Life-History Data for the Funnel Weavers Eratigena Agrestis And

345 Life-history data for the funnel weavers Eratigena agrestis and Eratigena atrica (Araneae: Agelenidae) in the Paciﬁc Northwest of North America Samantha Vibert,1 Maxence Salomon, Catherine Scott, Gwylim S. Blackburn, Gerhard Gries

Abstract—The life history of the funnel weaver Eratigena agrestis (Walckenaer) (Araneae: Agelenidae) is not well studied despite its widespread occurrence in Europe and its establishment and spread in the Pacific Northwest of North America since its introduction in the early 20th century. We report phenology and life-history data for E. agrestis and another co-occurring funnel weaver, Eratigena atrica (Koch), in two study sites in British Columbia, Canada. The most notable difference in phenology between the two Eratigena species was the timing of emergence: E. atrica spiderlings emerge in the fall whereas E. agrestis spiderlings emerge in the spring. Surprisingly, the contrasting densities of E. atrica in the two study sites and the presence of the western black widow spider, Latrodectus hesperus Chamberlin and Ivie (Araneae: Theridiidae), in one study site had little effect on the life history of E. agrestis. This unexpected finding may be explained by (i) low overall competition pressure in the study habitats, (ii) conspecifics and heterospecifics exerting equivalent competition or predation pressures; and/or (iii) aggregations of heterospecifics providing benefits that offset costs associated with any competition.

Introduction duellica;Bolzernet al., 2013), with a focus on E. agrestis. These two congeneric species were The life history of an individual is defined by introduced to the coastal Pacific Northwest of North traits that describe its growth, reproduction, and America from Europe a century ago (Crawford and survivorship. These traits, in turn, are affected by Vest 1989). Their range has since expanded across an individual’s biotic and abiotic environment this region into a variety of habitats (Vetter et al. (i.e., temperature, humidity, food availability, and the 2003). Eratigena atrica, in particular, is often found presence of competitors or predators). Interpreting near or inside houses in its introduced range. the ecology, evolution, or behaviour of a species Because specimens of both species are relatively requires knowledge of its life history. Yet, for many large, brown, and highly mobile, they are regularly organisms detailed life-history accounts are lacking. misidentified as brown recluse spiders, Loxosceles Here, we report phenology and life-history data reclusa (Gertsch and Mulaik) (Araneae: Sicariidae) for two species of funnel weavers (Araneae: (Bennett and Vetter 2004). Reports that E. agrestis Agelenidae): the hobo spider Eratigena agrestis bites can cause necrotic lesions have resulted in (Walckenaer) (formerly Tegenaria agrestis; negative perception by the public, despite being Bolzern et al., 2013) and the giant house spider unsubstantiated (McKeown et al. 2014). It is of Eratigena atrica (Koch) (formerly Tegenaria interest to better understand the life cycle and life

Received 31 May 2016. Accepted 22 October 2016. First published online 13 February, 2017.

S. Vibert,1 G. Gries, Department of Biological Sciences, Simon Fraser University, 888 University Drive, Burnaby, British Columbia, V5A 1S6, Canada M. Salomon, Department of Biological Sciences, Simon Fraser University, 888 University Drive, Burnaby, British Columbia, V5A 1S6, Canada; and Department of Biology, Douglas College, 700 Royal Avenue, New Westminster, British Columbia, V3M 5Z5, Canada C. Scott, Department of Biological Sciences, Simon Fraser University, 888 University Drive, Burnaby, British Columbia, V5A 1S6, Canada; and Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Scarborough, Ontario, M1C 1A4, Canada G.S. Blackburn, Department of Biological Sciences, University of Alberta, 11455 Saskatchewan Drive, Edmonton, Alberta, T6G 2E9, Canada

1Corresponding author (e-mail: [email protected]). Subject editor: Christopher Buddle doi:10.4039/tce.2016.73

