Amphibians are declining globally, and the fungal pathogen Batrachochytrium dendrobatidis (Bd), which causes the disease chytridiomycosis, has been the culprit in many of these declines. In the Mediterranean region of Baja California, Mexico—a biodiversity hotspot—native amphibians are also in decline, and Bd is present. To determine which factors best predict Bd infection prevalence and intensity in Baja California anurans, we conducted Bd sampling at three disparate sites at varying elevations. We observed an overall Bd prevalence of 68%, and found that species was an important predictor of pathogen prevalence and burden (load) both across and within sites. Pathogen prevalence and load increased positively with elevation. The highest Bd load we detected was 139,188 zoospore equivalents (ZE) in a western toad ( Anaxyrus boreas )—more than 10 times the pathogen burden at which chytridiomycosis is lethal in some species. We observed high Bd prevalence (84 – 99%) and Bd loads (> 44,000 ZE) in the California red-legged frog ( Rana draytonii)—a cause for concern in this threatened anuran, especially at La Grulla—the highest elevation (2050 m) site for this species across its range. We sampled museum specimens to determine how long Bd has been present in the region, and detected the pathogen as early as 1932. Contemporary Bd prevalence and load in Baja California are considerably higher than would be expected given similar habitats and the same community composition as in southern California, USA. We therefore highlight the importance of historical context and comparative analyses in examining contemporary Bd dynamics, as well as adhering to strict biosecurity measures when implementing reintroductions and translocations.

The US and Mexico share a common history in many areas, including language and culture. They face ecological changes due to the increased frequency and severity of droughts and rising energy demands; trends that entail economic costs for both nations and major implications for human wellbeing. We describe an ongoing effort by the Environment Working Group (EWG), created by The University of California's UC-Mexico initiative in 2015, to promote binational research, teaching, and outreach collaborations on the implications of climate change for Mexico and California. We synthesize current knowledge about the most pressing issues related to climate change in the US-Mexico border region and provide examples of cross-border discoveries and research initiatives, highlighting the need to move forward in six broad rubrics. This and similar binational cooperation efforts can lead to improved living standards, generate a collaborative mindset among participating universities, and create an international network to address urgent sustainability challenges affecting both countries.