 2007). These variables are freely available at 30 arc-seconds (approx. Given that we only considered continuous environmental vari- 1 km) resolution from the WorldClim data set Version 1.4, ables, and the lowest number of samples within a single group

TABLE 2. Putative Nepenthes prey capture mechanism groups modelled in the study

Group Peristome Wax Viscoelastic fluid No. of species No. of populations

Small peristome Small Any Any 41 1098 Large peristome Large or medium Any Any 53 881 Waxy Any Yes Any 56 1170 Waxless Any No Any 20 422 Viscoelastic Any Any Yes 33 460 Non-viscoelastic Any Any No 56 1505 ‘Dry’ syndrome Small Yes No 29 1010 ‘Wet’ syndrome Large or medium No Any 18 416 ae6o 13 of 6 Page Moran

TABLE 3. Results of Maxent models for putative prey capture mechanism groups in Nepenthes ta.— al. et Permutation importance (%) by group

Bioclimatic variable* Large peristome Small peristome Waxy Waxless Viscoelastic Non-viscoelastic ‘Wet’ syndrome ‘Dry’ syndrome lmt n atr ehns nPletoia ice plants pitcher Palaeotropical in mechanism capture and Climate BIO1 (annual mean temperature) 04.90 0 0 0.85 0 0 10.72 BIO2 (mean diurnal temperature range) 10.66 13.03 9.92 8.11 4.85 8.91 2.51 10.77 BIO3 (isothermality) † 8.51 10.37 7.95 4.96 0 7.05 0 7.66 BIO4 (temperature seasonality)‡ 0.40 0.23 0.20 0.21 0.81 0.27 0.39 0.40 BIO5 (maximum temperature of warmest month) 0 0 0 0 0.01 0 0.90 0 BIO6 (minimum temperature of coldest month) 9.10 0.69 1.40 9.46 6.50 5.11 14.37 0.31 BIO7 (annual temperature range) § 16.49 11.72 7.88 12.72 6.45 10.97 0.77 8.24 BIO8 (mean temperature of wettest quarter) 16.51 17.12 22.50 9.59 27.73 18.58 10.98 16.27 BIO9 (mean temperature of driest quarter) 0 0 0 0 0 0 0 0 BIO10 (mean temperature of warmest quarter) 1.31 7.55 10.11 11.10 0.17 13.86 12.95 12.22 BIO11 (mean temperature of coldest quarter) 0 0 0 0 0 0 0 0 BIO12 (annual precipitation) 9.56 9.37 12.14 5.53 18.32 7.13 9.42 8.52 BIO13 (precipitation of wettest month) 1.12 1.09 0.76 1.60 0.88 1.08 2.81 0.62 BIO14 (precipitation of driest month) 8.18 2.54 3.49 13.29 4.36 7.10 18.39 5.00 BIO15 (precipitation seasonality) } 0.60 1.70 0.94 0.09 5.62 0.05 0.72 0.10 BIO16 (precipitation of wettest quarter) 16.43 14.24 16.99 17.04 21.17 15.37 21.60 12.33 BIO17 (precipitation of driest quarter) 0.07 3.79 2.51 3.52 0.62 2.21 2.42 3.18 BIO18 (precipitation of warmest quarter) 1.06 1.55 3.10 2.75 1.27 2.19 1.77 3.32 BIO19 (precipitation of coldest quarter) 0.00 0.11 0.10 0.03 0.38 0.12 0 0.31 Test AUC** 0.81 0.77 0.77 0.82 0.86 0.77 0.83 0.80

* Bioclimatic variables are in bold if permutation importance is ≥5 % for any group. † Isothermality is a measure of diurnal temperature range compared with annual temperature range, calculated as: (BIO2/BIO7) × 100. ‡ (Standard deviation) × 100. § (BIO5–BIO6). } Coefficient of variation of precipitation.

** AUC: area under curve, a measure of model performance. A value of 0.5 represents a model that performs no better than random; a value of 1 represents a perfect fit.

