Tag Archives: seed dispersal

Seed dispersal in an empty forest

Scientific thinking works like this:  First, we make observations about the natural world.  Next, based on those observations we can make inferences, or hypotheses, that are possible explanations for our observations.  Finally, we can predict what will happen if we somehow alter the set of circumstances upon which our observations and hypotheses are based.  The last step is to conduct tests to see if the hypotheses and predictions are true.  This is how scientists go about learning how the natural world works.

Let’s do this with seed dispersal and see what we can learn.  Based on previous blogs, let’s start with the observations.

OBSERVATION #1:  Plants don’t fly.

Um, yeah.  I’m a reasonable guy, and I can buy that one.  Because plants don’t fly, some species in the rainforest use sheer trickery to get animals to eat their fruits and disperse the seeds all over the land where they will grow into new plants.  Think of this as survival of the fittest, plant style.  Doesn’t sound familiar?  Read some earlier blogs.

OBSERVATION #2:  The forest needs animals.

Um, wait a minute.  Isn’t it the other way around?  Without a forest the animals don’t have a place to live, therefore animals need the forest.  Right?  Perhaps, but the opposite is also true.  What we learned in two earlier blogs is that without animals like toucans to disperse seeds, many forest plants, from lofty trees to pesky mistletoes, don’t stand a chance of ever spreading their genes into a next generation.  So yes, the forest needs animals, as in winged and legged dispersers of seeds.

OK, because plants don’t fly, the forest needs animals.  Now the scientist in me asks, “What will happen to the rainforest if the animals were to disappear?”  Maybe the seed dispersers go on strike or something.  For a scientist with too much time on his hands it is simply too much drama.  Even better, it’s the subject of today’s blog, and we will give our drama a name, “Seed dispersal in an empty forest.”  I bet you can’t wait!

But first, let’s take a trip to the rainforest and meet the cast.

White-faced monkey, Cebus capuchinus, seed disperser
White-faced monkey (Cebus capuchinus). Monkeys are major dispersers of large seeds. Monkeys love to eat fruits, they clamber all around the forest canopy, and they are messy eaters who drop stuff they don’t want, like seeds. Photo © Roy Toft.
Central American agouti, Dasyprocta punctata.
Central American agouti (Dasyprocta punctata). Agoutis are rodents in the tropical forest that eat seeds, unlike other animals that are after the fruit. They carry seeds in their mouths, some get dropped here and there, and one day a lucky seed carried away by an agouti will grow into a nice sapling and later a large tree. Photo © Roy Toft.
Toco Toucan, Ramphastos toco, seed disperser
Toco Toucan (Ramphastos toco). Toucans have large bills for a reason. They use them to reach out and pluck fruits from trees. But they don’t poop the seeds out; after flying through the forest they cough them up. Many seeds actually require this type of treatment. They are evolved not to germinate unless exposed to the acid in animal stomachs. Photo © Roy Toft.

Now let’s skip ahead and visit some predictions about the empty forest.

PREDICTION #1:  If we lose our seed dispersers from a particular forest, there will be a decline in plant species with large seeds that rely on animals to “fly” their seeds for them.   Makes total sense.  Without seed dispersal those large seeds will fall under the parent tree where most will rot in piles, and the few survivors will choke each other out in a battle for sunlight.

PREDICTION #2:  In the empty forest, there will be fewer sapling trees, period.  This one is more complicated, so let’s think about it.  Seeds that are transported away from the parent tree are more likely to live, and most of those that fall under the parent will perish.  Without animal dispersers to help out, few seeds in this forest ever get to travel to safe sites, and fewer ever germinate and grow into trees.  So, overall we predict fewer total saplings than in a nearby forest with all its animal dispersers still in place.

Now for the weird part – I’m not making this stuff up!  This little drama has actually happened in real life and scientists are watching the whole thing.

Screeeech! – Rewind!  The forest lost its animals?!?  Yes.  In developing countries in the tropics like Brazil, Colombia, and Peru, food is hard to come by.  When a new road punches through the rainforest for logging, mining or oil exploration, humans move in, and hungry humans fan out with rifles and eat the fauna.  Agoutis, toucans, monkeys – that’s what is on the menu.  Given enough time, a small community of people can totally eradicate the larger wildlife species in the forest.

And come to find out, when animals disappear from the forest, the whole forest gets turned on its head.  I didn’t have to make up today’s predictions; I borrowed them from Dr. John Terborgh at Duke University.  Terborgh and his team counted animal seed dispersers and saplings in two forests in Peru.  The forests are only 90 km apart and the only difference is that in one the large wildlife that disperse seeds are basically extinct due to hunting, and in the other no hunting takes place.

