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.

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.

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

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.”

 

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.

 

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.

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?