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The influence of abiotic and biotic factors on the geographic distribution of spider sociality : insights… Hoffman, Catherine 2014

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 The influence of abiotic and biotic factors on the geographic distribution of spider sociality: insights from a factor exclusion and transplant experiment    by  CATHERINE HOFFMAN  A thesis submitted in partial fulfillment of the requirements for the degree of  Master of Science  in  The Faculty of Graduate and Postdoctoral Studies (Zoology)    The University of British Columbia (Vancouver)  December 2014  © Catherine Hoffman 2014     	   ii  Abstract  Species ranges, which are manifestations of species ecological niches in space, are generally determined by gradients of abiotic and biotic factors. In group-living organisms, not only the properties of individuals, but also those of their groups, should interact with environmental challenges and opportunities to determine a species range. Social and subsocial spiders are well known for having distinct geographical distributions. Intriguingly, subsocial species in the genus Anelosimus are absent from the lowland tropical rainforest where social congeners thrive. Previous studies have attributed this absence to increasing rain intensity and predation, particularly by ants, closer to the rainforest. After confirming that these factors do indeed increase in intensity approaching the lowland tropical rainforest, I test these hypotheses by transplanting nests of the subsocial Anelosimus elegans from its native lower montane cloud forest (1000m) to the lowland tropical rainforest (400m). At both locations I performed a fully factorial ant and rain exclusion experiment and monitored colony survival over time. I found that survival was lower in the lowlands, but improved by the exclusion of rain and ants. At the native higher elevation habitat, in contrast, colony survival did not differ between treatments and controls, confirming that neither intense rains nor predation are factors that negatively impact colony survival in the native habitat. At both locations, large colonies were able to build more webbing, suggesting that larger groups with limited dispersal may benefit from reduced per capita web maintenance in addition to increased predator protection. These findings would explain why subsocial Anelosimus, with small single-family groups and dense webs, have been unable to colonize the lowland tropical rainforest where their social congeners thrive.     	   iii  Preface  Catherine Hoffman and Leticia Avilés designed this research project. Catherine Hoffman collected the field data with the assistance of Philippe Fernandez-Fournier, Marc-Antoine Leclerc, Mark Robertson, and Esteban Calvache. Analyses were done by Catherine Hoffman with supervision from Leticia Avilés.    	   iv  Table of Contents  Abstract…………………………………………………………………………………………. ii Preface…………………………………………………………………………………………. iii Table of Contents.……………………………………………………………………………… iv List of Tables……………………………………………………………………………………. v List of Figures…………………………………………………………………………….……. vi Acknowledgements……………………………………………………………………………. vii Dedication…………………………………………………………………………………….. viii Chapter One: Introduction………………………………………………………………………. 1 Chapter Two: Increased rain intensity and predation risk exclude subsocial Anelosimus spiders from the lowland tropical rainforest. 2.1 Synopsis…………………………………………………………………………….. 9 2.2 Methods……………………………………………………………………………. 10 2.3 Results………………………………………………………………………………14 2.4 Discussion………………………………………………………………………….. 19 Chapter Three: Conclusion………………………………………………………….…………. 22 Bibliography…………………………………………………………………………………… 28                      	   v  List of Tables  Table 1: Number of sets and total number of spider baits at each elevation…………….…..… 15   Table 2: Parametric survival analysis with a Weibull distribution of colony survival performed       separately in the Lowlands and Source locations……………………………………………… 17   Table 3: The effects of location, log group size, and rain exclusion and their interactions on the quantity of new webbing built by spider colonies. ……………………………………………. 18	   vi  List of Figures  Figure 1: Average rainfall rate (mm per minute) decreases as elevation increases on the  eastern slope of the Andes in Ecuador………………………………………………………….15  Figure 2: Results from Kaplan-Meier survival analysis of the probability that baits remain undiscovered by ants overtime. ……………………………………………………….. 16  Figure 3a&b: Survival probability of colonies in each treatment group between the two  transplant locations…….………………………………………………………………………. 17  Figure 4a&b: The average rating of new web quantity for each nest against its log group…… 18           	   vii  Acknowledgements  This work was funded by James S. McDonnell Foundation (USA) and NSERC Discovery grants to L. Avilés (Canada), with additional funding from a UBC Zoology Graduate Fellowship and a BRITE Fellowship to C.Hoffman.  For assistance in the field I would like to thank Marc-Antoine Leclerc, Philippe Fernandez-Fournier, Mark Robertson, and Esteban Calvache. I would like to acknowledge Jatun Sacha Biological Station, Yanayacu Biological Station, and the Sumaco National Reserve for logistical support during my field work.   For insightful comments and discussions on this project I would like to thank Leticia Avilés, Ruth Sharpe, Gyan Harwood, Philippe Fernandez-Fournier, Jennifer Guevara, Angélica Gonzalez, Megan Bontrager, Jill Jankowksi, & Amy Angert.   For their support, advice, encouragement, and patience during this endeavor, I am indebted to Christopher Price, my friends, and my family.    	   viii  Dedication   I dedicate this thesis to my parents, Linda and Dean Hoffman, for their encouragement and guidance in my life.            	   1  Chapter One: Introduction  As most species live in a small area compared to the vast expanses of sea and land, the questions of where species live and why have long interested naturalists and scientists (Darwin 1859; see also MacArthur 1972, Sexton et al. 2009). A species’ range is often considered a manifestation of the ecological niche in space and an expression of a species’ adaptations to the environment (Holt 2009; Hargreaves et al. 2014). The range of a species contains areas across the landscape where births are higher than deaths, including non-contiguous areas connected by dispersal (Sexton et al. 2009; Kubisch et al. 2014). Understanding the causes and consequences of a species range is intricately linked to questions of fitness and adaptation to the environment (Bridle & Vines 2007; Sexton et al. 2009). Despite the fact that all species have some range limit, we have neither a complete knowledge for any particular species nor a clear general picture of the factors constraining range limits (Gaston 2009). Questions surrounding species distributions, range limits, and adaptation capability are becoming increasingly important as changing climate causes range shifts and contractions ultimately putting species at risk of extinction (Thomas et al. 2004).  