Signs of Spring 6: Snail Kites and Rapid Evolution

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(Great thanks to Ty Bauman for permission to use several of his photographs in this week’s blog. Ty and his wife Linda have a bird/travel blog that is full of absolutely stunning pictures! I recommend it highly!)

Photo by T. Bauman

I have written about snail kites (formerly called the “Everglades snail kite”) (Rostrhamus sociabilis plubeas) before. They were one of the species that was hammered by Hurricane Irma when it hit the Florida peninsula last September (see Signs of Fall 7, October 19, 2017). All forty-eight of the snail kite nests around Lake Okeechobee were destroyed by Irma, and biologists were worried about the lasting impact of this reproductive disaster on this locally, very endangered sub-species.

But the snail kite had many more stresses prior to this latest hurricane. For example, this bird is a very specialized predator that feeds primarily on a single species of freshwater snail called the Florida apple snail (Pomacea paludosa). The importance of these snails to the snail kite is reflected in the very anatomy of the kite itself. The snail kite has evolved a highly specialized, sharp-pointed beak that is curved and aligned perfectly to allow it to pull the soft-bodied Florida apple snail out of its shell with great ease and efficiency. Now the snail kite does, at need, take and eat other prey items including crayfish and small fish, but apple snails are its overwhelmingly obligatory food of choice.

The snail kite also needs a very open vegetative habitat so that it easily locate the apple snails and then swoop down unimpeded and grab them at water level. They then take the captured snail to a perch to eat it.

There have been a number of impacts on the snail kite’s habitat that have interfered with both its food supply and its hunting strategies. Exotic snail species including the island apple snail (Pomacea maculata) have invaded the snail kites’ marshes and caused declines in the vital Florida apple snail. The island apple snails are two to five times larger than the Florida apple snail and are not as easily taken or de-shelled by a typical snail kite. Most of these invasive snail species are thought to have entered these Florida wetlands after being released by careless, tropical aquaria owners.

USGS, Public Domain

The Everglades is a vast, tropical wetland that once covered almost all of southern Florida. In wet seasons water flows slowly in a sixty mile wide front from Lake Okeechobee southward, down to Florida Bay. The flatness of the water surface hides the complex topography of the underlying, limestone bedrock. Small lifts or slight valleys in the bedrock can generate vastly different conditions for vegetative habitats. The open waters of the ponds over the deeper sections can transition into the wet prairies of slightly shallower sites, or the saw grass marshes, or the still only slightly drier “islands” of hardwood or pine forest in the shallower sections of wetland. This complex patchwork of wetland habitats supports and sustains a rich diversity of both plant and animal life.

Photo by Ty Bauman

The present day Everglades represents about half of its original area in southern Florida. Much of the primal wetland has been drained for agriculture and other human uses. Ongoing drought in Florida has also caused great loss of previously untouched wetlands and marshes. This shrinkage of essential habitat has greatly affected the snail kite. More insidiously, though, pollution from sewage and septic systems and nutrient runoff from agricultural fields has stimulated inappropriate plant growth and expansion of invasive plant species (like cattails and water hyacinths) throughout the remaining Everglades. This denser vegetative system makes the snail kite’s style of hunting less efficient and less productive.

In 2000 there were 3500 snail kites in Florida. By 2007 there were only 700 snail kites left. There was also a surge in the numbers of the larger, invasive island apple snail (coupled to a precipitous decline in the Florida apple snail) starting in 2004. Many scientists felt that this would be the final straw for the snail kites.

But they were wrong!

Photo by Ty Bauman

In the last ten years the population of snail kites has increased to 2000 birds. Further, these present day birds are different from the typical snail kite of a decade ago. In the past ten years the snail kites have gotten larger (on average 8% larger and at maximum 12% larger!) and their beaks have gotten bigger, too! Their larger body size and bigger beaks have enabled them to much more easily take and eat the larger island apple snails! Scientists like Robert Fletcher, Jr. of the University of Florida have been studying these changes in the snail kites and feel that they represent a classic example of Natural Selection at work. In the original population of snail kites there were some individuals that were larger than average. Those individuals (and their offspring) took advantage of the new abundance of the larger, exotic snails and fed extensively upon them. With each breeding cycle the larger kites had more food resources and produced more offspring, until the entire size profile of the population changed. That evolution could occur over such a short time period was surprising, but underlying logic of Nature Selection is compelling (Dr. Fletcher’s work is published in the November 27, 2017 issue of Nature, Ecology and Evolution).

We hope that the Irma-induced disruption of the snail kite’s breeding will not seriously sidetrack the recovery of its Florida population. Hurricanes are natural phenomena, and native species have persisted though a long history of these massive storms. The human generated stresses in the Everglades, though, may be more than what the native species can handle. That the snail kite has been able to re-model itself in the face of such a radical and rapid  transformation of its food supply is an incredibly hopeful sign. We wish the snail kite bon appetite and continued good escargot hunting!





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