Assessing risk for butterflies in the context of climate change, demographic uncertainty, and heterogenous data sources

Abstract

Ongoing declines in insect populations have led to substantial concern and calls for conservation action. However, even for relatively well-studied groups, like butterflies, information relevant to species-specific status and risk is scattered across field guides, the scientific literature, and agency reports. Consequently, attention and resources have been spent on a miniscule fraction of insect diversity, including a few well-studied butterflies. Here we bring together heterogenous sources of information for 396 butterfly species and 1,004 subspecies to provide the first regional assessment of butterflies for the 11 western US states. For 184 species, we use monitoring and other observational data to characterize historical and projected trends in population abundance; for another 212 species (for which sufficient observational data are not available), we use exposure to climate change, development, geographic range, host breadth and other factors to rank species for conservation concern. We also organize information relevant to subspecific risk and prioritize a top 50 subspecies for further attention. A phylogenetic signal is apparent, with concentrations of declining and at-risk species in the families Lycaenidae and Hesperiidae. A geographic bias exists in that many species that lack monitoring data occur in more southern states where we expect that impacts of warming and drying trends will be most severe. Legal protection is relatively uncommon among the taxa with the highest risk values: of the top 100 species, one is listed as threatened under the US Endangered Species Act and one is a candidate for listing; of the top 50 subspecies, 15 have federal legal protection and one is under review for protected status. Among the many taxa not currently protected, we highlight a short list of species in decline, including Vanessa annabella, Thorybes mexicanus, Euchloe ausonides, and Pholisora catullus. Notably, many of these species have broad geographic ranges, which perhaps highlights a new era of insect conservation in which small or fragmented ranges will not be the only red flags that attract conservation attention.

Estimating butterfly population trends from sparse monitoring data using Generalized Additive Models

“…Ecologists are harnessing new and previously overlooked data sources to better understand populations in our changing world (e.g., Gotelli et al 2021, Ellwood et al 2022, Di Cecco et al 2023), and smoothing splines have proven a useful tool in inferring population and phenology trends from insect monitoring data (Hodgson et al 2011, Wepprich et al 2019, Stemkovski et al 2020). However, rare and at-risk species are frequently under-represented in comparative studies of insect trends, due in part to the limited and messy data available for such species and the data requirements of standard methods (Forister et al 2023). We share our recommendations in the hopes that they will facilitate analyses of previously unusable data, and better inform our understanding of insect population dynamics and phenology.…”

“…However, the flexibility of smoothing splines leaves them prone to overfitting when working with sparse data, as is common for some types of population monitoring data. The simple solution of dropping years or populations with limited data will bias inferences made from estimated trends (Didham et al 2020), particularly since populations with more limited data have been found to be disproportionately at-risk species (Forister et al 2023). Here we offer specific approaches – pre-processing steps, model structures, and post-hoc analysis steps – that perform well for sparse monitoring data.…”

Additive and interactive effects of anthropogenic stressors on an insect herbivore

“…The Melissa blue butterfly, L. melissa (family Lycaenidae) is distributed across much of the American west in a population structure that is patchy with low gene flow among individual locations [43]. Regional monitoring indicates that L. melissa is among a group of at-risk species, and that the family Lycaenidae in general contains a high concentration of declining species [45]. Additionally, although not occurring in the west, the related Karner blue butterfly ( Lycaeides samuelis or Lycaeides melissa samuelis) is a federally listed endangered species [46].…”