There are multiple ways that a range limit may be formed and maintained. In the simplest case, a range edge may exist where there is a physical dispersal barrier like a mountain range, desert, or ocean (Gaston 2003). However, more complex models show that environmental heterogeneity and/or evolutionary dynamics can also give rise to stable range limits without a geographic barrier (reviewed in Sexton et al. 2009). When there is no environmental heterogeneity across the landscape, an edge may exist where Allee effects on small populations do not allow growth necessary for range expansion (Lewis & Kareiva 1993; Keitt et al. 2001). Additionally, multispecies interactions can create edges through priority effects, where a novel 	   2  species cannot invade the habitat occupied by a particular species, and through reproductive interference between species (i.e. hybrid sterility, Case et al. 2005). For models that incorporate environmental heterogeneity, a range edge may occur where the fundamental niche or realized niche ends due to changes in the underlying environmental gradients (Hutchinson 1957, Sexton et al. 2009). Furthermore, a stable range limit may form when significant gene flow from the center of the range to the periphery hampers the ability for edge populations to adapt to edge conditions and extend beyond the range (Kirkpatrick & Barton 1997).  Despite the variety of causes of range limits, a common starting approach is to determine the underlying environmental heterogeneity associated with a species distribution. These environmental parameters are often delineated into gradients of abiotic and biotic factors that vary across the landscape (Brown et al. 1996; Gaston 2009; Sexton et al. 2009; Kubisch et al. 2014). Abiotic limits include climatic variables such as temperature, precipitation, and variability in these factors (Gaston 2003), with numerous examples of their effects from the plant and animal world (Cahill et al. 2014). Biotic limits can be shaped by interspecific interactions incorporating the presence of consumers, competitors, predators and parasites (Gaston 2003), although other definitions include intraspecific factors (i.e. Allee effects & local adaptation) as biotic limits (Kubisch et al. 2014). A recent review identified competition as the most frequently studied biotic limit and found that predation and parasitism were understudied (Sexton et al. 2009). Biotic limits are often considered stronger constraints at the lower limits of the range such as in the tropics or at low elevation (Schemske 2009; Hargreaves et al. 2014; but see Cahill et al. 2014). Sexton et al. (2009) go on to point out that there are many examples of negative correlations between species abundances and competitors or predators/prey, but these relationships may be a result of an additional underlying abiotic gradient. Transplant studies that 	   3  experimentally alter the biotic limit (i.e. removal or addition of competitor, herbivore, predator) can show clear evidence for the role of biotic interactions in structuring range limits (Bruelheide & Scheidel 1999; Williams et al. 2010; Stanton-Geddes et al. 2012; this study). Abiotic and biotic limits can obviously both influence a range, but few studies have experimentally shown the simultaneous influence of both factors on limiting a range (Sexton et al. 2009; Hargreaves et al. 2014). Determining the interaction between and relative importance of abiotic and biotic factors will be key to fully understanding range limits (Jankowski et al. 2013).   Although many taxa of plants and animals have been studied in the context of range limits, social or group-living animals have yet to receive considerable attention. Group-living organisms offer a unique opportunity to examine the effects of abiotic and biotic gradients since not only the properties of individuals, but those of their groups will interact with the environmental challenges and opportunities to determine a species’ range. In turn, the range limits of group-living organisms should provide insights on the conditions that favor group versus solitary living (Emlen 1982, 1991; Avilés 1999).  The ecological constraints hypothesis is a mechanism that has been suggested to explain why social organisms are often associated with harsh or unpredictable environments. The hypothesis was originally proposed for cooperatively breeding birds that remain at the nest to help with brood care, but can be extended to other taxa (Emlen 1982;1991). Four risk factors were proposed to affect the decision: cost of dispersal, probability of establishing a new nest, probability of finding a mate, and likelihood of successful reproduction (Emlen 1982). These risks are often affected by ecological conditions such as intense predation pressure, breeding constraints, strong competition, and unpredictable resources and can help explain why some individuals forgo dispersal, remain at the natal site, and may eventually evolve permanent group-	   4  living (Emlen 1982; Brockman 1997; Avilés 1999; Hatchwell & Komdeur 2000). Ecological conditions that favor group-living can vary across landscapes and ultimately favor colonization by group-living organisms of habitats where solitary individuals fail to replace themselves (Avilés 1999). The eusocial mole rats (Family: Bathyergidae), for instance, occur in extremely arid areas where foraging costs are prohibitive for solitary individuals, whereas solitary congeners occur in areas with greater precipitation and resource abundance (Jarvis et al. 1998). Likewise, in a recent global analysis, Jetz & Rubenstein (2011) found that cooperatively breeding birds are associated with higher climatic variability, specifically variability in precipitation, as in Sub-Saharan Africa, parts of the Amazon basin, and Australia.  Ecological challenges and opportunities that promote sociality may also be associated with abiotic and biotic gradients across latitude and elevation. Social arthropods, for instance, tend to be associated with lower latitudes and elevations, although this pattern is not universal (Purcell 2012). A recent review also defines four general abiotic and biotic factors with gradients along latitude and elevation that may affect social arthropod distribution: (1) climate, (2) predation,  (3) prey size & availability, and (4) parasites/pathogens (Purcell 2012). Each of these factors may favor social species in unique ways. Higher temperatures and less seasonality at low latitudes and elevations may allow for multiple generations to overlap, thus facilitating some forms of sociality (Hunt & Amdam 2005). Additionally, increased precipitation intensity and variability at low latitudes and elevations may be associated with sociality in group-living spiders (Purcell & Avilés 2008; Majer et al. 2013). Predation is often considered more intense in the tropics compared to temperate zones (Schemske et al. 2009). As defense against predation is a major benefit of group-living, groups may thus be favored at low latitudes. An abundance of large insects at lower latitudes and elevations may be required in order for large social spider 	   5  groups to form (Powers & Aviles 2007; Guevara & Avilés 2007), as the cooperative capture of larger insects would compensate for the decline in the number of prey per capita captured stemming from the scaling differences of web surface area and colony size (Yip et al. 2008).  