TY - JOUR
AU - Lubin,, Yael
AB - Abstract Density-dependent dispersal is a common dispersal strategy, mainly as a mechanism of escaping decreased fitness associated with high intra-specific competition. However, in group-living species, high density is expected to be beneficial for the individual, at least up to a certain threshold. A possible mechanism for maintaining an optimal density is negative density-dependent dispersal. In order to examine this hypothesis, we studied the effect of colony density on growth, dispersal and prey capture under different diets in the colonial spider Cyrtophora citricola (Forskål, 1775) (Araneidae). Colonies of C. citricola often reach high densities but the spiders are also capable of living solitarily. Previous studies showed that indirect benefits related to prey capture and predator defense may arise from colony-living, despite the lack of direct cooperation. We found that dispersal propensity of spiders decreased with increasing colony density, and that the effect was strongest when prey abundance was high. Additionally, site tenacity of spider hatchlings increased with greater density of adult females in colonies. Both results support a negative density-dependent dispersal strategy. As expected, body mass of spiders increased with density, suggesting that fitness increases with density (Allee effect). Variance in body mass was higher within dense colonies than among solitary spiders, therefore it is likely that spiders in the colony differ in their prey capture success, and consequently in body mass. This interplay between Allee effect, dispersal strategy and individual fitness may have an important role in the life history and distribution of colonial spiders and of other group-living species. INTRODUCTION Dispersal is a key mechanism in the dynamics of populations and can influence their genetic structure and gene flow, spatial distribution and many other processes (Kokko and López-Sepulcre 2006, Ronce 2007). These various spatial, demographic, and evolutionary consequences of dispersal at the population level are a result of the ultimate and proximate causes of dispersal at the individual level. Studying these causes is therefore essential to understand dispersal (Bowler and Benton 2005; Clobert et al. 2009). From the individual’s point of view, the goal of dispersal is reaching a higher quality site and thus increasing its fitness—but dispersal is often also extremely risky and costly. High energy expenditure, increased mortality and the risk of dispersing into a less favorable site, all are common costs of dispersal (Bonte et al. 2012). The decision of when, where and how far to disperse is therefore a critical decision in the life history of the individual, and there should be strong selection pressure acting to optimize it. Conditional dispersal strategies allow organisms to adjust their dispersal behavior according to both environmental and internal conditions such as temperature, food availability, or reproductive state—and thus to react to spatial and temporal heterogeneity in their environment (Ims and Hjermann 2001; Bonte et al. 2012). This plasticity—the ability to make an informed decision on whether the current site is inadequate, or whether current conditions are suitable for dispersing—generally makes conditional dispersal strategies more beneficial than fixed ones (Ims and Hjermann 2001; Bowler and Benton 2005; Ronce 2007). Perhaps one of the most common and studied conditional dispersal strategies is density-dependent dispersal. In many species, an increase in local population density induces emigration, mainly as a mechanism of avoiding the intensifying competition over resources (Bowler and Benton 2005; Matthysen 2005). However, organisms are not always expected to suffer a decrease in fitness as a result of increased population density. Allee effect is a positive effect of density on the per-capita growth rate, which is directly related to fitness. Allee effect was observed in solitary organisms, usually at very low densities, where a slight increase in density increases the chance of finding a mate (Allee 1931; Courchamp et al. 1999). Due to the fitness benefits of living in a group, group-living animals are expected to experience a stronger Allee effect than solitary species and over a wider range of densities. Despite the costs of increased density, an overall increase in fitness may be possible in these species thanks to benefits such as increased foraging efficiency, increased predator vigilance and cooperative defense (Krause and Ruxton 2002; Kramer et al. 2009), up to a certain threshold, above which the costs of crowding will exceed these benefits. In this range of densities, in which the fitness is expected to increase with density, the conditional dispersal strategy may change. In this case, it is probably more beneficial to maintain large groups and emigrate if density is too low, preferably to join larger groups, i.e. exhibiting negatively density-dependent dispersal. A relationship between group-living, Allee effect and negative density-dependent dispersal has been suggested previously (e.g. Travis et al. 1999, Bowler and Benton 2005), but this has received very little