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Sunshine A. Van Bael is a biologist, a doctoral candidate at the University of Illinois who’s looking for insights into one of the more heated ongoing debates in the field of ecology.

Scientists who study nature’s complex webs of interaction have been busy arguing about what is the more important factor in keeping much of the world green.

Note, for example, that from time to time in East Africa and the Middle East, especially when wars disrupt national and international pest control efforts, huge swarms of locusts hatch and eat all plants in their path. So what keeps Panama’s insect pests from doing the same with what remain of our forests?

The question is not just limited to green terrestrial zones, but also to marine environments. It gets complicated when there are multiple, sometimes interacting, “trophic levels” --- food chains ultimately dependent on the same source of nourishment.

One point of view is “top down” --- the forest remains green because birds, bats, spiders and other predators eat the insects that would otherwise strip the vegetation bare. Another point of view is “bottom up” --- the forest is green because the resources that make the trees grow produce more foliage than the little arthropod grazers can eat.

Of course, all but the most dogmatic biologists recognize that in most complex ecosystems a combination of top-down and bottom- up processes are at work. Sure, in a barren desert where except for after a rare rainstorm hardly anything grows, the limiting factor will largely be bottom-up: with almost nothing to graze, lack of resources rather than predation limits the number of grazers. (But then there’s that story about the Mormon settlers around the Great Salt Lake getting wiped out by a plague of locusts, until a flock of birds came into the picture.) But in a rainforest, wouldn’t it be the other way around, with predators rather than the amount of underlying resources limiting the activities of the leaf cutter ants, caterpillars, termites et al?

Van Bael had a hunch that in a tropical rainforest, the top-down effects of predation would be the more important limiting factor on herbivorous arthropods. It’s not that she has particularly thrown her lot in with top-down or bottom-up theorists. “There are a whole lot of mechanisms by which herbivores can be limited,” she told a larger-than-usual audience at the March 11 free science lecture at the Smithsonian Tropical Research Institute’s Tupper Auditorium. She explained that the question she’s researching is not so much whether defenses or resources are the most important factors keeping a forest green, but when and where each factor prevails.

Down here she found the Canopy Crane Project an invaluable resource to test this hypothesis.

The canopy cranes are large construction cranes set up out in the bush, one of them in Panama City’s Parque Natural Metropolitano, another at the former Fort Sherman. The cranes work as elevators that do minimal damage to the forests’ flora and fauna, but permit scientist to observe close-up and conduct experiments in places that would otherwise be difficult or impossible to reach.

Fewer than 50 miles separate the two cranes, but they’re on different sides of the Continental Divide. Sherman’s Atlantic side climate is much rainier than the capital’s, more moisture watering the plant growth than in the metro nature park.

Sunshine Van Bael’s contribution to the top-down vs. bottom-up inquiry was to use the two cranes, some agricultural netting with a mesh large enough to exclude birds and bats but not bugs, and her powers of observation to test the effects of predation. The two sites feature different species of vegetation, and at each place there are both canopy and understory trees. Given those complicating factors, then, she set up bird- and bat-excluding nets around branches of both canopy and understory trees, three species in each park, and left other branches in their usual state. Then, over the course of one year, she counted bugs, noted where she found them, and noted the damage that their grazing had caused to tender new leaves.

This was not the first predator-exclusion experiment in a forest, nor even in a tropical forest. However, Van Bael’s work was unique in that it compared two nearby but different tropical forests and both canopy and understory trees in each place.

She found many things that one might expect.

In the canopy trees of Parque Natural Metropolitano, rainy season arthropods tended to be limited by the predation of birds and bats. In the understory, predation wasn’t a significant factor. In the dry season there wasn’t much predation in either forest level. On the branches from which birds and bats had been excluded, there were more caterpillars, which were less likely to have the real or simulated spines or hairs of the “electric” (actually, venomous) variety, were less likely to camouflage themselves and were less likely to hide on the bottoms of leaves. Noting that these data might suggest different things to different observers --- insect larvae might hang out under leaves rather than on top of them to get a little shade as well as to hide from birds, for example - -- Van Bael wielded Occam’s Razor and concluded that the differences in insect behaviors and appearances found between the open and netted branches probably indicate that predators cull the obvious food first.

The biologist was cautious about the significance of her data. Other factors, like the effects of parasites on herbivorous insects, arthropod migrations, the choices that butterflies and moths make about where to lay their eggs and the small spaces that she studied could have affected her experimental results. “These data may underestimate bird predation,” she warned.

Meanwhile, in the canopy over Fort Sherman, Van Bael found no significant difference between the exclosure and control branches she studied.

Although in the metro nature park there is a much larger and more diverse bird population, Van Bael found that bird forage at about the same rate in both places.

At Sherman, leaves stay on the trees longer, are tougher for insects or others to chew, and have higher tannic acid levels than are the norms in Panama City’s park.

Are the insects that eat Fort Sherman’s leaves more prone to parasitism and disease? Van Bael posed that question, leaving it for other biologists to research.

The bottom line was that Sunshine Van Bael’s hunch that more rain and biological diversity would mean more predation didn’t withstand the scrutiny of scientific experimentation. It’s not that simple.

Expecting to find something else, Van Bael found that predation is important where leaf production rates are higher, and that leaf palatability is also an important factor. “Bottom-up factors make it possible for predation effects to manifest themselves,” she concluded.

So should Ms. Van Bael feel embarrassed that her expectations didn’t match reality? Should she hang her head in shame for her failure to find the Ultimate Answer to Life, The Universe, and Everything?

In case you didn’t notice while studying science in school, Newton, Pasteur, Mendel and Einstein didn’t uncover all the secrets of the universe either. They just advanced humanity’s knowledge.

“It increases our understanding of biological communities,” Van Bael said of her experiment’s results.

This, after all, is the sort of thing that scientists do.

Also in this section:
Bottom-up or top down?
Reforestation underway in Bolivia's Tipuani River Valley
Global Alert: Atypical Pneumonia