A test of habitat selection at two spatial scales in a sit-and-wait predator : a web spider in the Namib Desert dunes

1. Distribution and web characteristics were examined for the dune-burrowing spider Seothyra henscheli in the Namib Desert to determine how these spiders relate to spatially heterogeneous habitat features that influence foraging costs and benefits, growth rate and survival. In particular, we set out to determine the relative importance of (i) habitat selection, (ii) site-dependent growth and survival, and (iii) restricted dispersal, to the occurrence of S. henscheli in different macro- and microhabitats. 2. Spider webs were mapped along transects that crossed dune regions with dense vegetation hummocks, steep dune plinths and bare, gravelly interdune plains. Spiders were more abundant in the hummocks than in the interdunes and plinths. Translocation of spiders from the hummocks to the interdunes and plinths confirmed that spider density was correlated with habitat quality. 3. In a comparison of web sites with dummy sites in terms of factors related to sand stability and food availability, it was found that, except for the avoidance of extreme conditions, web-site characteristics did not differ from dummy sites. Web geometry indicated that good sites were characterized by stable sand and abundant ants. However, many spiders occurred at poor sites when good sites were vacant nearby. 4. It is concluded that S. henscheli do not actively choose web sites, but have restricted dispersal. Site-dependent growth and survival may explain the pattern of local abundance. Site tenacity may be a result of the spiders' inability to predict site quality coupled with high costs of relocation. It is inferred that spiderlings tend to remain near their mother's site, which has a previous record of success. 5. Tolerance of variable conditions of dunes and the ability to sustain populations even at poor sites enable these spiders to occur in extreme desert conditions.