Effects of Dispersal and Local Dynamics on Spider Diversity (Araneae) in an Old Field System

Effects of dispersal and local dynamics on spider diversity (Araneae) in an old field system

Carol M. Frost

Department of Natural Resource Sciences Macdonald Campus McGill University Montréal, Québec, Canada

August 2008

A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science

Abstract

Processes at multiple spatial scales can interact to structure community diversity. I looked at the effects of both dispersal and local dynamics on spider diversity in young buckthorn trees in an old field system in southwestern Québec, Canada. I investigated the rate, mechanism, and timescale of spider re-colonization of emptied trees and found that the rate fluctuated, peaking in mid-June and early August. Preventing cursorial immigration only significantly reduced immigrant abundance of the jumping spider guild (Salticidae), but it reduced species richness and changed taxonomic composition of immigrants. At peak immigration times, four days was enough time for emptied trees to be fully re-colonized such that spider abundance, species richness, and composition matched that in undisturbed trees, suggesting an important role for dispersal in structuring spider diversity in this system. I used a diversity addition experiment to test whether spider diversity is limited only by dispersal and found that local dynamics also limit spider diversity. This work contributes to understanding the forces structuring arboreal spider assemblages.

1 Résumé Certains processus à des échelles spatiales diverses intéragissent avec la structure des communautés. Je me suis intéressée à la fois aux effets de colonisation et de dynamiques locales sur la diversité des araignées dans un champ à l’abandon peuplé d’arbustes situé au sud-ouest du Québec, au Canada. J’ai étudié les mécanismes, variations temporelles et taux de recolonisation d’arbustes préalablement vidés de leurs araignées et j’ai observé que le taux de recolonisation connaissait un pic d’activité entre la mi-Juin et le début Août. Seule l’abondance de la guilde des araignées sauteuses (Salticidae) a été réduite en empêchant la recolonisation terrestre. Cette manipulation a cependant diminué la diversité des espèces collectées et a modifié la composition taxonomique des colonisateurs. Lors des pics de recolonisation, quatres jours ont suffisaient pour permettre la recolonisation totale des arbres vides, de sorte que l’abondance, la richesse et la composition taxonomique correspondent avec celles des arbres témoins. Ceci suggère que la colonisation joue un rôle important dans structuration de la diversité de ce système. Des expériences d’ajout d’espèces ont permis de vérifier si la diversité des araignées est limitée seulement par la colonisation, et ont mis en évidence que des dynamiques locales intervenaient également. Ce travail contribue à une meilleure connaissance des processus structurant les assemblages d’araignées dans les systèmes arbustifs.

2 Table of Contents Page Abstract……………………………………………………………………………………1 Résumé…………………………………………………………………………………….2 Table of Contents………………………………………………………………………….3 Contributions of Co-author………………………………………………………………..4 Acknowledgements………………………………………………………………………..5 Chapter 1: Introduction, literature review, and objectives………………………………...6 1.1 Introduction………………………………………………………………………...6 1.2 Literature Review…………………………………………………………………..7 1.2.1 Community assembly and metacommunity theory…………………………..7 1.2.2 Empirical metacommunity studies…………………………………………...8 1.2.3 Foliage-dwelling spiders as a model metacommunity system……………...11 1.3 Objectives and thesis structure……………………………………………………14 Chapter 2: Effects of dispersal and local dynamics on spider diversity (Araneae) in an old field system………………………………………………………………………………16 2.1 Abstract…………………………………………………………………………...16 2.2 Introduction……………………………………………………………………….17 2.3 Methods…………………………………………………………………………...20 2.3.1 Study site…………………………………………………………………...20 2.3.2 Re-colonization rate, mechanism, and timescale…………………………..20 2.3.3 Diversity addition experiment……………………………………………...22 2.3.4 Data analysis……………………………………………………………….24 2.4 Results…………………………………………………………………………….27 2.4.1 Re-colonization rate and mechanism………………………………………27 2.4.2 Timescale of re-colonization………………………………………………29 2.4.3 Diversity addition experiment……………………………………………..34 2.5 Discussion………………………………………………………………………...37 2.5.1 Re-colonization rate and mechanism………………………………………37 2.5.2 Timescale of re-colonization………………………………………………38 2.5.3 Diversity addition experiment……………………………………………..40 2.5.4 Conclusion…………………………………………………………………41 Chapter 3: Summary and Conclusion……………………………………………………43 References………………………………………………………………………………..45 Appendix 1: Data from colonization rate and mechanism study………………………...51 Appendix 2: Data from timescale of colonization study…………………………………53

3 Contributions of Co-author

My supervisor, Dr. Chris Buddle is a co-author on the manuscript in Chapter 2, which is to be submitted for publication. Dr. Buddle helped with the conceptualization and design of the experiments and with the editing of the manuscript.

4 Acknowledgements

I would like to thank my supervisor, Dr. Chris Buddle (a.k.a. Rockstar Chris!), for helping to plan my project, for suggestions during my field work, for discussing my results with me, for editing my thesis, for lunchtime discussions about ecology, science, spiders, and other topics, and for his advice, encouragement, and enthusiasm throughout my time in his lab. From him I have learned a great deal about community ecology research, project design, scientific writing, and presentations. Chris Buddle provided financial support for me to attend two conferences, which were great opportunities to present my work for and interact with other students and professors in entomology. I would also like to thank my committee members, Dr. Terry Wheeler and Dr. Sylvie de Blois, for their thoughtful criticism of my proposal and experimental design. I was fully supported during this degree by an NSERC (Natural Sciences and Engineering Research Council of Canada) Canada Graduate Scholarship, and also received a McGill Entrance Scholarship and a McGill Alma Mater Student Travel Grant. I am grateful to Tara Sackett, Jean-François Aublet, Kathleen Aikens, Andrea Déchêne, and my mother for help with field work, to John Watson, Chris Cloutier, and Anne Murphy for the use of and help with the Morgan Arboretum whipper-snippers, and to Raphaёl and Léa Royauté for help with translation. Being part of the Buddle lab has been great fun, and I’d like to thank all the people whom I’ve spent time with there: Kathleen Aikens, Joey Bowden, Maxim Larrivée, Andrea Déchêne, Zach Sylvain, Tara Sackett, Alida Mercado, Charles Stephen, Annie Webb, Raphaёl Royauté, and Agnes Kwasniewska. Finally, I’d like to thank my family, friends, and room mates for encouragement, support, and discussion about my project; in particular, thanks to Mom, Dad, Michael and Peter Frost, Michael Pedruski, Glenna McGregor, Megan Wark, Natasha Prepas- Strobeck, Kristen Whitbeck, Audrey Wachter, Jean Lacasse, Jane Lee, Mylène L’Espèrance, and Vincci Tsui.

5 Chapter 1: Introduction, literature review, and objectives

1.1 Introduction Understanding processes that structure community diversity will be essential to predicting change due to anthropogenic habitat destruction, and in planning conservation initiatives in a wide range of systems (Holyoak et al. 2005; Ricklefs 1987), and is also of relevance in maintaining diverse assemblages of generalist predators for biological control (e.g., Samu et al. 1999). Processes structuring community diversity are currently viewed as a dichotomy of regional and local factors (Ricklefs 1987), and metacommunity theory is working toward a synthesis of how these factors interact (Leibold et al. 2004). There are currently four metacommunity paradigms, each of which fits certain natural systems, but for which a synthesis is lacking (Leibold et al. 2004). A fundamental distinction amongst the different paradigms is the different relative importance of processes operating at regional and local scales in structuring diversity. Many empirical studies have addressed the question of the relative importance of dispersal and local dynamics in structuring community diversity (see meta-analysis by Cadotte, 2006a). However, few studies have addressed this question in taxonomic assemblages other than plants and microorganisms (Cadotte 2006a). Studies in a wider range of taxonomic groups are needed in order to make general conclusions from this body of empirical work. In this thesis I examined the relative importance of dispersal and local dynamics in structuring spider diversity in an old field system. I had three objectives, the first and second addressing spider re-colonization of ‘empty’ local habitat patches, and the third addressing whether dispersal is the only factor structuring spider diversity in this system. My objectives were 1) to determine the rate and mechanism of spider re-colonization of emptied trees, and whether this differed by hunting guild; 2) to examine the time-scale of the re-colonization process by comparing spider diversity in re-colonized trees to that of undisturbed trees; and 3) to determine whether dispersal was the only factor limiting diversity, or whether local dynamics were also important.

6 1.2 Literature Review 1.2.1 Community Assembly and Metacommunity Theory A central question in community ecology is what determines a community’s limited membership (Putman 1994). Community structure may be determined by species traits that allow species to fill niches within the community (Pulliam 2000), and abiotic environmental factors and biotic interactions with other species can interact to determine an organism’s niche (Soberón 2007). Regional processes may also be important in influencing community structure, in that colonization of communities by dispersers may be the major factor structuring community composition (Cornell and Lawton 1992; Ricklefs 1987). It is likely that in many communities both local and regional processes are important, with k-selected species more strongly influenced by local processes and r- selected species, such as many arthropods, more strongly influenced by regional processes (Putman 1994; Cornell and Lawton 1992), but for any taxon, both processes may be important. The metacommunity concept is a framework for looking at the importance of processes operating at multiple spatial scales in influencing community structure and dynamics by considering individuals, local communities, and the metacommunity, a system of local communities connected by dispersal (Leibold et al. 2004). There are currently four metacommunity paradigms, each of which has been useful in describing dynamics of certain systems, but for which a synthesis is lacking (Leibold and McPeek 2006; Leibold et al. 2004). In each paradigm there is a different relative importance of the roles of regional and local processes. The first paradigm is the “patch dynamics” perspective, in which a competition/colonization tradeoff is assumed to maintain species diversity on a landscape of uniform habitat patches in an uninhabitable matrix (Leibold et al. 2004). Local dynamics cause local extinctions faster than dispersal can provide the full range of species that could theoretically coexist in a patch, so diversity is dispersal limited. In the “species sorting” paradigm, differences in species’ responses to abiotic environmental factors are assumed to “sort” species across environmental gradients (Leibold et al. 2004). Dispersal levels higher than that necessary to fill every niche will

7 not increase diversity because the patches are saturated, and diversity is limited by local interactions rather than by dispersal. In the “mass effect” paradigm (Shmida and Wilson 1985), environmental gradients, although assumed to be present, are not as important in structuring diversity in local communities as in the species sorting perspective, because there is such a high level of inter-patch movement that patches do not reach equilibrium because local dynamics are continually interrupted by immigration events causing source-sink dynamics (Leibold et al. 2004). The final paradigm is the “neutral” perspective. In this scenario all species are assumed to be competitively equivalent and habitat patches uniform, so that local community diversity is determined entirely by stochastic processes (Leibold et al. 2004). Initial theoretical work has shown that adding space to models of community dynamics adds a suite of new patterns and dynamics which had not been previously found in modeling community interactions (Hoopes et al. 2005; Holt and Hoopes 2005), and empirical studies have found that purely spatial factors, and not just environmental factors, often explain significant amounts of variation in community structure (Vyverman et al. 2007; Vanschoenwinkel et al. 2007; Beisner et al. 2006; Lekberg et al. 2007; Cottenie and De Meester 2005). Together these results suggest that considering multiple spatial scales will be important in the development of a unified theory of diversity. However, theoretical and empirical metacommunity research is still young; continued work is needed to achieve a synthesis of the four paradigms that has predictive value in natural systems (Leibold et al. 2004).

