SEASONAL HABITAT SHIFT IN AN INTERTIDAL WOLF SPIDER: PROXIMAL CUES ASSOCIATED WITH MIGRATION AND SUBSTRATE PREFERENCE

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Abstract
During most of the year, the wolf spider Pardosa lapidicina Emerton 1885 occupies tidal cobble beaches surrounding Narragansett Bay, RI, USA, but in late autumn part of the population moves into adjacent forest litter to overwinter. We monitored these movements with drift fences and pitfall traps from 1996–1999 and evaluated the possible roles of ambient temperature, rainfall, humidity and storm events. We tested substrate choice over the season as a proxy for migratory tendency, both in the laboratory and the field, focusing on the roles of temperature and photoperiod. The timing of peak migration differed among years (S.D. = 15.5 d). Minimum weekly temperature, weekly rainfall, percent relative humidity and storm events did not explain the variation in migratory times. However, significantly more spiders migrated during weeks with below-freezing temperatures than in weeks without them. Leaf litter, which has less variable temperatures than beach cobble, may provide a refuge from extreme temperatures during winter. Spiders maintained at cold temperatures in laboratory experiments chose leaves over beach cobble significantly more often than did those in warm temperatures. The time of year that spiders were collected also influenced their probability of choosing leaf substrate in the laboratory. Photoperiod, on the other hand, did not significantly influence substrate preference. This study helps to uncover how environmental cues influence seasonal movements across a habitat boundary.