The Andes from space. Photo copyright Calvin J. Hamilton, from his website, Views of the Solar System.

Of Mountains and Valleys

Another look into why South America has so many birds

The Andes: one of the defining geographical features of South America, filled with unique cultural and biological heritage — and a source of ornithological debate since the 1910s.

Why?

To answer this question, let’s return to the fascinating field of biogeography (which we have visited before here and here). Focusing on how species distributions change in space and time, biogeography is unsurprisingly interested in places where there are lots of species. If diversity is high, something has happened to generate it. And that something, whatever it is, probably started with geography.

In the world of biogeography, mountain ranges like the Andes are of particular interest. Rising mountains brings about new ecological opportunity at ever-higher elevations, providing resources for the taking by those species that could adapt to the uplift. This process eventually led to distinct communities in each elevational “band.”

For those species that couldn’t adapt to new elevations, the uplift of the Andes would have split up their once continuous lowland distribution. Unable to adapt to high elevations, the lowland species would had to heed the rising Andes as a barrier, becoming gradually disconnected with their counterparts on the other side. Simply enough, right?

4 bird species for whom mountains have played a role in their speciation. Clockwise, from top right, we have Dark-faced Ground-Tyrant, Millpo Tapaculo, Buff-breasted Mountain-Tanager, and a Flame Bowerbird just for the fun of it. All these photos are from the fantastic Nick Athanas. Seriously, go check out his stuff.

Well, it may not be that simple. Yes, the Andes split many bird distributions that were once continuous. But birds can fly. And the entire Andean range isn’t all the same high elevation. Where there are peaks, there must also be valleys and passes. Couldn’t the birds disperse across the range via these valleys?

Admittedly, assuming that the Andes was the main event that split up once-continuous bird distributions is an elegant model. It’s simple, it makes sense, and it can be applied to all sister species on either side of the Andes. If there’s anything that theory-builders want, it’s universality. Theoreticians want to be their field’s own Newton, discerning universal laws by which everything they study must abide.

Frank Chapman. Photo from the Encyclopedia Britannica.

In 1917, AMNH ornithologist Frank Chapman posited that once the Andes rose, the species splits it caused were final. He envisioned “a pre-Andean fauna, the Pacific portion of which has been cut off from that of upper Amazonia by the Andean uplift.” To him, it made the most sense to think of South American avifauna in terms of before and after the uplift of the Andes, basically denying that dispersal across the Andes occurred much if at all.

For a long time, this assumption held, in part because of its simplicity: the uplift of mountains leading to allopatric speciation. It’s perfect for textbooks.

Moving forward to the 1960s and 70s, the question of dispersal came to the fore. Jürgen Haffer, the creator of the theory of Amazonian forest refugia, wasn’t satisfied with Chapman’s model. Originally trained as a geologist, he realized that much of Western South America was covered by shallow seas before the Andean uplift. As such, there wouldn’t have been any continuous ranges split up by the new mountain range. Instead, the uplift of the Andes would have created new rainforest landscape to their west. Still supporting that the Andes resulted in speciation to the East and West, Haffer argued that the uplift on its own is not what caused it. Instead, he posited that dispersal through Andean valleys during climatically humid periods is what allowed allopatric speciation later. When the climate is generally more humid, lowland rainforest can expand upward into higher elevations, allowing lowland species to spread where they couldn’t before. With Haffer’s model, we expect periodic dispersal through the Andes to be the main cause of increasing bird diversity (diversification), not the uplift of the Andes itself.

In order to put Chapman’s model and Haffer’s model to the test, we need to look at species whose closest relative (sister species) is right on the other side of the Andes. If the molecular clocks in DNA show that speciation occurred around the same time that the Andes rose, Chapman’s simple model triumphs. If speciation has occurred episodically through history, with no correlation with the uplift of the Andes, Haffer’s dispersal model wins out.

Since 2000, a number of comprehensive analyses (see citations for just a few) of South American bird speciation have formed a scientific consensus: Haffer was on the money. The data support that speciation was initiated by dispersal across the Andes. Remarkably, a geological event at as massive a scale and as rapid a rate as the Andean uplift was not what had the most influence on bird diversification. Instead, those periods of time when birds were triumphant at traversing the new mountain range, when the climate was just right, had the most influence. It’s not the barrier itself. It’s the episodic cross of the barrier that counts.

On top, we have Figure 4 from Cadena et al. 2016, which compares present-day to past distributions of White-bearded Manakin (photos of male and female by Nick Athanas). On the left, the green circle highlights that during a humid stretch in the Pleistocene (21,000 years before present), White-bearded Manakins were probably able to cross the Andalucía Pass. On the right, this path, from A to B, is depicted (Yellow/orange depicts suitable habitat, while blue/purple is unsuitable). The bottom image portrays the historical potential distribution, showing that the pass was much more accessible then than now.

In spite of the elegant simplicity of Chapman’s model, I delight in Haffer’s dispersal model. Why? Because this model allows for idiosyncrasy; every species could have its own unique pattern of dispersal and diversification across the Andes. There may not be universality, but it makes for a richer story behind the vast diversity of birds in South America.

A diversity of stories to match a diversity of birds? Sounds about right to me.

A wonderful depiction of the diversity of birds, produced by the National Audubon Society in celebration of Noah Strycker’s 2015 World Big Year, in which he saw a record-smashing 6,042 bird species. Read his reflections on it here.

Citations:

Cadena, Carlos Daniel, Carlos A. Pedraza, and Robb T. Brumfield. “Climate, habitat associations and the potential distributions of Neotropical birds: Implications for diversification across the Andes.”Revista de la Academia Colombiana de Ciencias Exactas, Físicas y Naturales 40.155 (2016): 275–287.

Miller, Matthew J., et al. “Out of Amazonia again and again: episodic crossing of the Andes promotes diversification in a lowland forest flycatcher.” Proceedings of the Royal Society of London B: Biological Sciences 275.1639 (2008): 1133–1142.

Smith, Brian Tilston, et al. “The drivers of tropical speciation.” Nature 515.7527 (2014): 406–409.