1.2.2 Empirical Metacommunity Studies Microcosm experiments testing metacommunity theory have manipulated dispersal, competition (via manipulating resource levels), and/or predation, and looked at species diversity as the responding variable (eg. Cadotte 2006b; Cadotte et al. 2006; Kneitel and Miller 2003; Gilbert et al. 1998). Although microcosm studies do not show how dispersal, competition, and predation structure natural communities (Carpenter 1996), they allow manipulation of these processes, which is often difficult in more complex systems, and is very useful for developing theory about how each process in

8 isolation and in interaction with the others structures diversity. For example, studies using protozoans and rotifers by Cadotte (2006b) and Cadotte et al. (2006) supported the Mouquet and Loreau (2003) hypothesis that local community species richness shows a unimodal response to dispersal rate; however, Kneitel and Miller (2003) found that when they added a predator, local richness did not change in response to dispersal rate. These different studies have found dispersal, predation, and competition to be additive (Cadotte et al. 2006) or interactive (Kneitel and Miller 2003), making it difficult at this point to make generalizations about the relative importance of dispersal and local dynamics in structuring community diversity. Other empirical metacommunity studies have looked at the importance of regional versus local factors in structuring diversity in natural systems. Although this general ecological question is older than metacommunity theory (Mouquet et al. 2004), and not all of these studies have considered themselves “metacommunity studies”, the relative importance of dispersal and local dynamics in structuring communities is of great relevance in distinguishing among the four metacommunity paradigms. Numerous observational studies have set out to determine statistically the amount of variation in local diversity explained by environmental charactersistics (local processes) versus space (regional factors), and have generally found that both explained parts of the variation (Vyverman et al. 2007; Vanschoenwinkel et al. 2007; Beisner et al. 2006; Lekberg et al. 2007; Karlson and Cornell 1998). In an elegant study, Vandvik and Goldberg (2006) managed to assign every individual plant recruiting into emptied focal patches to local, site, or regional sources, and found that the dispersal-maintained component of plant diversity was 18-42% in seedling communities. Experimental studies have manipulated dispersal and looked at the effect on diversity. For example, McCauley (2006) looked at whether odonates in a series of eight artificial ponds were dispersal limited within one year by asking whether species richness decreased with isolation of the artificial ponds. She found that odonates in this system were dispersal limited, and that species richness was negatively related to isolation (McCauley 2006). Studying the same system, McCauley (2007) found that dispersal limitation (regional factors) was the primary filter on who colonized ponds, but that predation (local factors) may act as a second filter. Gehring et al. (2002) eliminated

9 dispersal of arbuscular mycorrhizal fungal spores by excluding the terrestrial vertebrates that usually disperse them, and found lower fungal diversity and abundance in vertebrate exclusion plots. Mouquet et al. (2004) manipulated seed rain into a plant community, adding seeds of species with different competitive abilities, and found that adding seeds increased diversity, but that competition was also important in structuring these communities, since only poor competitors responded positively to addition. In a meta- analysis of 23 dispersal manipulation studies, Cadotte (2006a) found that generally local communities are not saturated; as predicted by Cornell and Lawton (1992), diversity was limited by dispersal, not by species interactions. Some empirical metacommunity studies have addressed the specific objective of testing the four metacommunity paradigms in natural systems. The results of these studies have been different in different systems, and are therefore highly context- dependent. Cottenie and De Meester (2005) used regression analysis of the effects of spatial and environmental variation on cladoceran richness and zooplankton density in a system of interconnected ponds, and found that both explained significant amounts of the variation, which suggests that both mass effects and species sorting were occurring. Davies et al. (2005) concluded that mass effects were important in structuring beetle communities in fragmented eucalyptus forest in Australia. Van Nouhuys and Hanski (2005) described patch dynamics of a community module involving two host plants, a butterfly, two parasitoids, and two hyper-parasitoids. Miller and Kneitel (2005) experimentally introduced protozoan and rotifer species to inquiline pitcher plant communities to test for dispersal limitation. They found that some species were able to establish, and had therefore been dispersal limited, but that others were not, suggesting a combination of patch-dynamics and species sorting processes at work for different species. A study of meta-community dynamics of tree hole mosquitoes over 25 years in Florida found that habitat accounted for a significant proportion of the variation in mosquito density (Ellis et al., 2006)—a pattern consistent with the species sorting paradigm. However, they also found an occupancy/colonization tradeoff consistent with the patch dynamics paradigm, and they found substantial spatial and temporal species turnover, which is consistent with the patch dynamics and neutral paradigms, but not with the species sorting paradigm. Ellis et al. (2006) called for metacommunity theory to

10 make more specific predictions, since their results caution against fitting any one of the existing paradigms to their system. Holyoak et al. (2005) recommended that determining which paradigm best fits a study system should not be the primary objective of empirical metacommunity research, because many factors differ between the paradigms, and they suggest that studying the mechanisms driving dynamics will be more productive. Cadotte (2006a) noted that few experimental studies have tested the relative importance of dispersal limitation and local dynamics in structuring natural systems involving organisms other than plants, protozoans, zooplankton, algae, and microarthropods. Empirical studies are needed that will examine how local community processes are connected to larger scale regional dynamics by dispersal in other complex systems, for example, in macroarthropod communities. The work presented in this thesis is about the importance of dispersal and local dynamics in structuring foliage-dwelling spider (Arachnida: Araneae) diversity in an old field system in southern Québec, Canada.

1.2.3 Foliage-dwelling spiders as a model metacommunity system Foliage-dwelling spiders represent a diverse guild of generalist predators; Sackett (2007) collected 44 spider species in 11 families in the foliage of micro-orchards (each consisting of ten 3-year-old apple trees planted close together) in southern Québec, Canada. They are the dominant arthropod predator group in forest and agricultural systems (Ozanne et al. 2000, Samu et al. 1999), and there is theoretical (Holt and Hoopes 2005) and empirical evidence (Maloney et al. 2003) that as generalist predators they can decrease prey populations, including populations of pest species (Hoefler et al. 2006). Spider population dynamics can even influence plant population dynamics through a trophic cascade (Carter and Rypstra 1995; Halaj et al. 2000a). Spiders as a group are effective dispersers (Bell et al. 2005). They can disperse long distances by ballooning, a form of passive aerial dispersal in which spiders use friction of moving air against silk threads to remain aloft (Humphrey 1987). Spiders have been trapped at an elevation of 15 000 ft (4.6 km) above sea level (Glick 1939), and found at 22 000 ft (6.7 km) on Mount Everest (Davis 1933). At only 2 km above sea level wind speeds are often 10 to 20 m/s, at which a spider could balloon 72 to 144 km in

11 only two hours (Drake and Farrow 1988), and ballooning spiders have been caught at 880 km and 1300 km out at sea (Bell et al. 2005). Despite their potential for long distance dispersal (Bishop and Riechert 1990), most aerial dispersal by spiders is probably over much shorter distances (Meijer 1977; Coyle 1983; Van Wingerden 1980 (as cited in Weyman 1993)). Spiders disperse short distances aerially by spanning, i.e. releasing a silk thread that catches on a nearby structure (distances within a few metres) and then running along the thread (Morse 1993), or by dropping on a dragline, which blows in the wind until the spider on the end, still attached by a silk line to the original substrate, comes in contact with another nearby structure (Coyle 1983; Barth et al. 1991). Cursorial movement is an important mode of dispersal for ground-dwelling spiders (Hibbert and Buddle 2008), but for foliage-dwelling spiders in shrubs, although it contributes to spider diversity, it may not be as important as aerial dispersal (Ehmann 1994). Different spider species have different ballooning propensities (Bell et al. 2005). For example Lepthyphantes tenuis (Blackwall) females balloon almost continuously, perhaps as an ephemeral habitat life history strategy (Topping and Sunderland 1998), whereas there are many spider species which have never been observed ballooning in any life stage (Bell et al. 2005). Aerial collections made with sticky traps or nets towed by planes in both the Northern Hemisphere (Freeman, 1946; Dean & Sterling, 1985; Greenstone et al., 1987; Greenstone, 1990; Bishop, 1990; and Coulson et al., 2003) and the Southern Hemisphere (Greenstone et al., 1987) have found that the majority of the spider fauna in the air is usually from the family Linyphiidae, with large percentages also from the families Araneidae and Tetragnathidae, and in one case Thomisidae and Salticidae (Bishop, 1990), and usually minimal representation from the families Philodromidae, Clubionidae, and Theridiidae. Most spiders collected have generally been immature specimens, with significant numbers of adults collected from only the family Linyphiidae. However, spiders from “non-ballooning” species or life-stages can use short-distance aerial dispersal. Even large female Araneidae must use spanning in order to initiate construction of orb webs (Foelix 1996). Given their excellent long and short-distance dispersal potential, regional factors are likely important in structuring spider community diversity at multiple “regional”

12 scales. However, local abiotic and biotic factors also affect spider diversity (Wise 1993). Temperature and humidity are important in spider site selection (Samu et al. 1999), and vegetation structure has a major influence on spider diversity (Halaj et al. 2000b; De Souza 2004; Heikkinen and MacMahon 2004; Rypstra et al. 1999; Robinson 1981). Spiders are often food limited (Wise 2006), and prey availability can affect web site selection (Harwood et al. 2003) and the number of immigrating spiders that remain in a new habitat (Weyman and Jepson 1994). However, interspecific competition does not appear to be important among web-spinning spiders (Wise 1993), but may affect population growth rate of hunting spiders (Wise and Wagner 1992; Marshall and Rypstra 1999; Buddle 2002). Vertebrate predation and competition can affect spider population dynamics (Askenmo et al. 1977; Holmes et al. 1979; Marquis and Whelan 1994; Schoener and Spiller 1995), and ant intraguild predation and competition affects spider population dynamics in some systems (Moya-Larano and Wise 2007). Intraguild predation and cannibalism by spiders can be significant for hunting spiders; cannibalism and/or predation on other spiders can make up up to 20% of a hunting spider’s diet, although less than 3% for web-builders (Wise 2006). Although Bonte et al. (2006, 2004, 2003) have shown in detail how ground- dwelling grey dune spider diversity and species distribution can be explained by various landscape characteristics, such as patch size, connectivity, and stability, and by dispersal propensity of individual species, no study has experimentally compared the relative importance of dispersal and local dynamics in structuring spider communities. In this study, I used three experiments to determine how dispersal affects spider diversity throughout a season and whether local dynamics as well as dispersal are important in structuring foliage-dwelling spider assemblages. I looked at whether mechanisms presumed to operate at between-generation timescales (Wise 1993) can be seen at work within a generation. Foliage-dwelling spiders in small, isolated trees were used as a model system (Holyoak and Holt 2005) because spider habitat was not continuous in space, and individual trees could be defined as “local communities”.

13 1.3 Objectives and Thesis Structure My first objective was to determine the rate and mechanism of spider re- colonization of trees for which spiders were removed (i.e. ‘empty’ trees), and whether this differed by hunting guild. To accomplish this I repeatedly removed all spiders that immigrated into two groups of trees throughout the season: one group of trees which had Tangle Foot painted around the trunks to prevent cursorial immigration, and one group which did not. This treatment and control allowed me to look at the importance of cursorial as opposed to aerial dispersal, and from the control group I calculated immigration rate throughout the season. This experiment characterized spider dispersal by guild between habitat patches in this system throughout the season. My second objective was to examine the time-scale of the re-colonization process by comparing spider diversity in re-colonized trees to that in undisturbed trees. I carried out this part of the study by collecting spiders from an undisturbed group of trees on four of the dates when I also collected spiders from the aerial immigration and cursorial + aerial immigration groups of trees described above. I looked at spider abundance, taxonomic richness, and species composition in all groups of trees to determine whether four days was long enough for spiders to completely re-colonize trees, or whether spider diversity was dispersal-limited over this time period. My third objective was to determine whether dispersal was the only factor limiting diversity, or whether local dynamics were also important. I addressed this question by means of a diversity addition experiment, in which I removed all of the spiders from trees, added specific compositions of spiders to the trees, and three days later again collected all of the spiders on the trees. Under the null hypothesis that dispersal is the only factor limiting diversity, I expected the final spider abundance and taxonomic composition to be very similar to the abundance and composition of spiders placed on the trees, plus minor additions of spiders that had immigrated over the three days of the experiment. Under the alternative hypothesis that local dynamics also limit diversity, I expected the final spider abundance and taxonomic composition to be different than the treatment abundance and composition, and probably similar to the abundance and composition initially removed from the trees.

