529 B.C. Timm and K. McGarigal 22001133 NORNToHrEthAeSaTstEeRrnN N NaAtuTrUaRlisAtLIST 20V(o3l). :2502,9 N–5o3. 93 A Preliminary Assessment of the Ground-Dwelling Arthropod Community Composition in Six Common Dune Cover Types at Cape Cod National Seashore Brad C. Timm1,* and Kevin McGarigal1 Abstract - We provide a preliminary assessment of the ground-dwelling arthropod community composition in six common coastal dune ecosystem land cover-types at Cape Cod National Seashore. We captured 6815 individual arthropods representing 16 arthropod orders from 1008 terrestrial pitfall trap-nights. The most abundant orders were Hymenoptera, Diptera, Araneae, and Isopoda (76.1%, 8.5%, 5.5%, and 3.3% of total captures, respectively). There were differences in ground-dwelling arthropod community composition among the three early-successional and the three later-successional cover types, with the latter having a greater overall arthropod diversity and higher capture rates for a number of the major arthropod taxa captured. Our report is among the first communitywide analyses of arthropod community composition in coastal dune ecosystems of the northeastern US. The results from this study should be viewed as a preliminary assessment given that: 1) we employed a single trapping method (i.e., pitfall traps); 2) traps were only open during the late-afternoon to early morning hours, and only during the summer months; and 3) captured arthropods were classified only to order. We hope our report will inspire additional research of coastal dune arthropod communities. Introduction In many terrestrial ecosystems, arthropods comprise the greatest faunal species diversity, biomass, and number of individuals (Gaston 1991, Ponder and Lunney 1999, Wilson 1985). Arthropods are critical to nutrient cycling and play vital roles as decomposers, pollinators, predators, and prey in natural ecosystems (Greenslade 1992, Wilson 1987). Although the importance and relative abundance of arthropods in many ecosystem types has been well established and accepted by ecologists, surprisingly few studies assessing arthropod community composition and habitat associations in coastal dune systems of North America have been conducted (for exceptions see Fork 2010, Heckscher and Bartlett 2004, Mattoni et al. 2000). Given arthropods’ importance to biodiversity, understanding arthropod community structure and habitat associations are essential to identify and prioritize areas for conservation actions, and to understand and project impacts of humaninduced modifications to the natural environment (Kremen et al. 1993, Longcore 2003, Nakamura et al. 2007). In addition, because arthropods are a food source for many species (Hardy and Crnkovic 2006, Razeng and Watson 2012, Tahir et al. 2009, van Windergen et al. 1981, Vonshak et al. 2009), an improved understanding of arthropod abundance and habitat associations can provide insight 1Department of Environmental Conservation, University of Massachusetts-Amherst, Holdsworth Natural Resources Center, Amherst, MA 01003. *Corresponding author - timm@eco.umass.edu. B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 530 into habitat use and population dynamics of sympatric species. Finally, it has been suggested that terrestrial arthropod taxa may be very useful in ecosystem monitoring because they are diverse and abundant, they have rapid population growth rates and short generation times, and they are sensitive to minor changes in microclimate and microhabitat conditions (Andersen and Majer 2004, Mattoni et al. 2000, Schowalter 2006). Coastal dune faunal assemblages are typically dominated by arthropods (Gaylard et al. 1995, sensu McLachlan 1991), many of which have evolved behavioral adaptations (Boomsma and Isaaks 1982, den Hollander and van Heerdt 1981) and exhibit habitat specificities (Maes and Bonte 2006, Schirmel and Buchholz 2011) that enable them to survive in ecosystems that are characterized by wide microclimatic variability, high wind, and heavy salt loads. There has been some study of arthropod taxa in coastal dunes, (e.g., Araneae, Coleoptera, and Formicidae), but compared to other biota, arthropods have been understudied in these ecosystems. We report on a preliminary assessment of the community composition of ground-dwelling arthropods in a coastal dune ecosystem at Cape Cod National Seashore. Our study took place during the late afternoon to early morning from 20 June–21 September 2007. We assessed differences among six common land-cover types present in these dunes with respect to: 1) capture rates of the dominant (i.e., most abundantly captured) ground-dwelling arthropod orders throughout the study duration, and 2) the overall arthropod community composition (to the taxonomic level of order). Our goal is to enhance existing, yet limited knowledge of arthropod communities that are present in coastal dune ecosystems. The study was originally designed to assess ground-dwelling arthropod prey availability for two amphibian species (i.e., Anaxyrus fowleri Hinckley [Fowler’s Toad] and Scaphiopus holbrookii Harlan [Eastern Spadefoot]) present in the study landscape (Timm and