Can. Entomol. 149: 345–356 (2017) © 2017 Entomological Society of Canada Downloaded from https://www.cambridge.org/core. University of Athens, on 24 Sep 2021 at 10:54:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.4039/tce.2016.73 346 Can. Entomol. Vol. 149, 2017

history of these spiders in the wild, both for their soldanella Linnaeus (Convolvulaceae), Festuca potential management and to inform further, more rubra Linnaeus (Poaceae), Grindelia integrifolia detailed, studies of their ecology and behaviour. Candolle (Asteraceae), Linaria genistifolia We recorded life-history traits of E. agrestis and dalmatica (Linnaeus) Maire and Petitmengin E. atrica in natural habitats in southern British (Plantaginaceae), Lomatium nudicaule (Pursh) Columbia, Canada. We surveyed two sites to assess Coulter and Rose (Apiaceae), Poa Linnaeus variations in life history among populations of the (Poaceae) species, Polygonum paronychia two study species. The two study sites feature Chamisso and Schlechtendal (Polygonaceae), and marked differences in the relative abundance of Rumex acetosella Linnaeus (Polygonaceae). the two species, and of a third native species that Predominant plant species at Iona Beach are overlaps strongly in habitat use: the western black Carex macrocephala, Elymus Linnaeus (Poaceae) widow spider, Latrodectus hesperus Chamberlin species, Lathyrus japonicas Willdenow (Fabaceae), and Ivie (Araneae: Theridiidae). Comparisons Poa species, and Rumex acetosella, with patches across study sites offer insight as to how varying of the moss Racomitrium canescens (Hedwig) heterospecific interactions and predation pressures Bridel (Grimmiaceae). Both sites are relatively may affect life history. The life-history data we windy, and are susceptible to flooding during win- present for these introduced species are a critical first ter storms, and to disturbance by humans. step in understanding the impact they may have on Annual temperature profiles of Island View native arthropod communities. Beach and Iona Beach were similar over the course of the surveys at each site (Fig. 2), based on data from weather stations located within 6 km of each Methods site at similar elevations (Environment Canada 2015). In 2006, the highest average monthly Study sites temperatures were 17.8 °C and 18.7 °C (in July) for We studied populations of E. agrestis and Island View Beach and Iona Beach, respectively, E. atrica at two field sites separated by ~ 60 km in and the lowest average monthly temperatures were southern coastal British Columbia, Canada. The first 4.4 °C (in February and December) and 4.3 °C (in site (48°34'58'N, 123°22'20'W; elevation: 3–4m) February) for Island View Beach and Iona Beach, was located at Island View Beach on the Saanich respectively. Iona Beach received slightly more peninsula of southern Vancouver Island. The second precipitation over the course of the study period site (49°13'11'N, 123°12'53'W; elevation: 0.5 m) (1199 mm) than Island View Beach (883.3 mm). was located at Iona Beach Regional Park on the mainland, in the Greater Vancouver Regional Dis- Survey protocol trict (Fig. 1A–B). Both field sites are sand-dune For one year at both study sites, we conducted habitats featuring open sandy areas interspersed monthly surveys of driftwood logs to document with logs of driftwood. Eratigena agrestis and ecological and life-history traits of E. agrestis E. atrica commonly build their funnel-sheet webs and E. atrica. At each site, we followed spider beneath these logs. Both E. agrestis and E. atrica are populations within study areas that were compar- abundant at Island View Beach, but E. agrestis able in size and microhabitat characteristics predominates at Iona Beach (Fig. 1C). Latrodectus (Table 1). Logistical constraints limited our hesperus also builds cobwebs under driftwood logs survey to one year per site. At Island View Beach, at Island View Beach and can prey upon adults and we surveyed from May 2005 to April 2006, at juveniles of both Eratigena species (Salomon which point a major disturbance by vehicles 2011). compromised the integrity of the habitat and At both study sites, the sparse vegetation forced us to terminate the study. At Iona Beach, between logs consists of grasses, sedges, herbs, and we surveyed from May 2006 to April 2007. dwarf shrubs. Predominant plant species at Island In January 2007, an extensive and persistent snow View Beach are Abronia latifolia Eschscholtz cover prevented a survey of the Iona Beach site. (Nyctaginaceae),Ambrosiachamissonis(Lessing) The comparison among sites, therefore, is based Greene (Asteraceae), Carex macrocephala on the assumption that the data reflect general Willdenow ex Sprengel (Cyperaceae), Convolvulus annual patterns at each location (Fig. 2).

