Amphibians are the most imperiled class of vertebrates worldwide (Hoffmann et al. 2010) as a result of the synergistic effects of habitat conversion, wetland contamination, invasive species, disease, and climate change (Sodhi et al. 2008, Johnson et al. 2011, Adams et al. 2013, Bradley et al. 2019). Although numerous taxonomic groups have recently experienced human-caused biodiversity loss, amphibian population declines have been particularly severe (Stuart et al. 2004, González-del-Pliego et al. 2019). In North America, the highest amphibian biodiversity occurs in the southeastern U.S. and in the temperate rainforests along the west coast (Dodd 1997, Battaglin et al. 2005, Graham et al. 2010, McKerrow et al. 2018). These areas are subject to the same threats causing global declines in amphibian diversity and abundance, especially as deforestation, wetland conversion, and pollution continue to shrink available habitat (Mushet et al. 2014, Todd et al. 2014, Sievers et al. 2017).
In the Upper Missouri River Basin (UMRB), several amphibian species have already experienced population declines, including Blanchard’s cricket frogs (Acris blanchardi), Western toads (Anaxyrus boreas), and Columbia spotted frogs (Rana luteiventris; Burdick and Swanson 2009, Pilliod et al. 2015, Slough and deBruyn 2018). Many of the surviving wetlands in the UMRB are located within agricultural landscapes and are at high risk of being converted to cropland, even during exceptionally wet years (Johnston and McIntyre 2019). To feed over 10 billion people on earth (van Vuuren et al. 2013), agricultural production will likely need to increase by approximately 60% over the next four decades and will result in the drainage of prairie pothole wetlands with significant habitat loss, decreased habitat connectivity, and population declines for many midwestern amphibian species unless substantial conservation protections are enacted (Wright 2010).
Amphibian richness in the UMRB is already low, due to the arid conditions that limit the number of species able to exist under these conditions (Lannoo 2005). In response to elevated atmospheric carbon concentrations and resultant climate effects, amphibians in the U.S. may be shifting their distributions northwestward as they track temperature and precipitation conditions suitable to their sensitive physiological and phenological needs. Amphibian community assemblages in the UMRB may also be shifting as temperature and precipitation regimes continue to change. Yet how and to what degree climate and land use change will affect amphibian distributions in the central U.S. remains unquantified. To address these gaps, we developed predictive landscape models to describe projected distribution and assemblage changes in the UMRB under a range of future climate and land use scenarios.
Climate Change
The effects of climate change on amphibians are threatening the survival of numerous species by altering phenological cues for spring emergence (Buss et al. 2021) and shifting available temperature ranges surrounding biological processes (Fontaine et al. 2018), forcing species to rapidly adapt or migrate to remain within thermal optima (Enriquez-Urzelai et al. 2019). For example, amphibians have a bi-phasic (aquatic and terrestrial) lifestyle and many of their biological and reproductive processes rely on narrow environmental temperature and moisture ranges, and so amphibians are especially sensitive among vertebrates to climate induced stressors and abrupt changes in land use (Zellmer et al. 2020). Additionally, many species have limited capacities for long-range movements and are unable to escape current microclimates as they become increasingly uninhabitable. As distributions shift and community compositions change, species will likely encounter new stressors in the form of competition from native and non-native species, predation, and disease. Stressors stemming from novel community arrangements will likely result in altered species interactions and trophic changes (Williams and Jackson 2007, Brambilla et al. 2020).
BECCS
In addition to climate change stressors, land use changes in this heavily agricultural region are also altering the amount and quality of available habitat. One such land use change involves the conversion of land to grow biofuel crops. Bioenergy with Carbon Capture and Storage (BECCS) has been proposed as a means of mitigating climate change in the UMRB by cultivating bioenergy crops (e.g., switchgrass, canola, soybeans) that sequester atmospheric carbon into plant tissues, which can then harvested and converted into heat, electricity, liquid or gas fuels (“bioenergy”; Stoy et al. 2018). The carbon emissions produced during bioenergy conversion are captured and deposited in geological formations (“carbon capture and storage”), which could theoretically result in negative emissions and a reduction in atmospheric carbon. However, implementing BECCS would require dramatic land use changes that may further degrade amphibian habitat, water quantity and quality (Hu et al. 2020), while also contributing to biodiversity losses (Mushet et al. 2014, Baltensperger et al. 2020). Here we examine the influence that land use changes associated with biofuel cultivation may have on amphibian distributions in the UMRB.
Approach
As amphibian populations dwindle, it is paramount to identify how drivers of these declines are likely to change species distributions. Species distribution models (SDM), also known as ecological niche models, are commonly used to identify important biotic and abiotic factors that predict current species’ distributions as well as future distributions based on climate and land use change projections to identify spatially-explicit threats to species across broad landscapes (Oberhauser and Peterson 2003, Elith et al. 2006, Baltensperger and Huettmann 2015a, 2015b, Kandel et al. 2015, Baltensperger et al. 2020). We used the machine-learning algorithm, Random Forests (RF), which is adept at deciphering complex, non-linear, multi-variate relationships (Breiman 2001, Elith et al. 2006) to estimate current distributions of eight amphibian species across the UMRB using publicly available occurrence data attributed with 16 environmental and climatic predictors. We also projected future distribution changes using sets of ensemble climate predictions for 2060. Our models help to identify the spatial effects of climate change and land use scenarios on amphibian distributions in a highly modified landscape over time. We use results to identify species and geographic areas at the highest risk of distribution losses in the UMRB given different climate- and land-use changes over the coming decades.