Mary Lush together with Ros Gleadow produced an article on the pervasive consequences of increasing carbon dioxide levels on plants and animals in an area of native vegetation. It was published in the Autumn 2011 newsletter of Friends of Queen's Park (Lorne).
The taste of change
Queens Park is full of food. In one combination or other its plants and animals make up the characteristic diets of koalas, echidnas, kangaroos, parrots, wrens, ants, spitfires, scale insects, spiders and a myriad of others that live in or visit the park. It is a sort of wild food market for the wild.
Matters of taste
Animals have varied tastes. Some defy human notions of healthy eating by living on just one or two things. Koalas, with their insistence on eucalypt leaves, are one of the fussy eaters, but they are broad minded by comparison with scale insects which, although not popular with humans, are high on the shopping lists of pardalotes and ants.
Scale insects do have interesting tendencies including being legless for most of their lives – if they are female that is. They eat from just a few spots on the same plant by sinking a sort of drinking straw called a stylet into the bark and tapping the sap.
It must require considerable digestive skill to survive on sap alone and one suspects that scale insects walk a fine line nutritionally speaking.
Figure 1. Ants shopping for honeydew at the scale insect market. Scale insects conceal themselves underneath the pinkish sacs seen here, and secrete a sugary substance that ants harvest. By making themselves attractive to ants, scale insects make themselves unattractive to other animals because most animals avoid ants.
By comparison with scale insects, caterpillars have a varied diet - still only a single plant but they eat most of several leaves.
Koalas go a step further by eating leaves from many plants - provided they are all eucalypts. These diets however are still restricted and perhaps marginal - it is not for nothing that koalas are so lethargic.
At the other end of the range of eating patterns are animals that enjoy a varied diet. Take brush tail possums. They eat the leaves, fruits and flowers of many species and are not above including meat. Even so, it can be hard work for a possum to find enough food to maintain its body weight. Obesity does not loom large as a problem in the wild, because despite the apparent abundance of food its quality is often abysmal.
Figure 2. The gumleaf skeletoniser, a caterpillar, eats the whole leaf except for the veins (and sometimes them too). Hairs on these caterpillars can sting or cause rashes and help to make them unappetising.
Survival by stealth
If we were plants with minds we would see no wisdom in being either nutritious or delicious; either property would just ensure that we got eaten. Not surprisingly, wild plants have evolved to be as inedible as possible. Each plant species, and to some extent each plant, is unattractive in its own way. It may be tough and fibrous, low in protein, contain chemicals that interfere with digestive processes or toxins, or all of these and more.
So the Queens Park vegetable market is a perverse sort of place in which plants race to reach the bottom of the scale of desirability, while animals compete to overturn the strategies designed to repel them. It could be called a titanic struggle, but generally passes unnoticed because change is on an evolutionary scale – meaning slow.
The taste of climate change
Just like animals, plants need to feed. A plant’s constitution depends to some extent on the foods available to it. If we supplement the minerals available to eucalypts by applying fertiliser, their leaves tend to be both more nutritious (higher in protein) and less disgusting (lower amounts of particular chemicals), and as a consequence more attractive to a range of animals. If this was a plantation, the animals would be called ‘pests’.
Carbon dioxide, CO2, is also a plant food and one that is increasingly available as a result of human activities. This increase is well documented because of the secondary effects of CO2 on climate (e.g. higher temperatures, altered rainfall patterns). Is the increased availability of CO2 as a food in itself something to worry about? It seems that it is, and that changes in the composition of plants will have consequences for all living things including us.
Cyanide in one form or another is a toxin made by some plants to dissuade potential consumers and thus ensure a position low on the desirable foods list. Cyanides are familiar to us as the taste of bitter almonds. We don’t eat bitter almonds in large quantities so its presence is of no consequence, but people in parts of Africa rely on the cyanide-containing starchy tubers of cassava as a staple food. This situation of having populations dependent on toxic plants is only possible because cassava is processed before eating in ways that reduce its toxicity.
In CO2 enriched worlds, cassava makes more cyanide, leading to concern that traditional preparation methods may no longer be effective enough to avoid cyanide poisoning.
The situation that wild animals find themselves in is analogous to that of cassava-eating humans. Animals have evolved ways of dealing with normal levels of the toxins that are present in their foods. But they may be hard pressed to evolve ways of detoxifying higher levels of toxins in wild foods, or to adjust to the lower levels of protein (relative to carbohydrates) in plants that are another direct consequence of more CO2 in the air. We can be sure that there will be changes in the plant and animal populations of Queens Park as a consequence of rising CO2 levels, irrespective of how much the climate changes. We can be sure of this because the concentration of CO2 in the air affects the composition of plants. The changes in the populations of living things in Queens Park may be trivial or to the advantage of some species, and then again, for animals that subsist on diets that are already marginal, they could be harmful to the point of extinction.
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