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Fig. 1. (A) Map of the southern coast of British Columbia, Canada, with the locations of the ﬁeld sites: Iona Beach and Island View Beach. (B) Images of female Eratigena agrestis, Eratigena atrica, and Latrodectus hesperus. (C) Relative abundance of E. agrestis, E. atrica, and L. hesperus spiders under driftwood logs at each site. Relative abundance is based on all spiders found between May 2005 and October 2006 at Island View Beach, and between May 2006 and April 2007 at Iona Beach. Black and white map from SimpleMappr (Shorthouse 2010); Island View Beach (IVB) map imagery © Google 2015; Iona Beach (IoB) map imagery © Google 2014; spider photographs from Sean McCann.

Fig. 2. Mean temperature (°C) and total precipitation (mm) between May 2005 and April 2007 at Island View Beach (IVB) (Vancouver Island) and Iona Beach (IoB) (Greater Vancouver Regional District). Data were obtained from Environment Canada weather stations at the Vancouver International Airport and the Victoria International Airport, each situated < 6 km from the relevant study site.

Table 1. Summary of ecological characteristics of the study sites at Island View Beach and Iona Beach Regional Park (British Columbia, Canada).

Island View Beach Iona Beach Approximate size of study site 900 m2 600 m2 † ‡ Number of driftwood logs (i.e., potential spider microhabitats)* 68 123 Total surface area available under the logs* 28.94 m2 35.95 m2 Mean (± SD) surface area of the logs* 0.43 ( ±0.32) m2§ 0.29 (±0.32) m2 || || Spider community composition: total number (A) , % per site (B) , spiders per log (C)¶ AB CABC

Eratigena agrestis 538 23.4 0.66 756 83.4 0.51 Eratigena atrica 588 29.2 0.72 150 16.6 0.10 Latrodectus hesperus 839 47.4 1.03 0 0.0 0.00 * At the onset of the study. † Two logs went missing, 14 were moved to new locations. ‡ Eight logs went missing, ﬁve were moved to new locations. § The mean surface area under a log was signiﬁcantly greater at Island View Beach than at Iona Beach (Wilcoxon’s rank sum test: W = 5744, P < 0.0001). || Total number of individuals and % of the overall number of spiders per site over the survey period. ¶ Average number of spiders per log over the survey period.

At each site, we measured the length and 0.1 mm. We then returned the spiders to the exact width of each log to assess the space available locations under the logs where we had originally underneath. We turned over all logs that were at found them. We identified all juveniles to species least 0.30 m long and detached from the ground level based on somatic characters (Vetter and substrate, and surveyed these logs for the presence Antonelli 2002). To reliably compare growth of spiders. This length represents the minimum patterns of juveniles across species and field sites, log length used by E. agrestis, E. atrica, and we grouped measurements of spiderlings into L. hesperus (Salomon et al. 2010). At both sites, five size categories: 1 (2–4 mm), 2 (4–6 mm), park visitors occasionally disturbed logs. These 3(6–8 mm), 4 (8–10 mm), and 5 (>10 mm). We minor disturbances were typically restricted to confirmed the accuracy of species identifications by only one or two logs. When logs were disturbed, collecting 63 juveniles at Iona Beach and by rearing we returned the affected logs to their original them to adulthood in the laboratory. Voucher position, unless spiders had colonised the new specimens have been deposited at the Royal British underside. In the latter case, we surveyed the logs Columbia Museum (Victoria, British Columbia, in their new position. Canada). We collected 71 E. agrestis egg sacs at Island Variables measured View Beach in December 2005 to determine (i) the On each survey date and for each log, we number of eggs in each sac, (ii) the timing of recorded the following variables: (1) numbers of emergence, and (iii) the number of moults before E. agrestis and E. atrica, (2) sex and age class of the spiders reached adulthood. We focussed on each individual (juvenile, sub-adult (i.e., penulti- E. agrestis because E. atrica egg sacs were not mate instar), adult), (3) size (total body length) of available at the same time in the season. In a pilot each individual, (4) number of egg sacs, and study, we brought a dozen freshly deposited egg sacs (5) number of webs. We also recorded the number indoors in September. Three months later no of L. hesperus and their cobwebs present under the spiderlings had emerged from any of the sacs. This logs. In spiders, total body length is an index of both led us to suspect that E. agrestis egg sacs require an age and feeding history (Toft and Wise 1999; overwintering diapause. To maximise the chance of Uetz et al. 2002b). Using a glass vial, we achieving hatching success, we randomly assigned carefully picked up each spider and measured total each of the 71 egg sacs to one of two treatments. body length with calipers (the distance from the In the first treatment, we housed egg sacs outdoors chelicerae to the tip of the abdomen) to the nearest until mid-February 2006, after which average

