The Diversity and Distribution of Spiders (Arachnida: Araneae) Along an Outdoor – Indoor Habitat Gradient: Preliminary Findings from Piedmont Virginia

Abstract

Although the United States supports a considerable diversity of spiders, some aspects of spider habitat use and niche specialization are poorly documented. Specifically, little attention has been given to explore how urban development affects the diversity and abundance of arthropods. We sampled spiders along an outdoor – indoor habitat gradient at Longwood University to understand the impact of urbanization on species diversity and abundance. We found 50 taxa of spiders belonging to 43 genera and 16 families. Overall, the most abundant spider family across three sampling sites was Araneidae (orb-weavers; 18.2%) followed by Lycosidae (wolf spiders; 14.8%), Salticidae (jumping spiders; 13.6%) and Linyphiidae (sheetweb spiders; 12.5%). We found the highest species richness, spider abundance, and Shannon-Wiener diversity from Lancer Park (i.e. outdoors habitat), followed by the habitats associated with outside of the science center building (i.e. marginal habitat) and the lowest spider diversity inside the science building (i.e. indoors habitat). We also found a strong positive correlation between overall spider diversity and air temperature for outdoors and marginal habitats, but no correlation with relative humidity. Our study adds original knowledge about habitat use of spiders along an outdoor indoor habitat gradient and arthropod use of indoor biome. More importantly, our study stresses the need for more extensive systematic studies to fully understand how spatial and temporal variation of arthropod diversity and abundance may be influenced by alterations of habitats by humans through urbanization. INTRODUCTION Spiders are one of a few cosmopolitan groups of organisms utilizing a range of habitats from hot deserts to the cold Arctic (Foelix 2011) to urban habitats with man-made structures. Worldwide, there are about 48,000+ (World Spider Catalog 2019) formally described species of spiders including at least 3,800 species in North America (Bradley 2013). Although spiders are ubiquitous, little attention is typically given by the ecology research community, to study their diversity, biology, and ecology, possibly due to their small size, seemingly secretive behavior, lack of information on true diversity and 1 henkanaththegedarasm@longwood.edu Virginia Journal of Science Volume 70, Issue 3 Fall 2019 doi: 10.25778/ej65-br87 Note: This manuscript has been accepted for publication and is online ahead of print. It will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Virginia Journal of Science, Vol. 70, No. 3, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss3 Diversity and Distribution of Spiders 2 subjective fear of spiders in general. With that being said, the true diversity of spiders in the eastern United States may be poorly documented (Howell and Jenkins 2004). Spiders play important ecological roles in their habitats, mainly as predators and prey. They are important predators in the natural ecosystem (Foelix 2011, Mallis and Rieske 2011) and are typically generalist predators, many feeding on different terrestrial arthropods, but some are specialists (Mallis and Hurd 2005, Mallis and Rieske 2011). Agriculturally, spiders are very helpful in limiting the amount of pest populations in crops. It is estimated that spiders consume 400-800 million tons of prey annually (Mallis and Hurd 2005). Spiders are very efficient as natural pest control agents, hence some rice farmers in Asia do not use pesticides (Nyffeler and Benz 1987). Additionally, spiders have complex trophic networks and may belong to more than one trophic level based on their diet and size (Wise et al. 1999). For example, larger wolf spider species tend to prey on herbivores while smaller wolf spiders in leaf litter prey on detritivores, fungivores, and herbivores (Mallis and Hurd 2005). On the other hand, spiders are also a source of food for many larger organisms including birds (Rogers et al. 2012). Although the United States supports a considerable diversity of spiders, some aspects of spider habitat use and niche specialization are poorly documented (Howell and Jenkins 2004). A recent study concluded that eastern hemlock canopies were more diverse than deciduous canopies for spiders (Mallis and Rieske 2011). Mallis and Hurd (2005) reported 50 species of ground-dwelling spiders from a successional gradient of habitats in southwestern Virginia including habitat specialists and generalists. Smith et al. (2018) compared spider diversity between mesic and xeric habitats in Pike County, Alabama, and reported 82 species belonging to 24 families (Smith et al. 2018). Some relatively unexplored aspects of spider ecology are the use of man-made structures as habitat by spiders and how urbanization affects them. With the rapid expansion of human population, the impacts of urbanization generally cause loss of native species diversity (Blair 