Patterns of shrub-dwelling arthropod diversity across a desert shrubland-grassland ecotone: a test of island biogeographic theory

This study tested the applicability of Island Biogeographic Theory to arthropod assemblages on ‘islands’ of creosotebush (Larrea tridentata) across a desert shrubland-grassland ecotone in central New Mexico, U.S.A. We assumed that herbivorous arthropod taxa that specialize on creosotebush view these shrubs as ‘islands’ of suitable habitat in a ‘sea’ of grassland, and used the Island Biogeographic Theory to examine the observed plant and arthropod patterns. Specifically, we addressed predictions concerning the relationship among arthropod species richness and (1) shrub ‘island’ area, (2) distance between ‘island’ and ‘continent’, and (3) ‘island’ isolation. Using a pyrethrum insecticide, we defaunated 36 shrubs within three ecotonal areas (12 shrubs per area), defined as ‘continental’ (high shrub density), ‘archipelago’ (medium shrub density, close to the ‘continent’), and ‘oceanic’ (low shrub density, far from the ‘continent’). We assessed the initial arthropod distribution and then repeated the defaunation after two weeks to assess immigration patterns. The arthropods from both samples were catalogued and related to ‘island area’ (shrub volume), ‘island’ distance from the defined ‘continent’, and ‘island’ isolation (density). We found that in the initial sample, specialist herbivore species richness was (1) positively related to shrub volume, (2) negatively related to shrub distance from the ‘continent’, but (3) not significantly related to shrub density. Following the recolonization period, we found that specialist herbivore species richness was again significantly positively related to shrub volume, but was not significantly related to shrub distance from the ‘continent’ or to shrub density. We also examined the relationship between generalist predator arthropod (spiders and mantids) populations and shrub ‘island’ characteristics as well as specialist prey abundances on the shrubs. We found that predator abundances on individual shrubs were positively related to shrub volume prior to and following defaunation, negatively related to shrub density both initially and following recolonization, and positively related to the architectural complexity of host shrubs initially. We also found that predator population sizes on shrubs were positively correlated with abundances of specialist prey species. We concluded that our model conformed fully to the species richness–island area and species richness–island distance relationships of Island Biogeographic Theory, but only in part to the species richness–island isolation relationship. We also found that species-specific dispersal ability was a critical factor with regard to the observed species distribution patterns.