Of Windstorms and Bluebirds

It was a rather windy day today, so I did a little drive-by photography:

Eastern Cottonwood (Populus deltoides),
N. Unit, Illinois Beach State Park, Lake Co, IL 1/19/2014

This was taken at the site of our fall hawkwatch. We've named many of the trees that we see from that spot, so that we can point out birds to each other, and this one was named Tall Tree. The name wasn't given ironically -- at the time, it appeared to be the tallest tree in view. Then came July, 2011, and a record-breaking windstorm. It took down several of our trees in the North Unit of Illinois Beach, but the South Unit was hit very hard -- the park administrators estimated that half of the trees in the area were down. (The park as a whole encompasses 4600 acres, so that's a lot of trees.) The unit remained closed for 8 months as crews removed downed or partially downed trees. (Mostly in the campground and picnic areas -- in the nature area, they only worked close to trails.)

This was a once-in-a-lifetime event, a difficult sort of thing to study. But it opened up canopies in large areas of the park, produced a big pulse of downed wood, and overall changed many acres of habitat from Black Oak woodland to sand prairie and oak savanna. This was a major change in the ecological dynamics of a significant stretch of lakefront.

Now, here's another photo, from that same year:

Mountain Bluebird (Sialia curricoides), N. Unit, Illinois Beach State Park,
Lake Co, IL 11/12/2011

Prior to 2011, there had only been five records of Mountain Bluebird in Illinois. That's less than one every 20 years of records. In 2011, there were five more, which was frankly amazing -- who knows how many years we'll have to wait to see such an invasion again!

There has been a long history of ignoring rare events in ecological studies. Being rare, they're very hard to study, and attributing important aspects of an ecosystem to a rare historical event smacks of deus ex machina, without the justification of fiction. On the other hand, it's clear that the effects of an earthquake, hurricane, or volcanic eruption can indeed be a major factor in how an ecosystem functions for years afterwards. Ecologists today are beginning to work out methods of studying such events in a general sense. (1,2,3)

The same issue of rare, improbable events crops up in studies of evolution and biogeography. Alan de Quieroz addresses this in his recent work, The Monkey's Voyage, How Improbable Journeys Shaped the History of Life. (4) It's well worth reading, incidentally. Among other things, he points out how very different South America would look without one incredible voyage -- the crossing of the Atlantic Ocean (a narrower Atlantic Ocean back then, but still...) by a troop of ancestral monkeys. (Monkeys aren't normally thought of as sailors, for good reason!)

Of course, events that only occur once in a lifetime are hard to study, and it's easy to see why biologists might prefer to focus on the slow, ongoing processes of everyday life. But it's worth remembering that other species operate on other timescales. A White Oak may live 500 years -- what we think of as once-in-a-lifetime, a tree might have to deal with 5 or 6 times. Bristlecone Pines live for thousands of years, as do some lichens -- a 500-year drought is nothing new. And some ecosystems change on even longer timescales, which makes "rare" events commonplace.

Of course, quantifying these occurrences and their effects is a challenge, but it may be one that amateur naturalists (especially birders) can help with. Birds wander quite a bit, with a species' occurrence in a particular place being a function of distance, the bird's dispersal abilities and migratory status, and things like winds and stopping points along the way. (The number of observers looking for them does complicate things, of course.) That gives us a nice range of probabilities to compare and, perhaps, fit into a more general understanding of how rare events might impact a system.

(1) Jentsch, Anke, and Carl Beierkuhnlein. "Research frontiers in climate change: effects of extreme meteorological events on ecosystems." Comptes Rendus Geoscience 340.9 (2008): 621-628.

 

(2) Schwinning, Susan, et al. "Thresholds, memory, and seasonality: understanding pulse dynamics in arid/semi-arid ecosystems." Oecologia 141.2 (2004): 191-193.

 

(3) Frei, Christoph, and Christoph Schär. "Detection probability of trends in rare events: Theory and application to heavy precipitation in the Alpine region." Journal of Climate 14.7 (2001): 1568-1584.

 

(4) de Queiroz, Alan. The Monkey's Voyage, How Improbable Journeys Shaped the History of Life. (2014). Basic Books, New York.