Additionally, groups of predatory organisms are known to capture larger prey together than an individual could on its own (social spiders: Nentwig 1985; Rypstra & Tirey 1991, lions: Caraco & Wolf 1975). Lastly, disease transmission tends to be higher for social organisms in habitats with increased parasites and pathogens, leading to selection against group living (Cremer et al. 2007). However, some social organisms have evolved behaviors to mitigate the negative effects allowing them to occupy habitats where parasites and pathogens are common (Purcell 2012). Overall, a general pattern is hard to conceive since the abundance of parasites and pathogens depends on their host. Purcell (2012) suggests that more research needs to be done to determine the consequence of parasites and pathogens on group-living species ranges.  A few studies have addressed the range limits of group-living arthropods using some experimental manipulation, such as common garden rearing (Plateaux-Quénu et al. 2000), transplants beyond the range (Fernandez-Campon 2008; Purcell & Avilés 2008), or predator exclusion (Zammit et al. 2008). However, the number of studies remains small and no studies have simultaneously examined the role of both abiotic and biotic factors. Thus, I sought to experimentally understand how gradients of abiotic and biotic challenges across elevation interact with group-living in spiders to affect survival and determine a species range.  To examine the interaction between abiotic and biotic gradients and sociality, I used group-living spiders of the genus Anelosimus (Family: Theridiidae). Sociality in spiders is rare, occurring in less than 1% of described species (Avilés 1997). Species in the genus Anelosimus are ideal for examining the causes and consequences of group-living since congeners include 	   6  solitary, subsocial, and social life styles where sociality is presumed to derive from subsocial species with repressed dispersal (Avilés 1997; Whitehouse & Lubin 2005; Lubin & Bilde 2007). Solitary species are similar to most spiders where offspring disperse shortly after emerging from the eggsac. However, in subsocial species the offspring tolerate each other and cooperate in web maintenance and prey capture, but ultimately disperse from the natal nest before adulthood (Yip & Rayor 2013). Finally, in social species the spiders remain in the natal nest and cooperate in web maintenance, prey capture, and eventually show cooperative care of offspring (Uetz 1992; Avilés 1997; Samuk et al. 2012). Since individuals of the social species remain tolerant to each other and do not disperse, the social colonies consist of highly inbred individuals and colonies can range in size from several hundred to tens of thousands of individuals (Avilés 1997). Both subsocial and social spiders build dense, basket-shaped, three-dimensional webs that house adults and offspring. These webs are presumed to be energetically expensive because they require large amounts of silk output and serve as permanent living structures for the spiders.  Species with different social systems in the genus Anelosimus also show distinct geographic distributions across latitude and elevation, making them ideal to study the ranges associated with different levels of sociality. Social species tend to be found at lower latitudes and, within the tropics, lower elevations, whereas subsocial species are found at higher latitudes and elevations (Avilés et al., 2007; Guevara & Avilés, in prep.). The elevational pattern becomes apparent at low latitudes where social and subsocial species are both found. Here, social species are found at lower elevations whereas subsocial species are found at higher elevations, with a small area of sympatry in the middle of the range (Avilés et al. 2007). It is interesting to note that the elevational distribution pattern is repeated across multiple social and subsocial species, suggesting that the social system is fundamental to determining a species’ distribution.  	   7  Avilés et al. (2007) suggest that these contrasting distributions are the result of two separate processes. First, the absence of social species from higher elevations and latitudes may be attributed to a paucity of large insect prey in those areas (Powers & Avilés 2007; Avilés et al. 2007; Guevara & Avilés 2007). Given scaling properties of the tri-dimensional prey capture webs of these social spiders, the number of prey per capita declines with colony size (Yip et al. 2008). In areas where there is an abundance of large insects, however, the spiders are able to make up for this decline by capturing increasingly large insects as colony size increases, an effect that cannot be attained in areas where large prey are rare or absent (Powers & Avilés 2007; Guevara & Avilés 2007; Yip et al. 2008).  The second pattern, which is the focus of this study, is that subsocial Anelosimus are absent from the lowland tropical rainforest (Avilés et al. 2007; Purcell & Avilés 2008). A previous study has attributed this pattern to greater rates of predation and frequent heavy rainfall in this habitat, favoring group living over solitary living in this genus (Purcell & Avilés 2008). Specifically, Purcell & Avilés (2008) found that there were a greater proportion of ants in transplanted subsocial spider nests at lower elevations than at higher elevations. This aligns with studies that show higher ant abundance at lower elevations, particularly in the lowland tropical rainforest (Janzen 1973; Samson et al. 1997; Bruhl et al. 1999). The increased abundance of ants should correlate with increased predation pressure for social spiders, as ants are a major predator of social arthropods (Holldobler & Wilson 1990), and a particularly strong threat in the tropics (Jeanne 1979; Novotony et al. 2006; Schemske et al. 2009). The intense predation threat may favor large, social groups and exclude smaller, subsocial groups for several reasons.  Maternal death during offspring development, for instance, may leave the offspring unattended and thus favor nest sharing by multiple females (the maternal death hypothesis, Jones et al. 2007; Avilés et 	   8  al. 2007).  Larger social groups are also housed in larger nests, which allow more spiders to be away from the periphery where predation risk is greater (Rayor & Uetz 1993). Individuals in larger colonies would also benefit from decreased individual risk of predation (i.e. predator dilution; Uetz & Hieber 1994) and from early warning and predator detection (Uetz et al. 2002). Purcell and Avilés (2008) also found that rain was heavier in the lowland tropical rainforest than at higher elevations, which may be a problem for species that build dense, and presumably costly, three-dimensional webs (Avilés et al. 2007). Individuals in larger social groups would benefit from sharing the costs of web building, as appears to be the case for the social spider Agelena consociata (Riechert et al. 1986). Heavy and frequent rains should also reduce the time when prey can be captured. During rain events, for instance, orb weavers have been shown to build webs with less catching area or to cease web building activities altogether (Craig 1989).    