14 All three objectives were carried out in the same study site (an old field in southern Québec, Canada), and in one field season (2007). The work is written in manuscript format, and all three experiments are included together as one manuscript, and are in the second chapter of the thesis. The third and final chapter of the thesis is a general conclusion.

15 Chapter 2: Effects of dispersal and local dynamics on spider diversity (Araneae) in an old field system

2.1 Abstract The effects of both dispersal and local dynamics in structuring spider assemblages have not been simultaneously investigated. We considered isolated buckthorn trees in an old field in southwestern Québec, Canada as “local communities”, each with its own foliage-dwelling spider assemblage. To find the rate and mechanism of spider re- colonization of emptied trees, all spiders were collected every four days from May to September, with one October collection, from a group of trees for which cursorial immigration was prevented and a group for which it was not. Spider immigration rate fluctuated throughout the season, peaking twice (mid-June and late July/early August). Significantly fewer spiders in the jumper guild (Salticidae) immigrated when cursorial dispersal was prevented; the abundance of spiders in other guilds was not affected. Species richness was lower and taxonomic composition was different in trees for which cursorial dispersal was prevented. To investigate the timescale of re-colonization, spiders were collected from groups of undisturbed trees four times throughout the season on the same dates as the abovementioned collections, and spider abundance and diversity were compared between immigrant-removal and undisturbed trees. We found that at times of peak spider movement spider diversity was not dispersal-limited over a four day period, but at other times it was. We conducted a diversity addition experiment to test whether, at a time of year and over a time period when spiders had been found to be dispersal- limited, diversity was limited only by dispersal, or also by local dynamics. We removed all the spiders from groups of trees, added specific compositions of spiders, and after three days again collected all of the spiders from each tree. We found no difference in final spider abundance between treatments, and a significant difference between the final spider composition and that expected given the treatments plus background immigration, suggesting that dispersal is not the only factor limiting diversity, and that local dynamics must also limit spider diversity. Overall, this study suggests that processes acting at different spatial scales are important in structuring spider diversity, with a focus on processes acting over small spatial scales and short time periods.

16 2.2 Introduction The importance of regional factors in structuring communities has been acknowledged only relatively recently in the development of community ecology (Cornell and Lawton 1992; Ricklefs 1987), with previous explanations of community structure being based on local processes, such as local environmental characteristics and species interactions, that determined species’ niches (Soberón 2007; Pulliam 2000). Understanding the relative importance of local and regional factors in structuring community diversity within a system is of fundamental importance in determining the scales over which local communities are structured, and the mechanisms that allow species coexistence, and is an important basis on which the four paradigms of the metacommunity concept can be distinguished (Leibold et al. 2004; Holyoak et al. 2005). Metacommunity theory is working toward a synthesis of the way in which processes at multiple spatial scales structure community diversity (Holyoak et al. 2005), and the four paradigms (patch-dynamics, species sorting, mass effects, and neutral) (Leibold et al. 2004) provide a useful framework for interpreting results of empirical studies. Empirical studies investigating the relative importance of regional and local factors in structuring community diversity have been observational (Vyverman et al. 2007; Vanschoenwinkel et al. 2007; Beisner et al. 2006; Lekberg et al. 2007; Vandvik and Goldberg 2006; Cottenie and De Meester 2005; Karlson and Cornell 1998), or have experimentally manipulated dispersal by preventing immigration (Gehring et al. 2002), by manipulating local community isolation (McCauley 2007, 2006; Gilbert et al. 1998) or by adding diversity (Mouquet et al. 2004; Foster et al. 2004; Shurin 2000; Tilman 1997). In a meta-analysis of 23 dispersal manipulation studies, Cadotte (2006a) found that generally, local communities are not saturated; as predicted by Cornell and Lawton (1992), diversity was limited by dispersal, not by species interactions. However, few experimental studies have tested the relative importance of dispersal and local dynamics in structuring natural systems involving organisms other than plants, protozoans, zooplankton, algae, and microarthropods (Cadotte 2006a). Empirical studies are needed that will examine how local community processes are connected to larger scale regional dynamics by dispersal in other complex systems, for example, in macroarthropod communities. This study asked about the importance of dispersal and local dynamics in

17 structuring foliage-dwelling spider diversity in an old field system in southern Québec, Canada. Foliage-dwelling spiders (Arachnida: Araneae) are a diverse guild of generalist predators; Sackett et al. (2008) collected 43 spider species in the foliage of orchards and adjacent deciduous forests in southern Québec. Spiders are effective dispersers (Bell et al. 2005), and can travel long distances by ballooning, a form of passive aerial dispersal. Spiders have been trapped at an elevation of 15 000 ft (4.6 km) above sea level (Glick 1939). At only 2 km above sea level wind speeds are often 10 to 20 m/s, at which a spider could balloon 72 to 144 km in only two hours (Drake and Farrow 1988), and spiders have been found at 880 km and 1300 km out at sea (Bell et al. 2005). Despite evidence that they can re-colonize habitats from long distances (Bishop and Riechert 1990), most aerial dispersal by spiders is probably over much shorter distances (Meijer 1977; Coyle 1983; Van Wingerden 1980 (as cited in Weyman 1993)). Spiders disperse short distances aerially by spanning, i.e., releasing a silk thread that catches on a nearby structure (distances within a few metres) and then running along the thread (Morse 1993), or by dropping on a dragline, which blows in the wind until the spider on the end, still attached by a silk line to the original substrate, comes in contact with another nearby structure (Coyle 1983; Barth et al. 1991). Cursorial movement is an important mode of dispersal for ground-dwelling spiders (Hibbert and Buddle 2008), but for foliage-dwelling spiders in shrubs, although it contributes to spider diversity, it may not be as important as aerial dispersal (Ehmann 1994). Different spider species have different ballooning propensities (Bell et al. 2005). However, spiders from “non-ballooning” species or life-stages can use short-distance aerial dispersal. Even large female orb-web spiders (family Araneidae) must use spanning in order to initiate construction of orb webs (Foelix 1996). Given their excellent long and short-distance dispersal potential, regional factors are likely important in structuring spider community diversity at multiple “regional” scales. However, local abiotic and biotic factors also affect spider diversity (Wise 1993). Temperature and humidity are important in web site selection (Samu et al. 1999), and vegetation structure has a major influence on spider diversity (Halaj et al. 2000b; De Souza 2004; Heikkinen and MacMahon 2004; Rypstra et al. 1999; Robinson 1981).

18 Spiders are often food limited (Wise 2006), and prey availability can affect web site selection (Harwood et al. 2003) and the number of immigrating spiders that remain in a new habitat (Weyman and Jepson 1994). Vertebrate predation and competition can affect spider population dynamics (Askenmo et al. 1977; Holmes et al. 1979; Marquis and Whelan 1994; Schoener and Spiller 1995), and ant intraguild predation and competition affects spider population dynamics in some systems (Moya-Larano and Wise 2007). Intraguild predation and cannibalism by spiders can be significant for hunting spiders; cannibalism and/or predation on other spiders can make up up to 20% of a hunting spider’s diet, although less than 3% for web-builders (Wise 2006). Although Bonte et al. (2006, 2004, 2003) have shown in detail how ground- dwelling grey dune spider diversity and species distribution can be explained by various landscape characteristics, such as patch size, connectivity, and stability, and by dispersal propensity of individual species, no study has experimentally compared the relative importance of dispersal and local dynamics in structuring spider communities. In this study, we used three experiments to determine how dispersal affects spider diversity throughout a season and whether local dynamics as well as dispersal are important in structuring foliage-dwelling spider assemblages. We looked at whether mechanisms presumed to operate at between-generation timescales (Wise 1993) can be seen at work within a generation. Foliage-dwelling spiders in small, isolated trees were a good model system (Holyoak and Holt 2005) because spider habitat was not continuous in space, and individual trees could be defined as “local communities”. Our objectives were to determine the rate and mechanism of spider re- colonization of emptied trees and whether this varied by hunting guild, to examine the time-scale of the re-colonization process by comparing spider diversity in re-colonized trees to that of undisturbed trees, and to perform a diversity addition experiment to determine whether dispersal was the only factor limiting diversity or whether local dynamics were also important.

19 2.3 Methods 2.3.1 Study site The study site was an old field (100 x 1130 m), adjacent to the Bois-de-la-roche Agricultural Park, Montréal, Québec, Canada (N 45°26.337’, W 073°56.494’). The vegetation was comprised of tall grasses and non-woody plants, and saplings (generally < 2 m in height) of the common buckthorn (Rhamnus cathartica). This species is introduced to Canada (Kurylo et al. 2007), but was selected as a study species because it has become abundant (Kurlyo et al. 2007), and supports diverse spider assemblages. Rhamnus cathartica trees of approximately the same height and canopy volume were selected for study. Approximate foliage volume was measured for each tree with a tape measure by measuring the dimensions of the geometric shape most closely approximated by the canopy of each tree. Trees were at least 50 cm from other vegetation and at least 2 m from other trees used in the study. Grass and other vegetation immediately around all trees were kept trimmed so that they did not grow close enough to the trees to provide pathways into their canopies.

2.3.2 Re-colonization Rate, Mechanism, and Timescale To determine the rate and mechanism of immigration into assemblages of foliage- dwelling spiders, and to examine the timescale of the re-colonization process, 210 trees were selected and randomly assigned to one of three groups: aerial immigration only trees, from which spiders were removed every 4 days (35 trees); cursorial + aerial immigration trees, from which spiders were removed every four days (35 trees); and undisturbed trees, from which spiders were collected only once in the season (140 trees divided into 4 groups of 35 trees). The experiment was set up on 21 May 2007. Tangle Foot was applied to each aerial immigration tree, (and repainted throughout the season when necessary) on a strip of masking tape 48 mm wide, wrapped around the lower trunk, with the top of the tape at 30 cm above ground, in order to prevent cursorial immigration. Spiders were removed from each aerial immigration and cursorial + aerial immigration tree by shaking and beating the foliage over a 1 m2 beating sheet and collecting the spiders with an aspirator. Each tree was shaken until no more spiders fell off, then beaten, pausing to collect all the

20 spiders. Each tree was then visually inspected, and any remaining spiders or spider egg sacs were collected. Following set-up, collections were made from all trees in aerial immigration and cursorial + aerial immigration groups every four or five days until 4 September 2007, except for an 18 day period in August (Table 1). Two final collections were made on 22 and 26 October 2007. Data from the 21 May collection and from collections following long periods of no collection (21 August and 22 October) were not used. Collections were made from the undisturbed trees (as well as from the aerial immigration and cursorial + aerial immigration trees) on 10 June, 6 July, 3 August, and 4 September 2007. Spiders were identified to the lowest possible taxonomic level using Paquin & Dupérré (2003), Dondale & Redner (1978, 1982), and Dondale et al. (2003). Voucher specimens of adults have been deposited in the Lyman Entomological Museum (McGill University, Ste-Anne-de-Bellevue, Québec, Canada).

21 Table 1. Collection dates for trees of different treatment groups in the colonization rate, mechanism, and timescale study. Spiders were collected from aerial immigration and cursorial + aerial immigration trees on every collection date, whereas spiders were collected from undisturbed trees on only the dates indicated. Data from dates highlighted in gray were not used, since there had been long periods of no collection preceding these dates.

