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Fine-Scale Substrate Use by a Small Sit-And-Wait Predator

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Fine-Scale Substrate Use by a Small Sit-And-Wait Predator Behavioral Ecology doi:10.1093/beheco/arj053 Advance Access publication 22 February 2006 Fine-scale substrate use by a small sit-and-wait predator Douglass H. Morse Department of Ecology and Evolutionary Biology, Box G-W, Brown University, Providence, RI 02912, USA Downloaded from https://academic.oup.com/beheco/article/17/3/405/202064 by guest on 28 September 2021 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 naı¨ve, newly emerged crab spiderlings Misumena vatia (Thomisidae) occupied flowering goldenrod Solidago canadensis, their behavior differed markedly on inflorescences with rela- tively 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. [Behav Ecol 17:405–409 (2006)] oraging and predator avoidance are two of the most im- patterns on a homogeneous substrate, the flowering inflores- Fportant selective factors confronting most animals at one cences of goldenrod Solidago canadensis, a favored foraging or more stages in their lives (Dukas, 1998; Morse, 1980). site. Although they emerge from their natal nests with some Although the ideal solution for at-risk individuals is to resources in their yolk sacs, the spiderlings must soon begin to achieve success in both foraging efficiency and predator feed or they will starve (Morse, 1993, 2000; Vogelei and avoidance, the two may seldom if ever be maximized simul- Greissl, 1989). At the same time, allocation to predator avoid- taneously. Usually predator avoidance involves behavior or ance is a particularly important problem for spiderlings be- habitat use that depresses food intake, growth, and poten- cause they are vulnerable to a wide range of predators on tially fitness (Downes, 2001; Lima and Dill, 1990; Sih, 1982). the flowers at this time, especially juvenile jumping spiders Individuals might maximize their success either by seeking (ca. 1.5–10 mg) that frequent these sites (Morse, 1992). Once sites that improve their position in this trade-off or adapt they leave their natal nests, the spiderlings receive no maternal their behavior to the site currently occupied. If alternative protection (Morse, 1992). patches are not available or dangerous to access, they must While carrying out unrelated studies, I noted that spider- adjust their behavior to the site in which they find them- lings recently emerged from their natal nests captured large selves. Although experienced foragers may make behavioral numbers of small dipteran prey on certain goldenrod clones decisions on the basis of earlier encounters with predators at on which I placed them but captured virtually no prey on a site (Hugie, 2003; Jennions et al., 2003), totally naı¨ve for- other clones over a 2-h period. Spaces occurred between the agers, such as just-born young that fend for themselves with- flower heads of inflorescences on which spiderlings captured out parental care, must initially depend entirely on their prey, allowing them to move freely between exposed hunting innate capabilities (Morse, 2000, 2005). Because mortality sites and sheltered areas in the midst of the inflorescences. of young foragers is often particularly high (Curio, 1976; However, flower heads were so dense on other inflorescences Morse, 1992), early behavioral decisions may comprise an that after the spiderlings buried themselves, they did not ac- important factor in their survival and may also even have a cess the surface where their prey were aggregated. This system significant impact at a community level (Lima, 1998; Sinclair thus provided the opportunity to evaluate the effect of fine- and Arcese, 1995) if the vulnerable individuals provide an scale substrate characteristics on foraging behavior and change important resource for the predators. It is thus of particular in this performance over time. interest to investigate the newborns’ behavior on a fre- Therefore, I systematically investigated the role of flower- quented substrate to determine if their performances vary head density in determining the behavior of spiderlings on in response to the conditions experienced and to establish experimentally thinned and unthinned inflorescences of sev- whether their performance changes over time. eral clones, focusing on the following questions: (1) exposed Newly emerged crab spiderlings Misumena vatia (Thomisi- to hunting sites with likely different foraging and predator- dae) provide an excellent opportunity to test exploitation avoidance attributes, do naı¨ve spiderlings routinely use these sites in a way that varies with the fine structure of the inflor- Address correspondence to D.H. Morse. E-mail: d_morse@brown. escences? (2) Does this usage change over time? Naı¨ve spider- edu. lings prefer some flower substrates to others (Morse, 2000, Received 19 October 2005; revised 6 January 2006; accepted 15 2005), but (3) do they exhibit preferences for thinned or January 2006. unthinned goldenrod inflorescences that otherwise resemble each other? Ó The Author 2006. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: [email protected] 406 Behavioral Ecology METHODS Pelegrina(¼Metaphidippus) insignis, usually middle-instar juve- niles (88% of the jumping spiders on goldenrod at this time: Study area and subjects Morse DH, unpublished data). Juvenile P. insignis are older I carried out this work at the Darling Marine Center of the and larger than M. vatia spiderlings at this time, though as University of Maine, South Bristol, Lincoln Co., Maine, USA adults they are much smaller than penultimate and adult fe- (43° 57# N, 69° 33# W), in a 3.5-ha old field surrounded by male M. vatia and subject to predation by them (Morse, 1992). mixed coniferous-deciduous forest. The field, mown yearly in Much smaller numbers of similar-sized juvenile Eris militaris October, contains several grasses (Gramineae) and forbs. The and Evarcha hoyi (9 and 3% of the jumping spiders, respec- main forbs flowering during the period of this study, August, tively) also occupied goldenrod inflorescences in the study were goldenrods, asters (primarily Aster umbellatus), and wild area and likely also captured M. vatia spiderlings, though carrot Daucus carota. The study area is described in further my data on crab spiderlings killed are currently confined to detail by Morse (2000). P. insignis (Morse DH, unpublished data). Goldenrods (Solidago spp.) are the numerically dominant Downloaded from https://academic.oup.com/beheco/article/17/3/405/202064 by guest on 28 September 2021 forbs in the old fields and roadsides frequented by M. vatia (Thomisidae) in the vicinity of the study area. The common- Procedures est species in the study area and the one used in this study, S. canadensis, blooms from the latter half of July to early I surveyed the density of goldenrod flower heads on branches September at this site. S. canadensis is clonal and forms large of their inflorescences, randomly selecting fully flowering clusters of up to 75 or more flowering stems, ranging in height branches in the midst of inflorescences for analysis. I counted from 0.75 to 1.5 m. Their pyramidal yellow terminal inflores- the number of flower heads on a branch and measured the cences, approximately 12–25 cm tall, consist of several hori- length of the branch, permitting me to calculate substrate zontal branches, each bearing many (ca. 50–200) flower heads density. Ten branches from each of 10 clones were analyzed of approximately 2.5-mm diam and 4.5-mm height. A flower in this way. head is an urn-shaped structure containing large numbers of Initially, I placed a total of 32 individuals on the inflores- tiny flowers with a ring of ray flowers. Flower heads grow two cences of two goldenrod clones for 2 h. The density of flower to three abreast on the basal parts of the branches, but the heads differed twofold. At that time, large numbers of dance distal parts contain only a single row of heads. flies were visiting the goldenrods. The tiny, second-instar spiderlings (0.4–0.7 mg) emerge To evaluate further the performance of spiderlings on in- from their nests in leaves of the vegetation about 26 days after florescences with different densities of flower heads, I placed egg laying (Morse, 1993) and move rapidly on lines in search 10 individuals per brood on a randomly chosen flowering of satisfactory
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