temperatures increase (Fig. 2), and then moved and 0 L. hesperus spiders underneath 123 logs at them into a temperature-controlledgrowthchamber. Iona Beach, and a total of 538 E. agrestis,588 There, we gradually increased the temperature over E. atrica,and839L. hesperus spiders underneath the course of 10 days from 10 °C to 22 °C, in incre- 68 logs at Island View Beach. Consequently, the ments of 2 °C every two days. We changed night proportions and population densities of the three temperature in the same manner, but from 5 °C to study species differ among sites (Fig. 1C; Table 1). 16 °C. In the second treatment, we kept egg sacs The proportion of logs occupied by any of the three outdoors until spiderlingsemerged.Thisallowedus focal species was significantly higher at Island View to determine when emergence would occur under Beach than Iona Beach (Fig. 3). Across sampling natural conditions. We monitored both treatment months, the mean (± standard deviation) proportion groups daily until emergence occurred, recording the of logs occupied by spiders was 0.76 ± 0.09 at date of emergence and the number of spiderlings for Island View Beach and 0.40 ± 0.14 at Iona Beach each egg sac. Immediately upon emergence, we (χ2 = 22, df = 1, P < 0.0001). Of those logs that randomly selected four spiderlings per egg sac and were occupied, the number of spiders cohabiting placed each one into its own plastic Petri dish under them also differed between the two sites. At (60 × 10 mm; n = 120). In two instances, the brood Iona Beach, the majority of occupied logs (56 ± 8%) selected consisted of only three spiderlings, in which wasoccupiedbyonlyonespider,whereasonly case we selected all three. Three times per week, 28 ± 8% of occupied logs at Island View Beach had we provided water and a diet of vinegar flies only one spider under them (χ2 = 18, df = 1, (Drosophila melanogaster Meigen, Diptera: Droso- P < 0.0001). At Island View Beach, 49 ± 10% of philidae), house flies (Musca domestica Linnaeus, occupied logs were occupied by three or more Diptera: Muscidae), house crickets (Acheta spiders, whereas only 16 ± 6% of those at Iona domesticus (Linnaeus), Orthoptera: Gryllidae), and Beach were occupied by three or more spiders yellow mealworms (Tenebrio molitor Linnaeus, (χ2 = 22, df = 1, P < 0.0001). The maximum Coleoptera: Tenebrionidae), and noted the date of number of spiders found cohabiting under single each spider moult. We retained all laboratory-reared logs was also greater at Island View Beach (up to 14 spiders for use in experiments. cohabiting spiders) than at Iona Beach (up to 10 Two days after E. agrestis spiderlings first started cohabiting spiders). The mean (and median) maxi- to emerge in the laboratory, we transferred each mum number of cohabiting spiders across monthly brood to a separate Petri dish (150 × 25 mm). Each surveys was 10 at Island View Beach, compared brood was large and mobile, making it difficult to with a mean (and median) of six at Iona Beach. The count the number of spiderlings. To ensure data total area available under logs was similar at both accuracy, we counted each brood three times and sites, but the mean area under each log was calculated the mean number of spiderlings in significantly greater at Island View Beach (Table 1). each brood. At both sites, the size of occupied logs was greater We compared the total body length of adult than that of unoccupied logs (Island View Beach: E. agrestis females across field sites (at least W = 66, P = 0.01; Iona Beach: W = 632, 10 females per site) in September, October, P < 0.0001; data for August only – the middle of the November, and December. We could not assess mating season when occupancy is relatively high at reproductive rate for either species because it was both sites). At Island View Beach, the mean not possible to associate egg sacs with specific (± standard deviation) area under logs was females in the field, and because females did not 0.46 ± 0.33 m2 for occupied logs (n = 61) and produce egg sacs under laboratory conditions. We 0.18 ± 0.11 m2 for unoccupied logs (n = 7), and at measured the longevity of laboratory-reared indi- Iona Beach the mean area under logs was viduals that survived to maturity. We performed 0.42 ± 0.37 m2 for occupied logs (n = 69) and all data analyses in R 3.2.0 (R Core Team 2015). 0.13 ± 0.14 m2 for unoccupied logs (n = 54). The egg-laying season of E. agrestis and Results E. atrica overlapped, beginning in late August/ September and lasting for three months (Fig. 4). Over the 12-month survey period at each site, we Notably, the density of egg sacs doubled at both observed a total of 756 E. agrestis,150E. atrica, sites between September and December (Fig. 5).

