Using iNaturalist Observations to Evaluate the Extrapolation of Herbaceous Critical Load Models

Censky, Emma; Bell, Michael; Felker-Quinn, Emmi

doi:10.36967/2319384

Deposition of anthropogenic nitrogen (N) and sulfur (S) can harm ecosystems. Critical loads (CLs) have been calculated to quantify the levels of deposition that prevent harm to a wide range of species. CLs are used by the National Park Service (NPS) to evaluate vulnerabilities and create park-specific management plans. CL models of plant response to N and S deposition were developed for 198 herbaceous species surveyed at roughly 15,000 locations across the continental United States (Clark et al. 2019). The range of temperature, precipitation, soil pH, N deposition, and S deposition for each species across its surveyed range was summarized as a multivariate “environmental hull” (Clark et al. 2025). The environmental hull summarizes the growing conditions and pollution levels under which the N and S CLs are relevant for each herb species. In this study, we used iNaturalist observations to determine if the environmental hulls from the CL models overlap with the presumably broader distribution of several plant species. We selected the 12 herbaceous species with CLs most common in national parks based on Russell et al. (2022) and downloaded roughly 100,000 verified research-grade iNaturalist occurrences of those species. We compared the iNaturalist occurrences with each species’ environmental hull. Observations within the hull signify the ecological conditions of the record are similar to those used to determine the CL. Seven of the 12 species had over half of their iNaturalist occurrences within the environmental hull, indicating a match between surveys establishing CLs and broader species distributions. These, plus three additional species had >80% of their iNaturalist occurrences within one standard deviation of the hull, indicating similar environmental conditions across both datasets. Maianthemum canadense (Canada mayflower) was most aligned in the environmental conditions of its iNaturalist occurrences with the hull values of its CL. By contrast, Bouteloua curtipendula (side-oats grama) had most of its iNaturalist occurrences outside of its hull, indicating that the surveys used to determine the CL do not represent the full species range. Using verified citizen science observations in combination with environmental hulls allows us to better evaluate our level of confidence in applying the CLs for the associated herb species. iNaturalist also enhances our records of the locations of sensitive species, and land managers can use this additional information about the number of species in exceedance of their critical load.

Using iNaturalist Observations to Evaluate the Extrapolation of Herbaceous Critical Load Models

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