Collection Date Dates on which spiders were collected from undisturbed trees 21-May-07 25-May-07 28-May-07 02-Jun-07 06-Jun-07 10-Jun-07 Collected (1st group of 35 trees) 14-Jun-07 19-Jun-07 23-Jun-07 27-Jun-07 02-Jul-07 06-Jul-07 Collected (2nd group of 35 trees) 10-Jul-07 14-Jul-07 18-Jul-07 22-Jul-07 26-Jul-07 30-Jul-07 03-Aug-07 Collected (3rd group of 35 trees) 21-Aug-07 26-Aug-07 31-Aug-07 04-Sep-07 Collected (4th group of 35 trees) 22-Oct-07 26-Oct-07

2.3.3 Diversity Addition Experiment Fifty trees not used in the previous experiment were selected, and each assigned to one of five spider diversity groups: Standard (S), Richness added (R), Abundance added (A), Richness and Abundance added (RA), and Control (C). The numbers of individuals and families making up the “Standard” composition were representative of the numbers collected on some individual trees at this time of year in the first experiment, but were lower than the numbers collected on other trees. These numbers were limited by what spiders we were able to catch for use in the experiment. The R, A, and RA treatments were intended to “push the system” by increasing species richness and number of

22 individuals added to levels higher than those found on most trees in the first experiment. Abundance and species richness are two different aspects of diversity, and it was possible that one might be limited by local processes while the other was not, so we manipulated both. On 1 and 2 September the following spiders were collected from trees and tall non- woody plants in an old field adjacent to the study site: 90 Salticidae (mostly Eris militaris (Hentz)), 90 Linyphiidae (Frontinella pyramitela (Walckenaer)), 30 Thomisidae (Misumenops sp. and Misumena vatia (Clerck)), and 30 Dictynidae (a mixture of Dictyna bostoniensis Emerton, Emblyna sublata (Hentz), and E. phylax (Gertsch & Ivie), none of which were adult males. We collected from tall non-woody plants as well as from common buckthorn because we could not collect enough spiders from buckthorn trees alone, but we used only species that do occur on buckthorn, as found in the previous experiment.) Spiders were kept without food in individual vials (2 ml in volume) until placed on trees in specific combinations according to treatment group (Table 2). Each treatment combination of spiders was randomly assigned to a tree and was added after all the spiders already on the tree had been collected in an initial collection; the four or eight vials containing the spiders were attached to individual branches in the canopy. In trees to which zero or four spiders were being added, eight or four empty vials, respectively, were also added to control for any potential treatment effects caused by the tubes (e.g., potential habitat). Spiders were released by opening the vials. The experiment started on 2 and 3 September. After three days, spiders were collected from each tree using the method previously described, and preserved in ethanol for identification to family level (i.e., taxonomic level used to determine treatments).

23 Table 2. Compositions of spiders added to each tree in each treatment group for the diversity addition experiment.

Treatment No. Trees Spiders Initial Initial Added Richness Abundance Standard 10 2 Salticidae 2 4 (S) 2 Linyphiidae Richness 10 1 Salticidae 4 4 Added (R) 1 Linyphiidae 1 Thomisidae 1 Dictynidae Abundance 10 4 Salticidae 2 8 Added (A) 4 Linyphiidae Richness 10 2 Salticidae 4 8 and 2 Linyphiidae Abundance 2 Thomisidae Added (RA) 2 Dictynidae Control (C) 10 None 0 0

2.3.4 Data Analyses Re-colonization Rate and Mechanism Abundance data were standardized (to abundance /m3/day) by dividing the number of spiders that had arrived at the tree after four days by the approximate foliage volume of the tree and by the number of days since the previous collection. Data were non-normal, so Mann-Whitney tests were used to test for treatment effects (i.e. aerial and cursorial + aerial immigration) for each collection throughout the season. Overall average abundance of immigrated spiders for each treatment was calculated by totaling abundances across dates, then averaging across trees. We also tested for treatment differences in abundance at the hunting guild level, using Ehmann’s (1994) guilds (web- spinners, jumpers, pursuers, and ambushers), in order to be able to compare the results from this study with his 1994 study, which asked some of the same questions. Spiders in these four guilds use habitat differently (Foelix 1996), and their abundance might be differently affected by preventing cursorial access to trees. The web-spinner guild included spiders in the families Araneidae, Dictynidae, Linyphiidae, Tetragnathidae, Theridiidae, and Uloboridae; the jumper guild included spiders in the family Salticidae; the pursuer guild included spiders in the families Clubionidae, Philodromidae, and Lycosidae; and the ambusher guild included spiders in the family Thomisidae.

24 Abundances pooled over the entire season were calculated separately for spiders in each guild, and t-tests were used to test if there were treatment effects on abundance of each guild. When necessary, data were log-transformed to achieve normality, or the nonparametric Mann-Whitney test was used. Individual-based rarefaction was used to compare species richness between equal numbers of individuals from the two treatments, using the software program EcoSim 7.0 (Gotelli and Entsminger 2001). A difference was considered statistically significant if the 95% confidence intervals of one treatment did not overlap the mean richness of the other treatment as the number of individuals sampled neared the total number. Immature specimens which could not be identified to species were excluded. Taxonomic composition was compared between treatments using MRPP (multi- response permutation procedures) with the software PC-ORD V 4.17 (McCune and Mefford 1999). MRPP is a nonparametric method of testing for multivariate differences between two or more groups, which does not assume multivariate normality or homogeneity of variances (McCune and Grace 2002). MRPP was done using n/sum(n) weighting of groups and the Sørensen distance measure (McCune and Grace 2002) to test for a difference in composition between treatment groups. The p-value of the test statistic is reported, along with the A-statistic, which is the chance-corrected within-group agreement, and is a measure of effect size (McCune & Grace 2002). Immature specimens identified only to family were not excluded for this analysis, since they made up such a large percentage of the individuals collected (59.7%), and since there was no reason to believe that there was a treatment-related bias in the taxonomic resolution.

Timescale of Re-colonization Mean spider abundance/m3 on trees in each treatment was compared on each sample date using a Kruskal-Wallis test because of non-normality of the data, and Nemenyi post-hoc tests were done when there was a significant effect (Nemenyi 1963 (as cited in Zar 1999)). Richness pooled over all four sample dates was compared between treatments as above. MRPP was used to test for differences in taxonomic composition between treatments on each of the four sample dates separately. When MRPP results

25 were significant among all groups, pairwise comparisons between groups were done (Sackett et al. 2008).

Diversity Addition Experiment To compare final spider abundance between trees in different treatment groups, an ANCOVA was used to test whether there was a difference in final spider abundance between treatments, with initial spider abundance as a cofactor. This was done because the average initial abundance of spiders on trees of each treatment group was different, and we wanted to determine whether there was a treatment effect on abundance over and above the factors causing the initial difference in abundance. Final spider family composition was compared to the composition expected under the null hypothesis that dispersal is the only factor limiting spider diversity. The expected composition included all of the treatment spiders that had been added to each tree (since local dynamics were not assumed to be important in influencing them to emigrate), as well as spiders that had immigrated during the three days of the experiment. Data from the colonization rate and mechanism experiment were used to estimate the number and composition of immigrating spiders; the average spider abundance /m3/day arriving in cursorial + aerial trees on 4 September multiplied by the average foliage volume of all trees used in the diversity addition experiment (0.53m3), multiplied by 3 days, was the number of spiders expected to arrive. The family composition of these immigrants was expected to match the proportions of each family collected in the 4 September cursorial + aerial collection. Under the alternative hypothesis that local dynamics as well as dispersal limit spider diversity, the final composition was expected to be different from that expected under the null hypothesis, and was expected to more closely match the composition of spiders in the initial collection (before treatment application), since under the alternative hypothesis, these particular spiders would have been present initially because they had been selected by the operating local dynamics. A chi-squared test was used to compare the fit of the final composition to that expected under the null hypothesis and to the initial composition.

26 2.4 Results 2.4.1 Re-colonization Rate and Mechanism Overall, 2269 spiders from 11 Families (Appendix 1) were collected over the entire season; 83.7% of these were immature. Six species were determined from immature specimens and 44 species were determined from adult specimens. Abundance of spiders that had immigrated into the cursorial + aerial immigrant removal trees after four days fluctuated throughout the season and ranged from a rate of 0.3 to 3.3 spiders/m3/day (Fig. 1). Spider abundance after four days only differed significantly between treatment groups on three collection days throughout the season (14 2 2 2 June: Χ 0.05,1 = 4.25, p = 0.039; 27 June: Χ 0.05,1 = 4.62, p = 0.032; and 31 Aug: Χ 0.05,1 = 4.88, p = 0.027), with significantly more spiders on cursorial + aerial immigration trees. However, when spider abundance was pooled across collection dates for each tree and then averaged across trees within treatment, there were overall significantly more spiders collected from cursorial + aerial immigration trees than from aerial immigration only 2 trees (Χ 0.05,1 = 13.24, p = 0.0003).

4.5 Aerial 4 Cursorial + Aerial 3.5 3 2.5 2 1.5 1 0.5 0 Mean spider abundance/m^3/day (+/- SE) (+/- abundance/m^3/day spider Mean 6-Jul-07 1-Jun-07 8-Jun-07 5-Oct-07 13-Jul-07 20-Jul-07 27-Jul-07 3-Aug-07 7-Sep-07 15-Jun-07 22-Jun-07 29-Jun-07 12-Oct-07 19-Oct-07 26-Oct-07 10-Aug-07 17-Aug-07 24-Aug-07 31-Aug-07 14-Sep-07 21-Sep-07 28-Sep-07 25-May-07 Collection Date

Figure 1. Mean spider abundance/m3/day (+/- SE) collected on trees from which all spiders were removed every 4-5 days. Trees in the aerial group had a ring of Tangle Foot painted around the trunk to prevent cursorial immigration. Trees in the cursorial + aerial group had no Tangle Foot. There were collections on 21 May, 21 August, and 22 October 2007, for which the data are not shown; each data point shown here represents a collection four days after a previous collection.

When analyzed at the guild level, although in all guilds more spiders were collected from cursorial + aerial immigration trees than from aerial immigration trees, the difference was only significant for the jumpers (t0.05,2,68 = 3.765, p < 0.001; aerial mean abundance +/- SE = 6.96 +/- 1.17 spiders/m3/day; mean cursorial + aerial abundance = 3 11.42 +/- 1.18 spiders/m /day). (Web-spinners: t0.05,2,68 = 1.261, p = 0.212; pursuers: t0.05,2,68 = 0.289, p = 0.773; ambushers: U(2),35,35 = 612, p = 0.994.) Species richness was higher in cursorial + aerial trees than in aerial trees, since the 95% confidence intervals around the mean number of species did not overlap as the number of individuals sampled neared the total number (Fig. 2). For each treatment the number of species collected neared an asymptote as the number of individuals collected increased, suggesting that the sampling effort was sufficient to make proper comparisons of diversity. The total number of species collected on trees in aerial and cursorial + aerial treatment groups respectively were 38 and 48.

20 Aerial 10 Cursorial + Aerial Number of species (+/- 95% CI) 0 0 100 200 300 400 500 600 Number of individuals

Figure 2. Individual-based rarefaction curves showing the estimated number of species (+/- 95% confidence intervals) collected from aerial and cursorial + aerial immigration trees over the entire season.

MRPP found a significant difference in taxonomic composition between treatment groups (p = 0.0224), although effect size was very low (A = 0.00086). A species abundance distribution showed only minor differences in rank order of species between the treatments (Fig. 3). There was a drastic shift in taxonomic composition on 26 October

28 from that on all other sample dates; on this date, almost all spiders collected were large immature Tetragnathidae. Averaged over all sample dates previous to 26 October, the proportion of individuals collected that were Tetragnathidae was 3.5% for aerial and 1.4% for cursorial + aerial groups respectively, whereas on 26 October, these proportions changed to 74.5% and 90.5%. Spider abundance on 26 October was very similar to that on 4 September.

