The greenhouse diet
You may be familiar with the dangers of climate disruption as a result of human-induced increases of carbon dioxide (CO2)
and other greenhouse gases in the atmosphere. But recent scientific
discoveries hint at disastrous disruptive effects of increased CO2 concentrations on ecosystems - effects that are quite distinct from the climatic effects of this gas.
Carbon dioxide is the gas that plants use for photosynthesis, the process by which they produce carbohydrates for their growth. Increased atmospheric CO2 allows more rapid photosynthesis, and thus increased carbohydrate production and plant growth. But nitrogen uptake by the plant does not keep up with the increase in carbon uptake. Since protein synthesis demands nitrogen, plants that grow faster under high-CO2 conditions end up with a lower ratio of protein to carbohydrate than they'd have under normal atmospheric conditions. Hence, the quality of the plant as a protein source plummets. Insects feeding on the plant may have to eat more to get enough protein. The result: insects face increased malnutrition, starvation, attacks from predators, and overall death rates. Insects are crucial to most ecosystems, and their depletion could cause ecosystems to crash. The potential effects of protein-deficient plants on vertebrate herbivores, such as sheep and antelope, appear ominous as well.
David N. Karowe, an entomologist at Western Michigan University in Kalamazoo, fed cabbage white butterfly caterpillars leaves from plants grown in two different CO2 chambers: one in which the air matches today's CO2 levels (about a third higher than pre-industrial levels), and one in which the concentration of the gas was double that. Caterpillars that ate carbon-enriched leaves ate 40 percent more plant material than controls did. Yet they still did not acquire sufficient protein: the caterpillars' growth slowed by about 10 per cent, and it took them much longer than normal to develop into adults. When the adults finally emerged, they were smaller than those raised as caterpillars on leaves of plants grown at today's atmospheric CO2 levels.
Richard Lindroth of the University of Wisconsin-Madison found that gypsy moth caterpillars ate significantly more than controls when fed leaves of aspen, birch, and oak trees grown in high CO2 concentrations. They also grew into much smaller adults.
Lindroth says that caterpillars of both moths and butterflies generally tended to eat more, yet ended up smaller, when fed plants grown in elevated CO2.
Moth and butterfly caterpillar feeding studies are generally done in the laboratory, as it is very difficult to track individuals in the field. But other insects are less difficult to study in the wild. Peter D. Stiling of the University of South Florida and his colleagues studied leaf miners in nature. Leaf miners are easier to study in the field, as they spend their entire larval stage inside "mines" (small blisters carved out within leaves), which the miners depart when they become adults. It is easy to tell how much the larva has eaten by the size of its mine.
The mines also record the insects' fates. If the larva starves to death, the shriveled body can be found in the mine. Predation leaves telltale signs as well. After small predatory wasps make a meal of leaf miner larvae, they burst out of the mines, creating shotgun-like holes. Ants, spiders, and lizards rip open mines to devour the miners. And leaf miners that survive the larval stage slice a distinctive round or crescent-shaped escape hole, then drop to the forest floor and metamorphose into adults.
Stiling and his colleagues pumped CO2-enriched air into small artificial chambers in the field, and normal air into other such chambers. Oak leaves with leaf miner larvae were placed inside these chambers, which were open at the top to allow moisture and insects to get in and out. Insects in chambers with high CO2 levels dug out mines 20 percent larger than those in the normal-air control chambers, indicating that they had eaten much more leaf fiber. Yet autopsies showed that the leaf miners in the chambers with high CO2 levels were twice as likely to die of starvation as the insects living in the chambers with standard air. Since the leaves were not protein-rich enough to support them, they ate more and still starved.
In addition, compared with leaf miners in chambers with regular atmosphere, four times as many insects in the high-CO2 chambers were killed by parasitic wasps. Stiling speculates that bigger blisters might make the miners easier for the wasps to locate. And if a miner spends more time growing in the mine, its predators have more time to attack it.
A matter of chemistry
Increased CO2 may also interfere with the complex chemical systems by which plant and insect populations communicate.
Plants release chemicals into the air when an insect bite wounds them, and parasitic wasps can home in on these chemical signals. Some researchers think that increased atmospheric CO2 could prompt insect herbivores to telegraph their presence to their enemies by this mechanism: the more damage an insect does to a plant in its quest for protein, the more "distress" chemicals the plant will emit.
Parasitic and predatory wasps are also attracted by caterpillar droppings. Caterpillars raised in elevated CO2 levels produce up to twice the normal amount of waste, both because they are eating more and because the leaves are harder to digest. But the situation may not work to the wasps' advantage. Malnourished caterpillars cannot support as many parasites as caterpillars of normal size and nourishment can. Wasps that emerge from malnourished caterpillars tend to be smaller and lay fewer eggs.
