New Program for Studies of Environmental Niche Evolution

Late last year, Dan Warren, Michael Turelli, and I wrote a paper about niche evolution in which we developed new metrics and statistical tests for comparative studies of environmental niche models (ENMs). Our basic metrics permit quantification of similarity between ENM model projections generated by two or more populations. These metrics may either be explored in a phyogenetic context, or used in association with pseudoreplicated datasets to test two null hypotheses at opposite ends of the niche similarity continuum : (1) ENMs are identical and, (2) ENMs no more similar than expected by chance. Although our methods could work with several types of niche modeling algorithms, they are best suited to output generated by the maximum entropy method implemented in the program Maxent. We have now written a program of our own called ENMTools that interacts extensively with Maxent to implement the analyses discussed in our paper (more accurately, Dan Warren wrote a program and Michael and I beta tested it). Dan has done a masterful job with this Perl application, which presents as a simple GUI interface on any platform capable of running ActivePerl (including Linux, Mac OSX, and Windows). In addition to performing the methods we've already introduced, new functionality is being added constantly (although some of the coolest stuff is purposely left unexplained so we can publish the methods before they start getting used by others). Dan has set up a website and a blog to keep people informed about the latest developments, and hopes you appreciate his retro-internet stylings.

Worse Than Alligators in the Sewers

Writing in the latest New Yorker, Burkhard Bulger suggests that Florida is like Club Med for exotic tropical species, "an exlusive seaside getaway, far from the fang and claw of the usual tropical crowd." Exotic species have been checking in for decades, but the potential gravity of the problem didn't enter public consciousness until a group of Everglades tourists captured footage of an exotic Burmese python's epic battle with an alligator in 2003. Shortly thereafter, the hero of Bulger's piece - Everglades biologist Skip Snow - started to discover hatchling pythons, prompting state wildlife managers to quickly switch from telling him "no problem at all" when he raised concerns about Everglades pythons to telling him "you might as well give up". Although it may not yet be time to give up, I was surprised by the overly simplistic strategies supported some professional wildlife managers (e.g., "It's time to stop studying these things and start killing them"). I would hope that if we've learned one thing about invasive species it is that simple brute force extermination does not work, particularly in an area as large as the Everglades and when the strategy being employed is as simple as the intentional road-killing or "rapid-acceleration removal method" practiced by some Florida biologists. One source of scientific information with management significance comes in the form of recent climate envelope and niche modeling studies conducted by Rodda et al. (1) and Pyron et al. (2). Although Rodda et al.'s climate envelope models suggest probable expansion of Burmese Pythons throughout the southern United States, Pyron et al.'s niche modeling analyses suggest a much smaller potential range, and that concern about such expansion should not be a the top of the list of challenges facing managers of python populations. (Is this the first time niche modeling studies have recieved a nod on the pages of the New Yorker?) Of course, a range of phylogenetic and phylogeographic studies are also providing insight on the history, biology, and management of invasive species.

Although most Floridians seem more likely to be concerned about the safety of their house pets than the loss of an ecosystem, Bulger does a beautiful job driving home the profound significance of the latter when he suggests that some invasive species can "...change the way we see a place. A parrot in Miami is like a McDonald's in Kathmandu: a sign that you are everwhere and nowhere at once."

What the Hell is Niche Conservatism?

Niche conservatism is all the rage; virtually unheard of at the turn of the Millenium, 2008 has already seen nearly 30 papers on the subject. The only problem? Nobody seems to agree on what it is!

In a recent exchange in Ecological Letters, Jonathan Losos and John Wiens try to cut through the confusion. Let's begin with Losos' definition of niche conservatism as a pattern that "results when closely related species are more ecologically similar that [sic] would be expected based on their phylogenetic relationships" (emphasis mine). In advocating this definition, Losos argues strongly against the view that niche conservatism is a synonym for phylogenetic signal (i.e., the tendency for related species to be more similar than species drawn at random from a phylogeny). To make this equation, he argues, would defy the original meaning of the term niche conservatism and expands its scope so broadly as to render it meaningless. As Losos notes, some degree of ecological similarity is expected among closely related species even under the simplest models of evolution (e.g., Brownian motion resulting from shifting selective pressures or drift). Niche conservatism, meanwhile, results when species are more similar than expected under this simple model due to constraint by natural (stabilizing) selection, gene flow, pleiotropy or the absence of variation. Although Wiens agrees on this point, many recent studies claiming to support niche conservation involve nothing more than simple tests of phylogenetic signal. In Losos' view, these studies provide evidence that is necessary, but not sufficient, for the identification of niche conservation.

Although I've previously confounded niche conservatism and phylogenetic signal in my own work, I've come around to the view shared by Wiens and Losos. Another influential voice in the field -- Michael Donoghue (2008) -- seems to straddle the fence when he says "strictly speaking, it is not necessary to link PNC with the view that there are constraints on niche evolution, [but] I believe that it is the relative difficulty of making major ecological shifts that explains [patterns of niche conservation]." What do you all think?

If people are into this topic, perhaps we can try to tackle Losos and Wiens arguments about how and when niche conservatism should be studied, and whether it represents a pattern, a process, or both...

Literature Cited:
Losos, J. B. 2008. Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species. Ecology Letters 11:995-1003.

Wiens, J. J. 2008. Commentary on Losos (2008): Niche Conservatism Déjá Vu. Ecology Letters 11:1004-1005.

Losos, J. B. 2008. Rejoinder to Wiens (2008): Phylogenetic niche conservatism, its occurrence and importance. Ecology Letters 11:1005-1007.

Donoghue, M. J. 2008. A phylogenetic perspective on the distribution of plant diversity. PNAS 105:11549-11555.