Globalization and the 50-Year-Old Predicted Reorganization of Anole Biogeography

helmus_etal_fig2I caught an anole lizard and tossed it ten feet or so out into the water. To my dismay, it popped to the surface, swam expertly back to the shelter of the trees, and climbed up a mangrove trunk. Well, I continued, suppose a full hurricane blew an anole so far away on open water it couldn’t get back. Our little experiment shows that it could swim to the nearest islet if it were not too far away.

So wrote E.O. Wilson (1995 p. 271, Warner Books, NY) in his autobiography, Naturalist, reflecting on his island defaunation work with Daniel Simberloff. From his ‘little experiment’ (I can hear animal care committees cringing), Wilson postulated that anoles could, if they had to, disperse from island to island across open water. Whether anoles can cross water, however, isn’t that important. Rather, what’s important is that they rarely do. Anoles’ status as a symbol of island biogeography and adaptive radiation is largely due to the fact that isolation and the resulting low gene flow among islands set the stage for in situ speciation and adaptive radiation. In fact, much of what we (and island biogeography in general) owe to anoles, we owe because they don’t swim so well. And they don’t colonize new islands very often.

Or rather, they didn’t.

A new paper in Nature by Matt Helmus, with AA stalwarts Luke Mahler and Jonathan Losos, shows how human-mediated dispersal of anoles among Caribbean islands is reorganizing anole biogeography in a very predictable way. I suspect many who have worked on anole island biogeography, me included, have considered what to do about recent introductions and have often, like me, dropped them out of a dateset with the goal of trying to discern the ‘natural’ pattern. Helmus et al., however, saw the spate of recent anole introductions across the Caribbean as an opportunity, rather than a nuisance. Their great leap came from realizing that this reorganization of anole Caribbean biogeography should be predictable from the basic tenets of island biogeography theory.

Based on MacArthur and Wilson’s equilibrium theory, adaptive radiation theory and drawing on Losos and colleagues’ past work from 1993 and 2000, Helmus et al. predicted three patterns: 1) Species-impoverished islands (for their size) should have more exotics than more saturated islands, 2) The phylogenetic diversity of islands should increase due to exotic establishment, and 3) Human-mediated introductions should degrade richness–(geographic) isolation relationships. In short, they found evidence consistent with all of these patterns. Furthermore, they showed that economics that has replaced distance as the key determinant of island isolation. Needless to say, these are very exciting results that have supplied a key test, at biogeographic scales, of some classic theory*. It’s a must read.

This paper is also important because it shows how ‘blue skies’, curiosity-driven science can help us understand and, most importantly, predict how human activity will impact ecological systems. Did MacArthur and Wilson know, more than half a century ago, that their work would predict how increasing globalization and trade embargoes would affect modern biodiversity? I doubt it (cue someone pointing out in the Comments the exact line in the ETIB where they do predict this). However, regardless of whether they knew it at the time, this is exactly what their theory has done. As Helmus et al. state (p. 545): “Our results support the theory that it is the influence of geographic area and isolation on … speciation and colonization that fundamentally determine island biodiversity”. However, as they crucially find, what we now need to do is rethink how we define ‘isolation’. We can’t leave ourselves out of the equation any more. It’s economics, not geography, that matters now. Thus, not only does Helmus et al.’s paper test a long-standing theory, but it provides a clear example of the importance of fundamental scientific theory for understanding and predicting ecological dynamics in the ‘Anthropocene’.

In conclusion, the observation that humans are moving anoles — and other taxa — around faster than they could make their own way will come as a surprise to no one. But finding that the subsequent reorganization of life can be predicted by island biogeographic theory is fantastic (it should be pretty clear by this point that I like this paper. A lot). So if you haven’t read the paper, you should. I know it’s a terrible cliché to call a study ‘elegant’. So I won’t. I’ll call it damn elegant.

*I can’t help but mention that Helmus et al.’s findings were mostly based on good old-fashion OLS regression and ANOVA, and visualized using simple scatterplots – No fancy-shmancy statistical machismo here (phylogenetics aside). Just a clear set of predictions that could be parsimoniously tested. Chapeau.

Editor’s note #1: nice summaries of this paper have been written by Ed Yong’s Phenomena: Not Exactly Rocket Science blog and by Emily Singer in the new online Science magazine Quanta.

Editor’s note #2: The paper grabbed the cover of Nature.

Helmus et al. cover

This, in turn, joins a long list of recent science journal covers sporting an anole:

covers

About Adam Algar

Assistant Professor in Biogeography at the University of Nottingham (UK). My research focuses on how ecological and evolutionary process play out across geographical space, mostly on islands and lately, with anoles, but other taxa are okay too.

13 thoughts on “Globalization and the 50-Year-Old Predicted Reorganization of Anole Biogeography

  1. I haven’t read the paper yet, but have a burning question: I gather that the pattern of trade affecting dispersal of anoles is driven largely by Cuba, which has been economically isolated for a long time. But has there been equal sampling effort to search for invasive anoles on Cuba compared to other Caribbean islands? The restriction on travel to Cuba has likely prevented U.S. biologists from focussing their sampling on this island, but has there been enough sampling by local biologists for the authors of this study to be confident in the estimates of the number of invasives present in Cuba?

    1. Ambika, I had the same concern about Cuba driving the pattern while reading the paper. Especially b/c many of the pop sci pieces have highlighted the trade embargo (as does my title). But looking at the data, the pattern isn’t driven by a Cuban outlier (though I don’t have the paper in front of me to doublecheck), so it’s not just a Cuba effect.

      One aspect the authors didn’t examine in depth is what characterises source islands. presumably SR, but what else? Cuba has certainly supplied sagrei, despite its economic isolation, but I suspect sagrei escaped pre-embargo (can anyone enlighten me?) and since then how many of sagrei’ new colonisations originated from Cuba? I suspect AA readers can let me know.

      1. Great points! I have posted all of the data and code for the paper on Github. The code reproduces the results and with a little editing, one could easily check the leverage/relative importance of any bank on any pattern.

        https://github.com/mrhelmus/Helmus_Nature_2014_AnthIB

        Also see ED Fig. 4 and ED Table 5. Both are relevant in understanding variation in economic isolation across the Caribbean; we calculated the effect of economic isolation with the Greater Antilles removed, the effect remains.

  2. Sagrei got to Florida in the late 40s/early 50s, before the embargo. I believe that many of its subsequent invasions had Florida as the source. Sagrei is also native to the Bahamas and some of the Cayman Islands. Jason Kolbe’s work in Nature and Molecular Ecology uses molecules to chase the colonization history of sagrei and other anoles.

  3. Since we said many of the same things, I have a hard time understanding why the following chapter wasn’t cited. Although it didn’t specifically address anoles (although they were included), it did clearly make the case that commerce is a major player in this day and age.

    Powell, R., R.W. Henderson, M.C. Farmer, M. Breuil, A.C. Echternacht, G. van Buurt, C.M. Romagosa, and G. Perry. 2011. Introduced amphibians and reptiles in the Greater Caribbean: Patterns and conservation implications, pp. 63–143. In A. Hailey, B.S. Wilson, and J.A. Horrocks (eds.), Conservation of Caribbean Island Herpetofaunas. Volume 1: Conservation Biology and the Wider Caribbean. Brill, Leiden, The Netherlands.

    1. Many apologies!! Your work is a comprehensive overview of herp introductions in the Caribbean and I certainly will cite it in any future papers on the topic. Thank you for pointing me to it and apologies again for the oversight!

Leave a Reply

Your email address will not be published. Required fields are marked *

Optionally add an image (JPEG only)