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Material Type: Notes; Professor: Shi; Class: Seminar; Subject: Mathematics; University: William and Mary; Term: Spring 2006;
Typology: Study notes
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Cane toads (Bufo marinus) are
large anurans (weighing up to
2 kg) that were introduced to Aus-
tralia 70 years ago to control insect
pests in sugar-cane fields. But the
result has been disastrous because
the toads are toxic and highly
invasive. Here we show that the
annual rate of progress of the toad
invasion front has increased about
fivefold since the toads first
arrived; we find that toads with
longer legs can not only move
faster and are the first to arrive in
new areas, but also that those at
the front have longer legs than
toads in older (long-established)
populations. The disaster looks set
to turn into an ecological night-
mare because of the negative
effects invasive species can have
on native ecosystems
; over many
generations, rates of invasion will
be accelerated owing to rapid
adaptive change in the invader
,
with continual ‘spatial selection’
at the expanding front favouring
traits that increase the toads’
dispersal
.
Introduced to Queensland in 1935, cane
toads have since expanded their range to
encompass more than a million square kilo-
metres of tropical and subtropical Australia
.
We have radio-tracked toads (for methods, see
supplementary information) at the invasion
front 60 km east of Darwin and confirmed the
astonishing locomotor performance in these
animals, which move up to 1.8 km per night
during the rainy months — far further than
previously studied anurans
. Does this
remarkable ability result from selection for
enhanced dispersal during the toads’ Aus-
tralian colonization history?
The morphological trait most often linked
to locomotor ability in anurans is leg length,
both among and within species
. Our trials
(see supplementary information) confirm that
cane toads with relatively long legs are indeed
faster over a short distance (regressing time
taken to cover 1 m against residual leg length:
rǃǁ0.44, nǃ29, P<0.02). But, more impor-
tant, longer-legged toads moved further over
24 h (maximum displacement of radio-tracked
toads versus relative leg length: rǃ0.46, n
ǃ21, P<0.04) and over three days (rǃ0.58,
nǃ21, P<0.006; Fig. 1a). Longer legs there-
fore facilitate more rapid dispersal.
If the invasion process has been assisted by
the evolution of improved dispersal ability
among toads at the front, three consequences
would be expected. First, longer-legged toads
should be disproportionately common among
the first wave of arrivals at any site. As the toad
invasion front passed our study site, we mea-
sured relative leg lengths of all toads encoun-
tered over a 10-month period. Longer-legged
toads were the first to pass through, followed
by shorter-legged conspecifics (order of arrival
versus relative leg length: rǃǁ0.34, nǃ552,
P<0.0001; Fig. 1b). Longer-legged toads
therefore moved faster through the landscape.
Second, toads at the invasion front should
be longer-legged than toads from older popu-
lations. As predicted, longer-term historical
analysis within Queensland populations
shows that relative leg length is greatest in new
arrivals and then declines over a 60-year
period (Fig. 1c; rǃǁ0.23, nǃ139, P<0.008).
Third, the rate of progress of the toad inva-
sion front should increase through time. As
predicted, rates of frontal progress have con-
sistently increased (Fig. 1d; time
versus annual rate of spread, Pear-
son’s rǃ0.96, P<0.005). Toads
expanded their range by about
10 km a year during the 1940s to
1960s, but are now invading new
areas at a rate of over 50 km a year.
Accordingly, previous predictions
about the time course of future
expansion of the toads’ range
seri-
ously underestimate their actual
rates of movement.
These rapid shifts in toad mor-
phology, locomotor speed and
invasion velocity indicate that
conservation biologists and man-
agers need to consider the possi-
bility of rapid adaptive change in
invading organisms. If there is no
fitness disadvantage to individual
organisms at the invasion front,
evolutionary forces are likely to
fine-tune organismal traits in
ways that facilitate more rapid
expansion of the invading popu-
lation
. Hence, control efforts
against feral organisms should be
launched as soon as possible,
before the invader has had time to
evolve into a more dangerous adversary.
Benjamin L. Phillips, Gregory P. Brown,
Jonathan K. Webb, Richard Shine
School of Biological Sciences A08, University of
Sydney, New South Wales 2006, Australia
e-mail: rics@bio.usyd.edu.au
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(2004).
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(2005).
99–102 (2003).
(1985).
Supplementary information accompanies this
communication on Nature ’s website.
Received 24 November 2005; accepted 24 January
2006.
Competing financial interests: declared none.
doi: 10.1038/439803a
©200 6 Nature Publishing Group