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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.

GAPS IN EXTINCTION RESEARCH

We believe that there are four research gaps that must be addressed to improve our understanding of the consequences of species invasions for the future of native biodiversity. First, propagule pressure of exotic species must be better understood. To date, propagule pressure has been poorly studied in nearly all ecosystems. Some important attention has been paid to records of bird introductions (Cassey et al., 2004), but few other groups have received the same attention (Lockwood et al., 2005). In part, this is due to the difficulty of reconstructing records for groups where introductions have not been well documented. Nevertheless, for plants, a careful historical survey of seed catalogs and import records could undoubtedly provide critical insight on rates of introductions. Sec-



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