In this study, I build upon the work of Purcell & Avilés (2008), who provided a preliminary examination of these hypotheses. First, I quantified the predation threat and rain intensity gradients across elevation. Then, to determine the effects of rain and ants on colony survival, I performed a fully-factorial rain and ant exclusion experiment on nests of a subsocial Anelosimus species transplanted from its native habitat to its non-native lowland tropical rainforest.     	   9  Chapter Two: Increased rain intensity and predation risk exclude subsocial Anelosimus spiders from the lowland tropical rainforest 2.1: Synopsis To test the hypotheses of Avilés et al. (2007) and Purcell & Avilés (2008) that intense rains and ant predation in the lowland tropical rainforest exclude subsocial Anelosimus, I first quantified the gradient of rain intensity and ant predation threat along the eastern slope of the Andes. After verifying this pattern, I transplanted colonies from within their native range in the lower montane cloud forest (Source: 1000m) to a site in the lowland tropical rainforest (Lowlands: 400m). At both sites, I protected the nests either from rain, ground predators (ants), both, or neither. To achieve these treatments, I covered colonies from rain with a tarp and used a sticky barrier to prevent ants from entering the nest. As a control, I transplanted nests back at the Source and performed the same treatments there. I measured how long it took for colonies to become extinct at both locations. I predicted that (1) survival would be lower in the Lowlands and higher at the Source site. (2) In the lowlands, I predicted that nests with no treatment would have the lowest survival whereas nests with both ants and rain excluded would have the highest survival with single factor treatments intermediate. (3) Lastly, I predicted that there would be less effect or no effect of the rain and ant exclusions at the Source location.  I found that colony survival was lowest in the Lowlands but that excluding rain and ants increased survival at this location, supporting my predictions, even though survival did not increase to the same level as in the native habitat. In the native habitat there was an expected effect of age structure (group structure, related to colony size and dispersal tendency), such that smaller colonies composed of older age classes and closer to dispersal had shorter survival times. 	   10  In the Lowlands, however, the age structure effect was not as strong as the effect of rain and ant exclusion, indicating that the abiotic and biotic effects were the main limiting factors on survival in this habitat. My results show how adverse environmental conditions and intrinsic group demographics together exclude subsocial spiders from the lowland tropical rainforest.  2.2: Materials and Methods Rain intensity and predation pressure across elevation  Rain intensity and predation pressure were measured across elevation from approximately 400m to 2100m at four study sites. The lowest site was within the Jatun Sacha Biological Station, near Tena, Ecuador in the lowland tropical rainforest (S 1.07° W 77.61°, elevation: 400-430 meters, Napo Province). The next site was located along the road to Loreto, near Guagua Sumaco, Ecuador in the lower montane tropical forest (S 0.71° W 77.59°, elevation: 1000-1100 meters, Napo Province). There were two sites at different elevations in the cloud forest. The first was near Cocodrilo, Ecuador along the Baeza-Tena road (S 0.65°, W 77.88°, elevation: 1780–1950 m, Napo Province). The highest elevation site was located near Cosanga, Ecuador within the Yanayacu Biological Station (S 0.60 °, W 77.87 °, elevation: 2100m, Napo Province). At each location, I measured the amount of rainfall in an intense bout of rain and converted the amount to a rate (mm/min) depending on the duration of the rainfall (typically 30 minutes, unless rain was very light at higher elevations). I used 1-3 gauges per rain event and averaged the rainfall amount from all gauges. Data were collected between June-August 2013 and June-July 2014. I performed a regression of the average rain rate by elevation, weighted by sample size at each location.  To understand predation threat from ants, I used two types of high-protein baits: drained tuna and live spiders lightly glued on to the vegetation. Tuna is commonly used to attract 	   11  predatory or scavenging ants (Bito et al. 2011) and live arthropods or larvae are used in ant abundance surveys and to determine activity or predation threat (Jeanne 1979; Samson et al. 1997; Novotny et al. 1999; Armbrecht & Prefecto 2003; Sipos et al. 2013). Drained tuna was packed into 1.5 mL centrifuge tubes and between 8-15 tubes were tied to vegetation at a height of 1 to 2 meters. I checked the baits at least every 10 minutes for 2 or 3 hours and measured the time until each bait was found, indicated by an ant on the rim or within the tube. Wasps were found on some baits and these were included since wasps are a predators of spiders (Uetz & Hieber 1994; Rayor 1996; Uma & Weiss 2012; Blamires et al. 2013).  I used the local social and subsocial Anelosimus spiders as baits at each elevation (A. eximius at 400 and 1000m; A. elegans & A. guacamayos at 1780-1950m; A baeza at 2100m). Individual spiders were collected from nests and glued (Krazy Glue®, Westerville, OH, USA) onto small pieces vegetation and then affixed to plants at a height of 1 to 2 meters. Sets containing between 9 and 20 spider baits were set up either in the morning (8:30-10:15) or in the early evening (16:30-18:20). I monitored spiders at least every 2 hours during the day, but did not monitor the baits overnight. At the lowest elevation (400m), I monitored baits between 9-17 hours since baits were found quickly, but increased monitoring time to a full day (22-27 hours) at higher elevations where baits were found more slowly. During checks, I observed if there were ants on the bait or if parts of the body, particularly the abdomen and legs, were missing and noted these baits as found. Again, baits found by wasps were included. Data were collected between June-August 2013 (tuna baits only) and June-July 2014. For tuna baits, I performed Kaplan-Meier survival analyses of the time until baits were found with elevation as the explanatory variable and baits that were never found considered right 	   12  censored. I compared the proportion of spider baits found or removed by predators at each elevation weighted by the duration of the observation period to account for differences in time. Environmental factors and subsocial spider colony survival at local and transplant sites.  For the transplant study, the two study locations were the 400m and 1000m sites listed above and referred to as Lowlands and Source, respectively.  