Global patterns of plant invasions and the concept of invasibility

INTRODUCTION

The invasion of natural communities by introduced plants constitutes one of the most serious threats to biodiversity (Heywood 1989), and it seems that there is now no nature reserve in the world outside Antarctica that is without introduced plant species (Usher 1988). Exotic weeds in conservation areas are increasingly recognized as representing a major threat to the preservation of biodiversity (Humphries et al. 1991, Cronk and Fuller 1995, Luken and Thieret 1997, Schmitz et al. 1997), and can profoundly alter ecosystem structure and function (e.g., Hobbs and Mooney 1986, Braithwaite and Lonsdale 1987, Vitousek et al. 1987, Braithwaite et al. 1989, Cronk and Fuller 1995).

Invasions have long fascinated ecologists, and considerable attention has been brought to bear on the question of whether some ecosystems are more invasible than others (e.g., Crawley 1986, Fox and Fox 1986, Usher et al. 1988, Vitousek 1988, Cronk and Fuller 1995: 8-11, Williamson 1996: 26). To ask whether a region is more "invasible" than another is clearly to ask not simply whether it has more exotic species, but whether it is intrinsically more susceptible to invasion. It is not, however, easy to determine intrinsic susceptibility to invasion. A simple model illustrates this. Successful invasion of a natural community requires dispersal, establishment, and survival (Hobbs 1989), with the number of species in an area being determined by a balance between immigration and extinction. Most invading species fail to establish (Williamson 1996). Consider this simple equation:

E = IS (1)

which proposes that the number of exotic species E in a region is given by the product of the number of exotic species introduced, I, and the survival rate of exotic species in their new range, S. I and S can be further broken down into their components. For I,

I = [I.sub.a] + [I.sub.i] (2)

where [I.sub.a] is the number of accidental introductions (either as contaminants or by natural dispersal), and [I.sub.i] is the number of intentional introductions (for agriculture, as ornamentals, etc.). For S,

S = [S.sub.v][S.sub.h][S.sub.c][S.sub.m](3)

where [S.sub.v] is the rate of species survival after extinctions due to competition from the native vegetation, [S.sub.h] is the rate of species survival after extinctions due to herbivory and pathogens, [S.sub.c] is the rate of species survival after extinctions due to chance events at establishment (droughts, etc.), and [S.sub.m] is the rate of species survival after extinctions due to maladaptation (e.g., tropical species released into a temperate climate, terrestrial species released into an aquatic environment, etc.).

These simple equations provide a crude idea of the kind of mechanisms that contribute to determining the exotic richness of a region. When ecologists suggest that Region 1 is more invasible than Region 2, it seems self-evident that they are focusing on S in Eq. 1, and are proposing that the exotic species arriving in Region 1 are more likely to survive than those arriving in Region 2. From Eq. 1, however, it is clear that, to understand the underlying causes of exotic richness and to compare regions for invasibility, we need to know about both I and S. That is, we must control for the number of species introduced, I, before we can compare values of S. In practice, as I will show, this is often impossible, because we rarely know how many species have been introduced and have failed (see Simberloff 1989). As Williamson (1996: 55) pointed out, "Looking for real differences in invasibility requires looking at the residuals from the relationship between invasion success and propagule pressure."

Even if we could control for I, there are further complications before we can compare invasibilities. The invasibility of a region presumably consists of those properties of the region that affect exotic species survival S. The native biota affect [S.sub.v] and [S.sub.h], by definition, but these are both affected by the exotic biota too, because they represent the outcome of interactions between the native and exotic species. Invasibility will also be affected by the degree of disturbance that the region has undergone (e.g., Crawley 1986, Hobbs 1989, 1991, Hobbs and Huenneke 1992, Burke and Grime 1996), because it reduces the ability of the native vegetation to compete and so obviously increases [S.sub.v] for the invaders. Thus, an invaded region, relatively undisturbed and with a high native cover value, might be expected to give rise to a low [S.sub.v] among the invading species through competition, but this might be counterbalanced if the invaders are good competitors (raising [S.sub.v]) or repellent to herbivores (raising [S.sub.h]). If feral animals become abundant, disturbance increases and [S.sub.v] will rise, increasing E. The remaining quantities, [S.sub.c] and [S.sub.m], are likely to be affected only by the qualities of the invading species. [S.sub.m] is affected by definition, whereas [S.sub.c] might be raised if, for example, the invading species have persistent seed banks, allowing them to survive chance periods of hardship. Another hypothetical determinant of a region's invasibility is the ecosystem's resistance to invasion (Table 1; Williamson 1996: 193-196). This is a system property, relating to the way in which the community is structured, the strength of the interactions between trophic levels, etc. Thus, invasibility is an emergent property of ecosystems, manifested in the rate of mortality of exotic species, but at the same time potentially affected by the climate, the properties of the native species, the level of disturbance, and the ecosystem's resistance to invasion (Table 1).