Biol Invasions (2009) 11:1587–1593
DOI 10.1007/s10530-008-9408-x
INVASIVE RODENTS ON ISLANDS
The end of an 80-million year experiment: a review
of evidence describing the impact of introduced rodents
on New Zealand’s ‘mammal-free’ invertebrate fauna
George W. Gibbs
Received: 12 September 2007 / Accepted: 24 September 2008 / Published online: 17 December 2008
Ó Springer Science+Business Media B.V. 2008
Abstract Since separating from its super-continental origin 80 million years ago, New Zealand has
effectively been isolated from the impacts of terrestrial mammals. The arrival of Polynesians in 13th C
heralded the end of this era, with the introduction of
kiore, (Rattus exulans, or Pacific rat), which had farreaching effects on plant regeneration, survival of
small ground vertebrates, larger invertebrates, and
seabird breeding colonies. This paper reviews the
evidence available from raptor nest sites and Quaternary beetle fossils to summarise extinctions thought
to be caused by kiore in New Zealand. It also utilises
invertebrate comparisons between islands with
and without rats, or where rats have been eradicated,
in order to document the impacts of rats (R. exulans,
R. norvegicus) on invertebrate abundance, body
mass, and the behavioural responses of some large
New Zealand insects to the presence of rats. The role
of a ‘mammal-free’ evolutionary history is discussed.
Keywords Beetles Extinction Holocene fossils Kiore Islands Mammal-free evolution Rats
G. W. Gibbs (&)
School of Biological Sciences, Victoria University,
P.O. Box 600, Wellington, New Zealand
e-mail: [email protected]
Introduction
Apart from terrestrial microbats, the evolution of
New Zealand’s fauna and flora took place in the
absence of mammals. This resulted in what has
come to be regarded as a ‘naı̈ve’ biota that was illequipped for the ultimate invasion when humans
arrived (King 1984). Extinctions of large and small
vertebrates can be traced from an excellent Holocene fossil record (Worthy and Holdaway 2002;
Tennyson and Martinson 2006), on the other hand,
the invertebrates, because of the relative scarcity of
well-preserved Holocene material and the lack of
intensive study, have so far contributed disappointingly little to our understanding of the ‘before rats’
invertebrate fauna. This paper seeks and attempts to
open the few windows on past insect life that are
available. It reinforces the widely held view that it is
the larger species of flightless invertebrate that were
placed at risk when rats arrived (Ramsay 1978;
Campbell et al. 1984), but also finds that not all
large-bodied forms succumbed equally and it is
informative to examine ways in which some species
have survived in the presence of rats by possessing
‘pre-adaptive’ traits or perhaps by being able to
modify their behaviour. The paper is in two
sections; first the historical perspective based on
the Holocene record, followed by comparative
studies which contrast invertebrate populations on
islands with and without rats (including locations
where rats have been eradicated).
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Impact of rats
The historical evidence for insect extinctions
For 500? years, kiore (Rattus exulans) was the only
rodent in New Zealand. It invaded a land where the
rodent niche was empty and the fauna and flora
totally unprepared for defence against an active,
nocturnal, olfactory hunter. The prey fauna would be
expected to have evolved traits that kept them out of
the path of largely diurnal, ground-active, visual
hunters (avian and reptilian predators) and, more
particularly, from aerial raptors and insectivorous
birds. The larger insects and snails were highly
cryptic and usually nocturnal, features that perhaps in
themselves were not distinctively New Zealand. With
a scarcity of olfactory predators, many communicated
by means of strong pheromone odours (e.g. weta)
(Field and Jarman 2001).
In this situation, the un-checked colonising population of kiore must have exploded into all parts of
mainland New Zealand, with the exception of the
higher alpine zones, and eventually to certain islands
where canoe transport took them. Their impact on
small ground birds [14 species extinctions (Tennyson
and Martinson 2006)], seabird nesting colonies
(Worthy and Holdaway 2002) and frog and reptile
populations (Wilson 2004) is documented from the
Holocene vertebrate fossil record. Birds and lizards
of up to 200 g in weight suffered most at this time
(Holdaway 1999).
