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

4 thoughts on “Seed dispersal in an empty forest”

    1. Good question, Mindy. It’s a question that digs into poverty, and until we can improve social well being for so many people in developing countries this will continue to be a problem.

  1. Great story-telling, and fascinating, little-known facts about what keeps forests/jungles and animals together in the dance of health, abundance, and diversity. Keep ’em coming!

    1. Janina, thanks for the vote of confidence. I put a lot of thought into the blogs – nice to hear they are appreciated.

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