Curious Side Effects of Invasive Species

I am currently working as the field crew lead of the University of Florida’s Snail Kite monitoring project, which comes with several perks. The most obvious is that I have the daily privilege of getting to zip around on airboats through beautiful, remote, wetland habitat across the southern half of Florida. I spend hours a day observing and occasionally handling and banding Snail Kites, which are unique and beautiful birds that I can’t imagine ever tiring of watching. On our crew we sometimes joke about how they lack the intimidating presence of a typical bird of prey; they’re simply too goofy. While they spend their days deftly plucking apple snails from fresh water marshes, unthreatening to anything other than their mollusk victims (outside of the rare times they find an unlucky musk turtle to gruesomely excavate and consume), in this cruel and unforgiving world of ours they still get relentlessly mobbed by grackles and red-winged blackbirds, as if they were any other egg-stealing, fledgling-hunting, run-of-the-mill bird of prey. Their strained, nasal, almost mechanical calls are another incongruity separating them from their raptor relatives, but as with their other quirks, they are simply delightful. It’s definitely worth a Google if you’ve never heard the various calls they make, by the way.    

Young male Snail Kite with a musk turtle. ~99% of their diet is made up of apple snails but they will rarely catch other prey.

Getting back to the point, one of the other major benefits of my job is that I get to be part of one of the longest running wildlife monitoring projects in the country, and I can claim some association with the incredible wealth of research that biologists in the lab have been putting out about Snail Kites for decades. A few years ago, the UF Snail Kite lab published a paper that was exciting enough to generate a certain amount of mainstream attention. Cattau et al.’s 2018 paper, “Rapid morphological change of a top predator with the invasion of novel prey,” has been covered by all sorts of outlets including the New York Times. The paper is an elegant and reassuring example of what the daily toil of my field crew and the many field crews before us can contribute to. Cattau et al. analyzed the previous 11 years of meticulously collected nestling morphological data and coupled it with quantitative genetic analyses to show conclusively that since the invasion of the larger Chinese apple snail species into Florida in the late 2000s, Snail Kites had grown significantly larger beaks, talons, and tarsi. A fascinating result, especially given that in 2007, just 11 years before Cattau et al.’s paper, Snail Kite researchers Darby et al. published “Food-handling Difficulties for Snail Kites Capturing Non-native Snails,” in which they recount numerous observations of kites dropping the significantly larger Asian snails and otherwise failing to extract their meat. In a matter of a mere 11 years, researchers went from worrying about invasive snails acting as ecological traps that could hurt the kite population, to showing clearly that kites have adapted in a very short time span and are now actually benefiting from their new invasive prey.

A crucial and underrepresented detail I want to highlight from the paper is that the researchers make it very clear that these morphological changes come not quite from strict evolution but from phenotypic plasticity. The quantitative genetic analyses they used show that the genetic components of these traits did not significantly change over their study period. This suggests that “cryptic genetic variation,” e.g. preexisting variation in the population that is only made visible phenotypically by some novel environmental condition, is primarily behind the rapid shift. Researchers point to the increased overall growth rate in kite nestlings in areas with higher invasive snail availability as an indication that this richer food source may be triggering a shift to longer beaks and talons somewhat incidentally. They also highlight that these traits show a high degree of “additive genetic variance,” which is the genetic heritability component that is uncluttered by epistatic and dominance effects and otherwise known as “narrow-sense heritability.” This is good news for the kites since it means that real evolution in the form of genetic change may be easier/faster to generate in these traits, compared to a situation where more complicated dominance and epistatic hierarchies among alleles were involved and slowing down the adaptive process.  

Even though phenotypic plasticity is not technically evolution, this rapid morphological change is an incredible example of how the complex web of interactions between genes, phenotypes, and the environment is always changing and bringing about unique forms of life. Human beings are challenging wildlife in ways that go beyond habitat degradation and destruction and include the reshuffling of wildlife interactions via the promulgation of invasive species, and native wildlife must be resilient. In many cases these changes only bring devastation, but it is nice from time to time to read about a positive case, especially when it involves a species as unique and wonderful as the Snail Kite.

Citations:

Cattau, C.E., Fletcher Jr, R.J., Kimball, R.T. et al. Rapid morphological change of a top predator with the invasion of a novel prey. Nat Ecol Evol 2, 108–115 (2018).

Darby, Philip C., David J. Mellow, and Miranda L. Watford. Food-handling difficulties for Snail Kites capturing non-native apple snails. Florida Field Naturalist 35.3 (2007): 2.