350

Aerial 300 Cursorial + Aerial 250

200

150

100 Number of individuals

.) .) .) .) .) .) .) .) .) .) .) mitra .) hoyi nigra imm militaris abbotii phylax imm imm imm imm imm imm imm imm imm foliacea albidum imm imm . ( . ( . ( . ( . ( emertoni. ( murarium Eris . ( . ( differens tabulata displicata pygmaea emertoni sp. ( gibberosa vibransHentzia arabesca pyramitelasp. ( bimaculata autumnalis sp. ( spp spp spp sp.1 ( spp spp asperatus Evarcha Dipoena spp spp spp bostoniensis rufus Clubiona DictynaEmblyna unimaculatus Theridion Theridula Theridion Mangora Theridion Araniella ClubionaErigone Clubiona Neottiura CeraticelusThanatus Salticidae Larinioides Dictyna Salticidae Neoscona FrontinellaLycosidae Araneidae Linyphiidae Theridiidae Dictynidae Achaearanea Thomisidae Tetragnatha Misumenops Thymoites Philodromus

Species

Figure 3. Species abundance distribution of spiders collected over the entire season from trees to which aerial immigration only or cursorial + aerial immigration were allowed. Singletons and doubletons were not included.

2.4.2 Timescale of Re-colonization The total number of spiders collected from the undisturbed trees as well as the aerial and cursorial + aerial trees on the four dates on which all three groups of trees were sampled was 991, of which 83.2% were immature specimens. Adults were collected from 32 species, along with identifiable immatures from an additional eight species for a total of 40 species (Appendix 2).

29 On two sample dates, there were significant differences in spider abundance 2 2 between treatments (6 July: Χ 0.05,2 = 6.208, p = 0.045; 4 Sept: Χ 0.05,2 = 13.592, p = 0.001). Nemenyi post-hoc tests did not differentiate between treatments for the 6 July data, but for the 4 September data, showed that there were significantly more spiders on undisturbed trees than on aerial or cursorial + aerial trees, and there was no difference in abundance between the aerial and cursorial + aerial groups (Fig. 4b,d). On two sample dates there was no significant difference in spider abundance between treatments (10 2 2 June: Χ 0.05,2 = 2.558, p = 0.278; 3 Aug: Χ 0.05,2 = 2.819, p = 0.244; Fig. 4a,c)

30 24 24 a) 10 June 2007 b) 6 July 2007 22 22 20 20 18 18 16 16 14 14 12 12 10 10 8 8 6 6 4 4

Mean spider abundance/m^3 (+/- SE) (+/- abundance/m^3 spider Mean 2 2 Mean spider abundance/m^3 (+/- SE) (+/- abundance/m^3 spider Mean 0 0 Aerial Cursorial + Aerial Undisturbed Aerial Cursorial + Aerial Undisturbed

24 24 22 c) 3 August 2007 22 d) 4 September 2007 20 20 18 18 16 16 14 14 12 12 B 10 10 8 8 6 A 6 4 A 4

Mean spider abundance/m^3 (+/- SE) (+/- abundance/m^3 spider Mean 2

0 SE) (+/- abundance/m^3 spider Mean 2 0 Aerial Cursorial + Aerial Undisturbed Aerial Cursorial + Aerial Undisturbed

Figure 4. Mean spider abundance/m3 (+/- SE) collected on aerial and cursorial + aerial trees, from which all spiders had been removed 4 days previously, and from undisturbed trees, from which spiders had never before been removed (n = 35 for all groups) on a) 10 June, 2007 (p = 0.278); b) 6 July, 2007 (p = 0.045); c) 3 August, 2007 (p = 0.244); and d) 4 September, 2007 (p = 0.001). Trees in the aerial group had a ring of Tangle Foot painted around the trunk to prevent cursorial immigration. Trees in the cursorial + aerial group had no Tangle Foot.

31 Species richness did not differ between the undisturbed trees and the other groups of trees when richness was compared at a similar number of individuals, but cursorial + aerial trees had significantly more species than aerial trees (Fig. 5); however, the rarefaction curves for none of the groups reached an asymptote, suggesting that continued sampling would have increased species richness. The total numbers of species collected from each group of trees were 32, 21, and 29 from undisturbed, aerial, and cursorial + aerial respectively.

15 Aerial 10 Cursorial + Aerial 5 Undisturbed Number of species (+/- 95% CI) 0 0 50 100 150 200 250 Number of individuals

Figure 5. Individual-based rarefaction curves showing the estimated number of species (+/- 95% confidence intervals) collected from aerial and cursorial + aerial immigration trees and undisturbed trees pooled from collections on 10 June, 6 July, 3 August, and 4 September, 2007.

On 10 June and 6 July there were no significant differences in taxonomic composition between treatment groups (10 June: p = 0.4075; 6 July: p = 0.1800). On 3 August and 4 September there were significant differences (3 Aug: p = 0.0026, A = 0.0174; 4 Sept: p = 0.0293, A = 0.0142). Pairwise comparisons showed that only on 4 September were there significant differences between the composition in undisturbed trees and both aerial and cursorial + aerial trees; on 3 August there was no difference between composition in cursorial + aerial and undisturbed trees (Table 3).

32 Table 3. Results of MRPP tests for difference in taxonomic composition between treatment groups for each of the four collection dates in the timescale of colonization study. When there was a significant (α = 0.05) result for the test for a difference among all treatments, further MRPP tests were run between all pairs of treatments.

Date Comparison T-statistic p-value A-valuea 10-Jun-07 All treatments -0.062 0.4075 0.00057 06-Jul-07 All treatments -0.863 0.1800 0.0064 03-Aug-07 All treatments -3.656 0.0026 0.0174 cursorial+aerial vs. undisturbed -1.536 0.0787 0.0077 aerial vs. undisturbed -3.077 0.0088 0.0141 aerial vs. cursorial+aerial -3.005 0.0123 0.0173 04-Sep-07 All treatments -2.189 0.0293 0.0142 cursorial+aerial vs. undisturbed -2.175 0.0334 0.0134 aerial vs. undisturbed -2.892 0.0100 0.0164 aerial vs. cursorial+aerial 0.304 0.5318 -0.0031 aA-value is the chance-corrected within-group agreement, and is a measure of effect size: A = 0 heterogeneity within groups = expectation by chance A = 1 all taxonomic units are identical within groups A < 0 less agreement within groups than expected by chance

Species abundance differed between undisturbed and spider-removal trees mainly in that Larinioides sp. (Araneidae) immatures, Linyphiidae immatures, and Tetragnatha elongata Walckenaer (Tetragnathidae) were relatively more abundant in spider removal trees than in undisturbed trees, and in that the Dictynidae (immatures most notably, but also adults) were relatively less abundant in spider-removal trees than in undisturbed trees (Fig. 6).

33 140 120 Aerial 100 Cursorial + Aerial 80 Undisturbed 60 40 20 0 .) .) .) .) .) .) .) .) .) .) Number individuals of imm imm imm nigra imm imm imm imm imm imm abbotii vibrans imm . ( . ( . ( . ( . ( albidum phylax. ( . ( sublata florenstabulata murarium emertonimilitaris elongata sp. ( sp. ( pyramitela spp spp spp spp spp sp.1 ( spp spp Eris rufus bostoniensis Dipoena Emblyna Clubiona Theridion Emblyna Theridion Theridula Hypselistes Clubiona Dictyna Tetragnatha Lycosidae Salticidae Araneidae Larinioides Salticidae FrontinellaAchaearanea Dictynidae Theridiidae Linyphiidae Thomisidae Philodromus Species

Figure 6. Species abundance distribution of spiders collected from aerial and cursorial + aerial trees from which all spiders had been removed four days previously, and from undisturbed trees, pooled over 10 June, 6 July, 3 August, and 4 September, 2007. Singletons and doubletons were not included.

2.4.3 Diversity Addition Experiment There was no significant difference in spider abundance between treatment groups in the final collection (F4,48 = 0.80, p = 0.5331; Fig.7), nor was there a treatment*initial abundance interaction (F4,48 = 1.36, p = 0.2665). The final taxonomic composition (Fig. 8d) was significantly different from that expected (Fig. 8c) given the treatment composition (Fig. 8b) under the null hypothesis that dispersal is the only factor limiting 2 diversity (Χ 0.05,20 = 165.243, p < 0.001). We therefore reject the null hypothesis and accept the alternative hypothesis that local dynamics as well as dispersal limit spider diversity. The final taxonomic composition was also significantly different from the 2 initial composition (Χ 0.05,18 = 56.605, p < 0.001; Fig. 8a); however, the lower chi-squared value for the latter test indicates a better fit.

34 14 Initial 12 Treatment 10 Final 8 6 4 2 0

Mean spider abundance (+/- SE) Control Standard Abundance added Richness added Richness & Abundance added Treatment

Figure 7. Mean initial and final abundances of spiders collected from and treatment abundances placed on trees in the diversity addition experiment. Sample size was 10 trees in each treatment group except the richness added group, which had 9 trees. Spiders were collected initially, treatment spiders placed on immediately, and then 3 days later, all spiders were again collected in the final collection.

35 a) Initial collection b) Treatment 1 1 0.9 0.9 0.8 0.8 0.7 0.7 0.6 Thomisidae 0.6 Linyphiinae Thomisidae 0.5 0.5 Dictynidae Linyphiinae 0.4 0.4 Dictynidae 0.3 Salticidae Mean proportion Mean 0.3

Mean proportion Mean Salticidae 0.2 Other 0.2 0.1 0.1 0 0 Control Standard Abundance Richness Richness & Control Standard Abundance Richness Richness & added added Abundance added added Abundance added added

c) Expected under null hypothesis d) Final collection 1 1 0.9 0.9 0.8 0.8 0.7 0.7 Thomisidae 0.6 Thomisidae 0.6 Linyphiinae 0.5 Linyphiinae 0.5 Dictynidae 0.4 Dictynidae 0.4 Proportion 0.3 Salticidae 0.3 Salticidae Mean proportion Mean 0.2 Other 0.2 Other 0.1 0.1 0 0 Control Standard Abundance Richness Richness & Control Standard Abundance Richness Richness & added added Abundance added added Abundance added added

Figure 8. Mean proportion of spiders in each taxonomic group a) collected from trees initially; b) added to the trees in each group; c) expected in the final collection under the null hypothesis that dispersal is the only factor limiting diversity; and d) collected in the final collection (3 days after the initial collection was made and the treatments were applied). Sample size was 10 trees in each treatment group except the richness added group, which had 9 trees.

36 2.5 Discussion This study is one of the first to experimentally examine the effects of both regional and local processes on diversity of a macro-arthropod assemblage in a complex natural system. The results support the idea that processes operating at multiple spatial scales are important in structuring community diversity (Leibold et al. 2004), even over a time period shorter than one generation time.

2.5.1 Re-colonization Rate and Mechanism The first objective was to determine the rate and mechanism of spider re- colonization of small trees, and how this differed by hunting guild. The fluctuating re- colonization rate observed is supported by literature on changes in spider aeronautical behaviour throughout the year (Greenstone 1990, Bishop and Reichert 1990), with peaks coinciding with the hatch of spiderlings (Weyman et al. 1995). The immigration rate (0.3 – 3.3 spiders/m3/day) was higher than that found by Sackett (2007), who studied colonization of “micro-orchards” by foliage-dwelling spiders in southern Québec, and found a range from 0.007 to 0.82 spiders/m3/day re-colonizing one week after removal. This is likely because the trees in this study were closer to other trees and tall vegetation than were Sackett’s micro-orchards. Preventing cursorial dispersal significantly reduced immigrant abundance on only three days throughout the season, but over all sample dates combined there was a significant treatment effect. When analyzed at the guild level, the same trend could be seen for all guilds, but was only significant for the jumpers (family Salticidae). This result is supported by results from aerial collections in which few Salticidae were collected relative to other families (Freeman, 1946; Dean & Sterling, 1985; Greenstone et al., 1987; Greenstone, 1990; Coulson et al., 2003) but not by Bishop (1990), who trapped large numbers of ballooning Salticidae, or by Ehmann (1994), who found in a similar study to this one that immigrant abundance of jumpers along with pursuers was less affected by preventing cursorial dispersal than in other guilds. However, he noted that his result was surprising because jumpers and pursuers would seem to be the best adapted for cursorial dispersal.