With the invertebrates, the evidence is less clear
cut, but follows the same theme—some were more
susceptible than others and it was the larger,
ground-dwelling, flightless, nocturnal species that
suffered most (Ramsay 1978; Campbell et al. 1984).
The known data are derived mainly from the beetle
fauna, but there is a sense that we are detecting only
the tip of an iceberg. Indirect evidence rested for
many years on the observation that many of the
larger invertebrates have relict distributions on small
islands and are absent or rare on the mainland
(Worthy and Holdaway 2002). But there is direct
evidence as well. Today, Quaternary fossil material
is known from four types of sites: feeding deposits
of avian predators, caves and sinkholes, buried
forests that have succumbed to volcanic ash showers, and Pleistocene deposits where beetle remains
have been recovered.
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Two avian predators, laughing owl (Sceloglaux
albifascies), now extinct, and New Zealand falcon
(Falco novaeseelandiae), nest on overhanging rocky
outcrops where their nest debris can be preserved for
hundreds of years. These deposits, gleaned from 12
sites in Canterbury, South Island, have yielded over
40 beetle taxa (Kuschel and Worthy 1996). Interestingly, three species of large flightless beetles present
in reasonable numbers at the nest sites are no longer
known from the Canterbury regions where the raptors
hunted. That these generalist predators found the
beetles, suggests they were once reasonably common. Their extinction in Canterbury during the past
500–1,000 years is no more than coincidental with
kiore invasion, but all three species still occur today,
albeit with greatly restricted distributions, two of
them restricted to rat-free islands, the third in North
Island forests. Remains of the ngaio weevil, Anagotis
stephensis, were found in seven of the 12 laughing
owl sites examined in Canterbury, in one case with 13
individuals present (Kuschel and Worthy 1996).
Today this weevil occurs solely on rat-free Stephens
Island at the northern extremity of South Island. It is
large-bodied (length 23 mm) with larvae that are
thought to be wood-borers in a common coastal tree.
Likewise, a flightless click beetle, Amychus granulatus, (20 mm) was known only from three rat-free
islands in Cook Strait (Stephens, Maud, and North
Brother Islands), yet was present in the North
Canterbury laughing owl deposits (Kuschel and
Worthy 1996). Interestingly, this beetle has now
succumbed on North Brother Island (after 1960),
where the significant top predator is an expanding
population of tuatara (Sphenodon guntheri) (Gibbs
1999; Hoare et al. 2006), showing that rats are not the
only significant insect predators in New Zealand. A
third example of South Island extinction concerns a
smaller weevil, Ectopsis ferrugalis (16 mm), which
has wood-boring larvae in another common tree,
Pseudopanax arboreus. This cryptic beetle, with up
to 15 individuals found in one laughing owl site in
Canterbury, still survives in the southern half of
North Island where rats are numerous, yet on South
Island it only remains on two rat-free islands, one in
Cook Strait and one in Fiordland. These examples, by
themselves, cannot substantiate kiore as the sole
agent of extinction (Fig. 1).
The second category of evidence comes from
recorded extinctions of three large beetles in forests
Biological Invasions: special issue on rodents on islands
1589
Fig. 1 Hind leg of weevil Ectopsis ferrugalis from a late
Holocene deposit, Awatere valley, northern South Island, New
Zealand. This species is extinct on the South Island mainland.
Scale line 1 mm. M. Marra, with permission, Landcare
research NZ
of the central North Island at some time between the
Taupo eruption, AD1800, and the arrival of European
naturalists to document the fauna. Two were flightless weevils, one of which, Tymbopiptus valeas
(23 mm) (Fig. 2), is represented by well-preserved
parts recovered from a limestone sinkhole at Waitomo and from under the ash layer in a buried podocarp
forest deposit at Pureora (Kuschel 1987). Also
present in these same deposits were fragments of a
species of Anagotis of the kind associated with
Chionochloa tussock-grasses (Kuschel 1987). The
third beetle example was a large Ulodidae, Waitomophylax worthyi (25 mm) (see Leschen and Rhode
2004, 2002 for nomenclature), from the same sinkhole at Waitomo and dated at 1680–2024 year BP.
None of these beetles have been seen alive by
western scientists. The authors, when discussing
causes of extinction, suggest that of all possible
factors, introduced rodents are the most likely.
