Summary
Closely related species with overlapping geographic ranges encounter a significant challenge: they share many ecological traits and preferences but must partition resources to coexist. In Ontario, potentially eleven species of carrion beetles (Coleoptera: Silphidae) live together and require vertebrate carrion for reproduction. Their reliance on an ephemeral and uncommon resource that is unpredictable in space and time is thought to create intense intra- and interspecific competition. Evidence suggests that burying beetle species reduce competition by partitioning carrion for breeding across different habitats, temperatures, and seasons. Here, we test predictions of an alternative axis for partitioning carrion: vertical partitioning between the ground and forest canopy. We conducted a survey of carrion beetles from May to July 2016 at the Queen’s University Biological Station across 50 randomly generated points using baited lethal traps at zero and six metres. Ground traps yielded more species and individuals compared to those in the canopy, and the number of individuals and species caught increased through the season in both trap types. Ground and canopy traps were accurately distinguished by the presence or absence of three species: ground traps contained more Nicrophorus orbicollis and Necrophila americana, while canopy traps contained more Nicrophorus pustulatus. We trapped 253 N. pustulatus in the canopy, but only 60 on the ground. N. pustulatus is thought to be rare across its geographic range, but our results suggest it is uniquely common in canopy habitats, demonstrating a vertical partitioning of habitat and resources. Our results are consistent with N. pustulatus having diverged into canopy habitats as a strategy to coexist with closely related sympatric species when competing for similar resources. We still, however, do not know the traits that allow N. pustulatus to flourish in the canopy, exactly how N. pustulatus uses canopy resources for breeding, or the factors that restrict the expansion of other burying beetles into this habitat.
Methodology
Study site
We collected burying beetles in lethal traps baited with chicken on the Queen’s University Biology Station (QUBS, 44.5653, −76.322, 129 m) properties near Elgin, Ontario, Canada during the reproductive period from early May until late July 2016. We set traps at 50 block-randomized points across QUBS properties that are the subject of long-term studies of diverse taxonomic groups (birds, plants, and insects). These study points were originally chosen by randomly selecting GPS points that fell within the property boundaries, with the restriction that no point could fall within a body of water, and each point was at least 400 m away from all other points. Our study site includes areas of regrowth forest dominated by sugar maple (Acer saccharum) and ironwood (Ostrya virginiana), with some species of ash (Fraximus spp.), elm (Ulmus spp.), hickory (Carya spp.), and birch (Betula spp.), as well as basswood (Tilia americana), and areas of mixed forest with coniferous trees (Pinus spp., Thuja occidentalis, Tsuga canadensis) (Martin, 1994). The more mature deciduous and mixed forests reach an average height of about 24 m (Jones et al., 2001). Additional trapping locations at our study site include: areas of wet woodland composed mainly of eastern white cedar (Thuja occidentalis) and birch species, man-made conifer plantations, edges of small lakes and beaver ponds, forest edges, open fields that were once used for agriculture, or open rocky outcrops composed of scattered red oak (Quercus rubra), eastern white pine (Pinus strobus), red (Juniperus virginia) and common (J. communis) juniper, and a number of mosses, grasses, and lichen-covered rock (Martin, 1994).
Trapping methods
At each trapping location, we set two concurrent traps and collected them after 7 days: a pitfall trap in the ground, and a trap of the same design suspended six m above ground. We sampled each point twice: once in May/June, and once in July. We constructed our traps using plastic buckets approximately 35 cm deep and 17 cm in diameter. We filled the buckets with six cm (depth) of saturated saline solution to kill and preserve the beetles. We covered the top of each trap with a 35 cm2 piece of chicken wire. We baited each trap with one chicken wing wrapped in cheesecloth, suspended from the middle of the chicken wire using steel craft wire. The bait was frozen until deployed in traps without any prior thawing or ripening and suspended above the saline preservative. We covered each trap with a 30 cm2 plywood board to prevent rainwater from entering. We secured each ground trap by placing large rocks from each site on top of the plywood board in an attempt to deter vertebrate scavengers from disrupting the traps. Canopy traps were hung six m high in tree branches; the exact distance between the paired ground and canopy traps varied depending on the availability of soil for ground pitfall traps and trees for canopy traps (average = 4.8 m between paired ground and canopy traps with the largest distance being <20 m). Differences in the number of successful traps between the ground and canopy were caused by a greater disturbance of ground traps (N = 34), likely by vertebrate scavengers stealing the bait and/or pulling the trap from the ground. Three traps were also omitted because of trap failure due to human error in deployment. A total of 34 traps, mostly in the canopy, were successfully deployed and were undisturbed by vertebrates but did not collect any carrion beetles. These traps were included in tests for differences in the number of beetles and number of species between ground and canopy traps, but were omitted from subsequent classification analyses because they provided no information on carrion beetle community composition.