Don’t let the critics bug you: Benefits of increased CO2 outweigh risks from insects

Don’t let the critics bug you: Benefits of increased CO2 outweigh risks from insects
November 1, 2000

“Greening up.” The phrase expresses hope for the future. Yet detractors of the “greening up” paradigm—a belief backed up by literally thousands of articles in the scientific literature—remain.

Surely, critics argue, something will come along and spoil the benefits we have anticipated based on experiments on plant responses to the buildup of atmospheric carbon dioxide (CO2). What if, for example, the pumped-up plants of generations to come let down their guard against herbivores and lose out to an uncontrollable hoard of hungry insects? That would be a brown day indeed.

In fact, as countless published studies attest, the opposite seems to be true: CO2-enhanced plants generally increase their defenses against bugs and curtail the damage those unwelcome guests can do.

Two more articles bring more good news about the future of plants and bad news for the hungry herbivores of the insect kingdom.

Stiling and colleagues enclosed portions of a native scrub-oak community in Florida in open-top chambers and maintained atmospheric CO2 concentrations at the natural level of 350 parts per million (ppm) and 700 ppm. After one year of the experiment, the authors then focused on how elevated CO2 impacted the densities, feeding rates, and mortality of an herbivorous chewing insect known as leaf miner.

The researchers found that total leaf miner densities were 38 percent lower in the chambers where atmospheric CO2 was elevated.

Interestingly, although fewer in number, the hungry herbivores in the CO2-enriched chambers actually ate more than their bug-eyed counterparts in the natural-CO2 chambers. They feasted, to be sure. They even grew larger, but in the end they had a much higher mortality rate than the leaf miners in the natural-level chambers.

Why? In a word, wasps. It turns out that the leaf miner’s natural predator, the wasp, could more easily spot the oak-leaf-fattened leaf miners. A cooperative relationship between the plants of the scrub-oak community and the wasps that live there meant that the herbivorous leaf miners came up the losers in the elevated CO2 environment.

But what of the long-term CO2 enrichment impacts of herbivorous insects operating in a variety of ecosystems? Whittaker reviewed more than 30 articles, and it doesn’t look good for the six-legged herbivores. Indeed, only the nonchewing phloem feeders (such as aphids) seemed to increase their populations in elevated CO2 environments. Chewing insects (like the leaf miners) showed either no change or reductions in densities in CO2-enriched environments.

Whittaker’s review by no means leads to any prediction that herbivorous insects will destroy the biological benefits of higher levels of CO2. As Whittaker cautiously stated, "it is not known whether adaptation in insect-plant relations would occur over the decades required for this change [in CO2 concentration] to occur."

Even so, environmentalists (well-meaning and not so) tend to seize the notion that increased levels of CO2 in the atmosphere could result in a spread of insect pests that could disrupt or even destroy natural and managed ecosystems. Many popular presentations of the horrors of global warming and the greenhouse effect hold that argument as central to the threats our enhanced-greenhouse future promise us.

But when considering the relation between humans and plants, it is wise to take a lesson from our agricultural history. Agriculturists and other ecosystem managers have perfected their craft over literally thousands of years, and knowledge regarding ecosystem dynamics is increasing today at an exponential pace.

Technological developments--including irrigation to improve hydration, horticulture to refine plant type, and pesticides to control or eliminate unwanted insects--have caused crop yields to increase several-fold in the past century alone. And though it’s controversial, genetic engineering will no doubt continue to provide opportunities to look at herbivorous insects as another villain to be bested with a few tweaks to DNA here and there.

With science on our side, we are also better prepared to protect our forests and grasslands from migratory infestations that might otherwise decimate the plant life there.

Without question, future generations of humans can turn to ever-increasing resources for improving the relationship between insects and plants. Not only can we humans manage Nature, but Nature is pretty good at adapting to us humans.

As the research into leaf miners and CO2 makes clear, natural predators may come along and reduce insect impacts on plants, without human interference at all. Ecosystems have been changing throughout the long history of the Earth, and they will certainly change in the decades to come. There is one major difference: humans will have an ever-increasing ability to alter that change in any direction we consider desirable.

Nuisance herbivorous insects better not count on increased CO2 as an atmospheric colleague that grants them a competitive advantage in the global ecosystem. And nuisance environmentalists had better not count on that possibility as a reason to put stringent restrictions on CO2 output.

The hard scientific evidence is pointing the other direction.


Robert C. Balling Jr., Ph.D. is director of the Laboratory of Climatology at Arizona State University and coauthor of The Satanic Gases.


References

Stiling, P., et al., 1999. Decreased leaf-miner abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack. Ecological Applications, 9, 240-244.

Whittaker, J.B., 1999. Impacts and responses at population level of herbivorous insects to elevated CO2. European Journal of Entomology, 96, 149-156.