NO one knows how much organic diversity there is in the world. But alarm bells are ringing about the rate at which species are disappearing because of human activity on the planet. Birds offer a conspicuous example; and this is the season when migrating birds are especially visible - or not. ``Rain forests are the main repository of biological diversity on land,'' says Edward O. Wilson, professor of entomology at Harvard and curator of its Museum of Comparative Zoology, speaking at a recent MCZ seminar. Many migrating songbirds spend the winter in rain forests, so their disappearance may mean disappearance of these birds also. Songbirds depend on the food - such as insects, fruits and seeds - that rain forests provide.
``We're in the middle of an extinction spasm occurring in one lifetime,'' says Dr. Wilson. ``There has been nothing like it since the dinosaurs ... and it's all under our control!'' But in spite of his anxiety and that of many biologists, Wilson notes an attitude of ``thin optimism'' among some as well. There's hope for saving some of the rain forests - ``if there were a quick turnaround in attitudes, especially among national leaders,'' he says. No ideological conflict stands in the way, and the countries involved ``all see not just economic benefits but the need to prevent loss of an extremely important part of their national heritage in which they take great pride.''
Another sign of hope that Wilson sees is ``the hamburger connection,'' wherein North American health concerns led to reduced demand for beef and thus less forest cutting to provide grazing land in Latin America.
But conservation is a complex problem. Stanley Temple, professor of wildlife ecology at the University of Wisconsin, identifies three major issues in helping to save birds: Where and when is the trouble occurring? Where is the population limited? What event determines how many birds are present in the next breeding season?
There may be many plausible explanations for the decline of a particular bird population - restricted breeding habitat, excessive winterkill, inadequate reproduction. Scientists looking for solutions to the decline in songbird populations find that each species has its own particular difficulties.
Take the case of the Kirtland's warbler, now on the endangered-species list. This sparrow-size bird with a gray-blue back and a yellow belly has a song a bit like that of a house wren.
Dr. Temple explains how conservationists tried to reverse the bird's population decline. First, they thought perhaps the breeding habitat was too limited, so they began managing the jackpine stands in Michigan. No increase. Next came the question of brood parasitism: Cowbirds that laid their eggs in the warblers' nests were reducing the number of warbler offspring to 20 percent of normal. When the cowbirds were eliminated, the warbler population rebounded and stabilized but did not increase.
``The problem therefore had to be at the wintering area in the Bahamas,'' said Temple. Sure enough, they found that the coastal scrub on which the warbler depended was disappearing because of seaside development.
For small insectivorous birds - such as vireos and warblers - that land in Central America, deforestation there may not be as much of a problem as fragmented forest habitat on their northern breeding ground. For other birds, pesticides are the biggest hazard, reducing the supply of insects on which they feed.
But birds do have amazing durability. ``Thin optimism'' comes out again in Peter Ashton's comments. Dr. Ashton is director of Harvard's Arnold Arboretum. At the MCZ symposium, he said that ``for many centuries, migrant birds have had to go through extraordinary hurdles, and the kinds of birds are extremely diverse,'' as are their habitats. Different kinds of birds have been able to survive all sorts of hazards, some species being more adaptable than one might expect.
Various kinds of wood warblers, for example, winter in areas of Central America not fully occupied by resident birds. Other species congregate on riverbanks and damp places or evergreen forests in dry areas and seasons.
Ashton also held out hope for avian survival by observing that the ``most serious problems of change are in Central and South America, but those areas have been inhabited by very highly sophisticated cultures for a very long time and the birds must have got through that period because they take so long a time to evolve.'' In other words, birds have persisted despite human tampering with the environment.