Aphids, another common plant-eating insect, actually reproduce 10 to 15 percent faster under elevated CO2 conditions. The reasons for the increase are not fully clear. Aphids feed on phloem, vascular tissue containing the sugary sap that nourishes growing parts of the plant. This plant sap does not seem to be affected by CO2 concentration, so aphids are not subject to the problems discussed earlier.
Aphids seem to know what is good for them instinctively. Caroline S. Awmack, an ecologist at the University of Wisconsin-Madison, put aphids, one at a time, into a Y-shaped tube that gave insects a choice between dining on wheat grown in air with a normal concentration of CO2 or on wheat grown in air high in CO2. The aphids preferred the wheat grown in high-CO2 air.
However, sensible decisions for aphids can spell big trouble for plants. Awmack found that under high CO2 conditions, bean plants infested with aphids cannot grow flowers or new shoots. Thus, if global atmospheric increase in CO2 boosts aphid populations, bean crops could be catastrophically damaged.
Awmack may have found a point of vulnerability in the aphid. In normal conditions, when aphids are disturbed, they give off a pheromone as a chemical alarm that warns other aphids of danger. In experiments with high CO2, aphids stop making the pheromone in response to stress and even fail to respond to the hormone when researchers provide it. Such complacent insects would be easy prey for parasitic wasps and ladybugs.
John B. Whittacker, an ecologist at Lancaster University in England, and his colleagues have followed several generations of spittlebugs living in CO2-enriched chambers. Spittlebugs, unlike aphids, suck xylem sap. Xylem is vascular tissue that carries water and dissolved minerals.
The spittlebugs did poorly in high-CO2 environments. Instead of adapting to the bad food over time and over generations, the insects did progressively worse. Survival rates plummeted 27 percent over three generations.
But what about mammals? Scientists in New Zealand and Kansas are currently leading sheep to CO2-enriched pastures to see what happens to grazing mammals and their grasslands under high CO2 levels. Clenton Owensby at Kansas State University and his colleagues built greenhouses over patches of the Kansas prairie and pumped air with high CO2 levels into some and ambient air into others. They let sheep graze in the greenhouses, and then collected food from their throats before they digested it. As expected, the grasses grown in air with elevated CO2 levels had much less protein than those grown in standard air.
Cud-chewing animals differ from insects in that they eat less when their plants are grown in elevated CO2. That is because bacteria in their digestive tracts control how much food ruminants ingest. The bacteria digest plant matter, extracting maximum nutrients for their host, but the bacteria's digestive ability falls as the protein content of the food falls. The grazing herbivore thus soon fills its rumen to capacity and must wait for the bacteria to complete the digestive process before it can eat again.
More time between meals means more time on the farm before the animal gains enough weight to be marketable. Ranchers could feed their animals protein supplements, but that could greatly increase the price of meat.
It is not known, nor is it easy to study, what the effect of enhanced CO2 has on wild herbivores, such as deer, elk, antelope, and gazelle, but there is no reason to believe that they won't have similar problem, and they won't have the luxury of humans giving them protein supplements.
The effect of such stress on domestic and wild herbivores is not clear, but is potentially devastating to human food production and ecosystems. Certainly the collapse of an herbivore population on the African savanna, for instance, would be an environmental catastrophe. Predators would starve, and some areas would be overrun by weedy plants with no herbivores to control their growth.
The ecological impact of high CO2 as a result of its ill effects on insects is clearer. The effects on plants and insects are now reasonably well understood, and known to be serious. Insects constitute about three quarters of all animal species. Their ecological importance is far greater than that of vertebrates, and in fact than that of any other animal group. Insects pollinate a huge variety and number of economically important domestic and wild plant species, the latter being of paramount ecological importance. Their value as food for birds, bats, lizards, and other animals is immeasurable. Insects regulate plant growth, a far from trivial environmental service. The loss of a substantial portion of insect species could cause the collapse of entire ecosystem; indeed, the human race could not survive without insects.
The changes brought about by high CO2 levels as a result of differential responses of various plant species to these changes may cause a new set of plants to dominate ecosystems, as some plant species will adapt to high CO2 levels better than others. This will likely result in more weedy species, some of which tend to adapt better to high levels of CO2. Further, differences between classes of plant eaters in their ability to adapt to high-CO2 conditions could lead to shifts in the ecological balance among insects and the plants on which they feed. It will likely be harder for some plants than others to adapt to rapid increases or decreases in insect herbivore populations. All this could result in the extinction of many plant species, and the animals higher on the food chain directly and indirectly dependent on them.
David Seaborg is an evolutionary biologist. Seaborg founded and heads the World Rainforest Fund and the Seaborg Open Space Fund, a nonprofit foundation dedicated to saving open space, named after David's father, Nobel laureate Glenn Seaborg. He lives in the San Francisco Bay area.