Downloaded from from Downloaded http://aob.oxfordjournals.org/ at Royal Roads University Library on August 26, 2013 26, August on Library University Roads Royal at Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants Page 7 of 13

(n ¼ 378) was almost 5-fold greater than the acceptable minimum ‘permutation importance’ to investigate which bioclimatic of 80 observations (Elith et al., 2011), we were able to use the variables were most important for predicting geographical dis- product feature type for our model execution. This feature uses tribution of agiven group. This isa measureof decrease intrain- the product of pairs of continuous environmental variables, to- ing AUC (expressed as a percentage) resulting from randomly gether with the linear feature of each variable, to constrain the co- permuting values of a given variable during the training phase variance of the respective variablesto the empirical values, thereby of model development. The decrease in training AUC is in pro- incorporating interactions between predictor variables into the suit- portion to the importance of that variable to the model (Phillips, able habitat projections (Phillips et al., 2006). Additionally, the 2006). In addition, we compared the relative magnitude of im- default Maxent model parameters (regularization multiplier ¼ 1 portant variables between group pairs (large peristome vs. and convergence threshold ¼ 1025) were used. However, to de- small peristome, waxy vs. waxless, viscoelastic vs. non- crease the chance of the model over- or underpredicting suitable viscoelastic, and ‘wet’ syndrome vs. dry’ syndrome), to allow habitat for each group, we increased the maximum number of identification of variablesthat might be important in delimiting iterations from 500 (the default value) to 5000, to allow the suitable habitat for a given group. For each bioclimatic vari-

model a sufficient number of iterations for convergence. able, permutation importance values were summed for each Downloaded from When defining the environmental tolerances of the groups pair of groups. The relative contribution from each member examined in this study, it was important to consider potential geo- of the pair to the total was then calculated as a percentage, as graphic sampling bias within the occurrence data, given differing follows: degrees of accessibility between sampling locations. Forexample, a potential source of bias in species distribution modelling based % importance of bioclimatic variable to group A http://aob.oxfordjournals.org/ on presence-only data is the tendency to over-represent observa- =[PIA/(PIA + PIB)] × 100 tions close to roads and other means of access to a given area. Phillips et al. (2009) suggested a method to correct for sampling Where PIA and PIB denote permutation importance of a bias, by selecting background data with similar sampling bias to given bioclimatic variable to groups A and B, respectively. the occurrence data. Following the steps outlined in Phillips We considered variables with permutation importance values (2006) and Young et al. (2011), a bias file was generated using ≥5 % as important; the comparisons included only those vari- the smallest administrative unit (recorded as district, county or ables with values at orabove thisthreshold forat least one group municipality) for all countries in our study area. The geographic in each pair.