37 Rarefied species richness was higher on trees in which cursorial immigration was allowed, which agrees with Ehmann (1994), who found fewer species on aerial immigration shrubs than on cursorial + aerial immigration shrubs, and suggests that spider species differ in their aerial dispersal tendency (Bell et al. 2005). There was a significant difference in taxonomic composition between treatments, but the effect size was small (McCune and Grace 2002). For most species the difference in abundance between treatments was not very large, which suggests that foliage-dwelling spiders as a group move very effectively aerially (Sackett 2007). The shift in taxonomic composition on 26 October was interesting, and may indicate that the Tetragnathidae temporally separate their use of common buckthorn as habitat from use by other spider taxa. On 26 October, few buckthorn trees had any leaves remaining, and much of the tall non-woody vegetation in the field had fallen and lay horizontally over the ground, so it would have been harder for a spider to use spanning to enter the study trees. Since undisturbed trees were not sampled at this time, we do not know if other spiders were present in undisturbed trees but unable to immigrate into the study trees because of greater distances between study trees and nearby foliage due to the season, or if Tetragnathidae are active later in the season than other families of foliage-dwelling spiders.

2.5.2 Timescale of Re-colonization The second objective was to examine the timescale of re-colonization by asking how spider abundance, richness, and species composition of immigrants arriving at trees in aerial and cursorial + aerial treatment group trees compared to that of spiders in undisturbed trees at four times throughout the summer. The significant treatment differences in abundance on 6 July and 4 September, with more spiders on undisturbed trees than on either group of repeatedly disturbed trees suggests that at these times, spider abundance was dispersal-limited over the four-day time period, since after four days spider abundance on trees from which spiders had been removed had not reached the level on undisturbed trees. However, the lack of a significant treatment effect on 10 June and 3 August suggests that at these times, dispersal was high enough that spider abundance was not dispersal-limited over the four day time period, especially on 10 June,

38 for which the data do not even indicate a trend toward higher abundances on undisturbed trees (Fig. 4a). These two dates are at times during the summer when immigration rate was peaking (Fig. 1). The peaks in immigration rate in mid-June and late July/early August occurred as a result of spider phenology, because at these times spiderlings of many species were hatching and dispersing. The observed shifts in re-colonization time throughout one season suggest that the timescale over which a system is studied will affect conclusions about the importance of regional factors in structuring diversity. Had we collected spiders less frequently (e.g., once every seven days), it might have appeared that spiders in this system were never dispersal-limited, and had we collected more frequently than once every four days, it might have appeared that they were always dispersal-limited. The lack of a clear treatment difference in species richness suggests that high movement frequency is general to spider species in this system, as found by Sackett (2007), since undisturbed trees did not harbour significantly more species than spider- removal trees. Although effect sizes were small, the taxonomic composition data agree with the conclusions from the abundance data in that spider species composition was not dispersal- limited on 10 June (there was no significant difference in composition between treatment groups), nor on 3 August (no difference between undisturbed and cursorial + aerial groups), but was dispersal-limited on 4 September (composition on undisturbed trees differed significantly from that on aerial and cursorial + aerial trees). The 6 July composition data do not agree with the 6 July abundance data, in that there was no compositional difference between treatment groups, while there was a difference in abundance. Together the abundance and taxonomic composition data suggest that the timescale of spider re-colonization of emptied trees in this system changes throughout the season, but that at certain times (eg. 10 June and 3 August, 2007) spider diversity was not dispersal-limited, and four days was long enough for spiders to completely re-colonize trees from which they had been removed. At a more taxon-specific level, our results suggest that the Dictynidae may be poor colonizers, although they were abundant on undisturbed trees, and Larinioides sp.

39 (Araneidae), at least when immature, may be a better colonizer than competitor (Fig. 6), but apart from these potential competition-colonization tradeoffs, there is no evidence for this sort of tradeoff between spider species in this assemblage, suggesting that the patch- dynamics paradigm from metacommunity theory (Leibold et al. 2004) does not well describe this system. Rather, the most abundant species in undisturbed trees are also the most abundant species in the immigrant composition. This, along with the abundance, species richness, and taxonomic composition results, suggests that mass effects may structure the spider diversity in this system (Shmida and Wilson 1985; Leibold et al. 2004). This finding differs from Cadotte’s (2006a) generalization that local communities are usually dispersal limited, and is an example of how this mechanism of species coexistence, conceptualized as operating between generations (Shmida and Wilson 1985, Leibold et al. 2004), can operate over very short within-generation timescales.

2.5.3 Diversity Addition Experiment It was surprising that in the diversity addition experiment, spiders in the families Linyphiidae, Thomisidae, and especially Dictynidae generally left the trees on which they were placed, since spiders of these families and species had been collected from buckthorn trees throughout the season in the re-colonization rate, mechanism, and timescale experiments. Also, all of the individuals used had been originally collected from habitat similar to the experimental trees, some even collected from common buckthorn trees. It is unlikely that a significant number of these spiders left as a behavioural reaction to the disturbance of being placed in the tree, since only three individuals (all Thomisidae) of the 240 placed on trees were observed to dash out of their tubes and fall off of the trees. Most spiders did not exit their tubes as soon as they were opened, but at the final collection, none of the tubes on any trees contained spiders. The results from the diversity addition experiment suggest that over a time period (three days), in which spiders in this system were found in the timescale-of-re- colonization experiment to be dispersal-limited at this time of year, local dynamics as well as dispersal are important in structuring foliage-dwelling spider diversity. The abundance and richness of spiders added to trees did not affect the final spider abundance and composition, suggesting that local factors, such as microclimate and species

40 interactions (Wise 1993) influenced spiders in different families to emigrate at different rates within three days of being placed on a tree. McCauley (2007) found a similar result for colonization of artificial ponds by Odonata; pond isolation was not the only factor limiting diversity, but presence of predators within the ponds acted as a “secondary filter” on which species were able to colonize each pond. It is possible that the removal of potential spider prey while removing resident spiders previous to adding the treatment spiders influenced more of the added spiders to leave than would have left under normal conditions, since in the absence of other prey, the smaller Dictynidae and Linyphiidae may have fallen prey to the larger Salticidae, and this could explain their disappearance, rather than normal local dynamics. However, the Thomisidae added were often larger than the Salticidae, so were not likely preyed upon, yet also left more frequently than expected under the null hypothesis. Also, the foliage of the buckthorn trees produced a complex habitat, making it unclear whether or not the Salticidae would have found the other spiders to eat them (Huffaker 1958). It is unlikely that spiders sensed a prey deficiency on experimental trees and decided to leave before more prey arrived (Weyman and Jepson 1994), because few spiders were observed to exit their tubes immediately, so the decision to leave was probably not made rapidly. This is especially true for the Dictynidae and Linyphiidae, which would likely build webs and wait some time before deciding that there was not sufficient prey present (Harwood et al. 2003), by which time prey species may have re-colonized the trees. Nevertheless, it would be interesting to repeat this experiment also manipulating prey abundance to see if there is an interaction between dispersal and prey abundance in structuring spider diversity. This would be a first step in determining what local factors are important in determining spider diversity.

2.5.4 Conclusion This study found that in the old field system and season in which it took place, foliage-dwelling spiders moved very effectively aerially and at changing rates, and that at certain times of the season, they moved so frequently between trees that over a time period of just four days, mass effects appeared to structure spider diversity. At time periods when immigrant spider abundance was not peaking, four days was not long

41 enough for spiders to completely re-colonize emptied trees, but had sampling been done at longer intervals, it is likely that mass-effects would have appeared to be operating at these times too, suggesting that the entire field rather than individual trees act more as the ‘local community’ of metacommunity theory (Holyoak and Holt 2005). However, in a diversity addition experiment, we found that local dynamics also limited spider diversity, over a time period shorter than that in which dispersal limited diversity, and at the spatial scale of the individual tree. On a much larger scale, Bonte et al. (2006, 2004, 2003) have shown that spatial factors at the landscape level influence ground-dwelling spider diversity over between-generation time scales. Thus, all of the work to date on the relative importance of regional and local factors in structuring spider diversity suggests that processes determining diversity act at multiple spatial and temporal scales simultaneously (Levin 1992). This study highlights the importance of processes acting over small spatial scales and short time periods.

42 Chapter 3: Summary and Conclusion

In this study, I used three experiments to determine how foliage-dwelling spiders move between habitat patches, how dispersal affects spider diversity throughout a season, and whether local dynamics as well as dispersal are important in structuring foliage- dwelling spider assemblages. I found that spiders moved aerially between trees effectively, to the extent that in all but the jumper guild there was no significant difference in immigrant spider abundance in trees which had cursorial dispersal prevented and those which did not. There was, however, a difference in species richness and composition between treatments, suggesting that cursorial movement is also an important mechanism of spider dispersal in this system. Immigration rate into the small trees in the field fluctuated, with peaks corresponding to the hatch of spiderlings in many taxa. Concerning the timescale of re-colonization of emptied trees, I found that at certain times of year (10 June and 3 August), spiders were not dispersal-limited over a time period of only four days, whereas at other times (6 July and 4 September), spiders were dispersal limited over this time period; four days was not long enough for spiders to re-colonize to the extent that spider abundance and diversity matched that in undisturbed trees. In a diversity addition experiment I found that dispersal is not the only factor limiting spider diversity, but that local dynamics at the level of individual habitat patches also limit spider diversity. The nature of the local dynamics limiting diversity have yet to be worked out, but are likely a combination of abiotic factors and species interactions (Samu et al. 1999; Wise 1993). Defining the appropriate scale of the theoretical ‘local community’ is a problem in empirical studies looking at the relative importance of dynamics at multiple spatial scales (Holyoak and Holt 2005). Authors of these studies are generally interested in explaining large-scale patterns in diversity, since understanding at this spatial scale is most relevant to global concerns such as conservation and climate change (Leibold et al. 2004). However, Levin (1992) noted that there is no “correct” spatial or temporal scale of description; it is important to recognize that processes determining diversity act at multiple scales simultaneously. This study showed that processes at the scale of the

43 individual tree, and at the level of the entire field both affect foliage-dwelling spider diversity. On an applied level, for low-foliage-dwelling spiders, apart from the large spatial, long temporal patterns which are of interest in any species partly because understanding at this scale has remained the most elusive in ecology (Leibold et al. 2004), the spatial scales of most interest are the within- and between-field levels, the first of which was addressed in this study, and the time scales of most interest are the within- and between- season timescales, since these species are valuable generalist predators in agricultural systems (Maloney et al. 2003), which have disturbance cycles over these spatial and temporal scales (Samu et al. 1999). An understanding of processes structuring spider diversity within and between fields will be useful in planning how to best maintain this diversity in fields and field borders.