Quaternary fossil beetle community reconstruction
is a new field of investigation in New Zealand, first
published in 2000, but able to provide a third line of
evidence for invertebrate extinctions. Marra (2007)
reviews the findings from five sites, discusses Pleistocene climate implications and reviews the data on
extinctions. Although small beetles dominate her
samples, like the authors cited above, she concludes
that introduced predators played a role in extinctions.
The loss of South Island populations of the Pseudopanax weevil, Ectopsis ferrugalis (Fig. 1), mentioned
above, within the past 2,000 years, is attributed to
introduced predators (Marra 2007).
Fig. 2 Extinct weevil, Tymbopiptus valeas, from a sinkhole
deposit at Waitomo, North Island, New Zealand, dated
1,680 years BP. Drawn by D. Helmore. With permission,
Landcare research NZ
These invertebrate data by no means constitute
proof that rats have been the sole agents of invertebrate extinction in New Zealand. But it is clear that
the period of pre-European history, between the
arrival of Polynesians and the arrival of the first
European ships (i.e. from approx. 1250–1769 AD),
was a time when the native invertebrate fauna would
have been under pressure from a new type of
nocturnal olfactory predator. Kiore, Rattus exulans,
was the only rodent on the New Zealand archipelago
during this time period, it was having an impact on
small birds and mainland shearwater colonies
(Worthy and Holdaway 2002) and, although records
of invertebrate extinctions are sparse and opportunistic, the available evidence indicates that kiore
could have been responsible for extirpating a number
of large-bodied insects over this time period. Much of
this impact might have occurred during the early
explosive phase of their invasion, in much the same
way as the recorded extinctions due to ship rat
(R. rattus) on Big South Cape Island in southern New
Zealand (see Towns, this issue). The impact of kiore
was felt on both main islands, in dense rainforest and
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in mixed woodland environments, but not in the
alpine zone. Some of their prey victims survived on
offshore islands which remained rat-free.
Impacts of rats
Island comparisons
The New Zealand archipelago (over 600 islands) is
ideal for comparative island studies. Alien rat species
(three have invaded New Zealand islands) have
reached at least 113 (33.5%) of the 337 islands over
5 ha (Atkinson and Taylor 1992). Rats also occupied
numerous islands of less than 5 ha (Taylor 1989). In
addition, the species of rat on islands varied, some
islands received only kiore while others had ship rat
(Rattus rattus) or Norway rat (R. norvegicus), or
combinations of two species. Where comparative
studies have been made, it is again the large-bodied
ground-dwelling insects that have suffered most, yet
not all large-bodied species are equally susceptible,
some have adapted well to the presence of rats. Three
relevant studies are reviewed here, one on an island
group where kiore had been introduced to some but
not all the islands, a second comparing the ground
invertebrate fauna on two Fiordland islands, and the
third examining changes in behaviour of a common
tree weta when kiore were removed from an island.
Mercury Island group, Coromandel
The Mercury Islands comprise six relatively unmodified smaller islands ranging from 3 to 225 ha and one
larger island affected by farming and forestry. At
times of lower sea-levels, prior to 6,500 years ago,
they would have been connected (Hayward 1986),
suggesting that probably all shared a similar baseline
fauna. Sometime during the past 600 years kiore
reached all but the two smallest islands. A very large
(46–73 mm adult body length) tusked weta, Motuweta isolata, is known solely from rat-free Middle
Island (13 ha). It has never been seen on the other
islands in the group, where either kiore were present,
or the area was considerably smaller (Green Island,
3 ha). The absence of tusked weta on the islands with
rats provides circumstantial evidence that the rats
were the reason for their absence, rather than
unsuitable ecological conditions. That view has been
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substantiated by successful weta translocations following rat eradication. Kiore were eradicated from
four of the smaller Mercury Islands during the period
1986–1992 (Townes and Broome 2003), paving the
way for releases of tusked weta into suitable habitats
on Double Island, (33 ha) and Red Mercury (225 ha)
(Stringer and Chappell 2008), based on an understanding of their ecological requirements on Middle
Island (McIntyre 2001).
