Sean Higgins
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ABSTRACT Web spider diversity in the riparian forests of southwest Ohio by Terrence Sean Higgins Riparian forests are often the primary source of forest habitat in agricultural landscapes. I assessed the diversity of web spiders in riparian forests and hedgerows of SW Ohio. Samples were taken at the stream edge, in the center, and at the agricultural edge of riparian forests of three width classes (thin: 15-30m, medium: 45-60m, wide: >80m). Rarefaction and ANOVA compared diversity among forest types, and among the agriculture edge, center and stream edge. Detrended Correspondence Analysis compared the species assemblages among samples. Web spiders were significantly more diverse in riparian forests than in hedgerows. Thin riparian forests were most diverse of all forest types. While stream edges had the lowest diversity, their assemblages were distinct from other quadrats. My data suggest that the stream habitat plays a key role in structuring the adjacent terrestrial spider community, and that riparian forest corridors are important to regional spider diversity in agricultural landscapes. WEB SPIDER DIVERSITY IN RIPARIAN FORESTS OF SOUTHWEST OHIO A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science by Terrence Sean Higgins Miami University Oxford, Ohio 2003 Advisor: Dr. Ann Rypstra Reader: Dr. Tom Crist TABLE OF CONTENTS TABLE OF CONTENTS…………………………………………………………… ii LIST OF TABLES………………………………………………………………….. iii LIST OF FIGURES………………………………………………………………… iv ACKNOWLEDGEMENTS………………………………………………………… vii CHAPTER 1………………………………………………………………………... 1 CHAPTER 2: INTRODUCTION………………………………………………….. 8 METHODS………………………………………………................. 10 RESULTS…………………………………………………………... 14 DISCUSSION………………………………………………………. 16 CHAPTER 3: INTRODUCTION………………………………………………….. 30 METHODS…………………………………………………………. 31 RESULTS………………………………………………………….. 35 DISCUSSION……………………………………………………… 37 LITERATURE CITED……………………………………………………………... 52 ii LIST OF TABLES CHAPTER 2 Table 1 Spider families and species collected by quadrat type in riparian forests near Oxford, Ohio in August 2002…………………………… 26 Table 2 Results of ANOVA on web spider richness, abundance, Simpson diversity, and Shannon diversity in 5 x 10-m quadrats placed at the agriculture edge, forest center, and stream within replicated thin, medium and wide riparian forests…………………………………… 27 Table 3 Mean (± SE) abundance, species richness, Shannon diversity, and Simpson diversity of web spiders collected in riparian forests by landscape treatment (thin, medium, wide) and quadrat (agriculture edge, center, stream)………………………………………………… 28 Table 4 Mean (± SE) vegetation structure values as measured in riparian forests by landscape treatment (thin, medium, wide) and quadrat (agriculture edge, center, stream).……………………….……………. 29 CHAPTER 3 Table 1 Spider families, species, and relative abundance collected from riparian forests and hedgerows near Oxford, Ohio…………………… 47 Table 2 Mean (± SE) number of web spiders and species collected in riparian forests and hedgerow forests.…............................................................. 48 Table 3 Repeated-measures ANOVAs for web spider abundance (ln) and richness………………………………………………………………. 49 Table 4 Summary of vegetation structure results from 2-factor ANOVAs……………………………………………………………... 50 Table 5 Mean (± SE) vegetation structure values as measured in riparian forests and hedgerows by quadrat type.………………………………. 51 iii LIST OF FIGURES CHAPTER 2 Figure 1 The three major types of riparian forest habitat as designated by width, from the stream to the agricultural edge. Placements of the 5 x 10-m quadrats within each site are marked X…………………………………… 22 Figure 2 Species accumulation curves (±1 SD) for predicted spider species richness in riparian forests of three width treatments (thin, medium, and wide)…………………………………………………………… 23 Figure 3 Species accumulation curves (±1 SD) for predicted species richness at three quadrat locations (agriculture edge, center, stream) in riparian forests…………………………………………………………………. 24 Figure 4 DCA ordination of web spider species abundances collected from 18 quadrats.……………………………….................................................. 25 CHAPTER 3 Figure 1 Riparian forest sites compared with hedgerow sites of similar width. Locations of the 5 x 10-m quadrats are marked X……………….. 42 Figure 2 Means (± 1 standard error) of web spider abundance and species richness from quadrats sampled within hedgerow forests and riparian forests………………………………………………………………. 43 Figure 3 Species accumulation curves (±1 SD) for predicted spider species richness in riparian forests and hedgerow…………………………. 