Too many plants

We’re nearly finished with our transects here. Over the past month we’ve installed fifty 50 meter x 2 meter transects, tagging all plants with a diameter greater than 1 centimeter. Today we finished the last one – 5,419 stems in all. Nearly all of these are small – seedlings, or small vines. Large plants require disproportionately more resources, so competition means that many seedlings die before they can establish themselves.

Working in this forest has brought its share of challenges. Many locations are tangled messes of lianas (woody vines) that make it impossible to stand up straight. Walking through the forest is more like playing a game of Twister. Below is our crew, stooped over and sweaty.

Here is another view of the forest – lianas going everywhere. We have to make sure we don’t double-tag or no-tag every plant. This can be an interesting challenge in places like this.

There are several challenges working here. First, many species are covered in long thorns that snag onto clothing and pierce the skin.

Second, large spiders are very common – here the golden orb weaver, Nephila clavipes. Fortunately they don’t seem to bite people, but they enjoy constructing webs at face-height.

And finally, there are many ticks that fall out of the trees and latch onto the skin. We do regular tick checks during the day but still manage to find five or ten on us by each afternoon. Most dangerous is the tick bomb – a leaf holding a cache of dozens of ticks, which (when touched) release a crawling swarm onto the arm or neck.

Nevertheless, this is a beautiful forest – only sometimes do we think that there are a few too many plants and animals in it!

Plants, ants, and ant-eaters

Some animals challenge the human tendency to assume our superior nature. Leafcutter ants (Atta sp.) are one of these species. These ants explore the forest, choosing only the trees with the tastiest leaves. They then send thousands of workers to systematically cut small sections from each leaf. The harvested material is carried back to their nest along trails they build in the forest – long paths devoid of all vegetation, highways for ants. Once the leaves are safely in the nest, they are fed to a certain species of fungus which is cultivated underground by the ants – and it is this fungus which serves as the ultimate food source for the leafcutter ants. Not bad for a species with a head the size of a pin!

Below is a short video I made of these ants on one of their trails through the forest. Watch for a flowers. Ants can carry many times their body weight, because of how muscle power and volume scales with body size. Small animals are proportionately much stronger than large ones.

Of course, ants sometimes meet untimely ends of their own. Here is an anteater, Tamandua sp. climbing up a liana, looking for arboreal ants. I surprised this animal, which was resting hidden in a vine tangle just next to my head – it made a dash for the canopy just as I pulled out the camera. This forest is full of surprises…

Treehugging

A fundamental property of an organism is its size. Ecologists like to measure trees’ diameters because they are good predictors of a tree’s biomass and thus its role in the ecosystem. Measuring tapes work well, but can become unwieldy on large trees. The hug then becomes the unit of choice.

Here is a large Jacaranda copaia – at least three hugs around.

And here is a tree larger yet – a thoroughly unhuggable Ceiba pentandra rising up into the sky. This is BCI’s “big tree” which merits its own symbol on maps of the island. I felt so small and young, looking up into its canopy.

Around the lab clearing

Only a small part of Barro Colorado Island has been cleared for laboratory and dormitory facilities. Here’s a short tour of the places we stay in – a small foothold against the living forest.

First, the island’s docks, where ferries and supplies come and go. The forest guards (guardabosques) who protect against poaching and other activities have a base here too.

Next, the cable car that lifts supplies to the higher points of the lab clearing. We walk this staircase each day on the way to working in the forest.

Our laboratory facilities, with equipment strewn about and a wayward guayaba hiding in the corner.

Another view of the lab, as early morning sun streams in.

And finally the view out our window – ships passing through the Canal in the early morning.

Warning signs

How best for an animal to defend itself against attack? Some succeed by virtue of their large size or ability to fight (think of a bear). But others take a more subtle path through the production of painful or poisonous chemicals. Predators prefer to avoid these animals, because consuming them is worse than consuming nothing. Of course, this sort of chemical defense generally requires advertisement – it does an animal no good to be highly poisonous if the predator only finds out after eating you. I say generally, because you can also imagine a situation in which the predator has a bad experience with one individual animal (who dies) but then avoids other individuals of the same species, producing a group-level benefit. And you can also imagine other situations where harmless animals mimic others that are truly dangerous, with the aim of deceiving predators… the natural world is baroque in its variations.

Here are two examples of Panamanian animals that rely on chemical defenses. The first is Dendrobates auratus, commonly known as a poison dart frog. The blue-green coloration indicates its ability to secrete a highly toxic chemical, probably derived from its diet of ants.