Fig. 3. Log occupancy data for Island View Beach and Iona Beach. Bars show the proportion of logs that were occupied by spiders (combined data for all three focal species: Eratigena agrestis, Eratigena atrica, and Latrodectus hesperus). Coloured sections of bars indicate the proportion of occupied logs with a given number of spiders cohabiting beneath them.

Fig. 4. Life cycles of Eratigena agrestis and of Eratigena atrica at Island View Beach and Iona Beach. Each tick on the scales represents a month. Each arrow indicates the month in which the associated event begins to occur in the ﬁeld.

In the field, E. atrica spiderlings emerged in the fall, spiderlings at Island View Beach are not available whereas those of E. agrestis emerged in the spring. due to uncertainties in the identification of small At Iona Beach, second-instar (i.e., just emerged) specimens during the first month of study at Island E. atrica spiderlings were present in large numbers View Beach. By the time we conducted the from September to December, whereas second- phenology survey at Iona Beach, we were able to instar E. agrestis spiderlings were present almost distinguish E. agrestis spiderlings from those exclusively in July. At Island View Beach, of E. atrica. It is important to note that these data thenumberofsecond-instarE. atrica spiderlings represent densities of only those spiders that we (2–4 mm size category) began to increase in successfully detected and captured. We may have September and peaked in December (Fig. 6). failed to detect spiders because they were hidden Comparable data for second-instar E. agrestis from view in deep crevices within logs, or because

Fig. 5. Phenology of Eratigena agrestis and Eratigena atrica at Island View Beach and Iona Beach. The graphs represent the densities (numbers of individuals per m2 of available habitat, i.e., driftwood logs) of juveniles, adult females, sub-adult and adult males, and the number of egg sacs for both species and ﬁeld site. Note that the y-axis scales differ across graphs. Data were collected between July 2005 and June 2006 at Island View Beach, and between May 2006 and April 2007 at Iona Beach. The egg sacs spun by E. agrestis and E. atrica are very similar and could not be identiﬁed to species level.

they moved into deep crevices or the surrounding in these two treatment groups implies that emer- vegetation when we first flipped logs over, or else gence is mediated, at least in part, by ambient because they were located in nearby vegetation temperature. rather than under logs (especially likely for small A mean (± standard deviation) of 64.73 ± 24.76 juveniles after dispersal). As detection probability (range: 17–124) E. agrestis spiderlings emerged increases with size, we almost certainly under- from the egg sacs reared indoors. A mean of estimated the numbers of small juveniles compared 60.52 ± 26.70 (range: 2–115) spiderlings emerged with larger size classes. from the egg sacs left outdoors. Data from the E. agrestis egg sacs we collected We monitored moulting patterns for the 120 at Island View Beach and reared in the laboratory E. agrestis spiderlings that we housed singly showed differences in emergence date based on immediately after emergence from the egg sac. The our treatments. Egg sacs that were kept outdoors first moult took place within the egg sac after had spiderling emergence between 31 May and hatching and before emergence. Males matured 12 June (i.e., greater than five months after after 8–12 moults and females after 7–9 moults collection). Of the 38 eggs sacs we first kept (Table 2). A total of 23 males reached maturity, outdoors and then moved indoors, spiderlings between July 2006 and April 2007. Unexpectedly, emerged from 26 sacs between 17 and 20 March, males generally underwent more moults than which is less than four months after collection and females before maturing. These unusual results may one month after we had moved them indoors. be due to rearing conditions in the laboratory. Many The contrasting emergence times of spiderlings males died before maturing, and we speculate that a

