Behavioral Ecology Advance Access originally published online on February 22, 2006
Behavioral Ecology 2006 17(3):405-409; doi:10.1093/beheco/arj053
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Fine-scale substrate use by a small sit-and-wait predator
Department of Ecology and Evolutionary Biology, Box G-W, Brown University, Providence, RI 02912, USA
Address correspondence to D.H. Morse. E-mail: d_morse{at}brown.edu.
Received 19 October 2005; revised 6 January 2006; accepted 15 January 2006.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Substrate choice is one of the most important decisions that sit-and-wait predators must make. Not only may it dictate the prey available but also the cover for the predator which may conceal it from prey or its own predators. However, while on a particular substrate the behavior and use of that substrate may vary widely. When naïve, newly emerged crab spiderlings Misumena vatia (Thomisidae) occupied flowering goldenrod Solidago canadensis, their behavior differed markedly on inflorescences with relatively sparse and densely packed flower heads as well as on experimentally thinned and unthinned inflorescences. Initially, the spiderlings most often hunted at the thinned sites and hid among the dense flower heads at the unthinned sites, a difference that disappeared in all broods tested after 2–3 h, possibly because of the growing hunger of the initially concealed individuals. Prey capture (dance flies) in the thinned sites initially significantly exceeded that in unthinned sites but subsequently did not differ. However, spiderlings encountered their principal predator, the jumping spider Pelegrina insignis, significantly more often on unthinned than thinned inflorescences. Even though usage patterns initially differed strikingly, spiderlings did not differ in their rates of quitting the two types of sites. These results suggest a trade-off between foraging and predator avoidance that changes in response to increasing hunger over time.
Key words: crab spider, foraging, Misumena vatia, predator avoidance, substrate use.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Foraging and predator avoidance are two of the most important selective factors confronting most animals at one or more stages in their lives (Dukas, 1998
; Morse, 1980). Although the ideal solution for at-risk individuals is to achieve success in both foraging efficiency and predator avoidance, the two may seldom if ever be maximized simultaneously. Usually predator avoidance involves behavior or habitat use that depresses food intake, growth, and potentially fitness (Downes, 2001; Lima and Dill, 1990; Sih, 1982). Individuals might maximize their success either by seeking sites that improve their position in this trade-off or adapt their behavior to the site currently occupied. If alternative patches are not available or dangerous to access, they must adjust their behavior to the site in which they find themselves. Although experienced foragers may make behavioral decisions on the basis of earlier encounters with predators at a site (Hugie, 2003; Jennions et al., 2003), totally naïve foragers, such as just-born young that fend for themselves without parental care, must initially depend entirely on their innate capabilities (Morse, 2000, 2005). Because mortality of young foragers is often particularly high (Curio, 1976; Morse, 1992), early behavioral decisions may comprise an important factor in their survival and may also even have a significant impact at a community level (Lima, 1998; Sinclair and Arcese, 1995) if the vulnerable individuals provide an important resource for the predators. It is thus of particular interest to investigate the newborns' behavior on a frequented substrate to determine if their performances vary in response to the conditions experienced and to establish whether their performance changes over time.
Newly emerged crab spiderlings Misumena vatia (Thomisidae) provide an excellent opportunity to test exploitation patterns on a homogeneous substrate, the flowering inflorescences of goldenrod Solidago canadensis, a favored foraging site. Although they emerge from their natal nests with some resources in their yolk sacs, the spiderlings must soon begin to feed or they will starve (Morse, 1993, 2000; Vogelei and Greissl, 1989). At the same time, allocation to predator avoidance is a particularly important problem for spiderlings because they are vulnerable to a wide range of predators on the flowers at this time, especially juvenile jumping spiders (ca. 1.5–10 mg) that frequent these sites (Morse, 1992). Once they leave their natal nests, the spiderlings receive no maternal protection (Morse, 1992).
While carrying out unrelated studies, I noted that spiderlings recently emerged from their natal nests captured large numbers of small dipteran prey on certain goldenrod clones on which I placed them but captured virtually no prey on other clones over a 2-h period. Spaces occurred between the flower heads of inflorescences on which spiderlings captured prey, allowing them to move freely between exposed hunting sites and sheltered areas in the midst of the inflorescences. However, flower heads were so dense on other inflorescences that after the spiderlings buried themselves, they did not access the surface where their prey were aggregated. This system thus provided the opportunity to evaluate the effect of fine-scale substrate characteristics on foraging behavior and change in this performance over time.
