Data from: Mixed responses of tenebrionid beetles to solar energy development in the Mojave Desert

Data files for manuscript titled "Mixed responses of tenebrionid beetles to solar energy development in the Mojave Desert".

Contains 2 .csv files: Metadata and Master Data that contains plot IDs, abundance, richness, Shannon values, and species composition.

Abstract from paper:

Solar energy development causes land-use change and habitat alteration that may affect desert ecosystems. Tenebrionid beetles have evolved to exploit desert environments and heavily contribute to ecosystem functionality in aridlands, yet their species-specific, ecological responses to solar energy development are unknown. Our objective was to elucidate effects of solar energy development decisions at a solar power facility (392 MW) on a tenebrionid beetle community in the Mojave Desert. Seven years post-construction, we used pitfall traps to collect tenebrionid beetles for one month in treatments representing variably intense site preparation practices and conservation measures, including blading (i.e., bulldozing), mowing, and establishment of undeveloped patches in solar fields, replicated across three power blocks comprising the facility and in undeveloped control sites surrounding the facility. Blading caused significant, deleterious effects on tenebrionid beetle abundance, species richness, and diversity. Ordination revealed apparent overlap of non-bladed treatments and controls, suggesting that intermediate levels of disturbance from less intensive solar energy development decisions may serve to minimize negative effects of solar energy development on tenebrionid communities. Anepsius delicatulus and Triorophus laevis, the two most commonly collected tenebrionids, were significantly more abundant in non-bladed treatments in the solar facility than in undeveloped controls; thus, solar facilities with minimized disturbance to vegetation may provide preferred habitat for certain tenebrionid species. Our results demonstrate the value of understanding variable responses of insects to solar energy development decisions in deserts to inform real-world adaptive management of natural resources during the renewable energy transition.

Methods from paper:

Study area

We conducted the study at a concentrating solar power facility (Ivanpah Solar Electric Generating System) and surrounding undeveloped desert scrubland. The facility has a gross capacity of 392 MW; it consists of 173,500 heliostats (347,000 individual mirrors) and three power towers, comprising ~1289 heliostat-covered hectares. The facility is located at the base of Clark Mountain, San Bernardino County on a bajada at elevations ranging from 855 m to 1075 m in the Mojave Desert of California, United States. The facility was constructed in 2011 on a 1400-ha tract of previously undeveloped Mojave Desert creosote bush scrub in the Ivanpah Valley near the Mojave River corridor, the Mojave National Preserve, and Mesquite and Stateline Wildernesses. The Ivanpah Valley is geologically characterized by piedmonts, intersecting active and inactive alluvial fans and channels, and terminal playas. Ivanpah Valley is a hot desert climate under the Koppen classification.

Experimental design

Within the facility, we designated each of the three power blocks (i. e., tower and associated heliostats) as replicated blocks. The area of each block is 3.66 km2, 4.33 km2, and 4.90 km2, respectively. Developers sited each block along the bajada; therefore, blocks shared similar attributes to one another and surrounding undeveloped desert. We defined treatments in each block representing three unique solar energy development decisions as follows: (1) bladed, intensive site preparation via blading (bulldozing) with above- and belowground biomass removed; (2) mowed, moderate site preparation intensity via mowing, aboveground biomass retained up to a height of ~0.30 m; and (3) “halo”, a pre-construction, plant-conservation decision that designated buffer zones around rare desert plants within the solar fields at the facility, which were roped off and left undisturbed (i.e., no site preparation, no heliostats), creating isolated habitat patches (average area =22 m2). Bladed treatments had shade from heliostats and little vegetation cover, mowed treatments had intermediate shading from heliostats and regrowth of desert shrubs but no cacti or Mojave yucca (Yucca shidigera), halo treatments had limited shading from heliostats and intact plant species structure, cover, and composition (e.g., shrubs and cacti) similar to controls. Individual heliostats track the sun throughout the day, creating spatiotemporally variable shade patterns on the ground.

We randomly established 15 plots in each of the three treatment units in blocks (five plots per treatment per block, treatment plots =45). We designated 15, replicated controls in undeveloped creosote bush scrub immediately surrounding the blocks that comprised the facility, creating a total of 60 plots; one plot in mowed and halo treatments, respectively, and two plots in controls were compromised (e. g., uprooted by an animal) and thereby excluded from the study. We situated control plots along five transects randomly selected from a set of superimposed grid points laid over a map of the facility with spatial analysis software (QGIS). Each transect contained three plots located at 250 m, 500 m, and 1 km from the boundary of the nearest block. We chose the upper limit of plot distances from landscape features based on visual analysis of satellite imagery, coupled with distance measurements taken with the “distance matrix” tool in QGIS; our assessment indicated that plots established at a distance greater than 1 km from the facility would be confounded by other landscape features potentially affecting the distribution of beetles, including Clark Mountain to the north (e.g., elevational plant community shift) and a golf course and highway to the south.

Tenebrionid beetle sampling

We sampled tenebrionids seven years after the construction of the facility. We used pitfall traps to sample tenebrionids at 56 plots in the facility and control sites during one, continuous sampling period from April 5th to May 5th, 2018, encompassing a large portion of peak spring growing season for the year within adult activity periods. Our sampling effort was spread across over 9.5 km of the Ivanpah Valley, sufficiently overcoming natural heterogeneity in tenebrionid distributions concurrently with ants and non-bee insect flower visitors. Pitfall trapping has been deemed a reliable approach for sampling beetles in desert environments. Pitfall traps consisted of 0.47-L plastic containers with a diameter of ~8.5 cm filled with equal amounts of propylene glycol and water plus a drop of liquid dish soap to reduce surface tension. We placed the lip of each container at or slightly below ground level. We covered each pitfall trap with an 81-cm2 roof constructed from corrugated plastic and staked 5 cm into the ground with 14-gauge baling wire to prevent rain from flooding the pitfalls and to minimize evaporation of the liquid within the pitfalls. Our sampling plots consisted of four pitfall traps situated at each corner (cardinal directions) of a 2-m × 2-m square centered on each plot (224 pitfall traps in total). At the conclusion of the sampling period, we collected tenebrionids from each of the four pitfall traps, combined them into one 60-ml Nalgene® bottles representing each plot labeled with plot locality data, and stored specimens in 70% ethanol. We identified tenebrionids using taxonomic keys, regional checklists, and images and observations taken of type specimens from museums around the world. We submitted voucher specimens of identified beetle species to the collection at the Hasbrouck Insect Collection at Arizona State University.