Fig. 6. Changes over time in population densities of Eratigena agrestis and Eratigena atrica juveniles of different sizes at Island View Beach and Iona Beach. The total body length of all juveniles was assigned to one of ﬁve size categories (1: 2–4 mm; 2: 4–6 mm; 3: 6–8 mm; 4: 8–10 mm; 5: > 10 mm). Data were collected between July 2005 and June 2006 at Island View Beach, and between May 2006 and April 2007 at Iona Beach.

sub-optimal diet was responsible for this (other December to April (Fig. 6). Spiderlings in the studies have also reported this, e.g., Toft and Wise larger size categories 3 (6–8mm),4 (8–10 mm), 1999). Furthermore, it is conceivable that only and 5 (>10 mm) were present mainly from May to especially slow-growing males were able to survive July, suggesting that they reach maturity after to maturity in the laboratory. Similarly, the 23 July. At Iona Beach, E. atrica spiderlings were females that reached maturity in the laboratory in much less abundant but their phenology in general July and August 2006 may also have been growing resembled that observed at Island View Beach. under unusual laboratory conditions. Indeed, only For E. agrestis at Island View Beach, the density a subset of females was able to mature, and did of spiders in the 4–6, 6–8, and 8–10 mm size so faster than is typical in the ﬁeld. The mean categories peaked in September, from December to (± standard deviation) longevity of laboratory- April, and from March to May, respectively (Fig. 6). reared spiders (n = 17 for each sex) that survived After May, most spiders presumably reached to maturity was 274 ± 83 days for males (range: maturity. Phenology data of E. agrestis at Iona 133–399 days), and 226 ± 52 days for females Beach and Island View Beach are highly compar- (range: 156–300 days). able (Figs. 5–6). These data show that (i) size-1 The developmental data of spiderlings are juveniles (size 2–4 mm) peaked in frequency in July summarised in Figures 5 and 6. For E. atrica (indicative of emergence the previous month), at Island View Beach, most size-1 spiderlings (ii) size-2 (4–6 mm) spiderlings were most prevalent (2–4 mm) were present in December, indicating over the winter months, (iii) size-3 (6–8mm) that they emerged the previous month (Fig. 5). juveniles peaked in frequency through April and Most size-2 (4–6 mm) juveniles were present from May, (iv) size-4 (8–10 mm) spiderlings were found

in the spring and early summer, and (v) mature numbers observed in October and November. spiders were present thereafter (Figs. 5–6). Strangely, we did not observe a similar pattern At both field sites, E. agrestis males seemed to at Island View Beach. There, from a peak mature before conspecific females (Fig. 4). While abundance observed in July, the number of adult we first observed adult E. agrestis males in July, females tapered slowly throughout the following with peak abundance in August, adult females first months. appeared in August and reached their greatest Box plots comparing the body size of adult abundance in October. By then, the number of adult E. agrestis females collected from September to E. agrestis males had already declined substantially. December (the only months where there were at At Island View Beach and Iona Beach, the number least 10 adult females at both sites) revealed no of adult E. atrica males peaked in July, then body size differences between individuals from declined sharply. At Iona Beach, E. atrica females different field sites (Fig. 7). started maturing in August, with the highest Discussion Table 2. Number of moults preceding maturity in laboratory-reared Eratigena agrestis individuals col- Overall, our phenological study of the two lected from Island View Beach (British Columbia, closely related funnel weavers Eratigena agrestis Canada). and E. atrica in their natural habitat revealed that their life histories are similar except that Number of spiderlings from each species emerge in different adults seasons. We also observed strong consistency in life-history traits across the two field sites. Below, Number of moults preceding maturity Females Males we discuss the implications of these results and 740suggest avenues for future research. 8112 983Life cycles of Eratigena agrestis versus 10 0 5 Eratigena atrica 11 0 8 The two study species differ in that E. agrestis 12 0 5 has a biennial life cycle, whereas E. atrica has

