By training I am a behavioral ecologist. That means that my background and experience help me understand what behavioral answers have evolved in response to the ecological problems animals face, like avoiding predators and finding a mate. So I am especially interested to learn why loons that are rearing chicks abandon them for periods of an hour or more to visit the neighbors, and why female territory holders are able to surrender their territory to a superior opponent and live for another day while male territory holders in the same predicament seem unable to sense the danger and often die in territorial battles. But such questions pale when compared to a single, burning question we have faced for the past year on the Loon Project: “What is causing the northern Wisconsin loon population to decline?”. That question has become a nagging source of unease that prevents me from feeling fully comfortable anywhere and at any time.
There are many possible reasons for the decline: the exploding eagle population, decreased fish numbers, human impacts like increased boating or angling. And, of course, climate change, which impacts temperature, rainfall, and extreme weather events, is the elephant in the room. Learning about and systematically eliminating each potential cause of the decline will require me to find and collaborate with other scientists who know about fish, eagles, human impacts, and climate. In other words, cracking this nut will force me far outside my comfort zone.
We have glimmers. My collaboration with Sarah Saunders showed us that increased rainfall, increased human settlement, and the North Atlantic Oscillation – a broad-scale climatic event that influences weather in the northern Hemisphere – are all linked to both lower breeding success and lower adult survival of our loons.
A month or so ago, Linda and her husband, Kevin, speculated that increased boat traffic on large lakes might be the cause of the reproductive decline of loons in Wisconsin. They reasoned that more big boats might churn up the water, reduce water clarity, and make it harder for loons to find their prey under water. Such a scenario might make chicks grow more slowly now than 25 years ago and cause higher chick mortality.
Water clarity has always been a prime suspect among factors likely to influence loon survival and breeding success. As visual predators, loons must be affected by water clarity. Right? Yet we have no evidence to date that clarity affects loons. Brian Hoover’s recent paper, for example, showed that juvenile loons try to forage on lakes similar to their natal one in pH, but not in clarity. Our analysis from several years ago showed that young loons tend to settle on breeding lakes similar to their natal one in overall size and pH – but, again, water clarity is not a factor. Moreover, a glance at our study lakes shows that loons survive well and produce chicks on lakes that range from crystal clear (20 feet of visibility or more) to very murky (4 feet or less of visibility). If loons live and breed successfully on lakes that vary so greatly in clarity, perhaps clarity simply does not matter at all.
Nudged by Linda and Kevin to look once more at water clarity, I finally had some success. When two new collaborators at Rensselaer Polytechnic Institute provided me with thirty years of water clarity data based on satellite overpasses from my Wisconsin study lakes – and I plugged those data into my statistical models – suddenly clarity mattered. To be specific, mean water clarity during July was a significant predictor of chick mass. Clear water produced fatter loon chicks! Furthermore, chick survival decreased significantly in cloudy lake conditions.
Wait……what does this pattern mean? If you are like me, you think of water clarity as being constant or static for a lake. That is, you consider Two Sisters Lake as a very clear lake and Pickerel Lake as a murky lake. And you are correct. But those lakes – all lakes – fluctuate in clarity seasonally, annually, and even over days or weeks. Runoff events caused by rainstorms reduce clarity, for example, because silt and other materials are carried by streams into lakes. So you can have a bad few weeks or month for clarity on a lake that is generally quite clear. And a very clear lake can gradually become less clear over the years. The new satellite data are showing us that such short-term fluctuations in water clarity are associated with lower chick mass. It is a conceptual leap, but the obvious interpretation here is that short-term losses in water clarity impair foraging by loons and reduce the amount of food they are able to provide for their chicks.
It is early days. My collaborators are refining their estimates of water clarity from the satellites for northern Wisconsin and promise improvements by October. Meanwhile, I am left to ponder two things. First, water clarity in northern Wisconsin has declined over the past ten years, as the featured graph shows. Second, if recent declines in lake clarity really do hurt loons’ ability to catch prey for their chicks, what can a single loon researcher do about it?