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Context. Feral cats (Felis catus) are a threat to biodiversity globally, but their impacts upon continental reptile faunas have been poorly resolved. Aims. To estimate the number of reptiles killed annually in Australia by cats and to list Australian reptile species known to be killed by cats. Methods. We used (1) data from>80 Australian studies of cat diet (collectively>10 000 samples), and (2) estimates of the feral cat population size, to model and map the number of reptiles killed by feral cats. Key results. Feral cats in Australia's natural environments kill 466 million reptiles yr⁻¹ (95% CI; 271-1006 million). The tally varies substantially among years, depending on changes in the cat population driven by rainfall in inland Australia. The number of reptiles killed by cats is highest in arid regions. On average, feral cats kill 61 reptileskm⁻² year⁻¹, and an individual feral cat kills 225 reptiles year⁻¹. The take of reptiles per cat is higher than reported for other continents. Reptiles occur at a higher incidence in cat diet than in the diet of Australia's other main introduced predator, the European red fox (Vulpes vulpes). Based on a smaller sample size, we estimate 130 million reptiles year⁻¹ are killed by feral cats in highly modified landscapes, and 53 million reptiles year⁻¹ by pet cats, summing to 649 million reptiles year⁻¹ killed by all cats. Predation by cats is reported for 258 Australian reptile species (about one-quarter of described species), including 11 threatened species. Conclusions. Cat predation exerts a considerable ongoing toll on Australian reptiles. However, it remains challenging to interpret the impact of this predation in terms of population viability or conservation concern for Australian reptiles, because population size is unknown for most Australian reptile species, mortality rates due to cats will vary across reptile species and because there is likely to be marked variation among reptile species in their capability to sustain any particular predation rate. Implications. This study provides a well grounded estimate of the numbers of reptiles killed by cats, but intensive studies of individual reptile species are required to contextualise the conservation consequences of such predation.
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How many reptiles are killed by cats in Australia?
J. C. Z. Woinarski
A,J
, B. P. Murphy
A
, R. Palmer
B
, S. M. Legge
C
, C. R. Dickman
D
,
T. S. Doherty
E
, G. Edwards
F
, A. Nankivell
G
, J. L. Read
H
and D. Stokeld
I
A
NESP Threatened Species Recovery Hub, Research Institute for the Environment and Livelihoods,
Charles Darwin University, Casuarina, NT 0909, Australia.
B
Department of Biodiversity, Conservation and Attractions, Locked Bag 104, Bentley Delivery Centre,
WA 6983, Australia.
C
NESP Threatened Species Recovery Hub, Centre for Biodiversity and Conservation Research,
University of Queensland, St Lucia, Qld 4072, Australia.
D
NESP Threatened Species Recovery Hub, Desert Ecology Research Group, School of Life and Environmental
Sciences A08, University of Sydney, NSW 2006, Australia.
E
Deakin University, School of Life and Environmental Sciences, Centre for Integrative Ecology (Burwood Campus),
Geelong, Vic. 3220, Australia.
F
Department of Environment and Natural Resources, PO Box 1120, Alice Springs, NT 0871, Australia.
G
Nature Foundation SA Inc., PO Box 448, Hindmarsh, SA 5007, Australia.
H
University of Adelaide, School of Earth and Environmental Sciences, SA 5000, Australia.
I
Department of Environment and Natural Resources, Berrimah, NT 0820, Australia.
J
Corresponding author. Email: john.woinarski@cdu.edu.au
Abstract
Context. Feral cats (Felis catus) are a threat to biodiversity globally, but their impacts upon continental reptile faunas
have been poorly resolved.
Aims. To estimate the number of reptiles killed annually in Australia by cats and to list Australian reptile species known to
be killed by cats.
Methods. We used (1) data from >80 Australian studies of cat diet (collectively >10 000 samples), and (2) estimates of the
feral cat population size, to model and map the number of reptiles killed by feral cats.
Key results. Feral cats in Australias natural environments kill 466 million reptiles yr
1
(95% CI; 2711006 million). The
tally varies substantially among years, depending on changes in the cat population driven by rainfall in inland Australia. The
number of reptiles killed by cats is highest in arid regions. On average, feralcats kill 61 reptiles km
2
year
1
, and an individual
feral cat kills 225 reptiles year
1
. The take of reptiles per cat is higher than reported for other continents. Reptiles occur at a
higher incidence in cat diet than in the diet of Australias other main introduced predator, the European red fox (Vulpes
vulpes). Based on a smaller sample size, we estimate 130 million reptiles year
1
are killed by feral cats in highly modied
landscapes, and 53 million reptiles year
1
by pet cats, summing to 649 million reptiles year
1
killed by all cats. Predation by
cats is reported for 258 Australian reptile species (about one-quarter of described species), including 11 threatened species.