space of an administrative unit was included in the bias file if an at Royal Roads University Library on August 26, 2013 observation fell within the unit. By including the bias file, Maxentsampledthedefault10 000pseudo-absencesfromthecol- lection of small-scale geographic units where observations were RESULTS recorded, rather than from the entire prediction area. Model evaluation Finally, for model evaluation, the random test percentage was set at 30, so that 70 % of the observations for a given group were Model evaluation statistics for group predictions are presented in randomly selected as a training data set and the remaining 30 % Table 3. Mean AUC values were consistently high, ranging from were used for model testing. Model fit was evaluated by the 0.77 to 0.86. A high AUC value indicates a high level of model widely used area under the curve (AUC) of the receiveroperating accuracy: a value of 0.75 means that 75 % of the time a characteristic (ROC), which represents the probability that the random sample from presence predictions will have a score model classifier will correctly identify a randomly chosen true greater than a random selection from absence predictions presence (Fielding and Bell, 1997; Fawcett, 2006). The AUC across all available probability thresholds. Therefore, an AUC is a threshold-independent measure of model performance that value of 0.5 indicates a random predictor, and values between ranges from 0 to 1; avalue of 0.5 represents a model that performs 0.5 and 0.6 are generally considered indicative of a failed no better than random, whilst 1 is maximally predictive. To model (Fawcett, 2006). Overall, groups with capture mechan- evaluate model performance further, the replicates option was isms predicted to be associated with wet environments (large set at 10 so that the selection of testing data and calculation of peristome, waxless, viscoelastic and ‘wet’ syndrome) have AUC values were repeated. Therefore, final model performance slightly higher AUC values than groups predicted to occur in was evaluated with the mean AUC for each group over the ten drier or more seasonal environments (small peristome, waxy, replicates. Similarly suitable habitat (in terms of the climate en- non-viscoelastic and ‘dry’ syndrome; Table 3). velope) for each group was displayed as the average projected For each group, graphical representations of modelled habitat probability of presence across the ten replicates. Although the suitability are presented in Fig. 1A–H. Probability of habitat Maxent parameter settings listed above were constant for all suitability is plotted on a colour scale, where grey represents groups, each was modelled and evaluated individually. values of 0–0.1 and dark blue represents values of 0.9–1. In Graphical outputs of the model for each group were produced general, the groups putatively associated with perhumid condi- using the ArcMap V.10 software package (Esri Inc., Redlands, tions (large peristome, waxless, viscoelastic and ‘wet’ syn- CA, USA). drome; Fig. 1B, D, F, H) show less extensive areas of probable habitat suitability than do those associated putatively with less humid conditions (small peristome, waxy, non-viscoelastic and Evaluation of bioclimatic variables as predictors of occurrence ‘dry’ syndrome; Fig. 1A, C, E, G). The most restricted area of Maxent provides several methods for quantifying the rela- suitable habitat is associated with the viscoelastic group tive contribution of each variable to the model. We used (Fig. 1F). ae8o 13 of 8 Page

Probability of Moran A Small peristome B Large peristome C Waxy D Waxless habitat suitability N 0–0·1 0·1–0·2 0·2–0·3 — al. et 0·3–0·4 0·4–0·5 0·5–0·6 0·6–0·7

0·7–0·8 plants pitcher Palaeotropical in mechanism capture and Climate 0·8–0·9 0·9–1

E Non-viscoelastic F Viscoelastic G Dry syndrome H Wet syndrome N

F IG. 1. Probabilityof habitat suitability foreight putative Nepenthes prey capture mechanism groups, modelled from 19 bioclimatic variables using Maxent. Probability is mapped by colour, from grey (0–0.1) to

dark blue (0.9–1). Scale bar ¼ 1000 km.

Downloaded from from Downloaded http://aob.oxfordjournals.org/ at Royal Roads University Library on August 26, 2013 26, August on Library University Roads Royal at Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants Page 9 of 13

Bioclimatic variable importance A Min temp. coldest month (BIO6) Of the 19 bioclimatic variables used in the models, two pro- Precip. driest month (BIO14) duced permutation importance values of zero for all models, i.e. Annual temp. range (BIO7) they contributed no useful information to account for the observed Precip. wettest quarter (BIO16) distributions: mean temperature of the driest quarter (BIO9) and Annual precip. (BIO12) meantemperature of thecoldestquarter(BIO11;Table3).Anadd- Mean temp. wettest quarter (BIO8) itional six bioclimatic variables produced values of ,5 % for all Isothermality (BIO3) groups, essentially contributing no useful information to the Mean diurnal temp. range (BIO2) models: temperature seasonality (BIO4), maximum temperature Mean temp. warmest quarter (BIO10) of the warmest month (BIO5), precipitation of the wettest month 100 50 0 50 100 (BIO13), precipitation of the driest quarter (BIO17), precipitation % Large peristome % Small peristome of the warmest quarter (BIO18) and precipitation of the coldest B quarter (BIO19; Table 3). Conversely, three variables were im- Mean temp. wettest quarter (BIO8) portant in informing the model across all groups: mean tempera- Annual precip. (BIO12) Downloaded from ture of the wettest quarter (BIO8), annual precipitation (BIO12) Isothermality (BIO3) and precipitation of the wettest quarter (BIO16), which produced Mean diurnal temp. range (BIO2) high permutation importance values (≥5 %), ranging from 5.5% Precip. wettest quarter (BIO16) to as high as 27.7 %. The remaining eight bioclimatic variables Mean temp. warmest quarter (BIO10) Annual temp. range (BIO7) produced values that varied across groups and, therefore, http://aob.oxfordjournals.org/ provide insight into the differential influence of climatic factors Precip. driest month (BIO14) Min temp. coldest month (BIO6) between them (see below). 100 50 0 50 100 % Wax present % Wax absent Comparisons of bioclimatic variable importance for habitat C suitability between groups Precip. seasonality (BIO15) Annual precip. (BIO12) There are discernible patterns of differences in permutation Mean temp. wettest quarter (BIO8) Precip. wettest quarter (BIO16)