The predictions turned out to be true.  There are fewer saplings in the empty forest, and of those saplings the scientists found, almost none grew from large seeds dispersed by animals.

This is bad news for biodiversity.  The preservation of biodiversity depends on natural balance.  Once we lose the trees with large fruits that animals eat, this forest will never again be home for toucans and monkeys.  Next we lose the animals that eat animals, like jaguars and eagles.  A rainforest without wildlife just isn’t a rainforest anymore.  It’s quiet, lonely, and just plain unnatural.  Biodiversity goes in the tank.

Animals need forests.  Forests need animals.  And so do we.

jaguar, Panthera onca
Jaguar (Panthera onca), largest cats of the Americas. Photo © Roy Toft.
Harpy Eagle, Harpia harpyja, The Peregrine Fund
Harpy Eagle (Harpia harpyja), largest eagle in the world. At the top of the food chain, they depend on the animals that eat seeds too. Photo © José de Jesús Vargas Gonzales of The Peregrine Fund.

Photo Credits

CWI thanks Roy Toft of Roy Toft Photography and Photo Safaris for another fine set of professional photos.

References – for more information.

Terborgh, J., G. Nuñez-Iturri, N. C. A. Pitman, F. H. C. Valverde, P. Alvarez, V. Swamy, E. G. Pringle, and C. E. T. Paine.  2008.  Tree recruitment in an empty forest.  Ecology 89(6): 1757-1768.  pdf

Seed dispersal in the rainforest canopy: mistletoe and the Lovely Cotinga

One of the tallest trees in the Neotropical rainforest, the mighty nutmeg, depends on toucans and rodents to disperse its seeds to places where they can grow. Here is a tree that can reach 150+ feet (45+ meters) in height, and without the help of rainbow-colored birds and bucktoothed mammals its seeds are doomed. What happens to small plants that grow in the forest canopy 100+ feet off the ground and don’t even have roots in the soil? Do they also need wildlife to disperse their seeds?

Mistletoe is great example of a plant that grows high up in the forest canopy and never touches the ground. Never touches the ground?? How does a plant do that? Epiphytes are those plants that grow on the surface of other plants. In a shady forest all the plants are competing for light. Some plants like the nutmeg tree win this little battle by growing head and shoulders above the rest. Other, smaller plants win by groing on the branches of trees, high up in canopy where they get all the sunlight they need. Orchids. Bromeliads. Cacti. These are just some examples of plants that bask in the sunlight on the branches of trees high in the rainforest. They get sun from above, water from the rain, and when leaves decompose around the roots that they grow along the surface of branches, they get their food. But what do these plants do with their seeds? And how did that puny plant get way up in the tree anyways? For a mistletoe, and lots of other plants, the answer is birds again.

Here is one of my favorite birds of the Neotropical rainforest, the Lovely Cotinga. This bird oozes cool! It’s a day-glow, neon turquoise blue with a plum purple throat. I bet astronauts can see these dudes from space.  And yes, the actual name of the bird is “Lovely Cotinga.”

A male Lovely Cotinga perches like a jewel atop the rainforest. Photo © James Adams of the Lodge at Pico Bonito.
A male Lovely Cotinga (Cotinga amabilis) perches like a jewel atop the rainforest. Photo © James Adams of The Lodge at Pico Bonito.

 

And here is the object of cotinga desire, Psittacanthus rhyncanthus, a species of mistletoe. There is nothing a cotinga loves more than to chow down on mistletoe fruits.

 

Mistletoe berries (purple fruits lower left) are the gas that light up a male Cotinga like this one. Photo © James Adams of the Lodge at Pico Bonito.
Mistletoe berries (purple fruits lower left) are the “juice” that light up a male Cotinga like this one. Photo © James Adams of The Lodge at Pico Bonito.

And herein lies our rainforest intrigue. The mistletoe, like the nutmeg, is a trickster. It puts out hundreds of small fruits that birds like the Lovely Cotinga love to eat, but this time the joke is on the birds. After a cotinga or other unsuspecting bird goes in for a tasty meal of fruits it finds out that its gullet is full of seeds, seeds that are so sticky, so gooey, it’s like having a mouthful of glue globs. To rid itself of these seeds the cotinga has to literally wipe its face across the surface of a tree branch until the seed sticks to the branch. Voila! Just like that a mistletoe is born. From branch, to bird, to branch, the mistletoe never leaves the forest canopy and is transplanted by an agent in electrified blue feathers. Stranger than fiction? That’s life in the rainforest.