Conclusions. Cat predation exerts a considerable ongoing toll on Australian reptiles. However, it remains challenging to
interpret the impact of this predation in terms of population viability or conservation concern for Australian reptiles, because
population size is unknown for most Australian reptile species, mortality rates due to cats will vary across reptile species and
because there is likely to be marked variation among reptile species in their capability to sustain any particular predation rate.
Implications. This study provides a well grounded estimate of the numbers of reptiles killed by cats, but intensive studies
of individual reptile species are required to contextualise the conservation consequences of such predation.
Additional keywords: conservation, diet, introduced predator, island, mortality, predation.
Received 10 November 2017, accepted 18 March 2018, published online 15 June 2018
Introduction
As with most other terrestrial vertebrate groups, many reptile
species across the world have declined or become extinct because
of invasive predator species, with susceptibility especially
pronounced for island-endemic species (Böhm et al.2016;
Doherty et al.2016). However, the impacts of introduced
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predators are generally not well resolved for continental reptile
faunas, because there is little information on predation rates or
on the effects of predation rates on the population viability of
any reptile species. In Australia, the signicance of predation
by introduced species upon reptiles is also difcult to evaluate
because the reptile fauna is incompletely catalogued. In addition,
population trends for reptile species are difcult to discern
because there are few substantial population monitoring
programs for reptiles (Meiri 2016; Meiri and Chapple 2016;
Woinarski 2018). Nonetheless, recent studies have demonstrated
severe impacts of introduced predators on at least local
populations of some Australian reptiles. For example, high
rates of predation on egg clutches by the introduced European
red fox (Vulpes vulpes) and pig (Sus scrofa) are implicated
in population declines of several species, some threatened,
of freshwater turtles (Spencer 2002; Spencer and Thompson
2005; Fordham et al.2006,2008; Micheli-Campbell et al.2013;
Whytlaw et al.2013; Freeman et al.2014; Fielder et al.2015;
Robley et al.2016) and marine turtles (Limpus and Reimer
1994). Predation by the red fox is also a recognised threat to some
threatened Australian lizard species (Nielsen and Bull 2016).
Furthermore, predation by the red fox has been implicated in
complex but substantial changes in some Australian reptile
communities (particularly suppression of native reptilian
predators; Olsson et al.2005; Sutherland et al.2011; Read
and Scoleri 2015). Introduced predators, probably primarily
the wolf snake (Lycodon capucinus), are also considered to
have caused the recent collapse of the endemic reptile fauna
of the Australian external territory of Christmas Island, with four
of the ve native lizard species becoming extinct, or extinct in the
wild, over the last decade (Smith et al.2012; Andrew et al.2018).
Here, we assess the extent of predation by the introduced
house cat (Felis catus) on the Australian reptile fauna. Cats
were introduced to Australia at the time of European settlement
(1788) and have since spread across the entire continent and
onto most large islands (Abbott 2008; Abbott et al.2014; Legge
et al.2017). They have caused severe detrimental impacts on
the Australian mammal fauna (Woinarski et al.2015) and are
a major source of mortality for Australian birds (Woinarski
et al.2017a). There has been far less consideration of their
impacts on the Australian reptile fauna. The most substantial
assessment was withina recent study of continental-scale variation
in the diet of feral cats (Doherty et al.2015). That study reported
that: (1) reptiles formed a signicant component of the diet of cats
(average frequency of occurrence 24% in cat dietary samples);
(2) there was substantial geographic variation in the occurrence of
reptiles in cat diet, with a higher frequency of reptiles in cat dietary
samples from hotter, drier areas and in mid-latitudes; and (3) 157
reptile species had been recorded within the diet of feral cats in
Australia, including one threatened species, the recently extinct
Christmas Island forest skink (Emoia nativitatis).
Using a recent assessment of geographic variation in the
density, and hence total population size, of feral cats in
Australia (Legge et al.2017), information on the number of
individual reptiles within cat dietary samples (as distinct from
simply the percentage occurrence of reptiles in dietary samples),
and a more extensive compilation of cat diet information, we
extend the study of Doherty et al.(2015) to evaluate the annual
tally of reptiles killed by cats in Australia and the spatial variation
in this number. This study complements, collates information
from many of the studies reported in and adopts a similar
analytical approach to, a recent assessment of the number and
species of birds killed by cats in Australia (Woinarski et al.
