Assessing the importance of arthropod abundance, community composition, and habitat structure as determinants of habitat quality for Cactus Wren (Campylorhynchus brunneicapillus)

Background/Question/Methods

Cactus wren (Campylorhynchus brunneicapillus) in Southern California’s Coastal Sage Scrub (CSS) have experienced drastic population declines in recent decades. While habitat loss and fire are considered key drivers, populations within protected habitat are vanishing. Several lines of evidence, including greater nestling survival and brood size with food supplementation, suggest food limitation may be contributing to these declines in locations where fire has been absent. Food limitation might occur due to declining arthropod abundance and/or alteration of arthropod community composition that reduces its suitability as food.

To evaluate this hypothesis, we quantified arthropod abundance and community composition across eight cactus wren territories at three sites in Orange County, California.  Within territories, arthropods were sampled from nine different habitat components (including several native shrub species, exotic grassland, and bare ground) by vacuum, pitfall traps, and visual surveys.  Sampling was conducted during nest establishment and laying of eggs, feeding of nestlings, and the initiation of fledgling foraging.  All arthropods were identified to Order or Suborder and measured for biomass estimates.  Fecal samples were also collected to identify wren diet. Reproductive success of wrens was documented in the same eight territories and attempts were made to observe foraging patterns and prey capture.

Results/Conclusions

Arthropod abundance and community composition differed among sites, habitat components, and sampling time points.  Arthropod community composition also differed by sampling method, indicating that a single sampling method is inadequate to detect the larger community. Nearly two-thirds of all insects were non-native argentine ants (Linepithema humile) and aphids (Aphis spp.), although arthropods from over 20 different orders were represented across all samples.  Large arthropods (greater than 1 cm) were greatest on two native shrubs, California buckwheat (Eriogonum fasciculatum) and Mexican elderberry (Sambucus nigra), indicating they may be important components of wren diet.  Both annual, invasive habitat components, mustard (Brassica spp.) and annual grasses, hosted large numbers of arthropods during the initial sampling but abundances decreased dramatically during periods of nestling feeding when resource requirements are likely to be greatest.   This study of arthropods across multiple time points and spatial scales will permit subsequent tests of associations between arthropod communities, wren diet, and reproductive success.   Understanding the links between these should enhance our understanding of habitat quality for cactus wren and ultimately improve efforts to restore habitat and conserve the species.