Hot Off the Wires [1] - establishment success in introduced birds

Subject: Hot Off the Wires [1] - establishment success in introduced birds
From: Laurie & Leanne Knight <>
Date: Mon, 12 Nov 2001 21:40:13 +1000
I've received the following item from the author [pdf] and converted the
substantive part to text for people to ponder ...


letters to nature

NATURE |VOL 414 | 8 NOVEMBER 2001 | 195

Determinants of establishment success in introduced birds

Tim M. Blackburn* & Richard P. Duncan2

* School of Biosciences, University of Birmingham, Edgbaston, Birmingham
B15 2TT, UK

2 Ecology and Entomology Group, Soil, Plant & Ecological Sciences
Division, PO Box 84, Lincoln University, Canterbury, New Zealand


A major component of human-induced global change is the deliberate or
accidental translocation of species from their native ranges to alien
environments1,2, where they may cause substantial environmental and
economic damage3,4. Thus we need to understand why some introductions
succeed while others fail. Successful introductions tend to be
concentrated in certain regions2, especially islands and the temperate
zone, suggesting that species-rich mainland and tropical locations are
harder to invade because of greater biotic resistance1,5±9. However,
this pattern could also reflect variation in the suitability of the
abiotic environment at introduction locations for the species
introduced3,9±11, coupled with known confounding effects of nonrandom
selection of species and locations for introduction8,12±14. Here, we
test these alternative hypotheses using a global data set of historical
bird introductions, employing a statistical framework that accounts for
differences among species and regions in terms of introduction success.
By removing these confounding effects, we show that the pattern of avian
introduction success is not consistent with the biotic resistance
hypothesis. Instead, success depends on the suitability of the abiotic
environment for the exotic species at the introduction site.

Historical records provide a valuable resource for investigating reasons
for introduction success or failure, but firm
conclusions have proved difficult to draw from such data3,8,13 for two
reasons14. First, patterns of success are
confounded because species were not randomly assigned to introduction
locations, and because some locations will
have received disproportionately more invaders, or more good or poor
invaders. Second, individual introductions cannot
be regarded as independent data points in a statistical analysis.
Instead, introduction outcomes are likely to be correlated
because the same species was frequently introduced to many locations,
and because most locations were subject to
several introductions15,16.

To overcome these problems, we modelled the success or failure of all
known historical bird introductions using a
generalized linear mixed model (GLMM), including as random effects
variables that coded for the clustering of
introduction events within species, higher taxa and biogeographic region
of introduction (Methods). This allows us to
control for differences in introduction success rate among species and
regions, and to account for the nonindependence
of introductions by modelling the covariance among introductions of the
same species to different locations, among
introductions of species within higher taxa, and among introductions to
the same biogeographic region.

Having controlled for the above effects, there is no relationship
between introduction success and either the latitude of
introduction or whether the introduction was to an island or mainland
location. Thus, within biogeographic regions,
speciesrich locations (for example, low latitude, mainland) are as easy
to invade as species-poor locations (for example,
high latitude, island)8,17. There was significant variation in success
rate among biogeographic regions but the ordering
of regions by their ease of invasibility was not consistent with the
biotic resistance hypothesis. Two of the most
species-rich regions, the Afrotropics and Central/South America were
ranked among the easiest to invade. Hence, our
results find no support for an effect of biotic resistance on
introduction success8,13. The traditional perception, that
species-poor islands and temperate locations are easy to invade,
probably reflects the greater total number of avian
introductions to these locations14. 

Introduction success was significantly greater when the difference
between a species latitude of origin and its latitude of
introduction was small, and when species were introduced to locations
within their native biogeographic region.
Locations at similar latitude and in the same biogeographic region are
more likely to share climatic and habitat features
in common6,18, showing that introduction success is enhanced if species
are matched with suitable environments19. All
else being equal, species with larger geographical range sizes should
have a wider environmental tolerance20, or use
more widespread resources21, and so have a higher probability of
encountering an abiotic environment at a new
location that allows successful introduction22. Consistent with this
prediction, geographical range size is also a
significant correlate of introduction success for global bird
introductions. These results support the view that physiological
tolerances are likely to be at least as important as biotic interactions
in determining the responses of species to global
climate change23,24.

Variation in introduction success unaccounted for by the fixed effects
in the model can be decomposed into variation
due to differences among species at different levels of the taxonomic
hierarchy, differences among biogeographic
regions (see above) and residual variation. This decomposition revealed
significant unexplained variation among
species, but not among taxonomic groups at the genus level or higher
(Table 1, random effects). This shows that
unmeasured species-level traits associated with introduction success
must be phylogenetically labile, varying even
among closely related species. By the same token, characteristics
typically shared by related species, including several
life history traits hypothesized to predict introduction success (body
mass, generation time and population growth
rate3,25±27), can explain only a small and non-significant amount of
variation in global introduction success. To check
this, we ran our analysis again, including familytypical

values for three traits (body mass, clutch size and incubation period;
see Methods) as fixed-effect predictors in the model.
None of these family-level traits were significant determinants of
introduction success, as predicted by the decomposition
of unexplained variance among avian taxonomic levels (Table 1, random

In addition to variation in specific characteristics that affect
invasibility, significant differences among species in
introduction success will almost certainly reflect variation in
introduction effort (the number of releases or individuals
released at each location)3. Although data are not available for most
locations, our analyses account for variation in
effort in two ways. First, species with a larger geographical range size
tend to have been introduced more often and in
greater numbers because they were more readily obtained for
introduction14,22. Hence, the significant effect of
geographical range size on introduction success may reflect introduction
effort in addition to abiotic tolerance, both of
which are required to account for the significant relationship between
geographical range size and success in Australian
bird introductions22. Second, coding species identity as a random effect
in the model controls for differences among
species, including variation in introduction effort.

Determining the causes of introduction success is important for
improving our ability to identify and screen out
environmentally and economically damaging biological invaders3,28. Our
results show that for birds the outcome of
introductions is not predicted by general features of locations related
to biotic resistance (such as latitude), and that the
success of a species cannot be predicted from that of its relatives.
Instead, the importance of phylogenetically labile
species-specific factors, such as geographical range size, and
event-level effects related to environmental suitability,
such as the match between latitudes of origin and introduction, suggest
that success depends on the particular
combination of species and location. This may help to explain why
general features of invaders have been hard to
characterize3,13,26, but shows that an understanding of introduction
success is possible nonetheless.
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