Species description Anelosimus elegans is a subsocial species ranging from Southern Mexico to Peru with most records coming from elevations of 1000-3100 meters (Agnarsson 2006). In Ecuador, the species ranges from approximately 900 – 2600 meters (Avilés et al. 2007). A. elegans has an unbiased sex ratio and colonies typically contain one female and her offspring, with individuals dispersing as subadults to form new colonies (Iturralde 2004). Transplant and treatment protocols  I used a fully-factorial rain and ant exclusion design where No Rain treatments had a tarp (100 cm x 64 cm) placed above the nest and No Ant treatments had a disk (diameter: 12 cm) covered in Tanglefoot (Tanglefoot Company, Grand Rapids, MI, USA) to block out insects, reapplied as necessary to maintain its stickiness. There were 63 nests transplanted to the Lowlands with 16 nests per treatment, except the No Rain & Ants treatment that had 15. As a control, there were 60 nests transplanted within Source (15 nests per treatment). Due to a limited population of nests, I reused 8 nests originally transplanted within the Source site that had high survival.  The previous procedures apply to these colonies with 4 transplanted to the Source and 4 to the Lowlands.  Nests of A. elegans were collected from sites between 900-1100 meters along the “Via Loreto” road (see coordinates above). Intact nests and substrate were clipped from the original 	   13  plant and tied to a potted plant with careful consideration to avoid spider escape. I counted spiders in each nest by instar on the same day they were collected and arranged the nests into blocks of four by age structure (composition of adults, subadults, and juveniles). Age structure is highly correlated to group size (R2=0.79, log-transformed group size to achieve normality), as younger nests contain more spiders. I thus had a wide variety of group sizes as well. Each block was assigned to a location and treatments were randomly assigned (see above). Nests were transplanted the following afternoon and set-up far enough apart (approximately 4 meters) to avoid overlap with existing colonies or other transplants.  Monitoring protocol We censused nests on the first night after transplant and then each morning (7:00-11:00) and night (17:00-21:00) for 3 days to prevent any premature dispersal events and transplant failure. Nests were then monitored only at night every 2 to 3 days for the remainder of the 28 days. Three observers did censuses such that more than one person observed each block during monitoring. During censuses, we counted the spiders by instar and noted any new or eclosed egg sacs. We checked for newly founded nests (propagules) around the original nest, which are the result of dispersal and colonization events. Censuses stopped after three consecutive checks with no spiders found. We quantified the amount of new webbing on the plant and/or original nest using the following scale (1-no new webbing, 2-less than five lines, 3-more than five lines but not a full basket, 4- significant new webbing and new basket and/or moved totally to new plant). Statistical analyses   I calculated colony survival time as the difference between the time a nest was transplanted and the first of three consecutive checks with zero spiders. Age structure of each nest was based on which instar was most abundant during the initial census. To calculate colony 	   14  size, I used the maximum number of individuals found at any time during the census period, as censuses are more likely underestimates than overestimates (Tufiño 1997). Offspring eclosing from eggsacs during the transplant period were not included in the counts. I performed a parametric survival analysis with a Weibull distribution with the full model containing the effects of Location, rain exclusion, ant exclusion, and their subsequent two and three-way interactions on colony survival. Colony age structure was also included as a covariate since it correlates with colony size and with the likelihood that individuals will disperse from the group. Since, individually, effects of rain and ant exclusions and their interaction were non-significant given my sample size, I pooled the colonies with at least rain or ants excluded, hereafter referred to as Treated, to compare with the untreated control colonies. Finally, as there was an interaction between Location and Treated (df=1, X2=3.87, p<0.05), I ran analyses for the effects of Treated and Age Structure separately at each Location. Colonies that survived to the end of the observation period were right-censored.   To examine the effects of location, rain exclusion, and group size on web building, I averaged the rankings for quantity of new webbing for each colony and performed a linear mixed effects model with location, cover, log group size, and their two-way interactions as fixed effects and colony ID nested within block, as a random effect. The response was weighted by survival time. All analyses were done in JMP version 10.0.2 (SAS Institute).  2.3: Results Rain intensity and predation pressure across elevation Rain intensity significantly increased with decreasing elevation (R2=0.97, F1,2=71.6, p=0.014; Figure 1). Tuna baits were found faster at lower elevation than at higher elevation (Log-Rank: X2= 114.6, df=3, p<0.0001). Specifically, the baits at 400m were found the fastest, with 	   15  50% of baits found within 60 minutes whereas 50% of baits at 1000m were found within three hours, and baits at the two highest elevations were found the slowest, with almost 90% of baits left undiscovered (Figure 2). The proportion of spider baits found also decreased as elevation increased (Table 1, df=3, X2=642.7, p<0.0001).  Table 1: Number of sets and total number of spider baits at each elevation. Proportion found indicates the number of baits with ants on them, evidence of ant damage, or totally removed.    Elevation  Number of sets Total Number of Baits Proportion Found  400m   6   60   0.82  1000m   2   40   0.72  1800m   3   40   0.48  2100m   2   34    0.26   Figure 1: Average rainfall rate (mm per minute, during intense bouts of rain) decreases as elevation increases on the eastern slope of the Andes in Ecuador. Number of rain events measured at each location is as follows: 400m (n=17), 1000m (n=21), 1800m (n=3), 2100m (n=4). Error bars represent mean +/- standard deviation.  	   16   	  Figure 2: Results from Kaplan-Meier survival analysis of the probability that baits remain undiscovered by ants overtime. Crosses indicate censoring. Number of baits used at each location is as follows: 400m (n=153), 1000m (n=110), 1800m (n=87), 2200m (n=50).  Environmental factors and subsocial spider colony survival at local and transplant sites.  I found that Location had a significant effect on colony survival such that colonies persisted longer at the Source elevation than in the Lowlands (df=1, X2=55.43, p<0.0001). In the Lowlands, colonies with at least rain or ants excluded had higher survival than controls (Figure 3a, Table 2), an effect that was absent at the Source (Figure 3b, Table 2).   