importance for habitat suitability between the groups putatively at Royal Roads University Library on August 26, 2013 associated with perhumid conditions and those putatively asso- Min temp. coldest month (BIO6) Precip. driest month (BIO14) ciated with drier environments (Fig. 2). Compared with the Annual temp. range (BIO7) small peristome group, the large peristome group is influenced Mean diurnal temp. range (BIO2) to a higher degree by minimum temperature of the coldest Mean temp. warmest quarter (BIO10) month (BIO6), precipitation of the driest month (BIO14) and Isothermality (BIO3) annual temperature range (BIO7). Conversely, the small peri- 100 50 0 50 100 stome group is more strongly influenced by the mean temperature % Viscoelastic fluid % Non-viscoelastic fluid of the warmest quarter (BIO10; Fig. 2A). The presence of wax is D highly influenced by mean temperature of the wettest quarter Min temp. coldest month (BIO6) (BIO8), annual precipitation (BIO12) and isothermality Precip. driest month (BIO14) (BIO3) compared with the absence of wax. Conversely, the Precip. wettest quarter (BIO16) waxless group is influenced more by annual temperature range Annual precip. (BIO12) Mean temp. warmest quarter (BIO10) (BIO7), precipitation of the driest month (BIO14) and Mean temp. wettest quarter (BIO8) minimum temperature of the coldest month (BIO6; Fig. 2B). Mean diurnal temp. range (BIO2) Habitat suitability for the viscoelastic group is highly influenced Annual temp. range (BIO7) by seasonality of precipitation (BIO15), annual precipitation Isothermality (BIO3) (BIO12), mean temperature of the wettest quarter (BIO8), pre- Annual mean temp. (BIO1) cipitation of the wettest quarter (BIO16) and minimum tempera- 100 50 0 50 100 ture of the coldest month (BIO6) compared with the % Wet syndrome % Dry syndrome non-viscoelastic group. Conversely, the latter is more highly F IG. 2. Comparison of relative magnitude of permutation importance (%) of influenced by isothermality (BIO3), mean temperature of the bioclimatic variables to the Maxent model between putative Nepenthes prey wettest quarter (BIO10), mean diurnal temperature range capture mechanism groups. Only those variables with permutation importance ≥ (BIO2), annual temperature range (BIO7) and precipitation of values 5 % for at least one group in each pair were evaluated. (A) Large peri- stome vs. small peristome. (B) Wax present vs. wax absent. (C) Viscoelastic the driest month (BIO14; Fig. 2C). Comparing the ‘wet’ and fluid vs. non-viscoelastic fluid. (D) ‘Wet’ syndrome vs. ‘dry’ syndrome. ‘dry’ syndromes, the former is influenced more by the minimum temperature of the coldest month (BIO6), precipita- tion of the driest month (BIO14) and precipitation of the wettest quarter (BIO16). Conversely, annual mean temperature DISCUSSION (BIO1), isothermality (BIO3), annual temperature range (BIO2), mean diurnal temperature range (BIO2) and mean tem- With a geographical range extending from Madagascar to New perature of the wettest quarter (BIO8) are more important for the Caledonia, Nepenthaceae is a successful and diverse carnivorous ‘dry’ syndrome (Fig. 2D). family. Nepenthes can be found from sea level to .3000 m Page 10 of 13 Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants elevation, on a variety of substrates (Clarke, 1997). Nepenthes since the wax-based capture mechanism does not require critical occur in both perhumid and seasonal tropical environments, levels of ambient moisture, in contrast to peristome-based capture. and the range of pitcher morphologies is striking, even to the casual observer. This diversityof pitcher morphology is mirrored Viscoelastic fluid by the range of nitrogen sources exploited, including bat excreta, leaf litterand arthropods (e.g. Moran et al., 2003; Di Giusto et al., Of the eight habitat models derived here, that for the viscoelas- 2008; Grafe et al., 2011; Pavlovicˇ et al., 2011). Yet, despite this tic group appears to be the most tightly constrained by climate, as wide range of pitcher structure and function, we detected a con- evidenced by the highly limited geographical extent of suitable sistent, large-scale pattern defining the relationship between bio- habitat. Bonhomme et al. (2011) noted the tendency for species climatic variables and prey capture mechanism. with viscoelastic pitcher fluid to be montane in habit, and this is corroborated by the distribution of suitable habitat in the current study. Theyalso suggested that the abundance of volant prey rela- Peristome tive to crawling prey in tropical montane environments favours