Fig. 7. Body length (mm) of adult Eratigena agrestis females at Island View Beach (IVB) and Iona Beach (IoB). The boxes depict the median, ﬁrst, and third quartiles. The whiskers represent the lowest and highest datum within the 1.5 interquartile range of the lower and upper quartile. Data were collected in 2005 at Island View Beach and in 2006 at Iona Beach.

an annual life cycle. Despite their different E. agrestis and E. atrica in relation to variation in life-history trajectories, the timing of many events weather conditions. Our laboratory data suggest is similar for both species. The mating seasons that ambient temperature influences the timing of take place in late summer and early fall, with emergence from egg sacs in E. agrestis. the first adult males appearing in July. Male abundance peaks in August and then declines Life-history traits of Eratigena agrestis and rapidly as mature adult males stop feeding. Eggs Eratigena atrica in ecologically contrasting sacs are produced from September to November. populations In both species, there is an intriguing mismatch Our study compared life-history traits of between the time when females mature and when E. agrestis across two field sites that feature strongly adult males are present: the maximum number of contrasting densities of E. atrica and L. hesperus adult females is reached only following the (Fig. 1), two species that are potential competitors or decline of male numbers. This pattern may be predators of E. agrestis. At Iona Beach, L. hesperus consistent with a mating system in which females is not present and the density of E. atrica is low; mate only once and males exhibit a strong conversely, at Island View Beach, L. hesperus is preference for virgin females (Stoltz et al. 2007), abundant and E. agrestis and E. atrica are found and/or a system in which there is first-male sperm at similar densities. As a consequence of these precedence. Males may also guard sub-adult differences in community compositions, it is females, as in some thomisid (Dodson and Beck conceivable that life-history traits of E. agrestis 1993) and araneid spiders (Fahey and Elgar differ based on the variation in potential competition 1997). A detailed analysis of the mating and predation pressure. However, none of the traits behaviours of each species during the peak mating we measured differed significantly between field season would reveal which of these male beha- sites. One potential explanation for this result is that viours is most prevalent. Furthermore, the fact that competition between the two Eratigena species and females mature late in the season raises the pos- predation risks are low in coastal habitats. However, sibility that they may reproduce only during the several lines of evidence suggest otherwise. mating season of the following year, provided First, all three species are generalist predators. they survive the winter. In a previous study of L. hesperus at Island View The major difference in life cycle between the Beach, Salomon (2011) found that this species two species concerns the timing of spiderling preys upon a broad range of arthropods, including emergence and juvenile growth. Eratigena atrica E. agrestis and E. atrica spiders. Although the spiderlings emerge from egg sacs deposited in diet of E. agrestis and of E. atrica has not yet early autumn after only a few weeks, whereas been specifically characterised, we observed both E. agrestis spiderlings overwinter in egg sacs and species capturing a variety of arthropod prey, such emerge during the spring. This means that most as Diptera, Isopoda, Lepidoptera, and Araneae. E. agrestis juveniles mature the following year. The prey spectra of these three spider species However, we were able to rear E. agrestis to appear to overlap broadly, even though maturity in just one summer in the laboratory, L. hesperus is capable of subduing much larger suggesting that under favourable conditions some prey compared with E. agrestis and E. atrica. early-emerging individuals may mature and mate Second, the three study species occupy the in their second year in the field. As a possible same microhabitats: the underside of driftwood consequence of the differences in life cycle logs. In late summer, when spider numbers were (biennial for E. agrestis, versus annual for at their peak, we frequently found all three species E. atrica), there could be substantial differences in under a single log, with L. hesperus inhabiting reproductive rates and annual survival between > 90% of logs surveyed at Island View Beach. the two species, especially if seasons vary across Salomon et al. (2010) documented the rapid years. For example, the delayed emergence of colonisation of experimentally introduced habitat E. agrestis might maximise survival under harsh logs by all three species at Island View Beach, winter conditions. An interesting direction for providing strong evidence that naturally occurring future research at the two study sites would be logs are a limited resource. Lastly, remains of to compare rates of reproduction and survival of E. atrica and E. agrestis spiders beneath

