It Takes Toucans for These Trees to Tango (Well)

Green-billed toucan (Ramphastos dicolorus), by Cláudio Dias Timm  via the Encyclopedia of Life.

Green-billed toucan (Ramphastos dicolorus), by Cláudio Dias Timm via the Encyclopedia of Life. Creative Commons info.

I’m fascinated by the relationships between species, particularly when you bring evolution into the picture. So I was pleased to meet a collection of birds like this charmer living along the southern coast of Brazil, courtesy of a recent Science article and its resulting coverage, and I wanted to share the acquaintance with all of you.

But first, let’s take a step back, this being lunch time, to…fruit!

Fruit, despite what the Supreme Court in 1893* and your grocery store‘s produce section would have you believe, is defined by the seed(s) it carries.** Fruit is basically a bribe to get animals to carry seeds away from their parent plant. Frugivores (fruit-eating animals) are one of the key techniques for getting seeds dispersed (others include wind, water, and passers-by). Scurry away, munch your treat, and happen to leave seeds lying around.

Of course, different animals prefer different types of fruit: a squirrel may carry off your blueberries but leave your watermelons alone. A key factor in determining whether a particular seed will be dispersed by a particular animal is size, whether of fruit or seed or both.

Euterpe edulis. Photo by João Medeiros via Wikimedia Commons. Creative Commons license.

Euterpe edulis palm. Photo by João Medeiros via Wikimedia Commons. Creative Commons info.

Take, for example, the palm Euterpe edulis, which lives in Amazonian rainforests along the Atlantic coasts. It’s a staple of these forests, and it feeds or houses about 60 species. Its seeds, nestled in a thin skin of fruit, rely on birds for success, both to get the seeds out of the pulp that surrounds them and to carry the seeds away from other individuals that would compete for resources. But will any bird do?

To find out, the team in Science collected 9000 seeds from 22 populations of palm trees: fifteen representing healthy ecosystems and seven where large-billed (wider than about a half inch) birds like toucans, toucanets, and cotingas have gone functionally extinct due to hunting and habitat loss, leaving primarily birds with smaller beaks. (A functionally extinct species may not have entirely disappeared from a habitat, but its population is too small to cover its ecological to-do list, so to speak.) The difference in who eats palm fruit in these two types of ecosystems is pretty impressive: small-beaked birds eat just a third of them in forests they share with large-beaked birds, but up to a whopping 98 percent where they rule the roost.

White-necked thrush (Turdus albicollis). Compare his beak to the toucan's. Photo by Carlos Henrique via the Encyclopedia of Life. Creative Commons info.

White-necked thrush (Turdus albicollis). Compare his beak to the toucan’s. Photo by Carlos Henrique via the Encyclopedia of Life. Creative Commons info.

But in tests on birds in captivity, the researchers found that small-beaked birds, like the white-necked thrush, never managed to successfully disperse seeds any larger than about a half inch (12 mm) across. So what does that mean on the ground in the forests where these small-beaked birds are all palm trees can rely on to take care of their seeds?

In the seven forests where white-necked thrushes and the like are left to their own devices, the vast majority of seeds the team found were less than 12 mm across. In three they found no larger seeds, and only a handful in the other four. In the forests where toucans and their like had a significant presence, every site had some larger seeds, and they made up at least half the sample at a handful of sites. Check out a picture showing two different groups of the seeds over at BBC.

That’s a pretty stark difference, but why does it matter?

In the short term, it matters because smaller seeds in this palm have been shown to produce smaller plants, and because smaller seeds don’t do as well when it’s warm and dry (and climate change will make that more common, not less).

It also matters because those numbers represent all the seeds the researchers found, not just the ones that eventually grew into trees or some other measure of success. The raw material of the plant’s next generation is noticeably different under the two conditions. It suggests that where there have been no large-beaked birds to disperse larger seeds, the smaller seeds have grown up in their stead and produced their own, generally smaller, seeds. And that, my friends, is evolution, with bird beak size being the evolutionary pressure.

Combine all of these factors and toucan-less forests may be looking at troubled times for one of their key tree species, a.k.a. troubled times for the forests. But unless you have a deep personal connection with Brazilian forests, you might not care.

Here’s another reason this research is cool: how quickly this all has happened. Human-driven changes to these forests in Brazil started in earnest in the 1800s. Their modelling, based on the breeder’s equation (which is sometimes sticky to apply to real life, like so many equations in biology), suggests that the difference in seed size observed between the two types of sites could have arisen in as little as 75 years. If you think evolution is slow and boring, don’t–new examples suggest rapid evolution is much more common than we’ve realized.

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*H/t to my high school lab partner, Sophia, for first introducing me to this case, a gift that just keeps on giving.

**Jed Bartlet would want me to point out that strawberries are the only fruit with seeds on the outside. In another example of the people of YouTube completely failing me, the clip is not online; you’ll have to read the episode transcript or I hear the series is on Netflix.

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