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.
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