L. hesperus webs indicate that both Eratigena (Whitehouse and Lubin 2005). The observed species are at least occasional prey of L. hesperus multi-species aggregations may be the result of (Salomon 2011; this study). habitat constraints, forcing individuals to build their An alternative explanation for the similarity in webs in already occupied microhabitats. Despite the life-history characteristicsacrossstudysitesisthat possibility for increased competition or predation conspecificandheterospecific individuals exert risk in densely populated microhabitats, such similar competition and predation pressures. In this co-inhabitation might also be beneficial. Support case, community composition would have no for this possibility is evident in group-living species bearing on life-history traits. This intriguing possi- of spiders, where individuals sometimes spin a bility suggests potential equivalence of community communal web, share web-attachment points, or members in terms of their effect on the life-history take advantage of the proximity of others when traits we measured. However, net competition or capturing prey. Indeed, group living has been predation pressures likely differed between the two shown to confer individual benefits in spiders, sites because spider densities at Island View Beach including greater foraging and reproductive success exceeded those at Iona Beach fourfold (Table 1). (Whitehouse and Lubin 2005) and protection from Studying the costs of competition and predation for predators (Uetz et al. 2002a). It is not known these spiders at Island View Beach and Iona Beach whether E. agrestis and E. atrica spiders benefit would provide estimates of resource availability and from forming conspecific or heterospecificasso- spider mortality. Such a study could be coupled with ciations under driftwood logs. It is possible that the experimental manipulations of spider density or presence of several funnel-webs under a particular resource availability to highlight the potential effects log increases prey retention under logs. of interspecific competition on life history. Aggregations may also form when spiders Finally, it is possible that competition or seeking suitable microhabitats use the presence predation alters life-history traits other than those of other spiders as an indicator of site measured here. However, we focussed on several quality (Schuck-Paim and Alonso 2001). Spider key traits such as phenology, growth, and adult aggregations such as those encountered at our size, all of which are expected to be constrained study sites may persist if benefits offset the costs by resource availability (Toft and Wise 1999; of searching for and settling in a profitable Oelbermann and Scheu 2002) or predation risk. microhabitat (Jakob 1991; Bilde et al. 2007). Future studies should accurately quantify also A study of microhabitat selection patterns at fertility by measuring the number of egg sacs Island View Beach and Iona Beach would provide produced and spiderling recruitment. These traits explanations for (i) the high spider densities we are affected by feeding regimes (Spence et al. observed in our study and (ii) the apparently 1996), and may therefore vary in accordance with stable E. agrestis life history across strongly competition or predation pressures. The data varying community compositions. presented in this study suggest that the life-history traits we document are not affected by strong Acknowledgements differences in community composition of close competitors or predators. However, these data are Robb Bennett introduced us to the spider popu- limited to two sites, and future work should lations featured in the study. The authors are compare a larger number of suitable sites with grateful for the logistical support provided by the differing community compositions. Such work Bennett family. They thank Sean McCann for the would be especially valuable for understanding spider photographs, and Chris Buddle and two the potential impact of introduced Eratigena anonymous reviewers for constructive comments species on native arthropod communities. on the manuscript. This research was conducted under a permit from the Greater Vancouver Potential implications of group living Regional District (Iona Beach) and with the The high population densities of spiders sharing permission of the Tsawout First Nation (Island microhabitats at Island View Beach are striking, View Beach). Funding was provided by a Natural considering that most spider species are highly Sciences and Engineering Research Council of territorial and aggressive towards other spiders Canada (NSERC) – Discovery Grant and by an

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