Biology Comics

Aliens

My immigrant friend gets defensive when I brake and start pulling up purple loosestrife from the roadside. Another dastardly invasive, I say! What’s wrong with it, he says? It’s pretty!

We’re the invasives of course. Even the Indians only came here 10,000-30,000 years ago, some walking across the Bering Strait with their wary companions the moose and buffalo, others following the fish and manatees of the kelp beds around the coasts of Siberia and Alaska in skin boats. For them it was a new world too, with tame mammoths on the steppe, giant sloths lumbering across the Great Plains, a fruit eating rhinocerous in the forests of Central America.

Hunters and gatherers tend to live in the natural world, though they change it. Agricultural people have been taking apart ecosystems for the last 10,000 years. Industrial people have been creating entirely new worlds for 200.

Vertebrates (except herbivores) access much of the energy in sunlight through insects, which have more protein then beef. Herbivores eat plants (transformed sunlight) directly.

Frogs eat mosquitoes, songbirds caterpillars, falcons dragonflies, many birds beetles, the Everglade Kite snails (an invertebrate, not an insect). Mice eat insects, invertebrates and plants and are eaten by foxes, coyotes, hawks, owls, weasels and men. (Mouse skeletons have been found in fossilized human dung.)

Green plants, the terrestrial transformers of sunlight (let’s ignore the bacteria and archaea), engage in chemical warfare with each other and with the insects that graze on them. (Maybe 20% of the leaf mass of a forest tree is lost to leaf eating insects in a summer.)

The trees and other plants defend themselves by producing so called secondary metabolites (those chemicals not involved in their primary metabolism, that of converting sunlight and carbon dioxide to carbohydrate). These glycosides, phenols, terpenes and alkaloids affect the taste, digestibility and toxicity of plant leaves (and other parts). A caterpillar biting into an aspen leaf begins the production of tannins that will in hours make the leaf indigestible to it. The release of secondary metabolites into the air warns nearby trees of an insect infestation; they also begin producing defensive chemicals.

The insects evolve methods of detoxifying what the trees produce. Some make use of the toxins: thus monarch caterpillars store the glycosides produced by milkweed; the bitter taste of the butterfly keeps the birds from eating it: chemical warfare carried to the next generation.

Insects and plants thus coevolve, for tens of thousands or tens of millions of years. Perhaps 90% of herbivorous insects are thought to be specialists on a few species of plants, which they have evolved the capacity to eat (detoxifying their secondary metabolites). The rest are generalists, that take their chances.

New plants (say, asian rhododendrons in eastern North America) have chemical defenses to which the local insects are not adapted. The new plant thus has an advantage over the natives. New insects, diseases (meeting organisms without immunity), fungi, predators (meeting defenseless populations) and parasites may have similar advantages. The European genotype of Phragmites (giant reed) is eaten by 5 species of insects in the northeastern U.S. and by 170 in Europe, with the result that it is replacing the native strain of Phragmites (eaten by numerous native insects) here. The plants are the same species (they can interbreed) but have different chemical defenses (and their inedible offspring will be selected for).

Over 400 arthropods (insects and spiders) eat the Melaleuca tree in Australia, where it is rare, but 8 eat it in Florida, where it is far too common. (Over time, native insects will learn to eat Melaleuca and also the European Phragmites; but the time may be long.)

Europeans had a similar effect on their own species in the New World, as they brought with them the crowd diseases of the Eurasian agriculturalists to which the native peoples of the Americas had no immunity. So, in seventeenth century opinion, “the good hand of God” cleared the New World of its native peoples. (Europeans suffered a similar fate in Africa, where people and disease had been evolving together longest; approximately half the Europeans emigrating to West Africa died of disease in a year.)

Anyway, the point is that native plants are more edible to native insects and so produce up to 4 times the insect biomass of nonnative plants, and 35 times more caterpillars (a primary food of songbirds). They support a far greater biomass of insect eaters above them.

So I pull the (inedible) purple loosestrife out of the swamp.

Not all native species are equal. A hundred years ago the chestnut was the primary nut producer in the eastern forest (its production dwarfing that of the oaks and hickories). Its mast supported turkeys, deer, mice, squirrels, bears, decomposers, buffalo and people, and the caterpillars that fed on its tasty leaves supported huge populations of songbirds. The chestnut was killed by an imported fungus and its place (partly) taken by the tulip tree (a native), which supports little wildlife.

How to construct an ecosystem?

The problem is breaking ecosystems apart. In intact ecosystems aliens may establish a niche but are less likely become invasive. Their flowers and fruit may be used by the natives, even if their leaves are inedible. (In 10,000 years the leaves will become edible.)

Against some introductions— some predators, fungi, bacteria, insects, parasites, amphibians—there is no defense. For some time, as ships and planes spread plants and animals around, the world will become poorer (until, say, elms develop resistance to Dutch elm disease, American toads to chytrid fungus, chestnuts to chestnut blight).

As the abundant world fades, few will remember it.

Eventually a new world will blossom.