44 References

Askenmo, C., A. von-Bromsen, J. Ekman, and C. Jansson. 1977. Impact of some wintering birds on spider abundance in spruce. Oikos 28: 90-94. Barth, F.G., S. Komarek, J.A.C. Humphrey, and B. Treidler. 1991. Drop and swing dispersal behaviour of a tropical wandering spider: experiments and numerical model. Journal of Comparative Physiology A 169: 313-322. Beisner, B.E., P.R. Peres-Neto, E.S. Lindström, A. Barnett, and M.L. Longhi. 2006. The role of environmental and spatial processes in structuring lake communities from bacteria to fish. Ecology 87: 2985-2991. Bell, J.R., D.A. Bohan, E.M. Shaw, and G.S. Weyman. 2005. Ballooning dispersal using silk: world fauna, phylogenies, genetics and models. Bulletin of Entomological Research 95:69-114. Bishop, L. 1990. Meteorological aspects of spider ballooning. Environmental Entomology 19: 1381-1387. Bishop L. and S.E. Riechert. 1990. Spider colonization of agroecosystems: mode and source. Environmental Entomology 19: 1738-1745. Bonte, D. L. Baert, L. Lens, and J.-P. Maelfait. 2004. Effects of aerial dispersal, habitat specialization, and landscape structure on spider distribution across fragmented grey dunes. Ecography 27: 343-349. Bonte, D., P. Criel, I. Van Thournout, and J.-P. Maelfait. 2003. Regional and local variation of spider assemblages (Araneae) from coastal grey dunes along the North Sea. Journal of Biogeography 30: 901-911. Bonte, D., L. Lens, and J.-P. Maelfait. 2006. Sand dynamics in coastal dune landscapes constrain diversity and life-history characteristics of spiders. Journal of Applied Ecology 43: 735- 747. Buddle, C.M. 2002. Interactions among young stages of the wolf spiders Pardosa moesta and P. mackenziana (Araneae: Lycosidae). Oikos 96: 130-136. Cadotte, M.W. 2006a. Dispersal and species diversity: A meta-analysis. The American Naturalist 167: 913-924. Cadotte, M.W. 2006b. Metacommunity influences on community richness at multiple spatial scales: a microcosm experiment. Ecology 87: 1008-1016. Cadotte, M.W., A.M. Fortner, and T. Fukami. 2006. The effects of resource enrichment, dispersal, and predation on local and metacommunity structure. Oecologia 149: 150-157. Carpenter, S.R. 1996. Microcosm experiments have limited relevance for community and ecosystem ecology. Ecology 77: 677-680. Carter, P.E. and A.L. Rypstra. 1995. Top-down effects in soybean agro-ecosystems – spider density affects herbivore damage. Oikos 72: 433-439. Cornell, H.V. and J.H. Lawton. 1992. Species interactions, local and regional processes, and limits to the richness of ecological communities: a theoretical perspective. Journal of Animal Ecology 61: 1-12. Cottenie, K., and L. De Meester. 2005. Local interactions and local dispersal in a zooplankton metacommunity. Pages 189-211 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Coulson, S.J., L.D. Hodkinson, and N.R. Webb. 2003. Aerial dispersal of invertebrates over a high-Arctic glacier foreland: Midtre Lovénbreen, Svalbard. Polar Biology 26: 530-537.

45 Coyle, F.A. 1983. Aerial dispersal by mygalomorph spiderlings (Araneae, Mygalomorphae). Journal of Arachnology 11: 283-286. Davies, K.F., B.A. Melbourne, C.R. Margules, and J.F. Lawrence. 2005. Metacommunity structure influences the stability of local beetle communities. Pages 170-188 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Davis, E.C. 1933. Another try at Everest. The Science News-Letter 23: 230-231. Dean D.A., and W.L. Sterling. 1985. Size and phenology of ballooning spiders at two locations in eastern Texas. Journal of Arachnology 13: 111-120. De Souza, A.L.T., and R.P. Martins. 2004. Distribution of plant-dwelling spiders: inflorescences versus vegetative branches. Austral Ecology 29: 342-349. Dondale, C.D., and J.H. Redner. 1978. The Crab Spiders of Canada and Alaska (Araneae: Philodromidae and Thomisidae). Ottawa, Ontario, Canada: Biosystematics Research Institute. Dondale, C.D., and J.H. Redner. 1982. The Sac Spiders of Canada and Alaska, Araneae: Clubionidae and Anyphaenidae. Ottawa, Ontario, Canada: Biosystematics Research Institute. Dondale, C.D., J.H. Redner, P. Paquin, and H.W. Levi. 2003. The Orb-weaving Spiders of Canada and Alaska (Araneae: Uloboridae, Tetragnathidae, Araneidae, Theridiosomatidae). Ottawa, Ontario, Canada: NRC Research Press. Drake, V.A., and R. A. Farrow. 1988. The influence of atmospheric structure and motions on insect migration. Annual Review of Entomology 33: 183-210. Ehmann, W.J. 1994. Organization of spider assemblages on shrubs: an assessment of the role of dispersal mode in colonization. American Midland Naturalist 131: 301-310. Ellis, A.M., L.P. Lounibos, and M. Holyoak. 2006. Evaluating the long-term metacommunity dynamics of tree hole mosquitos. Ecology 87: 2582-2590. Foelix, R.F. 1996. The Biology of Spiders. Oxford University Press. New York, U.S.A. Foster, B.L., T.L. Dickson, C.A. Murphy, I.S. Karel, and V.H. Smith. 2004. Propagule pools mediate community assembly and diversity-ecosystem regulation along a grassland productivity gradient. Journal of Ecology 92: 435-449. Freeman, J.A. 1946. The distribution of spiders and mites up to 300 ft. in the air. Journal of Animal Ecology 15: 69-74. Gehring, C.A., J.E. Wolf, and T.C. Theimer. 2002. Terrestrial vertebrates promote arbuscular mycorrhizal fungal diversity and inoculum potential in a rain forest soil. Ecology Letters 5: 540-548. Gilbert, F., A. Gonzalez, and I. Evans-Freke. 1998. Corridors maintain species richness in the fragmented landscapes of a microecosystem. Proceedings of the Royal Society of London B 265: 577-582. Glick, P.A. 1939. The distribution of insects, spiders and mites within the air. Technical Bulletin of the US Department of Agriculture 673: 1-151. Gotelli, N.J. and G.L. Entsminger. 2001. EcoSim: Null models software for ecology. Version 7.0. Acquired Intelligence Inc. & Kesey-Bear. http://homepages.together.net/~gentsmin/ecosim.htm. Greenstone, M.H. 1990. Meteorological determinants of spider ballooning: the roles of thermals vs. the vertical windspeed gradient in becoming airborne. Oecologia 84: 164-168.

46 Greenstone, M.H. C.E. Morgan, A.-L. Hultsch, R.A. Farrow, and J.E. Dowse. 1987. Ballooning spiders in Missouri, USA, and New South Wales, Australia: family and mass distributions. Journal of Arachnology 15: 163-170. Halaj, J., A.B. Cady, and G.W. Uetz. 2000a. Modular habitat refugia enhance generalist predators and lower plant damage in soybeans. Environmental Entomology 29: 383-393. Halaj, J., D.W. Ross, and. A.R. Moldenke. 2000b. Importance of habitat structure to the arthropod food-web in Douglas-fir canopies. Oikos 90: 139-152. Harwood, J.D., K.D. Sunderland, and W.O.C. Symondson. 2003. Web-location by linyphiid spiders: prey-specific aggregation and foraging strategies. Journal of Animal Ecology 72: 745-756. Heikkinen, M.W. and J.A. MacMahon. 2004. Assemblages of spiders on models of semi-arid shrubs. Journal of Arachnology 32: 313-323. Hibbert, A.C. and C.M. Buddle. 2008. Assessing the dispersal of spiders within agricultural fields and an adjacent mature forest. Journal of Arachnology 36: 195-198. Hoefler, C.D., A.Chen, and E.M. Jakob. 2006. The potential of a jumping spider, Phidippus claru, as a biocontrol agent. Journal of Economic Entomology 99: 432-436. Holmes, R.T., Schutlz, J.C., and Nothnagle, P. 1979. Bird predation on forest insects: an exclosure experiment. Science 206: 462-463. Holt, R.D. and M.F. Hoopes. 2005. Food web dynamics in a metacommunity context: modules and beyond. Pages 68-93 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Holyoak, M. and R.D. Holt. 2005. Empirical Perspectives. Pages 95-98 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Holyoak, M., M.A. Leibold, N. Mouquet, R.D. Holt, and M.F. Hoopes. 2005. Metacommunities: a framework for large-scale community ecology. Pages 1-31 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Hoopes, M.F., R.D. Holt, and M. Holyoak. 2005. The effects of spatial processes on two species interactions. Pages 35-67 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Huffaker, C.B. 1958. Experimental studies on predation: dispersion factors and predator-prey oscillations. Hilgardia 27: 343-383. Humphrey, J.A.C. 1987. Fluid mechanic constraints on spider ballooning. Oecologia 73: 469- 477. Karlson, R.H. and H.V. Cornell. 1998. Scale-dependent variation in local vs. regional effects on coral species richness. Ecological Monographs 68: 259-274. Kneitel, J.M., and T.E. Miller. 2003. Dispersal rates affect species composition in metacommunities of Sarracenia purpurea inquilines. The American Naturalist 162: 165- 171. Kurylo, J.S., K.S. Knight, J.R. Stewart, and A.G. Endress. 2007. Rhamnus cathartica: Native and naturalized distribution and habitat preference. Journal of the Torrey Botanical Society 134: 420-430. Leibold, M.A., M. Holyoak, N. Mouquet, P. Amarasekare, J.M. Chase, M.F. Hoopes, R.D. Holt, J.B. Shurin, R. Law, D. Tilman, M. Loreau, and A. Gonzalez. 2004. The

47 metacommunity concept: a framework for multi-scale community ecology. Ecology Letters 7: 601-613. Leibold, M.A. and M.A. McPeek. 2006. Coexistence of the niche and neutral perspectives in community ecology. Ecology 87: 1399-1410. Lekberg, Y., R.T. Koide, J.R. Rohr, L. Aldrich-Wolfe, and J.B. Morton. 2007. Role of niche restrictions and dispersal in the composition of arbuscular mycorrhizal fungal communities. Journal of Ecology 95: 95-105. Levin, S.A. 1992. The problem of pattern and scale of ecology. Ecology 73: 1943-1967. Maloney, D., F.A. Drummond, and R. Alford. 2003. Spider predation in agroecosystems: Can spiders effectively control pest populations? Technical Bulletin 190, Maine Agricultural and Forest Experiment Station, University of Maine, USA. Marquis, R.J. and C. J. Whelan. 1994. Insectivorous birds increase growth of White Oak through consumption of leaf-chewing insects. Ecology 75: 2007-2014. Marshall, S.D. and A.L. Rypstra. 1999. Spider competition in structurally simple ecosystems. Journal of Arachnology 27: 343-350. McCauley, S.J. 2006. The effects of dispersal and recruitment limitation on community structure of odonates in artificial ponds. Ecography 29: 585-595. McCauley, S.J. 2007. The role of local and regional processes in structuring larval dragonfly distributions across habitat gradients. Oikos 116: 121-133. McCune, B. and Grace, J. 2002. Analysis of Ecological Communities. MJM Software Design. Oregon, USA. McCune, B. and M.J. Mefford. 1999. Multivariate Analysis of Ecological Data Version 4.17. MjM Software, Gleneden Beach, Oregon, U.S.A. Meijer, J. 1977. The immigration of spiders (Araneida) into a new polder. Ecological Entomology 2: 81-90. Miller, T.E. and J.M. Kneitel. 2005. Inquiline communities in pitcher plants as a prototypical metacommunity. Pages 122-145 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Mouquet, N. and M. Loreau. 2003. Community patterns in source-sink metacommunities. The American Naturalist 162: 544-557. Mouquet, N., P. Leadley, J. Mériguet, and M. Loreau. 2004. Immigration and local competition in herbaceous plant communities: a three-year seed-sowing experiment. Oikos 104: 77- 90. Morse, D.H. 1993. Some determinants of dispersal by crab spiderlings. Ecology 74: 427-432. Moya-Laraño, J., and D.H. Wise. 2007. Direct and indirect effects of ants on a forest-floor food web. Ecology 88: 1454-1465. Nemenyi, P. 1963. Distribution-Free Multiple Comparisons. State University of New York, Downstate Medical Center. Ozanne, C.M.P., M.R. Speight, C. Hambler, and H.F. Evans. 2000. Isolated trees and forest patches: patterns in canopy arthropod abundance and diversity in Pinus sylvestris (Scots Pine). Forest Ecology and Management 137: 53-63. Paquin, P. and N. Dupérré. Guide d’identification des Araignées (Araneae) du Québec. Fabreries Supplément 11. Pulliam, H.R. 2000. On the relationship between niche and distribution. Ecology Letters 3: 349- 361. Putman, R.J. 1994. Community Ecology. Chapman and Hall, London, UK.