Therefore, I systematically investigated the role of flower-head density in determining the behavior of spiderlings on experimentally thinned and unthinned inflorescences of several clones, focusing on the following questions: (1) exposed to hunting sites with likely different foraging and predator-avoidance attributes, do naïve spiderlings routinely use these sites in a way that varies with the fine structure of the inflorescences? (2) Does this usage change over time? Naïve spiderlings prefer some flower substrates to others (Morse, 2000, 2005), but (3) do they exhibit preferences for thinned or unthinned goldenrod inflorescences that otherwise resemble each other?
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METHODS |
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Study area and subjects
I carried out this work at the Darling Marine Center of the University of Maine, South Bristol, Lincoln Co., Maine, USA (43° 57' N, 69° 33' W), in a 3.5-ha old field surrounded by mixed coniferous-deciduous forest. The field, mown yearly in October, contains several grasses (Gramineae) and forbs. The main forbs flowering during the period of this study, August, were goldenrods, asters (primarily Aster umbellatus), and wild carrot Daucus carota. The study area is described in further detail by Morse (2000)
. Goldenrods (Solidago spp.) are the numerically dominant forbs in the old fields and roadsides frequented by M. vatia (Thomisidae) in the vicinity of the study area. The commonest species in the study area and the one used in this study, S. canadensis, blooms from the latter half of July to early September at this site. S. canadensis is clonal and forms large clusters of up to 75 or more flowering stems, ranging in height from 0.75 to 1.5 m. Their pyramidal yellow terminal inflorescences, approximately 12–25 cm tall, consist of several horizontal branches, each bearing many (ca. 50–200) flower heads of approximately 2.5-mm diam and 4.5-mm height. A flower head is an urn-shaped structure containing large numbers of tiny flowers with a ring of ray flowers. Flower heads grow two to three abreast on the basal parts of the branches, but the distal parts contain only a single row of heads.
The tiny, second-instar spiderlings (0.4–0.7 mg) emerge from their nests in leaves of the vegetation about 26 days after egg laying (Morse, 1993) and move rapidly on lines in search of satisfactory dwelling sites. Due to the ubiquity of goldenrod at this time, many of them recruit to goldenrod, where they grow rapidly if small insect prey are abundant. If they fail to find a satisfactory site quickly, they usually balloon to an uncertain fate, which, given the prevalence of forest and water about the study area, will probably be an unsuccessful one (Morse, 1993).
I brought nests into the laboratory shortly before the spiderlings emerged and placed these nests in 7-dram vials (2.5-cm diam and 5-cm tall) at ambient temperature. The spiderlings emerged naturally from their nests. All spiderlings used in this study had emerged during the previous 1–3 days and none had previously fed. Neither had they experienced the substrates to which they were exposed in these experiments. However, at this stage, they routinely perform their full repertoire of substrate choice and dispersal behaviors (Morse, 2000). I selected spiderlings randomly from these broods.
The spiderlings fed primarily on small dance flies (Empididae sp.), which are periodically available in great numbers, such that at certain times a spiderling at a "fly" site may feed virtually ad libitum. These flies weigh 0.7–0.8 mg, and the spiderlings captured them with little apparent difficulty. In the absence of dance flies, the spiderlings fed primarily on other small, but less common, dipterans and thrips (Thysanoptera).
In that the spiderlings are sit-and-wait predators, it is important to define my use of "hunt" because they seldom preface an attack with searching or stalking tactics. Spiderlings were treated as hunting if their large raptorial forelimbs were spread widely, with the proximal segments roughly at right angles to the body, a pose illustrated in Hu and Morse (2004). In contrast, spiderlings noted as "hiding" held their forelimbs close to their body. Although this position might also characterize any spiderling not hunting, when in dense vegetation spiderlings could not spread their forelimbs to strike at prey.
Small jumping spiders are both the commonest and the most important predators of the spiderlings (Morse, 1992). Several species of jumping spiders occupy the goldenrod inflorescences. The commonest predator on the spiderlings was Pelegrina(=Metaphidippus) insignis, usually middle-instar juveniles (88% of the jumping spiders on goldenrod at this time: Morse DH, unpublished data). Juvenile P. insignis are older and larger than M. vatia spiderlings at this time, though as adults they are much smaller than penultimate and adult female M. vatia and subject to predation by them (Morse, 1992). Much smaller numbers of similar-sized juvenile Eris militaris and Evarcha hoyi (9 and 3% of the jumping spiders, respectively) also occupied goldenrod inflorescences in the study area and likely also captured M. vatia spiderlings, though my data on crab spiderlings killed are currently confined to P. insignis (Morse DH, unpublished data).