2017a,2017b). Our objectives are to: (1) examine environmental
and geographical variation in the number of reptiles killed by
feral cats; (2) derive an overall estimate of the number of reptiles
killed by feral cats annually; (3) attempt to contextualise this
estimate in relation to the overall number of reptiles in Australia;
(4) compare these assessments with the number of birds killed in
Australia by cats (Woinarski et al.2017a); (5) compile an
updated inventory of Australian reptile species known to be
killed by feral cats; (6) interpret the conservation consequences
and implications of these results; and (7) compare these results
with available information from other continents. Our main focus
is on feral cats in largely natural environments, but we also
include some information on predation of reptiles by pet cats and
by feral cats in highly modied environments.
This assessment is of some global signicance because the
Australian reptile fauna comprises more than 10% of the
global reptile complement, most Australian species (>90%)
are endemic (Chapman 2009) and because the conservation
management of reptiles has been notably neglected globally
compared with other vertebrate groups (Roll et al.2017).
Methods
Feral cats in natural environments
To determine the numbers of feral cats in Australia, and the
spatial variation in cat density, Legge et al.(2017) collated and
then modelled 91 site-based estimates of feral cat density from
largely natural sites widely and representatively scattered across
Australia. They concluded that the total number of feral cats in
largely natural landscapes in Australia was 2.07 million (varying
between 1.4 million in drought and average years to 5.6million
after prolonged and extensive wet periods in inland Australia).
We collated 82 results from Australian studies (with a
minimum of 10 cat scat or stomach samples per study) that
provided a quantitative assessment of the frequency of
occurrence of reptiles in cat stomachs or scats (Table 1). These
studies were widely spread across Australia (Fig. 1) and included
broad representation of Australian natural environments. In
total, 32 of these studies were also included in a previous
consideration of continental variation in the diet of feral cats in
Australia (Doherty et al.2015). Where multiple studies provided
estimates for the same location, these were averaged (weighted by
the number of samples examined). Several individual studies
reported information from more than one site or from the same
site in markedly different conditions; where the source data could
be readily resolved to site level, information from these separate
sitesor conditions was treated as distinct data pointsin our analyses
(as per Table 1), such that the total number of study site
combinations included in our analyses (98) exceeds the number
of included studies.
Although the frequency of occurrence of reptiles in the diet of
feral cats in Australia generally increases in warmer seasons,
consistent with seasonal variation in reptile activity (Paltridge
2002; Yip et al.2015), it was not possible for us to account for
seasonal variation in cat diet in this analysis because many of the
BWildlife Research J. C. Z. Woinarski
Table 1. Collation of accounts of the frequency of reptiles in the diet of feral cats in Australia (limited to those studies with >10 samples)
N, sample size; NG, relevant information not given in source; islandindicates island smaller than Tasmania. Abbreviations for Australian jurisdictions: NSW,
New South Wales; NT, Northern Territory; Qld, Queensland; SA, South Australia; Tas., Tasmania; Vic., Victoria; WA, Western Australia
Location % frequency of
occurrence of
reptiles in
diet (N)
% native
reptiles/all
reptiles
in diet
Mean no. of
individual
reptiles
in samples
with reptiles
Sample
type
Site type Source
Kanandah, Nullarbor, WA 42.1% (76) 100% NG Stomachs Mainland, natural Algar and Friend (1995)
Purple Downs, SA 100% (14) 100% NG Stomachs Mainland, natural Bayly (1976)
Farina, SA 55.0% (21) NG NG Stomachs Mainland, natural Bayly (1978)
Wedge Island, Tas. 1.5% (527) 100% NG Scats Island, natural Beh (1995)
East Gippsland, Vic. 4.5% (22) NG NG Scats Mainland, natural Buckmaster (2011)
Wet Tropics, Qld 0% (123) NG NG Scats Mainland, natural Burnett (2001)
Gibson Desert, WA 42.1% (19) NG NG Scats Mainland, natural Burrows et al.(2003)
Various sites, Tas. 8.8% (45) NG NG Stomachs Mainland, natural Cahill (2005)
Yathong, NSW 30.1% (112) 100% NG Stomachs Mainland, natural Catling (1988)
Collation across many sites, Vic. 2.5% (128) NG NG Stomachs Mainland, natural Coman and Brunner (1972)
Kakadu, NT 8.0% (49) NG NG Scats Mainland, natural Corbett (1995)
Erldunda, NT 55.3% (38) NG NG Stomachs Mainland, natural Corbett (1995)
Christmas Island 3.3% (92) NG NG Scats Island, natural Corbett et al.(2003)
SW Wheatbelt, WA 10.3% (39) NG NG Stomachs Mainland, natural Crawford (2010)
Ningaloo/Cape Range, WA 40% (10) 100% 3.00 Stomachs Mainland, natural F. Delanzy, T. Thomson and