A comparative island study in Breaksea Sound,
Fiordland
Norway rats (R. norvegicus) are known to have
infested remote Breaksea Island (150 ha), for over
100 years. When the density of invertebrates was
estimated on this, and a nearby smaller island (Gilbert
Island No. 6, 20 ha) which was free of all introduced
mammals, it was found that all taxonomic groups,
except small weevils, had declined (Bremner et al.
1984). The authors’ survey of indigenous invertebrates, which were heat-extracted from litter, moss
and rotten wood, showed that for every 100 invertebrates caught on Breaksea Island, 173 were found on
Gilbert No. 6, despite its much smaller area. Overall,
13 groups of invertebrates were significantly scarcer
on Breaksea than on Gilbert No. 6 (P \ .01), and
only one group, the flatworms, was commoner.
The widely held belief that the larger invertebrates
(in Bremner et al. 1984 study [7 mm) are more
vulnerable was supported in this study e.g. with
weevils the comparison of equal sampling effort
showed that where 151 large weevils ([7 mm) were
found on Gilbert No. 6, only 11 turned up on
Breaksea, despite the luxuriant state of their foodplants, whereas with smaller weevils (4–7 mm) the
numbers were 214 and 130, respectively. Carabid
beetles yielded 43 individuals on Gilbert No. 6, but
only one on Breaksea. Eight individuals of New
Zealand’s large stag beetle, Geodorcus helmsi, were
found on Gilbert No. 6, but only fragments on
Breaksea, suggesting that this species was recently
present there but could no longer be found alive.
Bremner et al. (1984) concluded that ‘predation by
Norway rats is having a devastating effect on the
population density of many invertebrate groups on
Breaksea Island’ and that the most vulnerable are
large beetles, weta, harvestmen and ‘some unexpectedly small species’.
Biological Invasions: special issue on rodents on islands
Behavioural adaptations of invertebrates
towards rats
The group of large orthopteran insects that are
referred to as ‘weta’ in New Zealand (Anostostomatidae and Rhaphidophoridae) display some
interesting behavioural characteristics in relation to
their survival in the face of rat predation. They were
recorded in Bremner et al. (1984) study cited above,
with 52 weta counted per 100 m2 on Gilbert No. 6
compared with 11 per 100 m2 on Breaksea Island.
Bremner et al. (1989) went on to test escape
responses of weta on several Fiordland islands with
and without rats, finding that where rats were present,
the insects displayed a significantly lower threshold
of stimulation, giving them more pronounced escape
responses.
Species in the genus Hemideina, that have earned
the epithet ‘tree weta’, are probably one of the best
examples in New Zealand of an insect that has been
‘pre-adapted’ to withstand the impacts of rats. These
large (45 mm), omnivorous but essentially herbivorous insects, have survived exceptionally well in
urban areas and in rat-infested shrubland throughout
much of the New Zealand mainland, whereas their
sister group, Deinacrida (giant weta), have become
extinct or been restricted to rat-free alpine areas and
offshore islands (see Gibbs 1998). The chief defence
of tree weta against rats lies in their choice of secure
refuges within the natural galleries left by woodboring insects or where fungal rot has created cavities
in standing wood [one alpine species utilises cavities
under stones on the ground, in the absence of trees
(Gibbs 2001)]. These refuges, with their narrow,
‘squeeze through’ entrance holes, are occupied by
day and, where the habitat is shared by rats, often
during much of the night as well (Field and Sandlant
2001). Tree weta are adept at sensing the presence of
conspecifics through substrate vibrations and acoustic
signals (Field 2001). It is likely that these sensors also
enable them to detect movements of predators and
hence remain in the security of their galleries when
danger threatens. An opportunity to test tree weta
behavioural responses to rats arose when kiore were
eradicated from Nukuwaiata Island (195 ha) in the
Chetwode Group, Pelorus Sound, Cook Strait (Rufaut
and Gibbs 2003).