the deployment of viscoelastic fluid rather than waxes. Their as- Downloaded from Bohn and Federle (2004) and Baueret al. (2008) demonstrated sertion was based on experimental testing of the effects of wax the profound reliance of peristome function on ambient moisture and viscoelastic fluid on insect traction. They demonstrated in Nepenthes species that rely on this structure as the primary that wax is a more effective mechanism for preventing escape trapping mechanism. The current study confirms that Nepenthes of ants, whereas viscoelastic fluid is effective for both ants and species employing peristome-based prey capture are generally flies. Further, the authors determined that an inverse relationship restrictedtoperhumidregions.AsFig.1demonstrates,habitatsuit- http://aob.oxfordjournals.org/ (similar to that between peristome and wax) exists between de- able for the large peristome group is considerably less extensive ployment of viscoelastic fluid and deployment of wax. This rela- thanthatforthesmallperistomegroup.Itisconfinedlargelytoperhu- tionship would account for the almost identical areas of suitable mid regions, notably the eastern Philippines, western Borneo, habitat modelled for the waxy and non-viscoelastic groups. It has western Sumatra and eastern Peninsular Malaysia. In contrast, been shown that even when highly diluted with water (as might habitat suitable for the small peristome group is much less geo- be the case with rainwater in montane and other perhumid envir- graphically constrained, and extends into regions with more sea- onments), Nepenthes viscoelastic fluid retains its prey-retentive sonal climates, both north and south of the Equator. properties (Gaume and Forterre, 2007; Bonhomme et al., 2011). The relative importance of individual bioclimatic factors to

