The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 5

these volumes explored issues from genetics to species attributes, community
assembly to biotic resistance, and disturbance to community attributes encour-
aging invasions. These two volumes mark the time when invasion ecology
became a legitimate academic discipline, metamorphosing from a research
interest of a few dedicated practitioners to one of the most frequent interests of
ecologists. The editors of these two volumes had the explicit goal of finding a
framework that would have allowed ecologists to predict which species
become invaders and which communities are likely to be invaded. However,
the editors noted in their conclusions that prediction was still some time off.
With his aptly-titled topic ' Biological invasions ', Williamson [35] contributed
the oft-cited 'tens rule', which focused on invasions as a series of stages (trans-
port, introduction, establishment, spread), and his insight into the importance
of 'propagule pressure' - the number of individuals or propagules introduced
to an area.
This brief history of thinking on NIS reveals that there have been two dif-
ferent interests by ecologists [5]. First, ecologists have tried to understand and
explore the processes and dynamics of invasions. In this case the primary inter-
est is the NIS themselves. The second interest is in using NIS as natural exper-
iments to test general ecological theories and concepts. In this case the primary
interest is theory. What follows is an expansion on these classic ideas of inva-
sion biology, incorporating an array of current theories and approaches, all of
which may lead to a deeper understanding of invasions and toward the ultimate
goal of predicting and preventing invasions by harmful NIS. However, the con-
cepts and theories explaining the success of NIS also potentially inform gen-
eral theories and concepts in ecology [1, 2].
Current ideas
To conceptualize current ideas in invasion ecology better, several authors have
proposed a stage-based approach ([35-39]; see also Chapter 2). This frame-
work breaks biological invasions into a series of consecutive stages, beginning
with transport and introduction, through the establishment of self-reproducing
populations, and ending with spread and abundance (Fig. 1). As first proposed
by Carlton [36], each stage entails a sub-sampling of individuals, such that
widespread, abundant species are the least likely end-point - many invaders
are introduced, but few establish, and even fewer become widespread or abun-
dant. Consistent with this framework, we use the term 'invasion success' in a
general sense, to describe success at any stage of the invasion process. We use
'establishment success', 'successful spread', or 'successful proliferation' to
refer to success at specific stages.
Much of the modern invasion ecology literature (since c. 1990) has focused
on identifying characteristics of successful invaders and habitats more suscep-
tible to invasion; these have been called 'invasiveness' and 'invasibility' char-
acteristics, respectively. Studies of invasiveness characteristics have taken a