MAN’S IMPACT ON ECOSYSTEMS
I yearn for a glimpse of what the plant and animal communities in Simi Valley were like when the Indians were the only human inhabitants of the area - a time before European colonization brought forth a whole series of ecological changes to our area. Native perennial grasses were largely replaced by exotic annual grasses, including wild oats, foxtail barley, red and ripgut bromes, and soft chess. Horehound, various Mediterranean mustards, thistles - the latest being yellow star thistle, wild radish, tree tobacco (from Brazil), tumbleweeds (Russian thistle), California (Peruvia pepper) trees, giant reed, castor bean, and hundreds of weedy species that are largely confined to urban and agricultural ecosystems now are parts of our new world. Animal invaders include the Virginia opossums, English sparrows (actually a North African bower finch), starlings, and hundreds of insects and other invertebrates. New invaders are arriving all the time. These changes are occurring everywhere - not just in Simi Valley. To a certain extent, we are developing a global ecosystem where once there was great complexity. While this assertion is somewhat overstated, the reality bodes profound changes and great loss of biodiversity in the world. All of this is brought by the hand of man.
Eastern hemispheric man’s move into the “new world” resulted in a 70-80% die off of the human population that was here. That die off was a tragic result of global travel in that American Indians did not have the ability to cope with eastern hemispheric diseases.
For most of human history, man has moved plants and animals around with them. Domesticated plants and animals moved with colonization and even a reverse flow of useful plants and animals was introduced to homelands where a benefit to local economies or ways of life could be seen. Tobacco, corn, potatoes, tomatoes, rubber trees, sunflowers and turkeys are examples of new world plants and animals that have had a global impact. Nineteenth Century “acclimatization societies” strove to populate America and Australia with European plants, birds and mammals. Most quickly died off. About 10% settled in quite nicely and about 10% of those spread wildly. Perhaps the best known of the latter is the impact of the introduction of rabbits into Australia, where they proceeded to have a devastating impact on native vegetation and the grazing economy. Only the accidental release of the European rabbit calicivirus disease in 1995 has at last resolved the scourge of the bunnies.
In Florida, one in every three or four plants is now a non-native species. Three exotic fig trees have been grown in Florida for over a hundred years. Only during the last 20 years or so have they begun to spread wildly into natural areas. The explanation is the arrival of the fig wasp that pollinates them. The Brazilian pepper was grown in south Florida for many decades as an ornamental, just as it is here. Then suddenly during the 1940s and 1950s, without any known reason, they spread into the wild.
In South Africa, ecologists anticipate the arrival of Varroa, a mite that parasitizes honeybees. It has already swept through Europe and North America, where it badly damaged commercial bee keeping and has devastated wild bee colonies. In South Africa, between 50% and 80%, depending on the ecosystem, of native flower species are pollinated by bees. The effect on ecosystems may be devastating.
One researcher has found that the greater the number of species there is in an ecosystem, the greater will be the number of exotic species that will be established. The same circumstances that favor a wealth of native species will favor a higher number of exotic species. Approximately 40 percent of the plant species present in north America, are present in California. Ours is the most species diverse region in north America.
Sometimes biological controls can be found. The tumbleweed arrived in southern California and much of the west during the 1880s. It had been introduced into central Canada with Russian red wheat. It was then carried by livestock, in their feces, via the railroad. Its effect was devastating. It choked out crops and then stacked up against fences, where it eventually burned - destroying wooden fence posts. Laws were passed making it a misdemeanor to have a tumbleweed on your property. Nothing worked. When I first visited Wood Ranch during the mid-1970s, it was difficult during August to walk over level land because of the harsh nature of standing tumbleweeds, which covered the fields. Then in the early 1980s a seed eating weevil was introduced from the steppes of Russia. Now, tumbleweeds have been effectively controlled.
Many introduced plants have been successful here because they have no known natural enemies. Our native insects and other animals don’t recognize them as food. The giant reed (Arundo donax) is a good example of this. It creates an ecological desert in our stream courses. Nothing eats it and nothing lives in it.
Sometimes natural enemies do arrive and those enemies threaten the continued existence of that particular plant species because there are no known biological control of the introduced species. The red gum lerps psyllid made its way to southern California in 1998 and by the summer of 2000 was devastating red gum eucalyptus trees. Dire predictions that most red gum trees would succumb to this insect within two to three years may or may not prove to be true. For now, there is no known biological control in this state.
This recombination ecology, assuming that frenetic international trafficking of biological components goes on for the next century or two, will result in a relatively small number of immensely successful species dominating nature everywhere. What have we wrought?
Mike Kuhn
11-13-04