Methods for: Control of arthropod abundance, richness, and composition in a heterogeneous desert city

The Phoenix metropolitan area is located in the Sonoran Desert (33degrees 30'N, 112degrees 10'W). Winter precipitation (December-March) averages by 82.5 mm (1954 - 2008, Phoenix Sky Harbor airport). Average annual air temperature is 22.7celsius (from 1948-2007). Summers are hot, with maximum temperatures reaching 40-50celsius, but winter temperatures have been as low as -8.3celsius (1950). In the last few decades, however, winter temperatures have rarely dropped below freezing for extended periods of time.

We chose nine sites for the manipulative experiments. Three desert sites were located in regional parks surrounding Phoenix (UP, WT and ES), three desert remnant sites were located within the city of Phoenix (DB and CS) or at the fringe of the city (SM), and three urban sites were located in human-altered landscapes, two of them located in elementary school yards (EX and ME) and one on the main campus of Arizona State University (SG). Desert habitats were relatively open landscapes with native perennial vegetation. Remnant habitats were also open landscapes and contained similar vegetation as desert, however, the two remnant habitats in the city were also close to non-native trees (e.g., Eucalyptus sp., Rhus lancea). The urban habitats were close to buildings and ornamental vegetation, most non-native to Arizona.

For our study we chose brittlebush, Encelia farinosa (Gray) (Asteracae), a native perennial shrub growing on uplands throughout the Sonoran and Mohave Deserts. Irrigated, it easily reaches 120 cm in height and width.

Experimental setup

On each site, 60 brittlebushes in five-gallon pots were spread out in a grid-like pattern, with 0.8 - 2.0 m between each plant. The different sites varied in area (UP~240 m2, ES~244 m2, WT~120 m2, SM~303 m2, CS~80 m2, DB~297 m2, ME~96 m2, EX~254 m2, and SG~72 m2, constrained by fences or buildings). The setup can be described as a split-plot design with habitat type as plots and all the other treatments completely randomized within each plot and replicated 3 times, for a total of 540 plants. The plants were obtained from local nurseries (seeds collected locally, all plants approximately seven months old). Plants were placed in the field in October-November each year to facilitate invertebrate establishment prior to first sampling in December. Half of the plants were enclosed in bird netting to exclude birds and other potential predators larger than the 1.9 cm mesh size. A cage consisted of three plant stakes positioned as an upside down cone enclosed with bird netting. The plants received three different water treatments; low, medium and high (levels determined in a trial season of 2006). Each plant received 2L supplemental water for 30 minutes at 6am and 2pm twice a week (low), four times per week (medium), and daily (high). Total supplemented water for 6 months was thus approximately low -192L, medium-384L and high-672 L. All the pots had drain holes, so the soil never remained saturated. Ambient precipitation per pot was calculated using pot diameter and mm precipitation; however, the supplemented water by far exceeded ambient precipitation both years. The plants were grown in pots (insulated, to moderate root temperatures of all plants since they were above ground) to exclude confounding effects of soil types in the desert versus the urban area. The pots contained soil consisting of 1/4 native top soil and 3/4 composted mulch, and two tablespoons of Osmocote 18-6-12 slow-release fertilizer to maintain a sufficient soil nutrient level the first year. The plants were about 7 months old, averaging 43.6 +/- 11.2 cm in height (average +/- SD) and 61.4 +/- 26.7 g in dry mass (estimated). In the second year, we re-used most of the plants and redistributed them randomly within the sites. Most of the plants were therefore older and taller the second year (57.1 +/- 20.3 cm, dry mass 139.2 +/- 93.6 g) and had a lower growth potential due to possible early root binding, reproductive maturity and less nutrients in the pot. Plant dry mass was estimated by measuring height and diameter monthly and compared to an equation developed from 360 plants from a trial season in 2006 (see Bang et al. 2010).

All desert and remnant sites were enclosed with poultry-netting fences to keep rabbits, ground squirrels and deer from feeding on the plants or chewing on the irrigation lines. All sites were visited once a week and fence and irrigation lines inspected. Arthropods were sampled monthly from December to May using a Vortis Insect Suction Sampler (Burkhart, UK) applied to each plant for ten seconds, covering approximately 60% of the outer foliage. This removes a standardized sample of the arthropod community inhabiting the plant, allowing remaining individuals to re-establish until next month. The arthropods were identified to family level and then further separated into morphospecies, and categorized into general feeding guilds based on various sources (e.g., Borror et al. 1989, Bartlett 2005). Herbivore damage was not systematically measured, but flowering phenology and pollinators were measured in a separate study (Neil 2008).