Thus, we might expect the production of viscoelastic fluid to be at Royal Roads University Library on August 26, 2013 the Maxent models also provides clues to climatic constraints associated with wetter habitats, and the findings of the current on peristome use. Compared with the small peristome group, study appear to support this idea. The results of the paired group habitat suitable for the large peristome group is influenced more comparison show that three of the five important bioclimatic by minimum temperature of the coldest month, annual tempera- variables for the viscoelastic group pertain to precipitation. In ture range and precipitation of the driest month. The first two contrast, only one of the five important variables for the non- variables could be considered pertinent with regards to montane viscoelastic group is based on precipitation; the rest are all climates (a large proportion of species in the large peristome temperature related. group are montane/sub-montane in habit); the last variable sug- gests a constraint with regards to periods of low moisture avail- ability, further supporting the idea that peristome-based capture ‘Wet’ and ‘dry’ syndromes is centred on perhumid regions. Given that the putative ‘wet’ and ‘dry’ syndromes are based on combinations of the individual prey capture mechanisms dealt with above, it is perhaps not surprising that the modelled Wax habitat suitability patterns closely match them, with the former It is evident from Fig. 2A and B that there is a reciprocal pattern being restricted to perhumid regions and the latter extending of relative importance of bioclimatic variables between peristome- into more seasonal regions. Of the three bioclimatic variables based capture and wax-based capture. The three bioclimatic vari- shown to be important to the ‘wet’ syndrome model in the ables identified above as important in the distribution of the large paired comparison, two are precipitation related; in contrast, all peristome group (minimum temperature of the coldest month, five of the variables shown to be important to the ‘dry’ syndrome annual temperature range and precipitation of the driest month) model are temperature related. arealsothethreemostinfluentialinthedistributionofthewaxless group. Similarly, the geographical range of suitable habitat is No evidence for edaphic influences on group distributions more restricted for the waxless group than for the waxy group. The maps for the small peristome and waxy groups are almost Climate is not the only determinant of the geographical distri- identical, as are those for the large peristome and waxless groups. butions of organisms. For plants, edaphic factors can play a crit- Given the previously noted inverse relationship between wax ical role in determining where a given species is likely to occur. production and peristome size (Bauer et al., 2012a; Benz et al., As with other taxa, Nepenthes are restricted to 2012), this finding is perhaps not surprising. The graphical nutrient-poor substrates (Givnish et al., 1984). There is a variety outputs from the models also confirm the prediction that species of such substrates available across the geographical range of using wax as a capture mechanism would be less confined to per- Nepenthes, and many species are restricted to one or more of humid conditions than those relying on a peristome-based strat- them. For example, N. rafflesiana is typically found growing egy. From afunctional viewpoint, this also makes intrinsic sense, on silica-derived podzols, N. bicalcarata occurs predominantly Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants Page 11 of 13 on peat-derived soils, Nepenthes faizaliana is restricted to lime- compounds (e.g. lignin and tannins) incurs higher construction stone outcrops, N. rajah grows on serpentine soils derived from costs (Poorter and De Jong, 1999; Villar et al., 2006; Osunkoya ultramafic bedrock, and Nepenthes sumatrana occurs on sand- et al., 2007, 2008). The peristome is a rigid, structurally rein- stone outcrops (Clarke, 1997, 2001). However, although soil forced region of the pitcher, and this rigidity is due to the incorp- type may play an important role in the distribution of individual oration of these more expensive secondary compounds. The Nepenthes species, it does not appear to be a factor in the distri- viscoelasticity of pitcher fluid results from the presence of long- bution of capture mechanisms examined in the current study. chain polymers (Bonhomme et al., 2011) the relative construc- Evidence for this is found in the five species mentioned above: tion costs of which have yet to be determined. The waxes pro- although each is restricted to a particular soil type, all are duced by Nepenthes pitchers are rich in primary alcohols and members of the large peristome group. Similar examples can very long chain aldehydes (Riedel et al., 2003, 2007). Waxes be found for the other groups. Therefore, we suggest that al- incur significantly higher construction costs than secondary though the distribution of Nepenthes as a genus (as well as indi- compounds such as tannins and lignin (Poorter and De Jong, vidual species, as highlighted above) may be constrained by 1999; Martı´nez et al., 2002). The inner walls of 22 climate and soil type, the geographical distribution of the three N. albomarginata pitchers produce .35 mgcm of wax Downloaded from major capture mechanisms can be explained largely by climatic (Riedel et al., 2007), which would, other factors being equal, variables alone. Nepenthes species employing peristome-based incur considerably higher construction costs than pitchers of or viscoelastic mechanisms are more restricted to perhumid waxless congeners. Therefore, we suggest that the reason that areas (including montane environments); in contrast, those wax has been lost independently from several clades over time employing wax-based capture are less constrained by moisture is probably due to the costs of production outweighing the bene- regime, and can successfully inhabit more seasonal environ- fits gained by its deployment in a particular environment. If the http://aob.oxfordjournals.org/ ments. less resource-costly peristome is as effective as wax in perhumid regions, it would be to a plant’s advantage to reduce or cease ex- pensive wax production to maximize net benefit. This idea is in Reasons for maintenance of multiple prey capture mechanisms agreement with the findings of Bonhomme et al. (2011), Benz in Nepenthaceae et al. (2012) and Bauer et al. (2012a):inNepenthes, there is a The current study has established that the wax-based capture strong reciprocal relationship between the use of wax and the de- strategy in Nepenthes facilitates carnivory in both perhumid ployment of alternative capture mechanisms. It is also in agree-