Procedures
I surveyed the density of goldenrod flower heads on branches of their inflorescences, randomly selecting fully flowering branches in the midst of inflorescences for analysis. I counted the number of flower heads on a branch and measured the length of the branch, permitting me to calculate substrate density. Ten branches from each of 10 clones were analyzed in this way.
Initially, I placed a total of 32 individuals on the inflorescences of two goldenrod clones for 2 h. The density of flower heads differed twofold. At that time, large numbers of dance flies were visiting the goldenrods.
To evaluate further the performance of spiderlings on inflorescences with different densities of flower heads, I placed 10 individuals per brood on a randomly chosen flowering branch of a goldenrod clone (unthinned) and 10 others from the same brood on a paired branch of the same clone from which I had removed alternate flower heads (thinned). After placement, I censused these sites at 10, 20, and 30 min to determine whether the spiderlings were hunting partly, or fully exposed, or sheltered under the flower heads and whether they had remained at the site on which they were placed. Thirteen broods were tested in this way.
To investigate the spiderlings' behavior over a longer period, I performed a similar experiment with censuses at 30 min, 1, 3, and 24 h, in which I again placed individuals on thinned and unthinned flowering branches of goldenrod inflorescences. As before, I used 10 sibs from the same brood on both unthinned and thinned branches, repeating this manipulation for 12 broods.
To evaluate the vulnerability of the M. vatia spiderlings to predation on different substrates, I used branches of goldenrod similar to the ones described above for this substrate-use experiment. I ran these branches in pairs, with one branch thinned to approximately two-thirds its normal density and the other branch unaltered. I added five newly emerged second-instar spiderlings to both branches, a density not infrequently found on sites near a nest. Five minutes later, I introduced a middle-instar P. insignis to both branches. Tests were run for 15 min, a period reflecting the time that a searching jumping spider of this size would likely remain on or in the vicinity of a goldenrod inflorescence branch. I recorded contacts or near contacts between jumping spiders and crab spiderlings, including predation, hunting method (roving and sit-and-wait), and evasive behavior of the spiderlings. I used different individuals and different goldenrod branches for each test. Spiderlings used in this experiment came from five broods and were randomly assigned, except that members of the same brood were used for each paired run on both thinned and unthinned branches. Jumping spiders were collected from goldenrod inflorescences in the field, using individuals ranging between 2.5 and 5.0 mg, the commonest size range present. Jumping spiders run on thinned and unthinned branches were paired for similar size in each replicate.
I used JMP Version 3.1 (SAS Institute, Cary, North Carolina, USA) to calculate the repeated measures multivariate analyses of variance (MANOVAs). Null hypotheses were rejected at p <.05. Means are accompanied by 1 SD (±SD).
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RESULTS |
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Lengths of branches, numbers of flower heads on a branch, and resultant density of flower heads differed markedly among representative S. canadensis branches from 10 randomly chosen clones in the study area. More than twofold range in length of the branches (3.3–7.9 cm range, mean = 5.3 ± 1.4 cm) is a consequence of both differences in inflorescence size and position within an inflorescence, which I did not explore systematically. Density of flower heads (number/branch length) differed over threefold (10.4–34.8 cm–1, mean = 19.4 ± 7.38 cm–1) and was not correlated with branch length (rs = .209, p > .5 in a two-tailed Spearman rank correlation coefficient).
Only spiderlings placed on the low-density clone captured prey during the initial observations (G1 = 15.42, n = 12, 20; p < .001, G test). In contrast to burying themselves in the clone with densely packed flower heads, spiderlings on the less dense inflorescences occupied the openings among the flower heads, where they were in intimate contact with the prey. Similar numbers of dance flies were present on the inflorescences of the two clones (19.3 ± 8.2 flies/inflorescence on the clone with no captures, 18.3 ± 7.9 on the clone with captures; six censuses with 10 inflorescences/census; T = 13; p > .6 in a two-tailed Wilcoxon matched-pairs signed-rank test), eliminating the likelihood of these differences in capture rates being driven by prey numbers.