1996, Gagne and Fahrig 2011). However, urbanization may also promote a few urban-adapted taxa and lead to biotic homogenization (Blair 1996). Overall, little attention has been given to explore how urban development affects the diversity and abundance of arthropods (McIntyre 2000, Shochat et al. 2004) despite the ubiquitous nature of arthropods in human dwellings. Spiders are one of many arthropod groups commonly associated with urban habitats and human dwellings. Shochat et al. (2004) showed that the transformation of a xeric natural habitat into an urban habitat caused reduced spider diversity and the establishment of a few spider taxa that can tolerate the new urban setting. Additionally, a recent study that analyzed the diversity of the indoor arthropod biome found that spiders represent nearly one-fifth of the indoor arthropod diversity (Bertone et al. 2016). Therefore, the differences in diversity and abundance of spiders may reflect the changes in trophic structure in human-altered systems (Shochat et al. 2004). In this study, we conducted a survey of spiders along an outdoor – indoor habitat gradient at Longwood University to understand the impact of urbanization on species diversity and abundance. Specifically, our goals included 1) comparing and contrasting Virginia Journal of Science, Vol. 70, No. 3, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss3 Diversity and Distribution of Spiders 3 spider diversity from three distinct habitats covering an indoor-outdoor habitat gradient, 2) exploring the relationships between environmental conditions and the diversity of spiders, and 3) generating a preliminary species list for Longwood University premises. We predicted that indoor habitats would support less diversity of spiders compared to outdoor and marginal habitats. Additionally, we expected a positive correlation between spider diversity and two environmental variables, temperature and relative humidity. MATERIALS AND METHODS Study Area This study was conducted at Longwood University in Farmville, Virginia (37.2972971,-78.3972648). We selected three specific habitats to represent an outdoor – indoor habitat gradient. We selected the lowland floodplain of the Buffalo Creek at Lancer Park as the outdoor habitat. This relatively small land area (0.12 km2) represents a diverse array of both aquatic and terrestrial habitats including a third order stream, a series of seasonal pools, several man-made ponds, eastern deciduous forests, grasslands and hedge habitat, and some buffer habitat with parking lots and roads. We specifically sampled grassy areas with shrub or tree margins at Lancer Park (i.e. outdoor habitat). Additionally, we sampled inside the Chichester Science Building as the indoors habitat including classrooms, lab spaces, and stairwells (i.e. indoor habitat), while habitats outside Chichester Science Building including walls, windows, and adjacent vegetation up to 5 m from the building (i.e. marginal habitat) (Fig. 1). We sampled all study areas in March and April of 2018. Outdoor and marginal habitats were sampled four times, but indoor habitats were sampled only twice due to logistical limitations. Virginia Journal of Science, Vol. 70, No. 3, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss3 Diversity and Distribution of Spiders 4 Figure 1. The major sampling locations for this study. The Lancer Park flood plain (A) served as the outdoor habitat and the Chichester science building (B) served as the indoor and marginal habitats. Field Data Collection and Spider Identification We collected spiders by opportunistic sampling (Motley et al. 2017) within each sampling location using an array of sampling methods during day time. Visual observations and hand picking were mainly employed in indoor habitats and additionally, sweep nets and beat sheets were used for outdoor sampling. These methods allowed us to collect spiders from diverse microhabitats. Sampling was conducted for two hours at Lancer Park and another two hours covering inside and outside of the Chichester Science building. All spiders were photographed and released back to the original capture locations. Environmental data such as temperature and humidity were collected at capture locations using the RockyMars ® RT36 temperature and humidity meter. Spiders were identified to Virginia Journal of Science, Vol. 70, No. 3, 2019 https://digitalcommons.odu.edu/vjs/vol70/iss3 Diversity and Distribution of Spiders 5 the lowest possible taxa (i.e. genus or species) using field guides and identification keys provided by Howell and Jenkins (2004), Gaddy (2009), Bradley (2013) and Ubick et. al. (2017). Data Analysis Overall relative abundance of spiders for each family was estimated by dividing the pooled number of individual spiders belonging to a given family by the total number of spiders. Shannon-Wiener diversity index (H’) and Simpson’s dominance index (D) were computed for each sample using the following formulae (Krebs 1999) to estimate overall diversity and dominance of spider communitie