Since subsocial spider individuals disperse as they reach the older age classes, it is not surprising that age structure had a strong effect on colony persistence at the Source elevation (Table 2).  Interestingly, age structure was only a minor factor in the extinction of colonies at the Lowlands site (Table 2), suggesting that factors other than age structure played a greater role at this site. Furthermore, no newly founded nests were seen in the lowland tropical rainforest whereas we identified an average of 2.35 new nests surrounding colonies under dispersal at the Source.  	   17  	    Figure 3a&b: Survival probability of colonies in each treatment group between the two transplant locations. Colonies with at least No Rain and/or No Ants had higher survival than control colonies (Rain & Ants) in the Lowlands, but there was no difference at the Source.  Table 2: Parametric survival analysis with a Weibull distribution of colony survival performed separately in the Lowlands and Source locations. Results show the log likelihood ratio tests for each factor. The factor “Treated” contains all treatments with rain and/or ants excluded versus the controls.  Location  Effect   DF Chi-square  p-value  Lowland (400m)       Treated  1 10.13  0.0015 Age Structure  4 12.47  0.0142 Source (1000m)       Treated  1 0.0101  0.7502    Age Structure  4 24.65  <0.0001   The effects of location, rain exclusion, log group size, and the interaction between location and rain exclusion significantly affected the quantity of new webbing produced by colonies (Table 3). At both locations, larger colonies were able to produce more new webbing. Colonies at the Source produced the same amount of new webbing regardless of whether they were covered from rain (Table 3, Figure 4b). In the Lowlands, in contrast, colonies that were 	   18  protected from the rain produced more new webbing than those exposed to the rain. Furthermore, there was a non-significant trend for larger colonies exposed to rain to build almost as much new webbing as those protected from rain in the Lowlands (Table 3, Figure 4a).  	    Figure 4a&b: The average rating of new web quantity for each nest against its log Group Size. Regression lines show all No Rain (solid line) and Rain (dashed line) treatments combined.    Table 3: The effects of location, log group size, and rain exclusion and their interactions on the quantity of new webbing built by spider colonies.   Effect     DF, DFDen  F-value  p-value  Location    1, 70.7   8.8   0.004 Log group size   1, 106.8  27.6   <0.0001  Rain exclusion   1, 74.1   8.5   0.005  Location: rain exclusion  1, 78.7   8.3   0.005 Location: log group size  1, 106.8  3.6   0.061 Rain exclusion: log group size 1, 87.8   0.57   0.451  	   19  2.4: Discussion   My results support the hypothesis from Avilés et al. (2007) and Purcell & Avilés (2008) that gradients of rain intensity and predation risk along the eastern slope of the Andes contribute to the exclusion of subsocial Anelosimus spiders from the lowland tropical rainforest. I also found that age structure correlated with colony survival at both locations, but this effect was much less marked at the lowland rainforest, showing that factors other than the dissolution of colonies due to dispersal were responsible for the demise of colonies at this elevation. Finally, larger colonies were better able to replace damaged web than smaller ones, showing how colony size may interact with adverse environmental factors to allow survival in certain environments.   I found that that rainfall is more intense and predation threat from ants is greater as one approaches the lowland tropical rainforest. Specifically, I found that average rainfall rate was five times higher in the lowland tropical rainforest than at the highest elevation. Likewise, I saw that baits were found the fastest in the lowlands with 50% of baits discovered by ants within one hour whereas many baits at the highest sites were never discovered. The speed and frequency at which baits were found is an indicator of ant predation across elevation with the lowland rainforest subjected to the highest threat of ant predation. This biotic gradient of predation may play a role in the distribution of other arthropod communities across elevation (Hodkinson 2005). I found that eliminating rain and/or ants increased survival of transplanted subsocial colonies in the lowland tropical rainforest, but not at the Source habitat. By eliminating ants, individual spiders may have avoided at least one major source of predation and survived longer allowing the colony to fare better. I also found that colonies covered from rain in the Lowlands were able to build more new webbing than those exposed to the rain, indicating that rainfall negatively affects web-building. Together, these results show that ant predation threat and the 	   20  effect of rain on web-building are important environmental stressors and that subsocial Anelosimus spiders are unable to cope with these factors in the lowland rainforest. The fact that the exclusion of ants and/or rain had no effect on colony survival nor did rain exclusion improve the amount of new webbing at the native habitat, indicates, as one would expect, that factors that exclude a species from a habitat outside its range are unimportant within the native range.  Although the treatments did improve survival in the Lowlands, survival did not improve to the same level as at the Source, indicating that factors other than those manipulated may also affect survival. For example, I could not exclude flying predators, like wasps, that could predate on or parasitize the spiders. It was also not possible to control for other environmental differences between the habitats, such as temperature and humidity, which likely play a role in the spiders’ physiological adaptation to their environment. However, the fact that populations of the closely related Anelosimus eximius occur in both locations and can be successfully transplanted between them (personal observation), indicates that the temperature and humidity differences between the habitats are not too large.  I also found that group structure and size modulated the role of the environment on colony survival, but differently at each location. Thus, age structure of colonies was the main explanatory variable at the Source, but had a much smaller effect in the Lowlands. This difference indicates that factors above and beyond dispersal lead to colony collapse in the Lowlands, re-emphasizing the strong role of environmental factors. Likewise, group size can modify the effect of the environment as larger colonies built a greater amount of new webbing, which in the Lowlands, resulted in uncovered larger colonies building almost as much new webbing as covered ones. Thus, large groups are buffered from environmental stressors because there are more individuals to maintain and repair the web and to warn of potential predators as 	   21  well as overall lower individual predation risk (Riechert et al. 1986; Uetz & Hieber 1994; Uetz et al. 2002). This may explain why only social Anelosimus species occupy the lowland rainforest (Avilés et al. 2007). Furthermore, at higher elevations where rain is less intense and there are fewer predators, social A. eximius have smaller colonies and a larger proportion of nests that contain single adult females (Purcell & Avilés 2007). Thus, there is ample evidence to suggest that large groups must be maintained to cope with the adverse conditions of the lowland tropical rainforest, but that selection for large groups is relaxed away from the rainforest.  