In the short-term experiment, individuals on the thinned branches of goldenrod inflorescences were significantly more likely to hunt actively than were those on the unthinned sites (Figure 1). This difference appeared clearly at the first census, 10 min after placement and remained relatively constant at 20 and 30 min. Numbers of hunting individuals remained stable over this period, about twice as high in the thinned sites as the unthinned ones (Figure 1). Individuals initially hunting tended to remain in hunting position throughout the 30-min period. Proportions of hunting individuals were significantly higher in the thinned sites over this period (F1,24 = 7.70, p = .01, repeated measures MANOVA with an arcsin transformation) but did not differ between broods (F2,23 = 0.10, p > .9, same test). In contrast to the original set of observations, few potential prey were present on the goldenrod during the short-term experiment. A single small dipteran was caught during both the experimental and control runs.
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Less than one-quarter of the individuals had left these sites by the end of the monitoring period (22.3% for the thinned sites and 19.2% for the unthinned sites). Neither the cumulative departures (Z = 0.408, p > .6 in two-tailed binomial test) nor the numbers gone at any of the census periods at the two types of sites (Z = 0.042–0.750, p > .4–.9, same tests) were significantly different.
Although the second, longer term, test confirmed that individuals on thinned goldenrod branches initially were significantly more likely to hunt at the surface of the inflorescences than those on the unthinned sites (at both 30 min and 1 h, Figure 2), this difference disappeared over time, with the results at 3 and 24 h revealing no differences in hunting/hiding behavior between the two types of sites (Figure 2). Increased numbers of individuals on the unthinned sites emerged to hunting positions in the 3- and 24-h censuses, such that the overall results from the unthinned and thinned sites did not differ (F1,22 = 0.87, p > .3, repeated measures MANOVA with arcsin transformation). As before, no significant differences occurred among the broods (F3,20 = 0.73, p > .5, same test). Roughly half of these individuals had left their sites at the end of 24 h (55.0% for the thinned sites and 48.3% for the unthinned sites). Neither the cumulative departures (Z = 0.629, p > .5 in two-tailed binomial test) nor the numbers gone at any of the census periods (Z = 0.147–0.641, p > .5–.8, same tests) differed significantly at the two types of sites.
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In contrast to the short-term experiment, considerable numbers of potential prey, mostly dance flies, were present during the longer experiment. The spiderlings caught significantly more prey over the period that behavior differed in the thinned sites (14) than at the unthinned sites (5) (p = .03 in a one-tailed binomial test), but no difference in prey capture occurred between these sites after the spiderlings' behavior at the two sites ceased to differ (p > .3, same test; thinned = 24, unthinned = 32).
Jumping spiders came in direct contact with 1.7 ± 0.21 spiderlings per run (total = 26 contacts) on the unthinned goldenrod before the spiderlings could bury themselves among the flower heads, compared to no recorded contacts on the thinned goldenrod (T = 0, n = 15, p < .001 in a two-tailed Wilcoxon matched-pairs signed-rank test). Three of these contacts resulted in captures, and in six other instances, spiderlings exhibited highly evasive behavior, jumping out of the vegetation and running, or hanging on lines below the vegetation for 2 min or more. More than six times as many spiderlings temporarily occupied the surface of unthinned goldenrod as thinned goldenrod when the jumping spiders were introduced (3.2 ± 0.17 versus 0.5 ± 0.16 [T = 0, n = 15, p < .001, same test]), a consequence of the time required for those on the unthinned sites to bury themselves within the dense flower heads, providing many more opportunities for jumping spiders to contact the spiderlings in the open. Most of these contacts took place over the first 5 min after the introduction of the jumping spiders (21 during 0–5 min and 5 during 5–15 min), before many of the spiderlings had disappeared amid the flower heads, a significant difference (Z = 2.942, p <.01 in two-tailed binomial test). Behavior of the jumping spiders differed markedly on the two substrates, with 10 out of the 15 constantly searching on the unthinned goldenrod and only one of 15 on the thinned goldenrod doing so (G = 12.99, df = 1, p < .01 in a G test).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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I designed these experiments to test differences in substrate use associated with subtle differences in substrate structure. Although substrate preferences are clearly demonstrated in patch-choice studies of a wide range of systems (Pyke, 1984
; Stephens and Krebs, 1986), the behavioral differences in this study demonstrated the ability of the naïve spiderlings to respond to differences at a finer scale than that usually investigated. The innate response of the spiderlings to flower-head density suggests that members of this population have regularly encountered fine-scale differences in substrate, as does the similar response of the different broods, including the time at which individuals on dense inflorescences shifted to hunting, a risky behavior under the conditions experienced. The spiderlings' similar rates of leaving the thinned and unthinned sites in both experiments strongly suggest a lack of preference for one inflorescence type over the other, in spite of their initial success in capturing prey on the thinned inflorescences and the danger of hunting on dense inflorescences. This pattern of substrate acceptability resembles that of their dance fly prey; however, the similar performance of the spiderlings in the short-term experiment, when prey were largely absent, indicated that the spiderlings' behavior was independent of their prey. Given the similar availability of dance fly prey at both thinned and unthinned sites, one would predict a greater rate of leaving sites with jumping spiders than sites without them (see Dukas, 1998; Lima, 1998). Thus, the basis for the similar rate of leaving thinned and unthinned sites is unclear.
