Web Relocation and Habitat Selection in Desert Widow Spider

Abstract

We investigated web relocation and habitat selection in Latrodectus revivensis (Theridiidae), a widow spider that constructs webs on scattered shrubs in the Negev desert. We used repeated observations of individual spiders' habitat preferences, movements to new web sites, growth, fecundity, and survival under natural and manipulated habitat conditions to examine relationships between movement, habitat, and components of fitness. Our main objectives were to determine the spatial and temporal patterns of movement, and the causes, costs, and fitness consequences of shifting habitat. Repeated censuses of individually marked females showed that web relocation in L. revivensis is associated with spider growth. Spiders tended to move larger shrubs after one or two molts, and the size of web components and web—site features scaled to body size. Moves occurred mainly in Spring (March—May), and most moves (74%) were by juveniles. The main cost of web relocation was increased mortality: there was a 40% chance of death during a move. A comparison of occupied shrubs with those available in the habitat indicated strong site selection involving seasonally varying and age—dependent preferences for particular shrub species, and a general preference for taller shrubs. Prey availability at shrubs (as measured by pitfall traps, and by prey remains in nests) varied spatially and was affected by shrub species and size. However these effects were small compared to habitat—wide temporal variation in prey availability, suggesting that web—site relocations would not result in substantially higher prey capture. Nonetheless, movement to larger shrubs resulted in improved spider body condition and, ultimately, greater reproductive success. The advantages of web—site relocation in this species may relate to architectural features of shrubs that provide support for the web, and minimize exposure by thermal extremes and to predators. However, experimental manipulation of exposure by trimming shrubs did not decrease spider fitness. We suggest that web—site selection in this species is less precise than expected because of the risk of dying during a move.