48 Ricklefs, R.E. 1987. Relative roles of local and regional processes. Science 235: 167-171. Robinson, J.V. 1981. The effect of architectural variation in habitat on a spider community: an experimental field study. Ecology 62: 73-80. Rypstra, A.L., P.E. Carter, R.A. Balfour, and S.D. Marshall. 1999. Architectural features of agricultural habitats and their impact on the spider inhabitants. Journal of Arachnology 27: 371-377. Sackett, T.E. 2007. Natural enemy ecology in apple orchards: spider colonization of orchards and effects of kaolin on the apple pest Choristoneura rosaceana and its natural enemies. Ph.D. Thesis. McGill University, Montréal, Québec, Canada. Sackett, T.E., C.M. Buddle, and C. Vincent. 2008. Comparisons of the composition of foliage- dwelling spider assemblages in apple orchards and adjacent deciduous forest. The Canadian Entomologist 140: 338-347. Samu, F., K.D. Sunderland, and C. Szinetár. 1999. Scale-dependent dispersal and distribution patterns of spiders in agricultural systems: a review. Journal of Arachnology 27: 325- 332. Schoener, T.W. and D.A. Spiller. 1995. Effect of predators and area on invasion: An experiment with island spiders. Science 267: 1811-1813. Shmida, A. and M.V. Wilson. 1985. Biological determinants of species diversity. Journal of Biogeography 12: 1-20. Shurin, J.B. 2000. Dispersal limitation, invasion resistance, and the structure of pond zooplankton communities. Ecology 81: 3074-3086. Soberón, J. 2007. Grinnellian and Eltonian niches and geographic distributions of species. Ecology Letters 10: 1115 – 1123. Tilman, D. 1997. Community invasibility, recruitment limitation, and grassland biodiversity. Ecology 78: 81-92. Topping, C.J. and K.D. Sunderland. 1998. Population dynamics and dispersal of Lepthyphantes tenuis in an ephemeral habitat. Entomologia Experimentalis et Applicata 87: 29-41. Vandvik, V. and D.E. Goldberg. 2006. Sources of diversity in a grassland metacommunity: quantifying the contribution of dispersal to species richness. The American Naturalist 168: 157-167. Van Nouhuys, S. and I. Hanski. 2005. Metacommunities of butterflies, their host plants, and their parasitoids. Pages 99-121 in M. Holyoak, M.A. Leibold, and R.D. Holt, eds. Metacommunities: Spatial dynamics and ecological communities. University of Chicago Press. Chicago. Vanschoenwinkel, B., C. De Vries, M. Seaman, and L. Brendonck. 2007. The role of metacommunity processes in shaping invertebrate rock pool communities along a dispersal gradient. Oikos 116: 1255-1266. Vyverman, W., E. Verleyen, K. Sabre, K. Vanhoutte, M. Sterken, D.A. Hodgson, D.G. Mann, S. Juggins, B. Van de Vijver, V. Jones, R. Flower, D. Roberts, V.A. Chepurnov, C. Kilroy, P. Vanormelingen, and A. de Wever. 2007. Historical processes constrain patterns in global diatom diversity. Ecology 88: 1924-1931. Weyman, G.S. 1993. A review of the possible causative factors and significance of ballooning in spiders. Ethology Ecology & Evolution 5: 279-291. Weyman, G.S. and P.C. Jepson. 1994. The effect of food supply on the colonization of barley by aerially dispersing spiders (Araneae). Oecologia 100: 386-390.

49 Weyman, G.S., P.C. Jepson, and K.D. Sunderland. 1995. Do seasonal changes in numbers of aerially dispersing spiders reflect population density on the ground or variation in ballooning motivation? Oecologia 101: 487-493. Wise, D. 1993. Spiders in Ecological Webs. Cambridge University Press. Cambridge, New York, USA. Wise, D.H. 2006. Cannibalism, food limitation, intraspecific competition, and the regulation of spider populations. Annual Review of Entomology 51: 441-465. Wise, D.H. and J.D. Wagner. 1992. Evidence of exploitative competition among young stages of the wolf spider Schizocosa ocreata. Oecologia 91: 7-13. Zar, J.H. 1999. Biostatistical Analysis, Fourth Edition. Prentice Hall. Upper Saddle River, New Jersey, U.S.A.

50 Appendix 1: Data from colonization rate and mechanism study

Table A-1. Number of individuals collected in each species, family, and guild from 25 May – 26 October, 2007 from 35 aerial immigration trees and 35 cursorial + aerial immigration trees. (Rows highlighted in gray indicate groups of immature spiders for which taxonomy is uncertain, some of which may be species represented in other rows in the table.)

Cursorial Guild Guild Family Species Aerial + Aerial Total Total Web- spinners Araneidae Araneidae spp. (immature) 93 91 184 Acanthepeira stellata (Walckenaer) 1 0 1 Araneus diadematus Clerck 0 0 0 Araneus thaddeus (Hentz) 0 1 1 Araneus trifolium (Hentz) 0 2 2 Araniella displicata (Hentz) 10 3 13 Eustala anastera (Walckenaer) 1 1 2 Larinioides sp. 61 61 122 Mangora gibberosa (Hentz) 24 29 53 Mangora placida (Hentz) 0 2 2 Neoscona arabesca (Walckenaer) 9 10 19 Dictynidae Dictynidae spp. (immature) 22 18 40 Dictyna bostoniensis Emerton 31 46 77 Dictyna foliacea (Hentz) 3 10 13 Emblyna phylax (Gertsch & Ivie) 5 9 14 Emblyna sublata (Hentz) 1 1 2 Linyphiidae Linyphiidae spp. (immature) 36 32 68 Agyneta fabra (Keyserling) 0 2 2 Ceraticelus sp. (immature) 2 1 3 Ceraticelus emertoni (O. Pickard-Cambridge) 5 2 7 Ceraticelus similis (Banks) 2 1 3 Erigone autumnalis Emerton 1 3 4 Frontinella pyramitela (Walckenaer) 2 6 8 Halorates plumosus (Emerton) 0 1 1 Hypselistes florens (O. Pickard- Cambridge) 2 2 4 Microlinyphia m. mandibulata (Emerton) 0 1 1 nr. Pityohyphantes sp. 0 0 0 Tetragnathidae Tetragnatha spp. (immature) 77 51 128 Tetragnatha elongata Walckenaer 0 1 1 Tetragnatha versicolor Walckenaer 0 2 2 Theridiidae Theridiidae spp. (immature) 17 23 40 Achaearanea tabulata Levi 5 4 9

51 Dipoena nigra (Emerton) 7 3 10 Enoplognatha ovata (Clerck) 0 2 2 Neottiura bimaculata (Linnaeus) 0 4 4 Theridion albidum Banks 10 7 17 Theridion differens Emerton 2 5 7 Theridion frondeum Hentz 0 1 1 Theridion murarium Emerton 56 49 105 Theridula emertoni Levi 83 52 135 Thymoites unimaculatus (Emerton) 1 5 6 Uloborus glomosus Uloboridae (Walckenaer) 0 1 1 1114 Jumpers Salticidae Salticidae spp. (immature) 217 313 530 Admestina tibialis (C.L. Koch) 1 0 1 Eris militaris (Hentz) 30 39 69 Evarcha hoyi (Peckham & Peckham) 1 4 5 Hentzia mitra (Hentz) 9 21 30 Pelegrina insignis (Banks) 2 2 4 Salticidae sp.1 10 30 40 Synageles venator (Lucas) 0 2 2 Tutelina similis (Banks) 1 2 3 684 Pursuers Clubionidae Clubiona spp. (immature) 186 152 338 Clubiona abboti L. Koch 3 12 15 Clubiona pygmaea Banks 2 3 5 Clubiona trivialis C.L. Koch 0 0 0 Lycosidae Lycosidae sp. (immature) 5 5 10 Philodromus rufus vibrans Philodromidae Dondale 32 22 54 Thanatus sp. 3 2 5 Tibellus oblongus (Walckenaer) 1 0 1 428 Ambushers Thomisidae Thomisidae spp. (immature) 10 2 12 Misumena vatia (Clerck) 0 2 2 Misumenops asperatus (Hentz) 4 12 16 Ozyptila distans Dondale & Redner 1 0 1 31 Unknown Unknown (immature) 5 7 12 Total 1092 1177 2269

52 Appendix 2: Data from timescale of colonization study

Table A-2. Number of individuals collected in each species, family, and guild on June 10, July 6, August 3, and September 4, 2007 from 35 Aerial immigration trees, 35 Cursorial + Aerial immigration trees, and 4 different groups of 35 Undisturbed trees. (Rows highlighted in gray indicate groups of immature spiders for which taxonomy is uncertain, some of which may be species represented in other rows in the table.)

Cursorial Guild Guild Family Species Aerial + Aerial Undisturbed Total Total Web- spinners Araneidae Araneidae spp. (immature) 15 16 38 69 Acanthepeira stellata (Walckenaer) 1 0 0 1 Araneus diadematus Clerck 0 0 1 1 Araneus thaddeus (Hentz) 0 0 1 1 Araneus trifolium (Hentz) 0 1 0 1 Araniella displicata (Hentz) 3 0 1 4 Larinioides sp. 19 22 20 61 Mangora gibberosa (Hentz) 1 1 1 3 Neoscona arabesca (Walckenaer) 2 1 2 5 Dictynidae Dictynidae spp. (immature) 7 2 34 43 Dictyna bostoniensis Emerton 8 7 19 34 Dictyna foliacea (Hentz) 0 2 1 3 Emblyna phylax (Gertsch & Ivie) 3 2 8 13 Emblyna sublata (Hentz) 0 0 4 4 Linyphiidae Linyphiidae spp. (immature) 10 7 7 24 Ceraticelus emertoni (O. Pickard- Cambridge) 2 1 2 5 Frontinella pyramitela (Walckenaer) 0 1 3 4 Hypselistes florens 0 1 3 4 nr. Pityohyphantes sp. 0 0 1 1 Tetragnathidae Tetragnatha elongata Walckenaer 10 4 6 20 Theridiidae Theridiidae spp. (immature) 5 5 16 26 Achaearanea tabulata Levi 2 0 3 5 Dipoena nigra (Emerton) 3 2 3 8 Neottiura bimaculata (Linnaeus) 0 1 1 2 Theridion albidum Banks 4 7 12 23 Theridion differens Emerton 0 0 2 2 Theridion frondeum Hentz 0 1 0 1 Theridion murarium Emerton 23 11 45 79 Theridula emertoni Levi 14 9 27 50 Thymoites unimaculatus (Emerton) 1 0 0 1 Uloboridae Uloborus glomosus (Walckenaer) 0 1 0 1 499 Jumpers Salticidae Salticidae spp. (immature) 39 74 127 240 Eris militaris (Hentz) 4 10 23 37 Evarcha hoyi (Peckham & Peckham) 0 1 0 1 Hentzia mitra (Hentz) 2 1 1 4 Pelegrina sp. 0 1 0 1 Salticidae sp.1 1 6 8 15

53 Tutelina similis (Banks) 0 1 0 1 299 Pursuers Clubionidae Clubiona spp. (immature) 47 29 70 146 Clubiona abboti L. Koch 1 2 6 9 Clubiona pygmaea Banks 0 1 0 1 Clubiona trivialis C.L. Koch 0 0 1 1 Lycosidae Lycosidae sp. (immature) 1 1 3 5 Philodromidae Philodromus rufus vibrans Dondale 6 1 5 12 Tibellus oblongus (Walckenaer) 0 0 1 1 175 Ambushers Thomisidae Thomisidae spp. (immature) 5 1 6 12 Misumena vatia (Clerck) 0 0 2 2 Misumenops sp. 0 1 1 2 16 Unknown Unknown 1 0 1 2 Total 239 235 515 991