study. The consequences of the Allee effect to the distribution of populations has been explored theoretically mainly in the context of habitat selection (referring to the immigration decision rather than emigration) and ideal free distribution, suggesting that the Allee effect should cause an aggregated distribution pattern (Fretwell and Lucas 1970; Greene and Stamps 2001). Negative density-dependent dispersal has been documented in various species, not necessarily group-living ones. It was observed in several species of insects, mainly Lepidoptera and Odonata (Kuussaari et al. 1996; Roland et al. 2000; Menéndez et al. 2002; Rouquette and Thompson 2007; Chaput-Bardy et al. 2010; Baguette et al. 2011), probably as a result of individuals using conspecific density as a cue for habitat quality. Conspecific attraction is a habitat selection mechanism common not only in group-living species (e.g. Sarrazin et al. 1994), but also in solitary and territorial ones (Stamps 2001), based on the assumption that presence of conspecifics implies that the habitat patch is suitable for settling. In some cases, observed negative density-dependent dispersal is promoted by fitness benefits of aggregation. For example, in many aposematic and distasteful Lepidoptera species the larvae are gregarious, and in the aposematic moth Zygaena filipendulae the adults as well display increased dispersal from low-density patches (Menéndez et al. 2002). In these species, fitness benefits of aggregation by the larvae (i.e. protection against predators; Stamp 1980) may also be true for the adult stage (Reader and Hochuli 2003). Negative density-dependent dispersal has also been observed in several colony-nesting bird species, where it is likely to be related to better protection against predators in larger, denser colonies (Birkhead 1977; Serrano et al. 2005; Kim et al. 2009). Cooperation among kin favors negative density-dependent dispersal in unicellular Tetrahymena (Jacob et al. 2016). In this study, we examined the effect of density on fitness and dispersal in a colonial spider species. Group-living in spiders occurs in 2 different forms: social form and colonial form. The colonial form is the less cooperative of the two, and includes species that form permanent aggregations of individual webs in which they catch their own prey and breed independently (Bilde and Lubin 2011; Avilés and Guevara 2017). Colonial spiders are thought to be foraging societies in which the main function of the colony is to increase prey capture (Whitehouse and Lubin 2005). Although they do not hunt cooperatively, the colony can provide foraging benefits by a ricochet effect (Uetz 1989), whereby insects bounce from one web in the colony to another until eventually captured inside the colony instead of escaping, or by the ability to use web-sites that are not available to a solitary spider (Rypstra 1979; Lubin 1980). Web building efficiency can be increased in a colony and silk production costs can be reduced due to shared frame threads (Buskirk 1975, Jakob 1991; Lloyd and Elgar 1997). The colony may also provide protection against predators and parasites (Uetz et al. 2002), thus also serving a protective function. However, similarly to other group-living animals, large colonies may suffer from greater visibility to predators and parasites and increased competition over resources (Lubin 1980; Hieber and Uetz 1990; Jakob 1991). We studied the effect of colony density on dispersal and fitness measures of the colonial spider Cyrtophora citricola (Araneidae), which lives both solitarily and in colonies of various sizes, sometimes at the same location (Leborgne et al. 1998). Colonies of C. citricola are composed of individuals of at various stages of development, from juvenile to adult. Previous studies of C. citricola have shown that both the presence of conspecifics or conspecific webs (Rao and Lubin 2010; Johannesen et al. 2012) and of prey leftovers in the webs (Mestre and Lubin 2011) induced young to remain in the group, but it was not known whether dispersal is indeed negatively density-dependent. It was also unclear whether there is a fitness benefit gained by colony-living in this species, and what it may be. In field observations, Leborgne et al. (1998) did not find differences in spider body size or prey capture success between solitary spiders and spiders living in colonies, but this was not studied experimentally. We therefore tested the general hypothesis of a negative density-dependent dispersal strategy mediated by an Allee effect in C. citricola by conducting a set of semi-field and laboratory experiments. In many spiders, dispersal occurs in the juvenile stage, often shortly after emergence from the eggsac (Weyman 1993). We investigated the effect of density of adult females on site tenacity of hatchling spiders in semi-natural conditions. If colonies increase in size due to juvenile philopatry, we expected hatchling spiders to respond to colony density with a negative density-dependent dispersal strategy. We predicted that hatchling site tenacity would increase as the adult female density increased. We then manipulated the density of juvenile C. citricola spiders in artificial colonies in laboratory