systems subject to sustained pressure by humans, appears widespread. Regier and Hartman (1973) provide an interesting account in which valued sport and commercial fish were exterminated in Lake Erie as a result of tremendous fishing pressure. These species were replaced by less valuable ecological equivalents. The substitute species still provide "services," though not of the same commercial value as those potentially contributed by the exterminated native species.
Other evidence indicates that what appears to be redundancy is not redundancy. For example, although two different species of zooplankton may feed on phytoplankton in lakes, one may be much more selective than the other, thus producing different ecological consequences. Selective feeders remove individuals of only certain species from a community. This gives the remaining species, those not chosen, a considerable advantage since their competitors are removed and the resources used by the competitors will almost certainly become available to the remaining species. This produces a marked alteration in community structure, as often occurs in rangeland cattle grazing when the most succulent plant species are removed and the less succulent species (e.g., thorny shrubs) become dominant or at least much more abundant. Nonselective feeding, on the other hand, is more equitable and is more likely to produce similar reductions in the various prey species. Thus, selective feeding is likely to alter the relative abundance of species present, whereas nonselective feeding is likely to have much less effect on the relative abundance of species.
2. How close is a crucial break point, or threshold, in the delivery of services by natural ecosystems?
There is good documentation of the increase in human population as well as the marked increase in technological and industrial activity, particularly energy consumption from fossil fuels. Since human population numbers, levels of affluence, and use of technology have increased dramatically in the past 10,000 years, and ecosystem services still appear adequate to support life (although regionally impaired here and there), one may ask whether delivery of ecosystem services is really an important problem?
3. To what degree are the ecosystem services of natural systems replaced by agroecosystems, managed forests, and even such things as vegetation on golf courses? As Harte (1993) notes:
When trees are cut and all or some of the wood and foliage is left to rot, the carbon in the tree is oxidized to carbon dioxide. Since about one-third of a tree, by weight, is carbon, a good deal of carbon dioxide can be produced when a large area of forest is felled. Even if the cleared land is planted with crops, the carbon that can be stored in cropland is vastly less than that in the forest it replaced.
4. To what degree do natural systems provide global, regional, or local services?