seldom members of the characteristic vegetation units as defined by the plant sociologists: they are marginal creatures living on the borderline of biota….” This general viewpoint has led to the following conclusion of Ruiz de la Torre (1985, p. 197): “Unlike the tropical rain forest, where most of the indigenous species can be conserved with climax formations under conditions of maximum stability, the Mediterranean region has been severely influenced by man and various other factors and is still very rich in species. Very few of these species are known to be part of Mediterranean climax vegetation. Most of them correspond to successional stages affected by either natural or artificial exploitation, and they should be conserved under the prevailing conditions of relative instability.”
As indicated above, plant diversity in Mediterranean-climate regions is among the world’s richest in terms of numbers of species, but there have been losses of species and continuing threats of extinction to many others. However, there have also been additions of new species to these and other regions of the world. As shown in Table 17–2, the floras of certain islands, ranging from subarctic to tropical, have been enriched half again by species from other biographic regions. In mainland Mediterranean-climate regions such as California, and even to a greater extent in South Australia, there are also substantial numbers of invading species that have become naturalized, many maintaining large and dominating populations. In these regions, as elsewhere, these invading species are not distributed uniformly in the landscape but are generally associated with ecosystems that have experienced human impact. Organisms other than plants are also being enriched by the addition of species in these climates. In California, for example, 49 species have been added to the 132 indigenous inland fishes (Moyle, 1976).
Thus in some cases, human disturbance can actually enrich biotic diversity. However, species counts in a given area give us little understanding of ecosystem functioning and how the invasions affect it. Some invaders may become the dominant species in the host-region ecosystem. Examples of this include a species of oat (Avena fatua) in the grasslands of California (Burcham, 1957) and brome grass (Bromus tectorum) in the intermountain West (Mack, 1986). Many of the invaders are pest species of one sort or another and may cause economic havoc. These species of course receive considerable attention, and their biology and community role is generally well known. However, we generally know little about the effects of most invaders on the ecosystem or, for that matter, the effects of most species on natural communities.
Are these invaders enriching biotic diversity? They are when considered in absolute numbers of species. In many cases, however, they are impoverishing the biota by leading to species exclusions (Race, 1982) or even to extinctions. The invaders are generally symptoms of an abused landscape, one that has been disturbed and has generally lost some of its original productive capacity. The successful introduction of exotic mammals has often resulted in greatly perturbed ecosystem function and losses of indigenous species. In general, new community types are being added to the original ones that in turn are being reduced in extent. The