conditions and examined the effect of the density on dispersal initiation, prey capture, aggressiveness, and proxy measures of fitness under low and high feeding regimes. Using densities similar to those encountered in colonies in nature, we predicted that the tendency to disperse should decrease with the increase in density, while the proxy measures of fitness, namely body mass and survivorship, would increase. The variance in body mass among individuals in the colony was used to assess whether all spiders benefit equally from colony-living. We also predicted that prey capture would improve with the increase in density, as it is expected to be a main benefit of coloniality in spiders (Whitehouse and Lubin 2005). Aggressive interactions, by contrast, are expected to be a major cost of coloniality (Jakob 1991); we predicted that their frequency should increase with density. Providing high or low abundance of prey enabled us to examine the interactions between prey availability and the density-dependency of fitness and dispersal. Colonial spiders are often found, and tend to aggregate more, in prey-rich habitats (Uetz et al. 1982; Mestre and Lubin 2011), and this could mean that the benefits of the colony are better realized under such conditions, perhaps due to a certain threshold of prey abundance that is required for sustaining a colony. An alternative hypothesis is that if colony-living indeed increases the individual’s prey capture rate, an increase in density will have a stronger effect on fitness and dispersal when prey are scarce. METHODS The study species Cyrtophora citricola (Araneidae) is an orb-weaving colonial spider that has a wide distribution in tropical and subtropical regions of the Old World (Levy 1997; World Spider Catalog 2017) and was recently found as an invasive species in several localities in the New World (e.g. Levi 1997; Víquez 2007; Starr et al. 2011). In Israel, it has undergone range expansion over the last 2 decades (Lubin Y, personal observation). It occurs in mesic to hyper-arid habitats, solitarily or in colonies of up to thousands of individuals (Mestre and Lubin 2011). The individual web is 3-dimensional, composed of a non-sticky, horizontal orb web, and an irregular barrier web above and below it, connecting it to other webs in the colony and creating a communal network. In their own capture area, spiders normally respond aggressively to conspecifics, but are more tolerant towards one another on the communal barrier threads (Lubin 1974). Female body size is highly variable, 7.5–13.3 mm in length, and males are much smaller, with average body length of 2.7 mm (Levy 1997). Adult density experiment Adult females and egg sacs were collected from a population in Kibbutz Gvulot, western Negev, Israel (31°12’46.14’’N, 34°28’9.49’’E). The adult females were placed on potted Negev acacia (Acacia pachyceras) trees that were trimmed to a height of about 1.5 m, inside a net-house on the Sede Boqer Campus (Ben-Gurion University). There were 3 density treatments: 1) control—no adult spiders, 2) low density—a single adult female spider, and 3) high density—5 adult female spiders. The trees were covered in mesh to allow web establishment and prevent the females from moving away. After adult colonies were established, the trees were uncovered, the spiders were fed, and hatchling spiders were placed on the same trees. In each replicate of the experiment, the hatchlings originated from 2 clutches mixed together. Forty-five hatchlings were randomly chosen and divided into 3 groups of 15 hatchlings each. Each one of the groups was placed on one of the 3 trees. The site tenacity of the hatchlings was assessed by recording the number of hatchlings on each tree during the following 5 days. The first 4 replicates of the experiment were achieved during October 2014, and the next 11 replicated during April–July 2015. Since the direction of wind can affect the dispersal of juveniles (Johannesen et al. 2012), the order of treatments assigned to the trees was reversed in each one of the 2 parallel rows of trees in the net-house, and switched between 2014 and 2015. Juvenile density experiments Egg sacs of C. citricola were collected from populations near Retamim (31°3’26.40’’N, 34°42’53.48’’E) and Kibbutz Gvulot (31°12’46.14’’N, 34°28’9.49’’E) in the western Negev, Israel. Each sib group, originating from a different clutch, was housed separately; siblings from a single hatched clutch were raised together until reaching the fourth instar. Twice a week they received springtails, Sinella curviseta (Collembola, Entomobryidae) “ad libitum”, and were lightly misted with water. Juvenile density experiments were conducted on spiders maintained on two diet regimes, a low-food diet (1 Drosophila melanogaster fruit fly per spider, twice a week) and a high-food diet (3 fruit flies per spider, twice a week). The low-food diet was shown to be adequate for survival and growth (Yip and Lubin 2016). Low-food experiment After reaching fourth instar (approximately 5 weeks), spiders were examined under a dissecting microscope and 12 juvenile female spiders were chosen from each clutch for each repetition of the experiment. It was sometimes difficult to accurately