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Integrating Ecological and Evolutionary Theory of Biological Invasions 81
cess represents a relative measure of average individual or population performance
in the introduced versus native environments. The response ratio,
R, is simply the ratio of performance in the introduced to the native environments
(R=P I /P N ), and provides a quantitative measure of demographic
change between the introduced and native environment. A response ratio of
1 indicates no change between the introduced and native range. A response
ratio greater than 1 shows positive change, and a response ratio less than 1
shows a negative change relative to the native environment. If P I and P N are
normally distributed, and P N is unlikely to be negative, then the log of the
response ratio (L) is approximately normally distributed (Hedges et al.
1999), making it a statistically tractable metric. Replicate measures of either
individual or population performance (e.g., average body size, fecundity,
seed set) are required to evaluate whether R differs significantly from 1 (or
L from zero) for a given species.
Strong invasion (R>1) can have one or more of several underlying causes,
such as important changes in the biotic environment (e.g., reduced competition
or reduced suppression by natural enemies), or intrinsic evolutionary
changes associated with the introduction. Quantifying whether strong invasion
occurs is simply a first fundamental step (that has often been skipped)
in coming to a more general understanding of the causes of biological invasions.
The response ratio (R) that we have defined for demographic change in
introduced species is linked to whether or not a species is labeled as “invasive”
(Fig. 6.1). Nevertheless, as labels of invasiveness typically are based on subjective
assessments of ecological and economic impacts, R will not perfectly predict
invasiveness. The outliers, however, are likely to be particularly interesting
cases.
Fig. 6.1 Relationship between the
response ratio and invasiveness.
When R=1, the populations in
the introduced range have not
experienced demographic
change (see text). Strong
invaders are those with R>1.
Weak invaders are those with
R=1. While some populations or
species may be considered quite
invasive and have R1 may not
be considered ecologically and
economically invasive, we predict
few species will lie in the
lower right quadrant