Fewer butterflies seen by community scientists across the warming and drying landscapes of the American West Item Type Article Authors Forister, M L; Halsch, C A; Nice, C C; Fordyce, J A; Dilts, T E; Oliver, J C; Prudic, K L; Shapiro, A M; Wilson, J K; Glassberg, J Citation Forister, M. L., Halsch, C. A., Nice, C. C., Fordyce, J. A., Dilts, T. E., Oliver, J. C., ... & Glassberg, J. (2021). Fewer butterflies seen by community scientists across the warming and drying landscapes of the American West. Science, 371(6533), 1042-1045. DOI 10.1126/science.abe5585 Publisher American Association for the Advancement of Science Journal Science (New York, N.Y.) Rights Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Download date 10/10/2021 20:50:30 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final accepted manuscript Link to Item http://hdl.handle.net/10150/657634 p. 1 1 2 3 4 Fewer butterflies seen by community scientists across the warming and drying landscapes 5 of the American West 6 7 M. L. Forister1*, C. A. Halsch1, C. C. Nice2, J. A. Fordyce3, T. E. Dilts4, 8 J. C. Oliver5, K. L. Prudic6, A. M. Shapiro7, J. K. Wilson6, J. Glassberg8 9 10 Affiliations: 11 1 Dept. of Biology, Univ. of Nevada, Reno, NV 89557, USA 12 2 Dept. of Biology, Texas State Univ., San Marcos, TX 78666, USA 13 3 Dept. of Ecology and Evolutionary Biology, Univ. of Tennessee, Knoxville, TN 37996, USA 14 4 Dept. of Natural Resources and Env. Science, Univ. of Nevada, Reno, NV 89557, USA 15 5 Office of Digital Innovation & Stewardship, University Libraries, Univ. of Arizona, Tucson, 16 AZ 85721, USA 17 6 School of Natural Resources and the Environment, Univ. of Arizona, Tucson, AZ 85721, USA 18 7 Center for Population Biology, University of California, Davis, California, USA 19 8 North American Butterfly Association, Morristown, NJ 07960, USA, and Dept. of BioSciences, 20 Rice University, Houston, TX 77251, USA 21 22 * Corresponding author. Email: [email protected] 23 p. 2 24 25 Abstract: 26 Uncertainty remains regarding the role of anthropogenic climate change in declining insect 27 populations, partly because our understanding of biotic response to climate is often complicated 28 by habitat loss and degradation among other compounding stressors. We address this challenge 29 by integrating expert and community scientist datasets that include decades of monitoring across 30 more than 70 locations spanning the western United States. We find a 1.6% annual reduction in 31 the number of individual butterflies observed over the last four decades, associated in particular 32 with warming during fall months. The pervasive declines that we report advance our 33 understanding of climate change impacts and suggest a new approach is needed for butterfly 34 conservation in the region, focused on suites of species with shared habitat or host associations. 35 36 One sentence summary: 37 Data from community scientists reveal complex, season-specific climate change effects on 38 declining butterfly populations. 39 40 41 42 43 p. 3 44 45 Main text: 46 Shifts in the structure and function of ecosystems in the Anthropocene pose numerous and poorly 47 understood threats to wild plants and animals and to human society (1). Of the changes being 48 tracked by ecologists, few are as potentially consequential as reductions in insect abundance and 49 diversity (2), with the status of pollinators being of particular concern (3, 4). While debate 50 continues on the magnitude and taxonomic scope of insect declines (5–7), there can be little 51 doubt that insects (like most other major groups) are responding to stressors that include habitat 52 loss, climate change, overuse of pesticides, and invasive species (8). However, the majority of 53 historical records of insect populations come from parts of the world, in particular densely 54 populated areas of western Europe (9–11), where habitat loss and degradation have been 55 pervasive, thus limiting the ability of researchers to separate the signal of climate change from 56 the effects of other stressors. Here we address this knowledge gap using community scientist 57 and expert-collected data, focusing on a region, the western United States, that is uniquely useful 58 for understanding the effects of climate change on insects because of warming and drying trends 59 (12) observed across land use gradients (from major cities to protected national parks) as well as 60 elevational and latitudinal gradients containing great habitat and climatic diversity. 