distinguish between juvenile males and females at the fourth instar (males have enlarged pedipalps while females do not). If one of the purported females turned out to be a male during the first week of the experiment it was replaced. If it was not discovered until later, it was left there but was not weighed at the end of the experiment. The 12 juvenile females were divided into three density treatments: 1) High density—8 spiders; 2) medium density—3 spiders; 3) low density—solitary. Each group of spiders was released into a glass terrarium (40 cm × 18 cm × 22 cm) with 5 vertical wooden sticks, one in each corner and one in the center, to support web building. Fruit flies released into the terraria were able to fly freely until they potentially got caught in the one of the capture webs. High-food diet The experimental setting was identical to the low feeding regime, but owing to a shortage of spiders, included only low density (1 spider) and high density (8 spiders) treatments. Thus, each clutch provided 9 spiders. Nineteen clutches were used for the low-food treatment, each providing a single replicate of the three treatments, and 17 clutches in the high feeding experiment. The experiments were performed during the spring and summer of 2014 and 2015. Dispersal propensity Dispersal trials were conducted one week after the beginning of each experiment. As a measure of dispersal propensity, we assessed whether spiders performed “tiptoe behaviour”, a pre-dispersal behavior that includes extending the legs, raising the abdomen and releasing silk threads from the spinnerets (Richter 1970; Bonte 2012). Tiptoe behavior is associated with both short-distance (bridging) and long-distance (ballooning) dispersal in many spider species, and thus often used to study factors affecting dispersal decisions (Weyman 1995; Bonte et al. 2003; Entling et al. 2011; Bonte et al. 2011). Cyrtophora citrícola spiderlings were observed bridging on silk in nature (YL), and a previous study showed that tiptoe behavior occurs in this species (Berner-Aharon, 2013). A trial was conducted once for each spider in both diet groups and all density treatments. Spiders were always fed on the day before the test. Spiders were taken out of the terrarium, one at a time, and placed at the base of a vertical wooden dowel (29 cm length) fixed in a Styrofoam and wooden base. The base was fixed inside a shallow container with water to prevent spiders from escaping by walking down the apparatus. An upward tilted fan placed 1 m away was used to create a weak flow of air (0.53 ± 0.12 m/s) and a light (40W bulb) illuminated from the opposite side. In a tiptoe event, the spider climbed to the top of the dowel and adopted the tiptoe position. It was not always possible to see the fine dispersal threads. A tiptoe event was recorded when the spider raised its abdomen and extended its legs. Multiple tiptoe events occurred when the spider interrupted the tiptoe posture with bouts of movement up and down the dowel. The number of tiptoe events was recorded during 5 min, a time period determined empirically as sufficient to observe several events, after which the spiders were returned to the terrarium. In case of actual bridging on a silk thread away from the dowel, the stopwatch was paused until the spider was returned to the dowel, and timing was continued. Prey capture We observed the spiders during a feeding event once per group in both diets. First, we measured the time until the first capture of a fly. Each terrarium was first given a single fly, and the time until it was captured was recorded. If it was not caught within 30 min the result was recorded as “no capture”. We also measured the probability of a spider to capture prey, i.e. whether each spider captured a prey or not, during the 30 min after providing all flies. Aggressive interactions such as stealing prey and chasing other spiders away were also recorded during the 30 min after feeding. Aggressiveness was recorded only in the spiders under low-food diet since the high-food diet did not include a medium-density level. Fitness We used 2 proxy measures to assess the fitness of the spiders: body mass and survival. After approximately 2 months, when most spiders had molted at least twice since the beginning of the experiment, spiders were weighed and the experiment was terminated. We used the measured body mass to compare both the mean body mass in the different density treatments and the variance in body mass. The survivorship of spiders was documented throughout the experiment. For technical reasons, in the low-food treatment, dispersal propensity data were collected for only 17 out of 19 clutches, and body mass data for 16 out of 19 clutches. Statistical analyses Adult density experiment The effect of adult density on site tenacity of hatchlings was analyzed using a repeated-measures generalized linear mixed model (GLMM; Bates et al. 2015), as the number of hatchlings on each one of the trees was counted each day for 5 consecutive days. The adult density treatment was included in the model as a fixed factor, and the replication number as a random factor. Pairwise comparisons between the levels of density treatment were done using