McGarigal 2010). Methods Study area The study was conducted in the Province Lands, a vast dune ecosystem encompassing approximately 1800 ha at the northern tip of Cape Cod, MA (≈42.05°N, 70.18°W1; Fig. 1). The topography is irregular with elevations ranging from ≈0–33 m above sea level, with a substrate comprised primarily of coarse-grained sand. Upland cover types throughout the study area vary spatially and include: non-vegetated (open sand) communities, Ammophila spp. (beachgrass) and Deschampsia flexuosa Trin (Wavy Hairgrass) grasslands, Hudsonia tomentosa Nutt (Woolly Beachheather)- and Arctostaphylos uva-ursi Spreng (Bearberry)- dominated heathlands, Cladonia spp. (reindeer lichen) patches, and deciduous and coniferous (predominantly Pinus rigida Mill [Pitch Pine]) shrublands and woodlands. The climate is mild with mean high temperatures ranging from 2.7 ºC in February to 25 ºC in August (NADP 2005). Mean annual precipitation is 110 cm with relatively little monthly variation (NADP 2005). 531 B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 Field methods Arthropods were captured using terrestrial pitfall traps in six common cover types (hereafter habitats or habitat types) found throughout the study area—1) creeping Pitch Pine edge: the interface between ground-creeping Pitch Pine branches and open sand; 2) Pitch Pine interior: the interior portion of an individual Pitch Pine shrub with a relatively open understory and complete canopy coverage; 3) deciduous shrubland: a patch of individual to several sparse deciduous shrubs, primarily Quercus ilicifolia Wangenh (Bear Oak) and Prunus maritima Marshall (Beach Plum); 4) Woolly Beachheather; 5) open sand; and (6) reindeer lichen. Pitfall traps each consisted of a single 300-ml-capacity plastic cup (top diameter = 7.25 cm, height = 10 cm) buried so that the top was flush with the ground surface and filled to a depth of 2 cm with a dishwashing soap and water solution to prevent escape by captured invertebrates. Traps were placed in each of the six habitats and kept closed for a minimum of 48 h prior to trapping to minimize the capture of any organisms due to the initial disturbance of the substrate. During a single night each week from mid-June to late-September 2007 (14 weeks total), we opened four groups of three pitfall traps in each of the six habitat types for a total of 1008 trap nights (14 nights x 72 pitfall traps per sampling night). During each night of trapping, the three pitfall traps in each group were situated ≈1.0 m apart from one another within the same habitat type, and each group was located 50–500 m away from any other group in the same habitat type during a given trapping night. A new trapping site was selected for each weekly sample, with a total of 14 sample sites during the study period (Fig. 1). Traps were open from the late afternoon through early morning to coincide with foraging activity patterns of Fowler’s Toads and Eastern Spadefoots. During each weekly sampling event, traps were open for ≈18 h (from ≈1530–930), after which all traps were collected and brought back to the laboratory for examination. Trap captures were sorted to order (with the exception of the Hymenoptera which were separated to family [Formicidae]). We pooled captures across all trap groups within each habitat type at each sample site for analysis (84 samples). Figure 1. Study area at Cape Cod National Seashore with the 14 weekly sampling locations denoted by black dots in the subset map. B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 532 Analysis We conducted a Kruskal-Wallis one-way ANOVA to test for among-habitat differences in captures across all six habitat types for each taxon with ≥50 captures. We then conducted a posteriori pairwise comparisons (Siegel and Castellan 1988) to test for pairwise differences in capture rates of individual taxa between habitat types. We conducted a non-metric multidimensional scaling (NMDS) analysis to detect groupings of sample sites using captures across all arthropod taxa and a Bray-Curtis dissimilarity matrix. Prior to running the NMDS analysis, we removed all taxa that we deemed to be insufficiently sampled, which we defined as being present in less than five of the 84 samples; insufficiently sampled taxa included Chilopoda, Dermaptera, Gastropoda, Homoptera, Opiliones, and Pseudoscorpiones. We conducted all analyses in the R computing environment (R Development Core Team 2012) and defined statistical significance as P ≤ 0.05. Results We captured a total of 6815 individual arthropods across 16 orders and one family (Formicidae) (Table 1). Formicidae (ants) accounted for 76.1% of the captures, followed in abundance by Diptera (8.5%), Araneae (5.5%), and Isopoda (3.3%;) (Table 1). Capture rates differed among habitat types for six of the seven taxa that had ≥50 captures (Table 2), which, when combined, accounted for ≈97.5% of all captures. There were a number of differences in the pairwise Table 1. Total captures for each arthropod taxon from six habitat types sampled by pitfall traps at Cape Cod National Seashore. Open Reindeer Woolly Deciduous Pitch Pitch Taxon sand lichen Beachheather shrub Pine edge Pine interior Total Acari 1 0 3 1 7 5 17 Araneae 22 39 25 42 117 129 374 Blattodea 8 1 1 22 28 16 76 Chilopoda 0 0 1 1 0 4 6 Coleoptera 8 13 19 14 17 13 84 Coleoptera larvae 3 1 1 2 4 22 33 Collembola 1 7 1 15 68 31 123 Dermaptera 0 0 0 1 0 2 3 Diptera 43 92 51 143 99 148 576 Diptera larvae 0 2 1 1 0 2 6 Formicidae 225 787 587 662 1774 1153 5188 Gastropoda 0 0 0 0 4 0 4 Homoptera 0 0 0 0 0 1 1 Hymenoptera* 5 10 1 10 3 2 31 Isopoda 1 8 2 118 63 32 224 Lepidoptera 1 3 2 2 4 2 14 Lepidoptera larvae 0 1 1 4 2 0 8 Opiliones 0 0 0 2 4 0 6 Orthoptera 1 6 6 1 11 12 37 Pseudoscorpiones 1 1 0 0 2 0 4 *Non-Formicidae. 