and more seasonal environments. If the wax-based strategy is ef- ment with the findings of the current study, with regards to the at Royal Roads University Library on August 26, 2013 fective over such a large geographical range, why are the other geographical distributions of wax-based vs. alternative prey mechanisms (large peristome and viscoelastic fluid) maintained capture mechanisms. To conclude, we suggest that climate is a in Nepenthes at all? There is a consensus that the ‘ancestral’ state strong candidate as the agent of the observed disruptive selection of Nepenthes pitchers is represented by a wax-based capture between wax-based and either peristome-based or viscoelastic strategy, and that this strategy has been lost independently in fluid-based capture syndromes in Nepenthes. several clades (Meimberg and Heubl, 2006; Gaume and Di Giusto, 2009; Bauer et al., 2012a). What might be the evolution- ary drivers of wax loss and the consequent adoption of alternative ACKNOWLEDGEMENTS strategies? One possibility is differential effectiveness of the We thank Royal Roads University fora grant to support the study, trapping mechanisms on different prey taxa: Bonhomme et al. B. Hawkins for material help, and A. Moran for reviewing the (2011) demonstrated that viscoelastic fluid was more effective manuscript. Authorcontributions: J.M., C.C. and L.C. conceived than wax in retaining volant prey. They also suggested that the study; C.C. collected the data; L.G. and J.M. analysed the volant prey are more prevalent than crawling prey in montane data; J.M. led the writing with contributions from L.G., C.C. habitats, thus favouring deployment of viscoelastic fluid. and L.C. L.G. constructed the figures. Two anonymous reviewers However, a discussion of possible prey specialization with helped to improve the manuscript considerably. regards to bioclimatic variables is beyond the scope of the current study. Another possibility is that there exists a cost differential LITERATURE CITED between wax-based and alternative strategies. Regardless of trapping mechanism, the production of pitchers incurs a cost to Bauer U, Bohn HF, Federle W. 2008. Harmless nectar source or deadly trap: Nepenthes pitchers are activated by rain, condensation and nectar. the plant. There is a quantifiable ‘opportunity cost’ associated Proceedings of the Royal Society B: Biological Sciences 275: 259–265. with producing a foliar structure that delivers sub-optimal photo- Bauer U, Willmes C, Federle W. 2009. Effect of pitcher age on trapping effi- synthetic performance. For example, pitchers of Nepenthes ciency and natural prey capture in carnivorous Nepenthes rafflesiana talangensis exhibit significantly reduced rates of photosynthesis plants. Annals of Botany 103: 1219–1226. (A ) and quantum yield of photosystem II (F ) compared Bauer U, Grafe TU, Federle W. 2011. Evidence for alternative trapping strat- N PSII egies in two forms of the pitcher plant, Nepenthes rafflesiana. Journal of with laminae (Pavlovicˇ et al., 2009). Additionally, some compo- Experimental Botany 62: 3683–3692. nents of the pitcher are more expensive than others to produce. Bauer U, Clemente CJ, Renner T, Federle W. 2012a. Form follows function: The construction costs of a plant organ such as a leaf can be morphological diversification and alternative trapping strategies in carniv- defined as the amount of photosynthate required to produce a orous Nepenthes pitcher plants. Journal of Evolutionary Biology 25: 90–102. unit weight of biomass (Poorter and De Jong, 1999; Villar Bauer U, Di Giusto B, Skepper J, Grafe TU, Federle W. 2012b. With a flick of et al., 2006). Simple compounds such as cellulose are relatively the lid: a novel trapping mechanism in Nepenthes gracilis pitcher plants. inexpensive to produce; however, synthesis of secondary PLoS One 7: e38951. Page 12 of 13 Moran et al. — Climate and capture mechanism in Palaeotropical pitcher plants

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