Life-history theory and management - Ecological Applications: Toward a Sustainable World - page 68

Physiological and behavioral fl exibility can be equated to possession of a broad (or
generalist) niche (Sections 2.4.2 and 3.2.1).

3.3.3 Separating
invasions into
sequential stages -
different traits for
each?

Species invasion is a complex process with several identifi able stages: (1) transport
from the native range; (2) release into a new location; (3) establishment of a self-
sustaining population in the new location; (4) spread beyond the site of fi rst estab-
lishment; and (5) impact in the receiving community. Both the invasive pine study
(Section 3.3.1) and that of the successfully introduced New Zealand birds (Section
3.3.2) fi t into category (4). Generally speaking, managers are most concerned about
species like these, regarding as truly invasive those that spread beyond their original
establishment site. However, to increase predictability in invasion ecology, Kolar
and Lodge (2001) suggest that species' characteristics should be examined in rela-
tion to all the stages of invasion.
Cassey et al. (2004) assessed the fi rst three of the fi ve phases - (1) transport, (2)
release and (3) establishment - in an analysis of all of the world's 350 parrot species.
Transporting and selling parrots is big business and populations that establish in
new locations are easily spotted and quickly recorded by ornithologists. For these
reasons parrots provide an excellent case study to determine species traits that cor-
relate with transport (species commonly transported outside their natural range
score 1, others 0), release (species intentionally released outside their range score
1, those transported but not released score 0) and establishment (the proportion of
releases leading to successful population establishment).
Cassey's team found that different sets of variables were related to the probability
that a species will enter each stage on the pathway through transport to establish-
ment (Table 3.2). Large species are more likely to be transported. More signifi cantly,
species that are transported and released outside their native range tend to be wide-
spread ones that are traded to be kept in captivity or as pets. Presumably this simply
refl ects the need for a predictable supply of desirable trade species. Threatened
species do not fi gure in the groups that are transported and released (note the nega-
tive correlation) - illegal trade in some rare species certainly occurs, but such indi-
viduals are not released into new areas.
Of more interest was the fi nding that species with broader diets and larger alti-
tudinal ranges (i.e. with broader niches) were more likely to establish in a new area,
as were those with longer fl edging periods, perhaps because of the head start gained
from an extended period of parental care. Migratory species, as with the New
Zealand birds, were less likely to establish. You might have predicted, on fi rst prin-
ciples, that migratory species would, like ruderal plants, possess some good
colonizer traits and be effective invaders. However, it turns out that the complex
requirements of migratory species - innate migration cues, learnt migration routes,
appropriate juxtaposition of seasonal habitats - do not suit them for invasions, at
least not those of the human-assisted kind.
Pysek et al. (2003) also have something to say about the fi rst three stages of inva-
sion based on their analysis of fi rst records of 668 alien plants in the Czech Republic
since the year 1500. Deliberately introduced species with utilitarian value (e.g.
culinary or medicinal) arrived before ornamental species, and 'accidental' arrivals
have been most recent. Moreover, species of European origin arrived earlier than
those from North America, with Asian and Australasian species turning up in more
modern times. In their analysis of the relationship between timing of fi rst records