How do rising C02 levels affect plants?

How do rising C02 levels affect plants?

Carbon dioxide and ecosystems.

Correction from the author: An earlier version of this column incorrectly stated that when Darwin visited Ascension Island, he observed no plants or even lichens. While reading a biography about William Dampier, the British pirate, explorer, and naturalist, I learned that when Dampier was shipwrecked on Ascension in the year 1701, he noted goats and a tree, among other organisms. So it was not true, then, that no plants lived there (the goats had to be eating something!).

When Darwin got there, well over 100 years later, he found a human settlement, lots of farm animals, grass and grasshoppers, and plenty of introduced chickens, guinea fowl, cats and rats, (but no trees) and who knows what else. Perhaps one corner of the island may have looked barren except for seabirds and their pests.

The statement about the short part of the food chain getting input from phytoplankton in the sea is still true — but rather beside the point, since it was based on a false report that the island was devoid of vegetation.

The Empire regrets the error, and the column has been updated below to reflect the change.

Plants are the base of nearly every terrestrial food chain. They take carbon dioxide from the air, process it by photosynthesis, making carbohydrates for building tissues and growing, and turning out oxygen as a by-product. Not only is the oxygen needed by virtually all living organisms (except for a special few in the deep ocean), plant tissues are consumed by a great diversity of herbivores and decomposers. These are, in turn, consumed by an array of predators and parasites, making complex chains of interactions.

Even in places where there are no plants at all, plants make some contribution to the food chains there. A famous example was attributed to Darwin when the Beagle stopped at Ascension Island in the mid-Atlantic Ocean. The nesting seabirds had fed at sea on fish — fish that ate invertebrates that ate plankton, including phytoplankton (i.e. plants) as well as zooplankton. Thus, aquatic plants made an indirect input to the terrestrial food chain.

[Yellow cedar denied protected status]

A similar case might be made for food chains in ecological communities in deep caves, where the local animal residents live in total darkness, have no eyes and no coloration. But water washes in debris from the surface, or if bats roost in the cave, the guano and debris support a nice community of bacteria and fungi, which may be eaten by springtails and isopods, which in turn are prey for salamanders. But before roosting, those bats had dinner of (probably) insects, and some of those bugs would have fed on plants that grew in the world above the cave.

All of those ecological relationships depend at least in part on carbon dioxide (processed by plants). But, alas, these is such a thing as too much, and there is now too much carbon dioxide in the atmosphere, due mostly to human activities. That excess creates the well-known greenhouse effect and global warming. We hear a lot about that these days, with good reason. But somewhat less well publicized are some chemical effects of all that carbon dioxide (which may be exacerbated by warming).

There are important effects of increased carbon dioxide on plants. It was formerly thought that the more carbon dioxide would be good for plants because they would photosynthesize more carbohydrates and grow better. But it turns out that there can be significant negative effects on the nutrient content of grains and potatoes (and probably more) that outweigh the potential growth benefits. The carbon dioxide now in the atmosphere is about 33 percent higher than it was in 1900. If it continues to increase until it doubles, the protein content of barley would decrease by about 15 percent, of wheat and rice about 10 percent, of potatoes by about 13 percent. Several B vitamins and minerals such as iron, zinc and magnesium would also be lower. Such decreases in nutrient value have obvious consequences for human populations, not to mention all the rodents, birds, and others that eat grass seed of various types. And what about the foliage and roots that are eaten by herbivores?

Thinking more broadly, we should be asking what must be the effects of more carbon dioxide on all the plant-based and plant-influenced ecological systems (including aquatic systems)!

Other important effects of increased atmospheric carbon dioxide are well documented in aquatic habitats. Carbon dioxide dissolves in water, increasing acidity. In the past two hundred years, sea water is reported to have become 30 percent more acidic — an increase faster than at any other time in the past fifty million years. Direct effects of higher acidity are seen in measurably thinner and weaker shells of shellfish, whose shells are made of calcium carbonate— which is eroded by acid. Weaker shells are more easily crushed or broken by waves or predators, and constant erosion means that clams, oysters and mussels don’t grow as fast. Baby oysters have to grow quickly and build a shell in the first two days after the larvae settle onto a rock, but acidified waters destroy the shell as it begins to form and the young oysters then die. Mussels attach to rocks by byssal threads, but those cannot hold on very well in acidified water. Sea urchins and sea stars, whose shells are made of another type of calcium carbonate, also produce weaker shells that are more vulnerable to damage.

Corals, too, have trouble building their calcium carbonate (a type called aragonite) skeletons in acidified waters. Corals grow upward and thicken their skeletons by stacking up crystals of aragonite. Thickening of the skeleton is particularly important in resisting or repairing breakage from storms and the nibbling of fishes and invertebrates that like to eat the living polyp inside. Coral reefs provide living spaces for many kinds of organisms, so their structure affects that whole ecological community.

Coralline algae, which grow flatly all over rocks, make mini-skeletons of a certain type of calcium carbonate that is extremely soluble in acid. In acidic conditions, these algae cover less are of the rock surfaces, allowing so-called turf algae to take over. The corallines provide settling surfaces for the polyps of corals and the larvae of various invertebrates, but the softer turf algae don’t. So the rock-living community changes, and again, there are community-wide effects of ocean acidification.

That’s a lot of negative effects of carbon dioxide in sea water (and it’s just a sample). Some organisms, however, apparently do OK: sea grasses reportedly do well, and the shells of crabs and other crustaceans may get even stronger in acid waters.

Fresh water takes up atmospheric carbon dioxide too. Increased acidity leads to decreased species diversity, greater vulnerability of some small invertebrates (e.g., water fleas or Daphnia) to predation, sometimes greater growth of moss and algae. Juvenile pink salmon rearing in acidified fresh water were smaller, less wary of danger, and less responsive to the chemical signals that would eventually lead them back to their home stream at spawning time. (Similarly, acidification affects some saltwater fishes too).


• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Wednesday.


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