A good day to contemplate mass mortality

27 01 2015

IMG_6387Snow is general across New England today. I can’t actually open my front door due to four foot snow drifts, but the view of the mounting piles and sideways blowing snow hints at the conditions. A couple of our Seanetters have written, pondering what they might find on their beaches once this blizzard winds down. Dennis Minsky closed his email to me with the words, “God help wildlife tonight.” Indeed, post storm, we do often see upticks in seabird mortalities. Species groups like alcids, already on a knife’s edge of survival during the harsher months, can be pushed over the edge by a bad storm and turn up in wrecks along the shores or even well inland. Storms can be one of the many stressors that may converge on wildlife and lead to mass death. A new paper out in the Proceedings of the National Academy of Sciences looked at patterns of these mass mortality events (MMEs) over decades and across multiple classes of organisms. Their findings do much to elucidate some causes of these events, but also to point out the challenges in tracking, reporting, and measuring them in wild populations across the globe. Some die-offs may simply be more conspicuous due to geography and timing, and others may remain unreported and undocumented simply because of the obscurity of where or when they occur. Some of it comes down to a variant of the tree-in-the-forest question: “if thousands of alcids die in the Atlantic, and no one is there to count them, does it make a sound?” The authors of the paper had to grapple with determining how many MMEs happen, and how may get reported in the scientific literature. ¬†Overall, these researchers have striven to account for these kinds of observation bias, and though it cannot be entirely eliminated, their general findings are quite interesting.

MMEs were rarely reported at all prior to the 1940s, so the authors of the study analyzed only the reports since then. As you can see in the figure below, the number of MMEs documented (and we know not all are documented) has increased in general–the colored bars show the number of events reported for each five year interval. The apparent declines in numbers of MMEs in most of these groups over the past decade is due at least in part to the delay between when an event occurs and when a scientific paper is finally published on that event, which can take years or even decades. The dashed lines show the upward trend in scientific papers published on MMEs, and it reliably increases across all groups.

Screen Shot 2015-01-27 at 11.59.51 AM

What these particular graphs do not show is the magnitude of each MME. Some die-offs are truly massive, and others are on a smaller scale. Further complicating the issue is the overall population size of the species. In a very common species, millions of individuals could die, drawing a great deal of human attention to the spectacle, and yet have little impact on the population’s overall viability. A very rare species might lose only a hundred of its number and face local or regional extinction without attracting much, if any, human attention. The differences in MME magnitude varies across different taxonomic groups too. In other words, the trends in magnitude of die-offs in amphibians do not hold for birds or mammals. The reasons for this are not simple or straightforward. There are several factors that may be at play, and sorting them out is challenging. The scientific community does shift in its foci over time, and while disease or toxic algal blooms may be in the spotlight for several years, interest in those can wane as scientists shift toward a focus on prey availabilty, or climate change as drivers. This is not either/ or of course, but these kinds of shifts do play a role. If we have not had entirely consistent attention on all the different potential causes of MMEs across all taxa, then we may have missed some significant ones while attention was turned elsewhere. In addition, there are the issues of scientific advances, which have made it possible to pinpoint causes of MMEs like algal toxins or viral diseases that simply could not be diagnosed fifty years ago due to lack of technological ability. These researchers also point out that, while scientists travel the globe in the course of their work, coverage is not uniform. Many more investigators are at work in Europe and North America, for instance, than in most regions of the developing world. These biases can lead us to miss entirely die-offs that happen in these less scrutinized regions.

What does appear clear is that, rather than pinpointing a single cause of wildlife die-offs as the main culprit, the number of events due to multiple stressors is likely to increase. Climate change can induce thermal stress on organisms not adapted to warming oceans, prey availability can threaten the survival of creatures already marginal in their foraging, and increasing severity of storms, droughts, and floods will converge on these species more and more often. Human-introduced pathogens will continue to pile on as well. What this paper tells us is that, in addition to doing whatever we can to address these threats, we must also make sure scientists are continuing to focus on mass mortality events. Interestingly, it is still the case that most wildlife mortality events are reported only in newspapers, and never make it into the primary scientific literature at all.

All the more impetus for me to finish up verifying all your volunteer data and get some publications out on seabird mortality on the east coast. I suppose a blizzard day is a good time to hunker down and do some of that work.




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