Thursday, June 20, 2013

A Trip to This Side of Paradise (Lower Alabama) and a Brief Discussion on Null Models in Ecology

By Brian Folt

    One of the perks of being a graduate student is that I have a somewhat flexible schedule (or so I like to think). For example, a few weeks ago, I was hanging out at home on a Tuesday night when my neighbor, another Auburn graduate student, asked me if wanted to join him on an impromptu trip to south Alabama the following day for some field research. At first, I was a bit hesitant, and the usual excuses crossed my mind (E.g., “Man, I don’t know, I’ve got a bunch of work to do”). ….But then I started thinking about all the incredible wildlife I could possibly see on a trip to the Lower Coastal Plain of Alabama. Thoughts of beautiful long-leaf pine forests, magnolia floodplains, and cypress swamps came to mind, the homes of incredible animals such as Gopher Tortoises, Indigo Snakes, Black Bears, Bobcats, Red-cockaded Woodpeckers, Black Swamp Snakes, or Rainbow Snakes. Whoa, giddy up, ¡vamos!

    In short sequence it was 5 AM Wednesday, and Jeff and I are leaving town, truck loaded with gear and to-go cups full of black coffee, hoping that my boss won’t drop by my empty office that day. We cruised through central Alabama, stopping briefly in Greeneville for the Shoney’s breakfast buffet (a tradition), and we quickly arrived in the Conecuh National Forest. It was a cool morning with clear skies and temperatures around 80˚F. We set out to accomplish our work, wistful to discover and encounter interesting natural history.


    Unfortunately, however, this trip would include neither Indigo nor Rainbow…but we did find practically the next best thing, a big female Pine Snake (Pituophis melanoleucus)! It was the first time I have observed this large and beautiful species in the field. Pine Snakes are thought to be associated with pine forests, where they eat a lot of Pocket Gophers (Geomys pinetis) and Cotton Rats (Sigmodon hispidus).

    Eventually, we scooped the snake up to measure length, identify sex, and to see if she had eaten recently. After a few minutes of holding her and marveling at her beauty, however, I was startled out of my reverie when the snake poked the back of my hand with her (apparently) pointed tail. “Hmmm…that’s strange, because I thought that behavior was typical of snakes that eat slimy food. That’s what Dave said on the blog,” I mused to myself. We have previously discussed this behavior on this blog in relation to the myth that ‘snakes will stab people!’ But, equipped with our natural history knowledge and field observations in mind, we hypothesized [in the comments] that some snakes may actually use pointed tails to detain and consume their favorite but slippery prey items, and that this behavior may be advantageous and adaptive for said snakes. With that hypothesis in mind, one would not expect this behavior from a Pine Snake, a species well known to dine on hairy mammals. 

    About half a century ago, our natural history hypothesis that some snakes use a pointed tail tip to detain slick prey would have fit the bill in the ecological community. In those days, ecological research typically occurred as follows: workers went out into the field, collected data, observed patterns, and wrote natural history papers describing and hypothesizing about what they saw. In our case, we described a pattern between snake tail poking and species that consume slippery prey, and we hypothesized that it is an adaptive behavior.

    This descriptive approach, known as induction, was more-or-less the status quo in those days, and much knowledge was generated about the ecology of plants and animals. However, in 1970s and ‘80s, some ecologists began to question the logical basis of their research, and the field underwent a progressive revolution. Folks finally began to realize that, in many cases, the pattern observed in the field was often no different than a pattern produced by random chance! Which is not very meaningful. To remedy this issue, researchers began to develop and implement new models that evaluated whether data were significantly different than expected by chance. As with all progress, these new ‘null models’ were resisted by some parties, and this topic was battled particularly fiercely by the community ecologists [perhaps notably while trying to understand whether competition between species influences what species persist in ecological communities]. 

    At this time, workers were forced to shift from simple interpretation of patterns to a hypothesis-testing approach (deduction) that often considered null models. In large part, this paradigm shift was spearheaded by a dominant group of faculty and graduate students at Florida State University, led in chief by Daniel Simberloff and Donald Strong. These guys were particularly visible, vocal, and hardnosed, both in the literature and at national meetings. While the slightly infamous reputation of this stalwart group quickly acquired them the moniker of the ‘Tallahassee Mafia’, these workers had a profound and lasting impact by challenging and improving the status quo in ecology. They helped turn ecology away from a descriptive and inductive science to a more rigorous and deductive method. 

    Anyway, allow me to reel this rant back in a semi-circle; returning to poking snakes, slippery foods, and our hypothesis. As we left the extremely diverse long-leaf pine communities of Lower Alabama that afternoon, I dozed off and on in the passenger seat, thinking on animal ecology, randomness, and null models. 

    Allow me to propose an alternative hypothesis, LAW readers: What if this morphology and behavior is not adaptive, but merely random? We only identified a handful of species exhibiting this behavior, which is quite paltry considering the total species richness of snakes (>3,500 species). Most of the snakes we mentioned are thought to be distantly related to one another (i.e., in different families; Rainbow Snakes: family Xenodontidae; Brown Snakes: Natricidae; Thread Snakes: Leptotyphlopidae, etc.). 

    Instead, what if tail poking behavior is randomly distributed through the snake world; and what if it randomly occurs on a few species that eat slick, slippery food? 

    Can we test our hypothesis?



Further Reading:


Means, D. B. (2006). Vertebrate faunal diversity of longleaf pine ecosystems. The longleaf pine ecosystem: ecology, silviculture, and restoration, 157-213 DOI: 10.1007/978-0-387-30687-2_6

Strong, Jr., D. (1983). Natural Variability and the Manifold Mechanisms of Ecological Communities The American Naturalist, 122 (5) DOI: 10.1086/284164

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