Bonferroni’s post hoc procedure. Juvenile density experiment The effect of density on the number of tiptoe events in 5 min, which is the measure for dispersal propensity, was analyzed using a GLMM. The model was constructed with a Poisson distribution (log link function) for count data (Bolker et al. 2009). The density treatment was treated as a fixed factor and the clutch identity (i.e. replicate) as a random factor. To account for overdispersion, an individual-level random factor was also included in the model (Harrison 2014). The significance of the overall effect of the density treatment was determined by a chi-square test, comparing the 2 nested models—with and without the treatment factor. This was necessary only in the low-food experiment, where there were more than two levels of density. Pairwise comparisons between the levels of the density treatment were done using Tukey’s post hoc procedure. The probability to capture prey was analyzed using a generalized linear mixed model (Bates et al. 2015). Since here the response variable is binary (spider succeeded or failed in prey capture), the model was constructed using a binomial distribution with a logit link function (Bolker et al. 2009). The significance of the overall effect of the density treatment was determined by a chi-square test, comparing the 2 nested models—with and without the treatment factor. The effect of density on the time to first prey capture was analyzed using Kaplan–Meier survival analysis, with density treatment as an explanatory variable. The arcsine-transformed proportion of aggressive interactions during prey capture was analyzed using a GLMM with the density treatment (medium or high) as a fixed factor and the clutch as a random factor. The body mass of spiders in each group (density treatment × clutch) was averaged, and the effect of the density treatment on the average body mass was analyzed using a GLMM, with density as a fixed factor and the clutch as a random factor. Pairwise comparisons between the levels of density treatment were done using Tukey’s post hoc procedure. The variance in body mass within the medium- and high-density groups was compared to the variance in body mass of solitary spiders using a bootstrapping method. Bootstrapping was necessary owing to the fact that low-density groups consisted of single spiders. For each density level separately, the coefficient of variation (CV, expressed as %) of each colony was compared to that of a random sample of the same number of solitary spiders, with 100 iterations for each group (overall between 1400 and 1700 iterations, depending on the number of groups). A paired t-test was used to determine whether the coefficient of variation significantly differed between the spider colonies and the random samples of solitary spiders. The effect of density treatment on survival was analyzed using a Cox-regression survival analysis, with both density treatment and clutch as explanatory categorical variables. We used Fisher’s combined probabilities test (Sokal and Rohlf 1981) to combine the probabilities of dispersal propensity and prey capture probability obtained independently from low-food and high-food diet tests. Results were analyzed using the programs Statistica 12 (Dell Inc. 2015), SPSS 22 (IBM Corp. 2013) and R Studio (R Core Team 2014). RESULTS Adult density experiment The site tenacity of hatchlings was significantly affected by the number of adult females on the tree (repeated measures GLMM: n = 17, F(2,47) = 52.6, P < 0.001; Figure 1). Pairwise comparisons revealed that the site tenacity of the hatchlings was significantly lower on trees with no adult females than on trees with one adult female (P < 0.001) and trees with 5 adult females (P < 0.001). There was a trend of higher site tenacity on trees with 5 adult females than on trees with 1 female. Figure 1 Open in new tabDownload slide The number of hatchlings that remained on trees with no adult females (open circles, dotted line), one adult female (closed circles, dashed line) and 5 adult females (closed squares, solid line) on them over the course of 5 consecutive days (n = 17 replicates each, mean ± 95% CI). Figure 1 Open in new tabDownload slide The number of hatchlings that remained on trees with no adult females (open circles, dotted line), one adult female (closed circles, dashed line) and 5 adult females (closed squares, solid line) on them over the course of 5 consecutive days (n = 17 replicates each, mean ± 95% CI). Juvenile density experiments After 1 week of experiencing different levels of density on the low-food diet, the spiders in the high-density groups displayed a trend towards a lower dispersal propensity, expressed as the number of tiptoe events in a 5-min interval (Figure 2a). The overall effect of density on dispersal propensity was marginally significant, when a generalized linear model that included the density factor was compared to a model that did not include it (χ2 test, comparing nested GLMMs: χ2 = 5.68, P = 0.058). Pairwise comparisons between the three density treatments did not reveal significant differences. When more food was available, this trend became more pronounced—on the high-food diet, spiders in the high-density groups