Life-history theory and management - Ecological Applications: Toward a Sustainable World - page 71

world of invasion success, that these species would encounter a match with appro-
priate environmental conditions.
Finally, the likely size of invader impact, assessed in terms of the abundance
achieved by populations, is not well predicted by species traits, but rather in terms
of prior invasion success and establishment effort.

3.3.4 What we know
and don't know about
invader traits

We know rather a lot about the relationship between species traits and invasion
success but, regrettably, only for a small proportion of the world's biota. The most
informative databases are those concerning taxonomic groups for which informa-
tion is available about species that have become successful invaders as well as those
that have not. This tends to apply to taxa that are important to particular segments
of society, and for which particularly good records have been kept - horticulturalists
and gardeners (pine trees and Czech plants), those in the pet trade (parrots), coloni-
als who wished to make themselves at home (New Zealand Acclimatization Society
- introduced birds) and angling organizations (freshwater fi sh). These databases are
enormously valuable, providing some of the best information we have to assess the
causes of invader success.
However, managers need to beware of unwarranted generalization. We do see
indications of predictability of invasion success for some taxa, related to high repro-
ductive output (e.g. pine seed production), fl exible lifestyles (bird behavior), broad
niches (e.g. parrots) and competitive strategies (e.g. Czech plants). But exceptions
to the 'rules' are common and there are many cases where no relationships have
been found, prompting Williamson (1999) to wonder whether invasions are any
more predictable than earthquakes. Commonly, the best predictor of invasion
success is previous success as an invader elsewhere. Looking on the bright side,
even this provides invasion managers with useful pointers when prioritizing poten-
tial invaders to their regions.

3.4 Species traits as
predictors of
extinction risk

Having discovered the life-history traits that are linked to the probability of invasion
or of success in restoration projects, I now turn attention to the suites of traits that
can be used to predict the vulnerability of native species to human pressures. This
question is vital to conservation biologists who must understand what makes certain
species prone to extinction.
Successful invaders naturally have much in common with successful restoration.
The only distinction is that we have different names for the species we do (restora-
tion) or don't (invasion) want to become established. In Sections 3.2 and 3.3 you
saw that traits associated with niche breadth and ecological fl exibility could some-
times predict the probability of successful establishment, as could suites of traits
associated with the r / K concept (small, rapidly reproducing vs large, slow-growing,
competitive organisms) or the CSR concept (particularly R species with good powers
of colonization and C species with strong competitive traits).
Vulnerability to extinction is a different kettle of fi sh because now we wish to
ensure that a species is not lost from its native environment in the fi rst place. The
list of human pressures that pose risks (and that are responsible for past extinctions,
both global and local) is long and diverse. It includes habitat loss and habitat frag-
mentation as a result of land-use change (e.g. pastoral grazing, forestry, freshwater
habitat disruption), habitat degradation as a result of pollution and overharvesting
of wild populations. Moreover, many species are confronted by more than just one