Data and code for: Moisture and competition constrain ephemeral resource quality for burying beetle reproduction

Shifts in abiotic factors such as temperature and moisture can change the availability of resources, especially under climate change. Both abiotic and biotic drivers can have profound, rapid effects on species distribution, survival, and reproduction. Little is known about how abiotic factors affect the availability of ephemeral resources. Burying beetles (Nicrophorus spp.) are specialist users of ephemeral resources, as their reproduction requires locating, defending, and burying a small carcass. Environmental moisture, such as coastal fog, could change how quickly carcasses dry out. We tested the role of carcass moisture and interspecific competition with a generalist scavenger, Heterosilpha spp., on reproduction by placing pairs of Nicrophorus guttula in field chambers with control and experimentally dehydrated mouse carcasses. Pairs that were given control of mouse carcasses were more likely to carry out reproductive behaviors and produce viable offspring than pairs that were given a partially dehydrated mouse. For those pairs that reproduced, competition limited the number of offspring. These results indicate that shifts in abiotic factors under climate change, along with biotic factors like competition, can reduce the availability of ephemeral resource patches for consumers.

Methods
Site. We conducted this experiment at the Bodega Marine Reserve in Bodega Bay, California (38.319061, -123.072443) between June and September 2022. The site is composed of coastal prairie, dunes, and rocky intertidal habitat. Summers are mild (average daily minimum temperatures are 9-14 degrees C, with average daily maximum temperatures 15-20 degrees C), with fog often present early in the mornings (mean relative humidity is 85-100%) (WRCC 2024). The coastal prairie is primarily composed of species such as bush lupine (Lupinus arboreus), seaside daisy (Erigeron glaucus), woolly sunflower (Eriophyllum spp.), and ice plant (Carpobrotus spp.) (Barbour et al. 1973).

Trapping beetles. We set up pitfall traps baited with mouse carcasses in the coastal prairie to collect N. guttula and Heterosilpha spp. for experiments (Figure 1B, Figure 1C, Appendix S1: Section S1). N. guttula is our focal species (Appendix S1: Section S1), a burying beetle that requires a small carcass that it buries belowground in order to reproduce. The Heterosilpha beetles are our competitors, as they will feed from carrion and other detritus, but do not bury carcasses. Most of the Heterosilpha beetles were likely to be H. ramosa, as H. aenescens is rare at this site. We will refer to “Heterosilpha spp.” as just “Heterosilpha” in the remaining text. We caught most of the beetles used in this experiment along a transect of 13 traps installed 12.5 meters apart on the coast bluffs, approximately 600m south of the Bodega Marine Lab. When collecting N. guttula, we sexed them, counted the number of phoretic mites currently attached, and took photographs to measure size in ImageJ. We usually visited traps in the mornings to collect the beetles that would then be placed into experimental chambers later that day.

Mouse carcass preparation. We purchased large frozen mice (mean = 21 g, range = 17-26 g, SD = 2 g) (Appendix S1: Section S2) from a commercial source (Gourmet Rodent via Petco.com). We dried half of the mice in a food dehydrator for 5-6 hours at 45 °C. On average, dried mice lost 25% (SD = 4%) of their weight in water. This corresponds to between 20 and 72 hours of mouse carcasses naturally drying out and decomposing at the site during the month of July (Appendix S1: Figure S1, Appendix S1: Section S3). Mice carcasses also tend to lose more water weight (dry out faster) during less humid and less foggy periods at the Bodega Marine Reserve (Appendix S1: Figure S2, Appendix S1: Section S3). While making these measurements of mouse carcasses naturally drying out at the site, we observed burial behaviors by Nicrophorus spp., fly presence, and evidence of vertebrate scavengers between 28 and 77 hours since the carcass became available (Appendix S1: Table S1). The overlapping timelines of these observations indicate that our 25% partially dehydrated treatment represents realistic conditions that burying beetles might experience at the Bodega Marine Reserve. All mice used in the experiment (partially dried and control) were refrozen after weighing to minimize further changes until they were placed into experimental chambers, where they thawed rapidly.

Setting up experimental chambers. We set up 63 experimental chambers (Figure 1D) between June 22 and August 10, 2022. We used 5-liter buckets (16 cm tall by 22 cm diameter) with modified lids that had a layer of wire mesh with quarter-inch openings and a layer of fine mesh to prevent beetles from getting in or out. A cinder block was placed over each chamber to prevent vertebrate disturbances (Figure 1E). Each bucket was filled with local soil and plants and buried to a depth of 8 cm. We placed experimental chambers along a series of transects (running approximately parallel to the shoreline) in the coastal prairie, approximately 25 meters apart. At each transect point, we haphazardly placed experimental chambers by throwing a small object into the air and digging a hole wherever the object landed. Each experimental chamber received a male-female N. guttula pair (Appendix S1: Section S4). We randomly assigned chambers to receive one of four treatments: (1) 14 received a control carcass and no Heterosilpha beetles, (2) 16 received a control carcass and a male-female pair of Heterosilpha, (3) 16 received a partially dried carcass and no Heterosilpha beetles, and (4) 17 received a partially dried carcass with a male-female pair of Heterosilpha.

Measuring reproductive output. We checked each chamber after about 39 days (range = 35-42 days, SD = 2), between August 6, 2022, and September 19, 2022, when the adult beetles that we started the experiment with had sufficient time to reproduce. We counted all larvae and pupae (Figure 1F). We also noted if the carcass was buried, even if there were no larvae or pupae present.

Analysis. We tested two univariate models to analyze these data. First, we analyzed the role of carcass dryness and competition from Heterosilpha on binomial reproductive success with a binomial generalized linear model (GLM) using the R packages lme4 and jtools (Long 2023, Bates et al. 2023). We also included the initial undried mass of the carcass, day of year, female size, male size, and total number of mites on adults as covariate predictors. The initial undried mass of the carcass was included to control for initial variation in carcass size. The day of the year that the experimental chamber was set up was included to control for seasonality. Female and male sizes were included to account for the effects of size on fecundity and/or competition between mates during the reproductive process. The total number of mites on adults was included to account for the effects of the mites as either competitors or mutualists. Second, we made another GLM with a Poisson distribution and a smaller dataset that only included experimental chambers that had successful reproduction. The model had a number of offspring as the response variable, with the same predictors as the previously mentioned model.