Few efforts have been made to evaluate responses of foragers to such fine structural differences in their habitat (Romero and Vasconcellos-Neto, 2005). The differences in substrate use involved behavior that facilitated both prey capture and predator avoidance (see Dukas, 2001a,b). Although the spiderlings responded to the subtle substrate differences, their dance fly prey apparently did not discriminate between the two sites, at least in ways that affected their abundance. Thus, the spiderling predators exhibited a more discriminating response to subtle differences in their substrate than did their prey. Such a predator-prey relationship is perhaps unexpected in a simple food-chain analysis, where specialization of the prey would be predicted to exceed that of the predator (Hairston et al., 1960) but is more likely to occur in sit-and-wait predators like M. vatia than in widely ranging predators (Rosenheim et al., 2004).
Crab spiderlings were particularly vulnerable when on top of the branches, as reflected by the high contact frequency of just-released spiderlings with jumping spiders on the unthinned sites. Jumping spiders were much less likely to burrow into unthinned than thinned branches and did not search intensively in the thinned branches, instead usually taking up their own sit-and-wait predatory mode similar to those of the spiderlings occupying these sites. Thus, they exhibited a foraging strategy that differed with the grain of the substrate and resembled that of the spiderlings. The perceived grain of this substrate should vary with the size of these jumping spiders, with resultant consequences for the crab spiderlings; however, I did not explore this aspect of their relationship.
The similarity of hunting patterns on thinned and unthinned sites at 3 and 24 h raises the question of whether the initial differences in behavior on thinned and unthinned inflorescences had a long-term effect because this difference lasted for only a short time, perhaps as little as 2 h. Clearly, the spiderlings' response to burrow into the unthinned inflorescences provided them with more protection than if they occupied the surface of an inflorescence. At-risk potential prey suffer higher mortality in unfamiliar circumstances (Altmann, 1998; Morse, 1980), and avoidance behavior should assume the greatest value at this time. However, their subsequent movement into the open, presumably in response to hunger, suggests a limit to this behavior and a trade-off between foraging and predator avoidance. As a result, these individuals should also incur a high predation rate, as strongly suggested by the jumping spider experiments.
The experimental manipulations effectively reflected the significance of fine-scale substrate differences on the decision-making and exploitation patterns of the spiderlings. The mean density of S. canadensis flower heads along an inflorescence limb (19.4 flower heads cm–1) considerably exceeds that of another common goldenrod in the study area, Solidago juncea (9.4 flower heads cm–1: Morse DH, unpublished data), that on average blooms slightly earlier, but whose flowering period overlaps extensively with S. canadensis. A common later flowering, but temporally overlapping, species, Solidago rugosa, has a flower-head density similar to S. canadensis (17.3 cm–1: Morse DH, unpublished data). Thus, one might predict an initially quantitatively different foraging pattern on S. juncea than on S. canadensis or S. rugosa, which would primarily affect the earliest emerging spiderlings. Spiderling emergence peaks during the flowering of S. canadensis, the commonest of the goldenrods, after the peak for S. juncea, and before the peak for S. rugosa, while the latest spiderlings emerge simultaneously with S. rugosa. Critical substrates available to the spiderlings thus not only vary spatially but also temporally in their physical characteristics.
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ACKNOWLEDGEMENTS |
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I thank W. Brim-DeForest and T. Jones who assisted in the field. R. Lutzy and R. Neff offered useful comments on an earlier draft of the manuscript. G.B. Edwards identified the jumping spiders. I also thank K. Eckelbarger, T.E. Miller, and other staff members of the Ira C. Darling Marine Center of the University of Maine for facilitating the fieldwork on their premises. Partially supported by the National Science Foundation IBN98-16692.
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