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Citation: Porensky, L. M., J. D. Derner, and D. W. Pellatz. 2018. Plant community responses to historical wildfire in a shrubland-grassland ecotone reveal hybrid disturbance response. Ecosphere 9(8):Abstract. Most ecotones include structural and taxonomic elements from both adjacent communities, but it remains unclear how these elements function and interact within ecotones. We investigated long-term plant community responses to wildfire in a 7000-km 2 ecotone between mixed-grass prairie and sagebrush steppe ecosystems, which have dramatically different historical fire regimes. We asked whether plant community responses to wildfire in the ecotone were more similar to mixed-grass prairie, sagebrush steppe, or a hybrid of these two. We sampled plant community composition at 70 pairs of transects located inside and outside of wildfires that burned from 1937 to 2012. We determined whether (1) wildfires predicted plant community composition, (2) plant community response to fire varied based on abiotic factors, and (3) effects of wildfire varied based on time since fire. Plant community responses to wildfire did not vary substantially across the study region, despite continuous plant community variation in response to abiotic factors. Overstory responses were characteristic of sagebrush steppe. Burned transects had <10% as much big sagebrush cover as unburned transects, and cover did not increase with time since fire. In contrast, understory plant community responses to fire were similar to mixed-grass prairie. Burned sites had high forb cover in the short term and perennial grass cover in the long term. Within an ecotone, different components of the plant community can maintain functional fidelity to their home ecosystems, despite being spatially juxtaposed. The idea of hybrid disturbance response may provide new opportunities for targeted management within ecotones.

Habitat loss and changing climate have direct impacts on native species but can also interact with disease pathogens to influence wildlife communities. In the North American Great Plains, black‐tailed prairie dogs (Cynomys ludovicianus) are a keystone species that create important grassland habitat for numerous species and serve as prey for predators, but lethal control driven by agricultural conflict has severely reduced their abundance. Novel disease dynamics caused by epizootic plague (Yersinia pestis) within prairie dog colonies have further reduced prairie dog abundances, in turn destabilizing associated wildlife communities. We capitalized on a natural experiment, collecting data on prairie dog distributions, vegetation structure, avian abundance, and mesocarnivore and ungulate occupancy before (2015–2017) and after (2018–2019) a plague event in northeastern Wyoming, USA. Plague decimated black‐tailed prairie dog populations in what was then the largest extant colony complex, reducing colony cover in the focal area from more than 10,000 ha to less than 50 ha. We documented dramatic declines in mesocarnivore occupancy and raptor abundance post‐plague, with probability of occupancy or abundance approaching zero in species that rely on prairie dogs for a high proportion of their diet (e.g., ferruginous hawk [Buteo regalis], American badger [Taxidea taxus], and swift fox [Vulpes velox]). Following the plague outbreak, abnormally high precipitation in 2018 hastened vegetation recovery from prairie dog disturbance on colonies in which constant herbivory had formerly maintained shortgrass structure necessary for certain colony‐associates. As a result, we observed large shifts in avian communities on former prairie dog colonies, including near‐disappearance of mountain plovers (Charadrius montanus) and increases in mid‐grass associated songbirds (e.g., lark bunting [Calamospiza melanocorys]). Our research highlights how precipitation can interact with disease‐induced loss of a keystone species to induce drastic and rapid shifts in wildlife communities. Although grassland taxa have co‐evolved with high spatiotemporal variation, fragmentation of the remaining North American rangelands paired with higher‐than‐historical variability in climate and disease dynamics are likely to destabilize these systems in the future.

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