Don’t believe it? You can see for yourself at places like The Lodge at Pico Bonito, Honduras, where Lovely Cotingas show up by the flockful every year during the rainy season from January to March. James Adams, a manager at the Lodge, witnessed an interesting dispute over some mistletoe between two cotingas.

An adult and a juvenile male Lovely Cotinga dispute a favorite perch. The tree branches are being killed by mistletoe plants spread by the contingas themselves. Photo © James Adams at the Lodge at Pico Bonito.
An adult and a juvenile male Lovely Cotinga dispute a favorite perch. The tree branches are being killed by mistletoe plants spread by the contingas themselves. Photo © James Adams at The Lodge at Pico Bonito.

James tells us: “Evidenced by the numbers of mistletoe stuck to this branch [note the little green leaves under the branch – that’s them], Cotingas have favored perches. And apparently they don’t like to share with just anyone. When this young male (note the maturing purple and blue plumage) flew in and tried to accompany this bright blue male, the older male would have none of it. And so a war of silent beak gaping ensued, with both parties opening and closing their beaks at one another, until the younger bird flew off.”

In other words, it appears from watching these colorful birds that they need the mistletoe just as much as it needs them.

Parting shot: Gotta love these guys, right?

Lovely Cotinga (Cotinga amabilis) in Pico Bonito National Park, Honduras. Photo © Roy Toft
Lovely Cotinga (Cotinga amabilis) in Pico Bonito National Park, Honduras. Photo © Roy Toft

Seed dispersal in the tropical rainforest canopy. How does your garden grow?

Seeds of the wild nutmeg (Virola kochnyi).  The red coating is a thin layer of fruit over a huge seed. Photo © David L. Anderson.
Seeds of the wild nutmeg (Virola kochnyi). The red coating is a thin layer of fruit over a huge seed. Photo © David L. Anderson.

Question: What do pumpkin pie, rainforest trees, and toucans all have in common? The answer, quite obviously, is nutmeg! Nutmeg is one of my favorite spices. I use it in pumpkin pie, hot chocolate, apple pie, and some Indian curry recipes. I don’t buy it in powder, though, but in whole seeds, which I grate into the foods I’m cooking. When I want a hot, wintery spicy dish, nothing satisfies better than nutmeg.

You might be surprised to find out that toucans agree with our liking for nutmeg. The seed in the photo above – that’s a wild nutmeg seed growing high in the canopy of the rainforest. When the fruit ripens the husk pops open exposing a plump seed covered in delicate tendrils of fruit. It’s the fruity layer that the toucans are after. A toucan will reach out with its long bill, pluck a seed, swallow it, and fly off through the forest, where it digests the fruit and spits out the seed. If you are lucky enough to find a big nutmeg tree with fruit it will literally be dripping with toucans.

Yellow-eared Toucanet (Selenidera spectabilis) getting ready to swallow a wild nutmeg seed.  Photo © James Adams.
Yellow-eared Toucanet (Selenidera spectabilis) getting ready to swallow a wild nutmeg seed. Photo © James Adams.

Now for the cool part. This toucan thing, flying away and spitting seeds in the forest, is all part of the nutmeg tree’s secret plan. If all those seeds were to fall directly under the parent tree and sprout, the seedlings would wither and die. They won’t get sun under a huge tree, and thousands of seedlings will choke each other out in competition for sun, nutrients, and water, and rodents will feast on seeds by the cheekful because, like toucans, they know where to find a good meal. In the battle for survival that happens every day in the rainforest, the tree needs to send its seeds far and wide if any are to become the next generation.

 

Down the hatch!  After the toucanet flies off it will cough up the seed.  With luck a new nutmeg will sprout.  Photo © James Adams.
Down the hatch! After the toucanet flies off it will cough up the seed. With luck a new nutmeg will sprout. Photo © James Adams.

 

Seedling of a wild nutmeg, "planted" by a toucan or other seed dispersal in the rainforest.  Photo © David L. Anderson.
Seedling of a wild nutmeg, “planted” by a toucan or other seed disperser in the rainforest. Photo © David L. Anderson.

In a natural arms race, nutmeg trees produce the thin fruity arils on their seeds to attract seed dispersers like toucans. Growing all the fruit costs energy, hence the arils are so thin that they are barely there. But it’s enough to lure in toucans by the droves. A nutmeg can’t grow its own garden of seedlings, raising them to nice young trees and planting them in the sweet spots in the forest where one day they’ll become living towers in their own right. Instead, they rig the game and get birds to help. A little bit of fruit for you, dear toucan, and seed dispersal for me, thank you very much.

 

In a future blog I’ll discuss how seed dispersal affects the biology of the entire forest community of trees, mammals, and birds.