Introduction
Ecological dynamics are often surprising (Doak et al. 2008). By now, many ecologists have realized that some unlikely events are likely to happen (Pielke and Conant 2003), and that systems might suddenly shift between alternate stable states (Beisner et al. 2003). Many examples of such shifts come from aquatic systems (Carpenter 2005, Petraitis and Dudgeon 2015), and examples from terrestrial systems are mostly focused on community and ecosystem level processes (e.g., Suding et al. 2004). In fisheries, large transitions in abundance have been driven by life history shifts (Persson et al. 2007). However, there are few examples of terrestrial populations that have made sudden shifts from abundance to near collapse in a few years. Thus, the operating paradigm in management of at-risk species – at least in terrestrial systems – is that declines in abundance will occur at a pace for which conservation biologists and managers have time to assess a species’ status and make recovery decisions over the course of processes that take years to implement.
Here, we explore sudden and dramatic changes in the abundance, distribution, and demography of monarch butterflies in the western USA. Rather than waiting for complete understanding, we describe these startling changes as they are occurring. We use simple calculations and available data to put these changes in the context of what we know about the current and likely future status of this population. We hope to inspire discussion about how we define population viability, and how we use ecology to guide management in the face of biological uncertainty.