In addition to an inherently small group size, subsocial spiders may be excluded from the lowland tropical rainforest due to their dispersal patterns. As a subsocial colony matures, it becomes composed of more subadults that tend to disperse from the nest (Powers & Avilés 2003). However, as I observed, and consistent with Purcell & Avilés (2008), these individuals fail to establish new nests, likely due to the inability of single individuals to maintain their web and survive predation in this habitat. Similarly, Avilés & Tufino (1998) found that single adult females of a local social spider had low survival, again emphasizing the environmental stress of this habitat on individual spiders. In this study, I obtained experimental support for the hypotheses developed in Avilés et al. (2007) and Purcell & Avilés (2008) that intense rains and predation threat from ants in the lowland tropical rainforest exclude subsocial Anelosimus spiders from this habitat. Additionally, I showed that group size and structure are important in determining the effects of environmental factors on survival.   	   22  Chapter Three: Conclusion  This research project provides connections among gradients of abiotic and biotic limits across elevation, ecological correlates of sociality, and species ranges. This study has several key findings including (1) a quantification of predation pressure by ants and of rain intensity across elevation, (2) an experimental approach to examining the effects of these abiotic and biotic limits on the range of subsocial Anelosimus, with (3) an explanation of how traits of social Anelosimus allow colonization of the lowland tropical rainforest unavailable to less social species and (4) integration of the ecological correlates of sociality with tenets of range limit theory.  The first key of this study was to determine the environmental gradients across the range of Anelosimus by quantifying rain intensity and predation threat. This understanding allows me to examine correlations between gradients and range limits, but also provided the basis for my experimental manipulations. I showed that ant abundance and predation threat is lower at higher elevations similar to patterns seen at other tropical locations (Samson et al. 1997; Bruhl et al. 1999; O’Donnell et al. 2011). However, to my knowledge, this is the first study to examine an elevational gradient of ant abundance in South America and may help explain community composition of arthropods along elevation that are subjected to ant predation (Hodkinson 2005). Although I focused on ants, wasps are another predator of spiders and it has been reported that predatory paper wasps are more abundant at higher elevations (Kumar et al. 2009). However, baits in this study were discoverable and sometimes taken by wasps, yet I still found that predation was overall lower at higher elevations confirming the important role of predation in the lowland rainforest. I also found that rainfall rate increased at lower elevations and was highest in the lowland tropical rainforest. In this habitat, warm, humid air is trapped by vegetation and builds up 	   23  throughout the day with a sudden release of heavy precipitation. Although elevation, per say, is not what is causing rainfall differences, it is correlated to changes in temperature and vegetation (Neill 1999a,b), which prohibit heavy rainfall. The gradient of rain intensity across elevation has not been examined in-depth, but some studies have hinted at an altitudinal gradient of annual rainfall. These studies actually show higher annual rainfall at higher elevations (Silver et al. 1999; Leuschner et al. 2007), which supports the hypothesis that rainfall intensity rather than total rainfall amount is the crucial difference in the lowland tropical rainforest.  My transplant study demonstrates how abiotic and biotic factors affect survival of a group-living species and the integration of these results with the effects of group size and demography (i.e age structure) provides a complete picture of why subsocial spiders are absent from the lowland tropical rainforest. Inherently small subsocial groups would be subjected to large web maintenance costs and be at a high risk of predation if they lived in the lowland tropical rainforest. Then, as the colony matured, individuals would disperse causing the original group to shrink in size. The shrinking size would subject any remaining individuals to even higher costs and predation risk. Additionally, individual dispersers would be unable to successfully colonize in this habitat, as I saw in my study. Thus, both the original group and the individual dispersers would be unable to cope with the adverse conditions of the lowland rainforest for species in this genus and a population would fail to persist.  By understanding the role of the environment and group size on the absence of subsocial spiders from the lowland tropical rainforest, I am also able to elucidate why and how their social congeners thrive in this habitat. Sociality in spiders is posited to derive from subsocial living where dispersal is suppressed (Whitehouse and Lubin 2005). With limited dispersal, social spiders maintain large colony sizes and reap the benefits of living in a group throughout the 	   24  colony lifespan. When dispersal does occur in social Anelosimus, either gravid adult females leave the nest to become established, singly or in small groups, or portions of the original colony disperse together, a process of “budding”, to maximize colonization success (Vollrath 1982). Synchrony of key events, like dispersing as a group, can increase success by eliminating Allee effects on groups who otherwise may not successfully colonize new patches (Friedenberg et al. 2007; Fernandez et al. 2012). Reduced dispersal and the use of collective dispersal are adaptations for social spiders to maintain a large group size and, therefore, thrive in the lowland tropical rainforest. By looking at subsocial spiders, this study was able to identify dispersal patterns and colony size as traits that would be under selection from environmental stress of this habitat, which are traits that closely aligned to the adaptations of social spiders.   In addition to elucidating the causes of the Anelosimus species’ ranges, I uncovered patterns in the distribution of abiotic and biotic factors, particularly precipitation and predation, which may be important for the distribution of other organisms, as well as ecological correlates of sociality in other arthropods. Annual rainfall is shown to affect social arthropod regional and local distribution (Murphy & Breed 2007; Picker et al. 2008) as well as global biogeography (Majer et al. 2013).  