showed significantly lower dispersal propensity than the spiders living solitarily (i.e. low density) (GLMM, Poisson distribution: n = 147 spiders, z-value = −2.26, P = 0.02; Figure 2b). For low-food and high-food diets combined, spiders in high-density groups had a significantly reduced dispersal propensity (Fisher’s combined probabilities test, χ2 = 13.5187, df = 4, P = 0.009). Figure 2 Open in new tabDownload slide The number of tiptoe behaviors spiders displayed after 1-week exposure to different densities at (a) low-prey diet (n = 193 spiders in 17 replicates), (b) high-prey diet (n = 147 spiders in 17 replicates, mean ± SE). Figure 2 Open in new tabDownload slide The number of tiptoe behaviors spiders displayed after 1-week exposure to different densities at (a) low-prey diet (n = 193 spiders in 17 replicates), (b) high-prey diet (n = 147 spiders in 17 replicates, mean ± SE). On the low-food diet, the probability that a spider would capture a prey was not significantly affected by the density. Models that did or did not include the density treatment as a factor did not significantly differ from each other (χ2 test for comparing nested GLMMs: χ2 = 0.003, P = 0.99, n = 431 observations). Similarly, on the high-food diet prey capture probability did not differ between high-density groups and solitary individuals (GLMM, binomial distribution: n = 163 observations, z-value = 1.81, P = 0.068; mean ± 1SE: 0.42 ± 0.12, 0.64 ± 0.04; for low and high density, respectively). Fisher’s combined probability for the 2 tests was not significant (χ2 = 5.397, df = 4, P = 0.26). On the low-food diet, the first fly was caught in <5 min in more than half of the high- and medium-density group trials, while in the low-density groups, fewer than one-third of the trials resulted in a capture within 30 min (Figure 3a). The capture of the first fly occurred faster in the high- and medium-density groups than in the low-density groups (Kaplan–Meier survival analysis, log rank test: n = 19 clutches, comparison of time to capture the first fly in low density versus medium and high density, respectively: χ2 = 5.22, P = 0.02; χ2 = 14.3, P < 0.001; Figure 3a). The difference in first prey capture time between the medium and high density treatments was not statistically significant (χ2 = 2.57, P = 0.10). Figure 3 Open in new tabDownload slide Proportion of prey (the first fly) remaining uncaptured during the first 30 min of observation in groups of different densities (low, solid line; medium, dotted line; high, dashed line), under (a) low-prey diet (n = 19 replicates), (b) high-prey diet (n = 17 replicates). Figure 3 Open in new tabDownload slide Proportion of prey (the first fly) remaining uncaptured during the first 30 min of observation in groups of different densities (low, solid line; medium, dotted line; high, dashed line), under (a) low-prey diet (n = 19 replicates), (b) high-prey diet (n = 17 replicates). A similar high capture rate was seen in the high-density group on the high-food diet (Figure 3b). In the high-food diet, the first prey was captured significantly faster in the high-density groups (Kaplan-Meier survival analysis, log rank test: n = 17 clutches, χ2 = 11.0 P = 0.001). The frequency of aggressive interactions in the low-food diet was not affected by the density treatment (GLMM: n = 19, F1,18 = 0.15, P = 0.71), and was low in both the medium and high-density levels (mean±SE: 0.026 ± 0.03 and 0.035 ± 0.03, respectively). By the end of the experiment, after approximately 11 weeks in the experimental conditions, the spiders in the high-density groups had significantly higher average body mass than the spiders in the low-density groups, in both the low-food diet (GLMM: n = 16 clutches, F(2,28) = 5.46, P < 0.01; Figure 4a) and the high-food diet (GLMM: n = 17 clutches, F1,15 = 69.8, P < 0.0001; Figure 4b). Figure 4 Open in new tabDownload slide The body mass of spiders (mg, mean ± SE) reared in different densities under (a) low-prey diet (n = 16 replicates), (b) high-prey diet (n = 17 replicates). Figure 4 Open in new tabDownload slide The body mass of spiders (mg, mean ± SE) reared in different densities under (a) low-prey diet (n = 16 replicates), (b) high-prey diet (n = 17 replicates). In the low-food diet, the coefficient of variation in body mass within both the high- and medium-density groups was significantly greater than in random samples of the same number of solitary spiders (1400 iterations, t(1399) = −13.0, P < 0.001; 1600 iterations, t(1599) = −56.7, P < 0.001; medium and high density, respectively). The difference in the coefficient of variation of high-density groups (77.2 ± 0.6%) and solitary spiders (41.7 ± 0.2%) was greater than the difference between medium-density groups (51.9 ± 0.8%) and solitary spiders (40.6 ± 0.6%). On the high-food diet, as in the low-food diet, high-density groups had significantly greater coefficient of variation in body mass than solitary individuals (1700 iterations, t(1699) = −87.9, P < 0.001), The difference in variation was even larger than in the low-food diet, as the variation within the samples of solitary spiders was reduced (68.1 ± 0.4% and 26.3 ± 0.2%; high density and solitary, respectively). Survival