44 Figure 4 Species accumulation curves (±1 SD) for predicted spider species richness in the edge and center quadrats of riparian forests and hedgerows………………………………………………………… 45 Figure 5 DCA ordination analysis of web spider species collected from 18 quadrats during the August sampling period.………………………. 46 iv ACKNOWLEDGEMENTS This research was made possible by private land owners all over Butler County, Ohio who granted full access to their forests, spiders, and fields for the purpose of ecology research. I thank Dr. Rypstra, Dr. Crist, Dr. Schaefer, Dr. Claussen, Dr. Buddle, Dr. Walker, Dr. Veech, D. Mefford, K. Sigler, and The Spider Lab of Miami University for their contributions to this work. Special thanks to the staff of the Hefner Zoology Museum Staff, The Spider Lab, Miami ecology professors, and fellow graduate students for enlightening my outlook on science and nature throughout the course of this study. Finally, thanks to my friends, family, and Karen for their infinite support. v WEB SPIDER DIVERSITY IN RIPARIAN FORESTS OF SOUTHWEST OHIO Chapter 1: Biological Conservation in Riparian Forests Biological Diversity Biological diversity, or biodiversity is variety in life. It can be measured at many scales of organization including the molecular level, the level of individual species, and the level of whole landscapes (Wilson 1992). Human alteration of the earth continues to cause an overall decrease in biodiversity, including loss of genetic diversity within populations, extinction of whole species, and homogenization of landscapes (Vitousek et al. 1997). Motives for biodiversity conservation include potential economic value derived from plants and animals, aesthetic and research value of species, and moral obligation to minimize species loss (Wilson 1992). Still, the majority of efforts to preserve biodiversity act at the level of conserving species (Folke et al. 1996). Loss of biodiversity includes more than just extinction of species; it includes changes in species composition, ecosystems, and landscapes that are caused by human development (Folke et al. 1996). For these reasons, further biodiversity conservation and research should focus at the level of species assemblages and communities. Landscape Arrangement and Biological Diversity The types and arrangement of habitat within a landscape affect biodiversity (Forman 1995). A landscape is generally defined as a kilometers-wide area of land, which encompasses a number of reoccurring local ecosystems (Forman 1995). MacArthur and Wilson (1967) advanced community ecology when they suggested that populations of species would go extinct in isolated islands in the absence of recolonization. Their island biogeography theory was later used to show how species richness varies among habitat patches that differ in size and isolation in fragmented landscapes. As landscape ecology emerged, it brought about an increased awareness of the importance of the spatial arrangement of habitats. Ecologists recognize that patches in fragmented landscapes are heterogeneous. Physical factors, including amount of sunlight, moisture, and temperature, can cause 1 variation in species assemblages within habitat patches (Forman 1995). Edges of habitat patches tend to have high biodiversity, due in part to the mixing of species from two adjacent habitats (Cadenasso and Pickett 2001). Dispersing organisms may also be more likely to encounter edge habitat (Fahrig and Paloheimo 1987). These factors cause habitat heterogeneity which influences overall biodiversity. The size of a habitat patch can affect its heterogeneity. Smaller habitat fragments tend to have higher species density (number of species per unit area), because of a greater edge to total area ratio (Palmer 1994). However, larger habitat fragments are likely to include higher numbers of species than smaller fragments because they comprise more total area (Forman 1995). Interior species, those found in the center of habitat patches, are also more likely to occur in larger habitat patches (Bender et al. 1998). Studies of community composition suggest that there is often a minimum patch size for the presence of certain interior species (Forman 1995). In fragmented landscapes, a disproportionate percentage of interior species tend to be rare (Gaston 1994), and interior species may be clumped into isolated patches (With and Crist 1995). In such cases, fragmentation and smaller habitat patch sizes are likely to cause loss in biodiversity through the extinction of rare species (Kruess and Tscharntke 1994, Golden and Crist 1999, Tscharntke et al. 2001). Landscape arrangement and fragmentation may disproportionately affect rates of predation and parasitism and the spread of invasive species (Kruess and Tscharntke 1994, Sakai et al. 2001). These results can alter ecosystem functioning by increasing herbivory, decreasing rates of