Future Of Plant Diversity On Islands

The composition of plant species on islands has been in a rapid state of flux during the past two centuries, because thousands of exotic plant species have been added to island floras. Despite efforts in some regions to control new introductions, we expect that still more exotic plant species will be added to islands over the next century. Even if colonization-based saturation is occurring and the probability of establishment is decreasing, ample introductions may have already occurred to compensate for a declining probability of naturalization. In fact, we see little evidence to support the conclusion that the flow of propagules to islands will be reduced in the near future. Many countries, such as the United States, currently lack or fail to enforce regulations ample to prevent many invasions from occurring (Lodge et al., 2006). Even in those cases where rigorous importation laws exist and are enforced, as in New Zealand, there will generally be a large ''bank'' of potentially invading species that are already present. For example, in New Zealand, >22,000 exotic plant species that have not yet become naturalized are grown in the country (Duncan and Williams, 2002). Many of these plants have the potential to escape cultivation and become naturalized components of the flora (Sullivan et al., 2005; Williams and Cameron, 2006). Further, global warming and changing environmental conditions many promote the establishment of many species that previously were unlikely to establish naturalized populations. The total number of plant species already present (but not yet naturalized) on most islands around the world has not been tallied, but the numbers are likely to be substantial [e.g., the estimate for Hawaii is «8,000 plant species (Eldredge and Miller, 1995)]. Given the substantial size of these species banks and the likely continued import of additional species, it seems likely that most islands will continue to see the addition of exotic species to their naturalized floras in the future.

The ultimate consequence of these exotic species additions for native diversity is still difficult to determine with certainty. We see three primary alternatives with respect to exotic plant invasions and their impact on native species. First, saturation may be unimportant for plant species or, at least, not important at or near the levels of diversity currently present

TABLE 5.2 Alternative Forecasts of Exotic Naturalizations and Native Extinctions of Plant Species on Islands

Scenario Naturalizations Extinctions

No saturation Many Few

Colonization-based saturation Few Few

Extinction-based saturation Many Many on islands. If this is true, then we might expect many more exotic species to be added without consequent extinctions of native plant species (Table 5.2). Second, if colonization-based saturation points are being approached, then we might expect rapid declines in the rate at which exotic species become naturalized in the future; importantly, we would also then expect few of the native plant species on these islands to go extinct (Table 5.2). Third, if extinction-based saturation points are being approached or have been exceeded but are masked by long times to extinction, then we would expect newly introduced exotics to continue to become naturalized and many native species to be on a pathway to extinction (Table 5.2). In each case, we predict an increase in naturalized plant richness but with different magnitudes and vastly different outcomes for native species extinctions. Unfortunately, on the basis of current data, we cannot distinguish among these dramatically different views of future change in island biotas. Fortunately, there are key types of data that could be acquired and key theoretical questions that could be explored that can help to distinguish among these alternatives. Such insight is critical to advancing ecological theory and informing our understanding of how best to use a limited number of conservation resources in preserving the unique biota of islands worldwide.

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