533 B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 comparisons between habitat types for capture rates of individual taxa (Figs. 2, 3); most of these differences were instances in which there were higher capture rates in later-successional habitat types than in early-successional habitat types The three early-successional dune habitat types (i.e., open sand, reindeer lichen, and Woolly Beachheather) were fairly well separated from the three latersuccessional habitat types (i.e., deciduous shrub, Pitch Pine edge, and Pitch Pine interior) along the first NMDS axis of ground-dwelling arthropod community composition (Fig. 4). The first axis represented a gradient from high Collembola Table 2. Results from the Kruskal-Wallis one-way ANOVA test for among-habitat differences in captures across six habitat types at Cape Cod National Seashore. Data are presented for each taxon with ≥50 captures. Taxon n df χ2 P-value Formicidae 14 5 28.81 less than 0.01 Diptera 14 5 18.36 less than 0.01 Araneae 14 5 34.74 less than 0.01 Isopoda 14 5 23.35 less than 0.01 Collembola 14 5 15.02 less than 0.01 Coleoptera 14 5 1.04 0.96 Blattodea 14 5 20.40 less than 0.01 Figure 2. Capture rates for five of the six most abundantly captured taxa in this study. Lines extending beyond the bars depict one standard error. Groups identified by different lowercase letters were significantly different (P < 0.05). B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 534 Figure 3. Capture rates of Formicidae in pitfall traps at Cape Cod National Seashore. Lines extending beyond the bars depict one standard error. Groups identified by different lowercase letters were significantly different (P < 0.05). Figure 4. Ordination plot of the non-metric multidimensional scaling of the arthropod community composition (retaining all arthropod captures) for each of the 14 weekly sampling events at each of the six habitat types sampled by pitfall traps at Cape Cod National Seashore. The location of the seven most abundantly captured taxa along the first two axes is also depicted in the plot. 535 B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 and Formicidae proportional abundance (negative NMDS sample scores) to high Coleoptera proportional abundance (positive scores); the majority of early-successional samples had positive scores whereas the majority of later-successional samples had negative scores. Due to the high relative abundance of Formicidae captures compared to all other captured taxa, we ran a second NMDS analysis excluding Formicidae. Results from this second NMDS analysis showed a similar magnitude of separation between the three early- and the three later-successional habitat types along the first NMDS axis of arthropod community composition (Fig. 5). The first axis was a gradient from high Isopoda and Collembola (and to a lesser extent, Diptera and Araneae) proportional abundance (negative NMDS sample scores) to high Coleoptera proportional abundance (positive scores); the majority of early-successional samples had positive scores, whereas the vast majority of later-successional samples had negative scores. Discussion We captured a large number and diversity of ground-dwelling arthropods throughout the six coastal dune habitat types sampled at Cape Cod National Seashore. There were significant differences in capture rates of individual arthropod taxa among the six habitat types. Results from multivariate ordination analyses Figure 5. Ordination plot of the non-metric multidimensional scaling of the arthropod community composition (excluding Formicidae, see text) for each of the 14 weekly sampling events at each of the six habitat types sampled at Cape Cod National Seashore. The location of six of the seven (i.e., excluding Formicidae) most abundantly captured taxa along the first two axes is also depicted in the plot. B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 536 revealed that three early-successional coastal dune habitat types were fairly well separated from the three later-successional habitat types based on ground-dwelling arthropod community composition. In general, later-successional habitat types had greater overall arthropod capture rates and greater within-taxon capture rates for several taxa compared to early-successional habitat types sampled. Other studies of coastal dune arthropods have also found among-habitat differences in capture rates of individual arthropod taxa (Fork 2010, Mattoni et al. 2000). These differences may be due, in