was high in all density levels and did not differ significantly among the three density levels. This was true for both the low-food diet (survival analysis, Cox regression: n = 19 clutches, Wald statistic = 0.23, P = 0.89; Figure 5a) and the high-food diet (survival analysis, Cox regression: n = 17 clutches, Wald statistic = 1.05, P = 0.31; Figure 5b). Figure 5 Open in new tabDownload slide Proportion of surviving spiders in different density groups (low, solid line; medium, dashed line; high, dotted line) over the weeks of the experiment, under (a) low-prey diet (n = 19 replicates), (b) high-prey diet (n = 17 replicates). Figure 5 Open in new tabDownload slide Proportion of surviving spiders in different density groups (low, solid line; medium, dashed line; high, dotted line) over the weeks of the experiment, under (a) low-prey diet (n = 19 replicates), (b) high-prey diet (n = 17 replicates). DISCUSSION The goal of this study was to examine the hypothesis that colonial spiders will exhibit a negative density-dependent dispersal strategy that is promoted by the benefits of group-living. We found that the site tenacity of hatchling spiders of C. citricola was positively affected by the density of adult females: hatchlings had a lower tendency to disperse when more females were present on the tree. Moreover, spiders in denser experimental groups had lower dispersal propensity, as measured by tiptoe behavior, than spiders that lived solitarily, especially when food abundance was high. Both these findings support a negative density-dependent dispersal strategy. This evidence for negative density-dependent dispersal further implies that we should expect an Allee effect that drives the reduction in dispersal propensity. Indeed, spiders that lived in a dense group had higher average body mass than spiders that lived solitarily, suggesting that there is a fitness benefit in colony-living in this species. The spiders’ survival, however, was not affected by the density of the group and the survival in all density treatments was high on both diets. This suggests that our experimental conditions were benign enough with both diet levels to not cause significant mortality. In nature, mortality is likely also to be caused by predators and parasites. The dense structure of the colony may provide a protection against some predators (Uetz et al. 2002), but it may attract others, such as parasitoid wasps, and accelerate the transmission of contagious pathogens (Hieber and Uetz 1990; Côté and Poulin 1995). The increased body mass in the high-density groups suggests that there is an advantage in prey capture when spiders are more aggregated, but the results of the prey capture observations are more ambiguous. The first prey indeed was caught faster in groups of higher density, but that more spiders in an equal volume of space captured the first fly more rapidly is not surprising. However, it does mean that any possible interference between spiders within groups of increasing density was not great enough to counteract this effect. Increased prey capture in dense groups was not supported when examining the effect of density on the individual’s probability of capturing prey. Only at high food abundance was there a non-significant tendency towards higher prey capture probability in the high-density groups. The differences in body mass among the density treatments, however, were observed in the low-food diet as well. Therefore, these differences could not be explained by whether or not spiders captured prey. Rather, since the variance in body mass was larger in the high-density groups than between solitary-living spiders, it is likely that some spiders had better prey capture abilities than others and captured more prey items, consequently growing much larger. This asymmetry in capture success could have resulted in higher average body mass in dense groups, despite there being no difference in the probability of an individual capturing a prey item. Differences in prey capture ability may be a function of a spider’s location in the colony, or differences in the initial body size. Rypstra (1979) observed differences among spiders within C. citricola colonies in the amount of prey they captured in relation to their distance from the edge of the colony, and an overall increase in the capture efficiency (defined as number of prey items captured by an individual spider divided by the number of insects that entered the colony) in larger colonies. Caraco et al. (1995) predicted, based on a mathematical model, that the mean number of prey captured by an individual will increase in larger spider colonies, and that the variance in individual prey capture will decrease, mainly due to stealing of prey. In the current study, the volume of the colonies was constant, yet the results of this model may still be relevant, as the differences in number of individuals in this volume still constitute different group sizes. The model did not take into account size differences among individuals, which the authors suggest may lead to unequal ability to steal prey, namely larger spiders will probably steal prey successfully more often than small spiders (Rayor and Uetz 1990; Jakob et al. 2000). The