Following rat eradication in 1993, the tree weta
population (Hemideina crassidens) was monitored
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over 5 years to determine their abundance and choice
of refuges, with the expectation that a notable
increase in weta density would occur. In fact, using
a method of gallery occupancy monitoring, there was
little change in density. The occupation of galleries
increased significantly over the first 3 years, but then
subsided again. However, the proportion of adults
increased and the adults occupied galleries with
significantly larger entrance holes and, moreover,
they used galleries that were closer to the ground. The
weta also spent less time sitting in their gallery
entrances, suggesting a more ‘relaxed’ attitude to
their vigilance against potential predators. To test
these behavioural adaptations, weta from rat-infested
and rat-free habitats, were set up in a series of
identical experimental cages where weta behaviour
could be observed throughout the night (Rufaut
1995). As expected, the weta from rat-infested places
(both mainland and islands) showed a significant
reduction in the duration of their activity periods with
a corresponding increase in their occupancy of
refuges compared with weta from rat-free islands.
The conclusion from tree weta studies is that these
particular invertebrates are able to sense the proximity of predators and modify their lifestyle
accordingly. Within one generation of rat removal,
the weta foraged for longer periods of the night.
These large flightless insects are, to some extent,
tolerant of rat invasion. On the other hand, other
species of weta (giant weta, e.g. Deinacrida rugosa
or D. heteracantha) that formerly occurred on the
main islands of New Zealand, succumbed rapidly to
the 19th C invasion by European rats (Gibbs 1998).
These species were larger-bodied and lacked the
protective lifestyle of tree weta (Hemideina),
although they had survived in the presence of kiore.
When the larger rats (Rattus norvegicus and R. rattus)
arrived, giant weta possessed no effective behavioural pathway for avoiding predation—their
particular lifestyle traits are incompatible with rats.
Discussion
Although New Zealand’s invertebrates have evolved
in an environment that lacked efficient mammalian
insectivores, the pre-human situation was far from
predator-free. Avian predators were clearly skilled
at utilising the invertebrate fauna, some specially
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adapted for the extraction of insects from refuge
holes (e.g. the extinct huia, Heteralocha acutirostris)
(Wilson 2004). Can we usefully speculate on the
likely outcome of a history where birds, rather than
small mammals, were the main insectivores?
In general, mammals are thought to rely more on
olfactory cues to seek prey, whereas birds, especially
raptors, use visual signals and have a poor sense of
smell (Worthy and Holdaway 2002). A preponderance
of diurnal insectivorous birds might tend to push
selection for crypsis to an extreme, not seen in
lands where the pressure comes from both birds
and mammals. Visually conspicuous flight could
well have been a more serious disadvantage in
New Zealand than elsewhere, indeed bright colours
in any context. On the other hand, nocturnally active
invertebrates are likely to have faced similar threats to
those elsewhere due to the presence of New Zealand’s
specialised mammal-like birds [kiwi show minimal
reliance on vision and increased reliance on tactile and
olfactory information (Martin et al. 2007)] as well as
terrestrial bats, lizards and frogs. However, the
employment of odoriferous pheromones for communication would possibly not have been penalised to the
extent that it would be in lands with rodents.
How well does the New Zealand invertebrate
fauna meet these assumptions? It is renowned for the
development of some large-bodied, nocturnal, flightless insects, often with strong odours (even to
humans). It is also known for its lack of colourful
diurnal insects such as butterflies, winged grasshoppers, wasps, bees and even large flies. It would be
overly simplistic to assume that avian predators were
the sole drivers of these characteristics but it is
perhaps not surprising that, along with the small bird
fauna, some invertebrates lacked the behavioural
finesse to avoid the first wave of rodent invasion.
Within the weta group for instance, we can see how
certain ‘pre-adaptive’ behavioural features, such as
the use of tree-hole refuges, have enabled certain
species to co-habit successfully with rodents while
others, that lack such protective behavioural features,
rapidly succumbed (Gibbs 1998). We will never
know the full extent of invertebrate extinctions in
New Zealand from the time kiore became established
but it is clear, from the present rat-free islands and
what can be deduced from the known extinctions, that
today’s fauna is less diverse in respect of the larger
invertebrates than it was during the Holocene.
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G. W. Gibbs
Acknowledgments I am grateful to the initiative of Don
Drake and Terry Hunt, University of Hawaii at Manoa for
organising the meeting devoted to rats, humans and islands.
The New Zealand Department of Conservation have provided
support and financial assistance for the author’s numerous
island visits. Cathy Rufaut has been an inspiration, while Chris
Green, Rich Leschen and Maureen Marra have helped with
data and illustrations. My thanks also to two anonymous
reviewers for their constructive suggestions.
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