Sep 26, 2015 by

Scientists are starting to examine the role the tiniest critters play in preserving ecosystems.

An earworm on an ear of corn. (Photo: Flickr)cornearworm=
Richard Conniff is the author of The Species Seekers: Heroes, Fools, and the Mad Pursuit of Life on Earth and other books.


Odd bits of recent wildlife news, mostly about very small and obscure species, have lately left me thinking about a game called Jenga. If you happen never to have played it, here’s how it works: The game consists of small wooden blocks, and you start by assembling them into a tower, with each level consisting of three blocks laid horizontally and the layers arranged crisscross to one another. On each turn, a contestant removes one block and places it on top, the point being to remove as many blocks as possible without causing the whole thing to collapse.

Believe it or not, something called the “Jenga hypothesis” has become an alternative model for understanding how ecosystems really work. In the conventional paradigm, keystone species are typically predators at the top of a food chain, and plenty of evidence has demonstrated that removing them can cause a “trophic cascade” of unexpected and sometimes catastrophic changes. It’s like removing the “keystone,” the central stone that holds up an architectural archway, and watching everything fall down. Take sea otters out of the coastal Pacific, for instance, and the populations of sea urchins, abalone, and other invertebrates they feed on predictably boom and demolish the kelp forests, and so on down the ecosystem.

But according to the Jenga hypothesis, first proposed in the journal Science in 2005, the keystone analogy is too static. It misses the highly dynamic character of real ecosystems, which change continually because of an endless and unpredictable host of factors. Jenga makes a better analogy for that complexity because of its constantly shifting weights and stresses.

Much as any individual block can decide the game, the Jenga hypothesis suggests, even the most obscure species—a humble plant-eater, say—can turn out under certain circumstances to be the keystone of an entire ecosystem. Sometimes you can remove a species from a habitat and other species make up for the loss, allowing the ecosystem to go on as if nothing has changed. In other circumstances, you pull out that same species and the tower comes crashing down.

The first of the studies that got me thinking about the Jenga hypothesis has to do with the copepod Calanus finmarchicus, a marine crustacean about the size of a grain of rice. There are a lot of these grains of rice in the North Atlantic, billions or trillions of them. European researchers have discovered that they swim down to a depth of more than half a mile for their winter hibernation. That’s deep enough that the water does not come into contact with the atmosphere, and the carbon dioxide the copepods release there is effectively sequestered, perhaps for thousands of years. The bottom line, the researchers report in Proceedings of the National Academy of Sciences, is that—good news—we can reasonably double the estimate of the amount of carbon being taken out of the atmosphere by the oceans. That is, unless circumstances change, for instance, because of the climate change that’s already happening. In that case—bad news—loss of Calanus finmarchicus might just become key to the collapse of the ecosystem called Planet Earth.

Given our tendency to think in hierarchical, top-down terms, it might be hard to accept that bottom-feeders can play a critical role in making the world work. But in truth, our own system of producing food depends on this reality. For another study recently published in PNAS, researhers carefully screened off cornfields by night to see if they could quantify just how much difference bats make in controlling agricultural pests. The screens were rolled back by day to provide free access to birds and other pest-eating species. Even so, the absence of bats by night meant that corn there had a higher level of fungal infestation, 60 percent more corn earworms, and 50 percent more kernel damage. The authors extrapolated their results—not counting the amount bats save farmers in avoided pesticide spraying—and calculated the benefit to the global crop at $1 billion annually.

The numbers are even higher for the agricultural work performed by honeybees and other pollinators. In the United States alone they contribute anywhere from $4 billion to $8 billion a year in essential services. Because pollinators (and also bats) are in sharp decline, farmers must often struggle to replace them. In China, they now have to hand-pollinate some crops.

But the Jenga hypothesis isn’t really about agriculture. It’s about the natural world in an age of global mass extinction caused by humans. Species are now disappearing at 10 to 100 times the normal rate of extinction, and that rate is likely to accelerate as climate change and ocean acidification kick in, and as early-wave extinctions cause knock-on extinctions among other species that depended on them. In Hawaii, for instance, the loss of a native pollinator species means conservationists must use a paintbrush to hand-pollinate one native shrub species to keep it from also becoming extinct. But you cannot hand-pollinate an entire planet.

The Jenga hypothesis asks us to think about each species—and even about each new oil well, each new housing development, each new logged forest that imperils a species—and ask: Is this the one? Is what I’m doing part of the great undoing? Is this the crucial block that will bring down the tower on us all?

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