mechanism behind the Allee effect found in our study is therefore likely related to greater prey capture overall in larger and denser colonies. Rypstra (1979) attributed the increased prey capture efficiency in larger C. citricola colonies to the large 3-dimensional structure of the colony and the larger amount of silk web, which deflect and confuse flying insects. The results of this study indicate that competition between spiders was perhaps not as strong and as costly as expected. Increasing the amount of food resulted in an increase in body mass both in solitary and high-density living spiders, but the increase was larger in the high-density groups. This suggests that there was competition between the spiders when prey were relatively scarce that was reduced when prey abundance was higher. However, since on both diets the mean body mass was higher in the high density, the competition did not seem to be strong enough to reduce the benefits of colony-living. As to aggressiveness within the colony, both Rypstra (1979) and Caraco et al. (1995) ascribe an important role to aggressive interactions such as stealing and intruding. While we observed these behaviors during prey capture and feeding, they were not very common and their frequency was not influenced by density. The interactions between spiders and the distribution of prey within the colony definitely require a more detailed study, as the patterns that emerged here imply that the colony has a complex economy where some spiders benefit from colony-living while others are negatively impacted by it. To better understand this inequality, it is necessary to address the question of how prey capture, fitness and dispersal propensity are related at the individual level within the colony. The interplay between the Allee effect and a negative density-dependent dispersal strategy demonstrated in the findings of this study reveals a possible mechanism of colony formation in C. citricola. The advantage of colony-living seems to be stronger, on average, when food is abundant, and accordingly dispersal is then more strongly inhibited by the increased density. This interaction between dispersal propensity and food availability suggests that C. citricola spiders have a higher tendency to group when prey is more abundant. Similar interactions between food abundance and grouping behavior occur in other group-living animals as well, and are often explained by a risk-sensitive foraging strategy (e.g. in birds, Ekman and Hake 1988). When food is abundant animals are expected to be risk aversive, since they are likely to meet their nutritional needs and thus can afford to avoid high variance in their personal food consumption. But when food is scarce animals tend to prefer a more risk-prone approach, where a high variance can provide a chance to exceed the starvation threshold (Caraco et al. 1980; Caraco 1980; Stephens 1981). Spider behavior was shown to be more risk prone when their food consumption was low (Lubin and Henschel 1996; Ainsworth et al. 2002). Risk-sensitive grouping behavior seems to apply to several species of colonial spiders that tend to aggregate more in prey-rich sites (Gillespie 1987; Uetz 1988; Rypstra 1989; Caraco et al. 1995). A tendency to aggregate at prey-rich sites supports the hypothesis of parasocial pathway to coloniality in spiders. Colonial spiders are thought to have originated in aggregations of solitary individuals around a resource (Whitehouse and Lubin 2005), that then evolved social features such as reduced aggression towards conspecifics. The distribution range and metapopulation dynamics in general, and of group-living species in particular, can be greatly influenced by the dispersal strategy (Bowler and Benton 2005). Negative density-dependent dispersal, such as shown here, is often ignored in this context, but some theoretical studies have found that negative density-dependent dispersal is expected to accelerate range expansions (Travis et al. 2009; Altwegg et al. 2013). The results of this study suggest that the intensity of such effects may vary with the availability of food. The consequences of negative density-dependent dispersal in group-living species under different conditions thus remain to be further explored. FUNDING This work was supported by the U.S.-Israel Binational Science Foundation (BSF) grant no. 2010178 [Y.L. and D.R.S.] and a scholarship from the Albert Katz School for Desert Studies, Ben-Gurion University of the Negev [L.V.]. We thank Iris Musli for assistance in the lab and field and Eric Yip for advice. We are grateful to S. Bar-David, O. Ovadia, I. Giladi and 2 anonymous reviewers for comments on the manuscript. 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Google Scholar Crossref Search ADS WorldCat Author notes " Handling editor: Jonathan Pruitt © The Author 2017. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
TI - Crowding leads to fitness benefits and reduced dispersal in a colonial spider
JF - Behavioral Ecology
DO - 10.1093/beheco/arx106
DA - 2017-09-01
UR - https://www.deepdyve.com/lp/oxford-university-press/crowding-leads-to-fitness-benefits-and-reduced-dispersal-in-a-colonial-JLC3rsJntk
SP - 1384
VL - 28
IS - 5
DP - DeepDyve
ER -