part, to the considerably different vegetation structure and microclimates among the habitat types we sampled. The later-successional habitat types, which were the only habitats that had significantly higher capture rates for individual taxa, were the most structurally diverse and likely had the most stable and moderated microclimates because of the extensive shading and wind protection afforded by the pine and shrub vegetative cover. Greater structural diversity may lead to increased arthropod species diversity (Blaum et al. 2009, Gardner et al. 1995) and abundance (Blaum et al. 2009, Langellotto and Denno 2004). The moderated microclimates in the later-successional habitats may also have the same positive relationships with arthropod species diversity and abundance (Bonte and Mertens 2003, Buchholz 2010). The combination of these factors helps explain our results. Interestingly, the pattern of increased ground-dwelling arthropod abundance in the later-successional habitat types is consistent with the pattern of habitat use and preference by Eastern Spadefoots in the study area (Timm et al. in press). The Eastern Spadefoot diet is comprised almost entirely of ground-dwelling arthropods (Pearson 1955, Punzo 1992, Timm and McGarigal 2010). The majority of arthropod captures were ants (Formicidae), which is consistent with the findings of previous studies in other coastal dune ecosystems (Cheli et al. 2010, Gaylard et al. 1995, Marshall et al. 2008). This prevalence is largely due to their overall abundance in dune ecosystems and their colonial nature, such that when they were captured in a location they were typically captured in relatively large numbers. In addition to comprising an impressive amount of biomass (≈15–20% of the global terrestrial animal biomass; Shultz 2000), the Formicidae is among the most diverse families of organisms known, which may explain the large number of ant captures we observed across all six habitat types sampled in our study. Though we did not classify ant captures below the taxonomic level of family, recent research from nearby Nantucket Island (Ellison 2012) suggests that ant species diversity in our study area may be relatively high. Given their apparent high relative abundance throughout the study area and their importance to ecosystem functioning (Folgarait 1998), further research on the ant fauna of the Province Lands dunes is warranted. For example, in our study landscape ants comprised the majority of the diet of subadult Fowler’s Toads (Timm and McGarigal 2010), a species which, in turn, comprises the majority of the diet of Heterodon platirhinos Latreille (Eastern Hognose Snake) in the Province Lands (R.P. Cook, Wildlife Ecologist, Cape Cod National Seashore, Truro, MA, unpubl. data); thus, ants clearly play a critical, if not a keystone role, in the food web dynamics of this dune ecosystem. 537 B.C. Timm and K. McGarigal 2013 Northeastern Naturalist Vol. 20, No. 3 Our study is among the first to analyze the ground-dwelling arthropod communities of coastal dune ecosystems in the northeastern United States, albeit at a coarse taxonomic resolution and a limited temporal scope. Our preliminary analyses indicate that these ecosystems may support abundant and diverse grounddwelling arthropods which are not equally distributed among habitat types. Given the critical roles that arthropods play in ecosystem function, the current knowledge gaps regarding coastal dune arthropod communities, and the projected impacts of climate change on coastal dune ecosystems (IPCC 2007), we hope that our research will provide foundational insight and will inspire additional research into this largely understudied coastal community assemblage. Scope and Limitations The results presented and discussed above must be interpreted in the context of the scope and limitations of our study. Pitfall traps at each sample site were only set for an ≈18-h period during the late afternoon through early morning hours (to coincide with foraging activity patterns of Fowler’s Toads and Eastern Spadefoots), thus the absolute capture numbers may be biased compared to the actual abundances at sample sites and may be skewed towards crepuscular and nocturnal taxa. Additionally, we only used pitfall trapping to assess arthropod communities, and this method estimates relative arthropod activity rather than absolute density, reflecting arthropod abundances and movement rates at sample sites (Mazia et al. 2006). However, it is accepted that pitfall traps can be used to effectively evaluate habitat associations as well as to establish relative species abundances within and among habitats (Mazia et al. 2006). In addition, we only classified captures to the taxonomic level of order (with exception to family in the case of ants). Classification to a finer taxonomic resolution (i.e., to family, genus, or species) would have allowed a more robust assessment of community composition and habitat associations. Finally, traps were only set during the summer; thus, it is unknown whether the patterns we observed extend beyond the summer timeframe. 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