Marine top predators as climate and ecosystem sentinels

I an era of unprecedented environmental change, developing a suite of tools for ecosystem monitoring is critical. This need is particularly urgent in marine ecosystems, given the rapid, climatedriven changes in marine populations and communities (Poloczanska et al. 2013). Comprehensive monitoring in marine ecosystems presents a challenge due to difficulties inherent in observing the highly dynamic ocean environment at relevant timescales. Traditional shipbased surveys are expensive, autonomous floats and underwater vehicles are still sparsely distributed, and remote sensing fails to capture threedimensional ocean structure. Furthermore, ecological monitoring in the open ocean is largely extractive and often involves lethal sampling of animal communities. In the undersampled marine realm, innovative and costeffective tools that can rapidly assess ecosystem responses to environmental change are vital. “Sentinel” species have been proposed as a means to provide information about unobserved components of the ecosystem (Zacharias and Roff 2001). Classic examples of sentinels include a domesticated variety of the canary (Serinus canaria), which was formerly used to monitor air quality in coal mines, and invertebrates, whose diversity has been used as an indicator of aquatic ecosystem health (Wilhm and Dorris 1968; Barry 2013). More recent studies show that vertebrate species can serve as sentinels of human health and environmental pollution (Bossart 2006; Smits and Fernie 2013), as well as coupled climate–ecosystem processes (Moore 2008). Useful sentinel species should integrate broader processes into rapidly interpretable metrics that reflect underlying ecosystem processes. Marine top predators (including certain species of predatory fish, seabirds, sea turtles, and marine mammals) have been proposed as ecosystem sentinels based on their conspicuous nature and capacity to indicate or respond to changes in ecosystem structure and function that would otherwise be difficult to observe directly (Figure 1; Bossart 2006; Boersma 2008; Moore 2008). Many marine top predators possess key characteristics of sentinel species, including (1) exhibiting clear responses to environmental variability or change (Sydeman et al. 2015; Fleming et al. 2016), (2) playing important roles in shaping marine food webs (Estes et al. 2016), and (3) indicating anthropogenic impacts on ecosystems (Sergio et al. 2008). Given these characteristics, there is a strong argument for using marine predators as ecosystem sentinels. Despite the contemporary use of marine predators as sentinels (relevant examples are listed in WebTable 1), the absence of a standardized framework for identifying sentinel Marine top predators as climate and ecosystem sentinels