61 The three datasets that we study are the Shapiro transect from Northern California (13), 62 the North American Butterfly Association (NABA) network of community scientist count data 63 (14), and the iNaturalist web platform (15) of contributed observations (Fig. 1). These three 64 sources encompass more than 450 species of butterflies and are complementary in that they 65 represent gradients of geographic coverage, temporal extent, and expertise; the three sources also 66 differ in coverage of urban and agricultural areas. The Shapiro dataset is expert run, the NABA p. 4 67 counts are generated by teams of volunteer or community scientists, and the iNaturalist records 68 are contributed by thousands of nature enthusiasts whose identifications are vetted by a machine 69 learning algorithm and at least two human experts. Previous work with the Shapiro data has 70 uncovered pervasive reductions in the density of butterfly populations across a relatively narrow 71 geographic area that includes large urban and agricultural areas (16). It has been an open 72 question as to whether similar population trajectories would be observed across wildlands and 73 natural areas (17). Estimating species-specific trends over time, we find that a majority of 74 species in each of the three datasets have downward population trajectories, ranging from 75 slightly downward trending to more severe reductions in abundance (Fig. 1D - F). Declining 76 taxa include, but are not limited to, wide-ranging species (e.g., the west coast lady, Vanessa 77 annabella; Fig. 2J) and butterflies that thrive in disturbed and degraded habitats, such as the 78 introduced cabbage white, Pieris rapae, for which decline has been reported from the 79 Midwestern United States (18). Overall, single-brooded species tend to be in slightly more 80 severe decline, but in general we find that life history traits (including geographic range size, 81 body size, and host specialization) have little explanatory power (Table S3). 82 Looking beyond individual species, we used the NABA data to investigate change in the 83 total number of individual butterflies and find a 96% probability that the total abundance of 84 butterflies is decreasing, with an estimated rate of 1.6% fewer individuals per year. That value 85 was estimated across 72 locations (and 262 species) with 10 or more years of data (Fig. 3A, Fig. 86 S1), in total spanning 42 years, from 1977 to 2018 (the average length of time series from 87 individual sites was 21 years; see Table S1). Using the same records to investigate geographic 88 variation in changing abundance, we considered indices of land use, static descriptions of annual 89 climate as well as season-specific rates of climate change and find the most influential predictors p. 5 90 to be indices of climate change (Table S4). Specifically, locations that have been warming in the 91 fall months have seen fewer butterflies over time (Fig. 3D), while warming in the summer 92 months is associated with the opposite effect (Fig. 3E). We hypothesize that warming in the 93 summer influences adult activity times directly and hence increases probability of detection, 94 while fall warming likely induces physiological stress on active and diapausing stages, reduces 95 host plant vigor or extends activity periods for natural enemies (19, 20). It is important to note 96 that the rate of warming is not homogenous across seasons, and warming is greatest in the fall 97 (Fig. S2). The difference between fall and summer warming is itself a predictor of changing 98 butterfly densities (Fig. 4). We also see a positive effect of increasing summer precipitation on 99 butterfly observations (Fig. 3G), which is likely associated with positive effects on nectar plants 100 and larval hosts; the western US has, however, been drying in recent decades (Fig. S2). We have 101 considered the possibility that results are affected by the abundance of the most common species, 102 but after excluding the 50 most abundant taxa we still estimate a decline of 1.8% with 93% 103 confidence (see Fig. S3 for a power analysis of overall decline, and Fig. 3 for the effects of 104 climate change and other predictors on more and less common butterflies). 105 Perhaps surprisingly, we do not detect an effect of proximity to either urban development 106 or agriculture (Fig. 3H and I), despite prior work illustrating that habitat loss and degradation 107 have severe negative effects on insects (21, 22). Pesticide applications in California's Central 108 Valley in particular have been implicated in recent declines (16). It should be remembered that 109 the NABA sites were not chosen with the goal of providing an unbiased sample of the landscape 110 (and the same can be said of the other datasets), and the median fraction of urban and agricultural 111 land around sites is less than 5 percent (Fig S4). The NABA sites do, however, include variation p. 6 112 in proximity to development, but these factors appear to be less important in comparison with the 113 influence of climate for the sites studied here. 114 In summary, we find that fewer individual butterflies are being observed across the 115 western US each year, with agreement from expert and volunteer datasets on that conclusion.