Trans-Continental Convergence II: Frog Communities in Three Corners of the Globe

Skip to the last three paragraphs for some advice on fieldwork from Dan Moen, author of the paper being discussed.

I’ve written before about trans-continental convergence in the morphology of some animals, and ended that post with the dream of collecting enough examples of the phenomenon to examine convergences in food chains or perhaps even whole ecosystems between the Neotropics and the Palaeotropics. The step in between an organism and an ecosystem, however, is a community, and I previously described a study showing convergence in communities of desert-dwelling lizards in two continents (Melville et al. 2006).

A recent study by Moen et al. (2013) describes another herpetofaunal example of community convergence, this time from three different continents–tropical forest frogs in Australia, China, and Colombia. Beyond adding to our compendium of trans-continental convergences, the authors investigate the evolutionary forces that have led to this uncanny similarity in what frogs look like in three far-flung corners of the globe. In particular, they ask the following questions:

• In addition to convergence in morphology, do these frogs also converge in microhabitat use and performance? For example, are frogs from different parts of the world that have converged upon semi-aquatic lifestyles also the best at swimming?
• Is convergence in these frog communities caused primarily by independent adaptive evolution in each continent, or have similar species dispersed incredibly long distances across the globe to reach their current distribution?

The answer to the first question is yes–frogs with similar lifestyles have, for the most part, similar morphologies, and perform better at tasks that are relevant to the microhabitat they occupy, when compared with frogs living in other microhabitats. For example, tree-living frogs are better at clinging to slippery surfaces (in nature, a leaf; in the lab, a non-stick pan) than terrestrial or aquatic frogs, and they achieve this superior clinging capacity by having larger finger and toe tips than frogs from other microhabitats. But for other aspects of performance, the relationships with morphology aren’t quite as simple. High jump speeds, for example, can be achieved by frogs with either longer legs or more muscular legs, but it seems tough to have both; across species, Moen et al. (2013) found a negative relationship between leg length and leg muscle mass.

The answer to the second question is both. At first I was confused by this–in my previous posts on trans-continental convergence, I suggested that the only way to achieve such patterns of similarity was by similar selection pressures exerted in similar environments on different continents. That assertion makes sense at the level of a single species–similar species that are also closely related are not usually convergent. But examples of convergence at higher organizational levels–communities or ecosystems–may not comprise only species-pairs that are individually convergent. Moen et al. (2013) find that dispersal has played a sizable role in shaping similarity between the frog communities in China and Colombia. Consider terrestrial frogs from the families Microhylidae and Bufonidae–in terms of morphology and performance,  species within these families in China and Colombia are more similar to each other than any other pairs of terrestrial frogs. In case this seems like an obvious result, note that arboreal Hylids in China and Colombia are, though closely related, no more similar to each other in morphology and performance than to other arboreal frogs.

But there’s plenty of adaptive divergence within clades and convergence across continents as well. Just look at the craziness in the Australian Hylids–diversification into every microhabitat!

Reptiles and amphibians seem to be good candidates for studies of trans-continental community convergence. We’ve already seen one example in desert lizards, and some ongoing work on geckos seems to be building another very interesting example. Now if someone would look at the question in snakes, which are rife with convergence, we’d have a shot at finding a convergent ecosystem! In all seriousness, that lizards and frogs occupy the middle of many food chains makes them excellent starting points from which to look both upwards and downwards, at both predators and prey, to broaden how we look at the role of dispersal and adaptation in creating similar communities and ecosystems across the world.

As must be apparent by this point, I’m a little obsessed with the patterns of trans-continental convergence. But I’m also in awe of the fieldwork that goes into producing a paper like this one. I asked Dan Moen about his experience doing fieldwork in three very different parts of the world. As one might expect, he found that fieldwork can be much more challenging in a place with an unfamiliar culture or language. Something as simple as where to buy basic hardware to set up a field experiment can involve a lot more thought in an unfamiliar place:

“The concept of going to a Home Depot to get anything I need for setting up the lab was out of the question [in China] – we instead went to the glass maker street, the wood street, the aquarium neighborhood, etc.”