Susan A. Foster
Trends in Ecology & Evolution, 1999, 14:5:190-195
Box 2. Spider-system test for local adaptive equilibria in fitness-linked behavioural trait
Optimal foraging theory and models of evolutionarily stable strategies (ESS) have been applied to population comparisons of fitness-linked behavioural traits in the desert spider, Agelenopsis aperta14. Spiders occupying arid habitats, such as desert grassland and desert scrub, conformed to optimal foraging predictions of a broad diet under food limitation. The ESS predictions of escalated fighting over exclusive use of limited foraging sites that also permit survival to reproduction were met in these populations. Observed behaviour deviated from the predicted phenotype in one population that occupied a narrow strip of riparian (woodland) habitat on either side of a spring-fed stream in the Sonoran Desert (USA). The site was surrounded by more arid evergreen woodland and desert scrub habitats. Because desert riparian habitats provide protection from temperature extremes and thus allow longer foraging bouts and higher prey availabilities, optimality models make the following two predictions: (1) that spiders occupying these habitats will have a narrow diet that incorporates only those food types that are most profitable (highest gain:cost ratio); and (2) that they exhibit only conventional (display) behaviour in the resolution of contests over foraging sites and associated territories. Although this 'riparian' behavioural phenotype was observed in spiders occupying a broad expanse of riparian habitat at the eastern extent of the A. aperta species range, the population occupying the restricted riparian habitat exhibited a breadth of diet and levels of fighting that were not predicted. Electrophoretic analyses of population genetic structure, drift-fence monitoring of spider movement between habitats and field experimentation have demonstrated that gene flow from surrounding arid-adapted populations prevents this island population of riparian A. aperta from adapting to local conditions.