Support for the role of predation as a biotic range limit for all organisms is sparse and understudied (Schemske et al. 2009 (latitudinal insect predation); Sexton et al. 2009). To my knowledge, this study is the first to experimentally connect predator abundance to the distribution of a social arthropod, despite evidence of the beneficial role of groups for predator protection (spiders: Henschel 1998; Unglaub et al. 2013; allodapine bees: Zammit et al. 2008). As more is understood about the role of predators in limiting species distributions, future studies should utilize predator exclusion to reveal the connection between predation threat and geographic ranges.  	   25   My results support the role of the environment and demography on range formation and I show an intricate connection between these fundamentals aspects of range limit theory and the ecology of sociality. At range edges species can be less abundant (Brown et al. 1996 but see Sagarin & Gaines 2002; Abeli et al. 2014) and less fit (Angert & Schmeske 2005). Small populations can limit ranges because Allee effects will enhance the effects of environmental gradients and disrupt vital cooperative behaviors (Keitt et al. 2001). Small groups (i.e. group Allee effect) may also affect distributions where group size and some performance parameter are correlated (Angulo et al. 2013). Group-living species, therefore, may be subjected to both group and population Allee effects at edges, explaining why and how edges form for social organisms.   Costly dispersal and colonization affect range limits and, in turn, sociality. Colonization is crucial to maintaining or expanding populations, but only occurs through dispersal. Thus, costly dispersal can limit species ranges since populations will fail to establish when colonization is unsuccessful (Angert et al. 2011; Kubisch et al. 2014). Costly dispersal is also a risk factor that may cause individuals to remain with the natal group and ultimately select for permanent sociality (Emlen 1991; Whitehouse & Lubin 2005). The evolution of permanent sociality would allow the related social species to extend into new habitats by eliminating costly dispersal, as explained with social and subsocial Anelosimus. This example shows how permanent sociality as an environmental adaptation can allow range expansion and, thus, social organism distributions should be examined in context of their solitary or less social congeners.  This study is an example of how to examine the dual questions of range limits and the ecological correlates of sociality using transplants and exclusion treatments. Transplant studies are especially useful to determine if range limits are equal to the niche limits or if range limits are partially dispersal limited (Hargreaves et al. 2014). For example, transplants beyond the range 	   26  that show low survival suggest that the range limit is more or less equal to the niche limit while transplants with high survival reflect a potential dispersal barrier or lag. In this particular case, there is no obvious dispersal barrier between transplant locations, such as a mountain range or large river, or any evidence of a recent change of conditions at either habitat.  Plus, the range of the co-occurring A. eximius includes both locations, suggesting enough time for the populations to have reached both locations. Thus, the fact that my transplants beyond the range failed to persist does suggest that the rainforest is in fact an unfavorable habitat for subsocial spiders. Additionally, I used a population from the elevational range edge, which is the population that would extend beyond the range if the proper adaptations to colonize this habitat existed (Bridle & Vines 2007; Hargreaves et al. 2014). Next, this study used exclusion of abiotic and biotic factors to test for their simultaneous effects. This approach, although straightforward, is used sparingly in range limit research. Studies that relate species abundances to environmental gradients are a good start, but stronger experimental connections are needed to ensure a cause and effect. Few studies have been able to simultaneously study an abiotic and biotic limit, which makes this study unique in its field (reviewed in Sexton et al. 2009). Although many biotic interactions are difficult to fully exclude (i.e. competitors), more studies utilizing an exclusion approach of two or more factors will help differentiate various effects. Lastly, I was able to compare the distributions of subsocial Anelosimus with their derived social congener. These species have a close phylogenetic history and similar morphology, which makes their group demographics, like colony size, the main difference between them. Comparing closely related species is a powerful way to understand how environmental gradients can cause range expansion through adaptation (Angert & Schemske 2005). To fully understand the role of sociality as an adaptation to an 	   27  environment, social organism distributions should be examined in context of their solitary or less social congeners.   Despite the strengths of this study, there are several limitations that can inform future research. First, I did not explicitly study the link between a group performance parameter (i.e. fitness) and group size. Avilés & Tufino (1998) have examined this relationship in a social Anelosimus species, but a closer look at the relationship between offspring success, probability of colonization, or other measures of fitness and group size in subsocial congeners would explain more as to why large groups are important. Secondly, although I observed and quantified differences in rain intensity and its effect on web-building, there is little direct evidence of the energetic cost of web building to the spiders. Future projects that examine the energetic and nutritional cost of web building in social and non-social spiders, as well as across a rain intensity gradient, will reveal the cost of web building and verify the expense of maintaining webs during heavy rains. Understanding the cost of web maintenance in the lowland tropical rainforest may also illuminate causes of distributions for other spiders with different types of webs.  As studies of range limits and range shifts burgeon, social animals will continue to offer an exceptional perspective on how the environment and species’ traits influence niches and distributions. Here I show that gradients of abiotic and biotic factors, modulated by group demographics, influence the range of a group-living spider. I also show that the environmental factors underlying range limit theory and those that explain the ecology of sociality are similar and may offer further explanation for the global biogeography of sociality.     	   28   Bibliography  Abeli, T., Gentili, R., Mondoni, A., Orsenigo, S., & Rossi, G. 2014. Effects of marginality on plant population performance. Journal of Biogeography, 41(2), 239–249.  Agnarsson, I., 2006. 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