TEXT E
All along the chain of biological evolution, the extinction of species appears to have been a stage in the process of adapting genetic lineages to changing environmental conditions. Although some catestrophic extinction occurred naturally, producing total loss of a genetic line, such catastrophes were comparatively rate. In modern times, however, human activities have altered the fundamental nature of this process, resulting in nearly total genetic losses.
It is not difficult to gain general agreement that man - induced increases in the endangerment and extinction of wildlife--whether due to habitat alteration or loss, pollution, insufficiently regulated hunting, or other factors--are undesirable. It is, however, more difficult to obtain consensus when consideration is given to the economic costs of correcting such trends, including natural habitat preservation, regulation of pesticides and other toxic substances, and wildlife and park management. Endangered species often are, in effect, competitors with humans for habitat and other resources which also provide other kinds of human use and need.
Measures needed to protect endangered species vary considerably in difficulty and cost. Of the approximately 400 inverterate species which at present appear to be threatened, for example, about one -third could probably be restored by such inexpensive means as modifying the boundaries of designated natural are- as, acquiring and protecting caves and other small areas which contain the particular species, and additional management of parks and refuges.
Another one - third of the endangered lower animal species are threatened principally by water pollution and could be protected by improved control, particularly of five southern rivers.
The remaining one - third of the 400 endangered shellfish species would be considerably more difficult to protect. These are threatened by complex factors, such as overcollecting, channelization, highway and housing development, dams introduced species such as the Asian snail, dredging, quarry washing, poor erosion control, and lowering of water table.
The identification of threatened species and other significant wildlife trends must precede any corrective! measures, and our knowledge base for making such identification is deficient in many respects. Our present lists of threatened species and subspecies are known to be incomplete, except in those geographical areas which contain habitats of species that have important commercial or sports harvest value.
It can be inferred from the passage that studies of endangered species ______.