The second example is an unknown insect species, here shown in caterpillar form. The apparently fuzzy body is actually a dense mat of stinging hairs. I hadn’t the courage to find out how much this one hurts to touch, but the red coloration is an obvious deterrent. Anyone have an identification?

 

 

What’s the point?

Many plants have elaborate mechanical defenses that deter herbivory. Bactris palms, for example, have a formidable spines covering the stems and leaves. For a large mammal, no part of the plant is safe to touch, but for small insects, defenses may be evaded. Evidently the cost of building spines is outweighed by the deterrent benefit. Walking through the forest one must be very careful to not trip and fall on these trees, and be very cautious whenever a heavily defended frond falls from the sky in a windy moment.

What is more puzzling to find is a tree with a large trunk and heavy investment in thorns. This tree, for example, is nearly a meter in diameter, and each thorn is a centimeter long. What benefit do these thorns serve? There are no large animals here, to my knowledge, that eat bark or new wood.

Zooming in, you can see the scale of these thorns. Perhaps they are an adaptation defending against a now-extinct species, but to me it seems like there should be strong natural selection against investment in useless defense mechanisms like this one. Do you have any ideas for the origin of these structures?

 

Monkey business

This forest is never quiet. Imagine an deep grunting coming from high in the canopy, in long pulses – this is the sound of howler monkeys. I made a recording today which you can listen to here. It is a very ominous sound if you don’t know what kind of creature is making it.

Howler monkeys are entirely vegetarian, and spend most of their time foraging high in the trees. Here is one about to pull a fruit off of a branch with its mouth.

A downside to working around monkeys is their behavior. One animal found our base camp in the forest and threw us a present of sorts, seen below.

Fortunately howler monkey excrement doesn’t smell so bad, because of its all-plant composition. This was a nice surprise for the morning!

Creatures of the island

Fieldwork on BCI is full of small joys and large surprises in the forms of animals that share the island with us. Here a few of the herpetofauna we’ve encounted in the last two days.

First, a lovely small frog – less than an inch long, but beautiful. The leaf litter here is very dry, because there has not been much rain lately, so it’s not very difficult to hear small things hopping around.

Second, a much larger creature. We were scouting out our second plot, about to cross a ravine, when one of us saw a large black creature moving very quickly across the ground. There being several species of deadly venomous snakes in Panama, we beat a hasty retreat. The snake (five or six feet long, we think!) quickly twined up a tree, and we decided to leave work at this site for another day…

Turns out it’s a non-venomous, but rather aggressive and territorial, tiger rat snake (Spilotes pullatus). I took this photo with a 200 mm lens from a very long distance before we knew what it was…

Forest dynamics

Trees come in many sizes – but some tend to be very large indeed. This is a Ceiba (Bombacaceae) with a commanding presence on Barro Colorado Island. If I were in this picture, I would only reach two thirds the height of the main buttress. Large individuals like this one have a disproportionate impact on the forest, out-competing others for resources like light and water. Winners stay winners, just as in human economies, it is very difficult for a small company to out-compete a large company even if its products are better.

Of course, everything with a beginning has an end – large trees eventually will die. In death, the impact of a large tree is tremendous – as it falls, it can destroy dozens of other large trees and countless of small ones. The resulting gap left by this demolition is full of light – a valuable resource for other plants. The survivors and new germinants compete heavily for light, resulting in a tangle of new vegetation – and ultimately, the growth of more large trees, to repeat the cycle!

Last week our team laid down a 50 meter transect line through one section of forest. When we returned, we couldn’t find the meter tape, nor did the forest look familiar. Some time over the weekend, this beast of a tree fell down, collapsing a large section of forest and burying our equipment. I am very glad we were not working at the time this fell down!

Through the Panama Canal

Panama – a narrow strip of land separating two oceans, until we were audacious enough to build a canal between them. The majority of this canal is Lake Gatún, which was formed by the damming of the Chagres river. What were once hills and valleys became islands and flat water. I’ll be spending two weeks on Isla Barro Colorado, an artificial island set aside for scientific research.

The day began with a wait for the Gamboa ferry. Large cargo ships were passing, only minutes apart.

Ferry terminal security, on a hot, muggy and sunny day.

I took a water taxi bound for the island. We were moving at more than 25 knots, except when slowing to negotiate the waves made by cargo ships.

The scale of these cargo ships is hard to describe. Look for the staircase to the right of the ‘C’ in this picture…

… and compare it for scale with this sailor on board that same ship!

After a half hour’s journey, I walked onto the island – my new home for two weeks. It is a welcome change from the Arizona desert.

I’ll be updating this blog every day with photos and stories of the research.