native species richness that the original native community no longer exists and has been supplanted by a new community dominated by one or more invasive species. Even the emergence of the new community does not guarantee the end of alteration. In the western United States the once dominant role of Bromus tectorum is being reduced locally through the more recent invasion of Centaurea solstitialis. Thus, one invader is being supplanted by another (R.N. Mack, personal observation).
In native forest ecosystems, insects and microorganisms are intrinsically involved in such processes as decomposition and nutrient cycling, maintenance of forest productivity, pollination, and food webs (Mattson and Addy 1975, Haack and Byler 1993, Gilbert and Hubbell 1996). Their influence on forest composition and structure occurs on genetic to landscape scales, but these organisms have evolved with their ecosystems over long biological periods. In contrast, introduced insects and microorganisms have no evolutionary history with the forest ecosystems that they have come to influence. Even though most foreign invaders are similar to indigenous organisms in how they feed or infect their hosts, the ecological changes that have resulted have been dramatic and cascading and have occurred over short periods (Gibbs and Wainhouse 1986, Oak 1998).
For example, forests since the invasion of the chestnut blight have often been dominated by oak species. Their increased abundance and continuity now provide ideal hosts for the introduced gypsy moth; for the increased expression of oak wilt, an indigenous vascular disease; and for drought-related declines (Liebhold et al. 1995, Simberloff 1996). Likewise, beech bark disease, caused by a nonindigenous scale insect and a pathogen, has led forests once composed of beech thickets or sites to be transformed to grass or shrub land–a change that can alter fire regimes (Oak 1998). Both maladies have affected wildlife populations that depend on beech nuts (Martin et al. 1961). A nonindigenous foliage feeder such as the gypsy moth may function like a native defoliator and appear to have minor effects on the forest, but the consequences of invasion include cumulative stresses on the host and alteration of the populations of other native herbivore species, and these effects can extend to other trophic levels.
What might be the ultimate, global result of mixing of the world’s biota? Brown (1995) has estimated, on the basis of species area relationships for continents, the worst-case scenario for the impact of free exchange of biotic material across former biogeographic barriers. The estimate assumed that continental drift could be reversed and that all the earth’s land could be reassembled in a single giant continent, but with the current climates and geological features intact. With these assumptions, there would be a massive decrease in species—65.7% in land mammals, 47.6% in land birds, 35% in butterflies, and 70.5% in angiosperms. Brown tempered that pessimistic assessment with a number of caveats. Colonization and expansion of ranges must ultimately decrease global diversity, but they tend in the short term to increase local diversity. The extent of the biotic enrichment varies, but many countries have 20% or more nonindigenous species