M. Vanderklift (unpubl.)
Dirk Hartog Island 71.4% (14) 100% NG Stomachs Island, natural Deller et al.(2015)
Oberon, NSW (natural area) 12.1% (33) NG NG Scats Mainland, natural Denny (2005)
Oberon, NSW (rubbish dump) 8.3% (48) NG NG Scats Mainland, modied Denny (2005)
Tibooburra, NSW (natural area) 45.8% (144) NG NG Scats Mainland, natural Denny (2005)
Tibooburra, NSW
(rubbish dump)
46.2% (119) NG NG Scats Mainland, modied Denny (2005)
Dwellingup, WA 7.1% (14) NG NG Stomachs Mainland, natural C. Dickman (unpubl.)
Katherine, NT 41.4% (29) 100% 2.75 Stomachs Mainland, natural C. Dickman (unpubl.)
Kellerberrin Durokoppin, WA 10.4% (48) 100% 1.20 Stomachs Mainland, natural C. Dickman (unpubl.)
Mt Isa Cloncurry, Qld 46.2% (26) 100% 2.17 Stomachs Mainland, natural C. Dickman (unpubl.)
Rottnest Island, WA 21.9% (32) 100% 1.14 Scats Island, natural C. Dickman (unpubl.)
Charles Darwin Reserve, WA 46.3% (123) NG NG Scats Mainland, natural Doherty (2015)
Burt Plain NT 36.4% (33) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Daly Basin bioregion, NT 18.2% (11) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Finke bioregion, NT 34.8% (23) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Great Sandy Desert, NT 25.0% (16) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Hamilton Downs, NT 72.7% (187) NG NG Scats Mainland, natural G. Edwards (unpubl.)
MacDonnell Ranges, NT 23.9% (109) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Mitchell Grass Downs, NT 17.7% (192) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Pine Creek bioregion, NT 7.1% (14) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Tanami, NT 18.0% (61) 100% NG Stomachs Mainland, natural G. Edwards (unpubl.)
Irving Creek and Hale River, NT 11.4% (35) NG NG Scats Mainland, natural Foulkes (2002)
Barrington Tops, NSW 2.0% (49) NG NG Scats Mainland, natural Glen et al.(2011)
Flinders Ranges, SA 14.0% (50) NG NG Stomachs Mainland, natural Hart (1994)
Great Dog Island, Tas. 51.6% (91) 100% NG Scats Island, natural Hayde (1992)
Flinders Ranges, SA
(before rabbit control)
15.2% (70) 100% NG Stomachs Mainland, natural Holden and Mutze (2002)
Flinders Ranges, SA
(post rabbit control)
13.1% (288) NG NG Stomachs Mainland, natural Holden and Mutze (2002)
Anglesea, Vic. 0.6% (159) NG NG Scats Mainland, modied Hutchings (2003)
Karijini NP, WA 17.0% (88) NG NG Scats Mainland, natural Johnston et al.(2013)
Flinders Ranges, SA 8.0% (60) NG NG Scats Mainland, natural Johnston et al.(2012)
Flinders Ranges, SA 29.2% (24) NG 1.29 Stomachs Mainland, natural Johnston et al.(2012)
Macquarie Island, Tas. 0% (41) NG NG Stomachs Island, natural Jones (1977)
Macquarie Island, Tas. 0% (756) NG NG Scats Island, natural Jones (1977)
Mallee, Vic. 13.0% (131) NG NG Stomachs Mainland, natural Jones and Coman (1981)
Kinchega NP, NSW 28.0% (65) NG NG Stomachs Mainland, natural Jones and Coman (1981)
Eastern Highlands, Vic. 3.0% (117) NG NG Stomachs Mainland, natural Jones and Coman (1981)
(continued next page)
Reptiles killed by cats in Australia Wildlife Research C
Table 1. (continued )
Location % frequency of
occurrence of
reptiles in
diet (N)
% native
reptiles/all
reptiles
in diet
Mean no. of
individual
reptiles
in samples
with reptiles
Sample
type
Site type Source
Phillip Island, Vic. 5.0% (277) NG NG Stomachs Island, natural Kirkwood et al.(2005)
Inland NE Qld 63.0% (169) 100% 2.61 Stomachs Mainland, natural Kutt (2011)
Lambert station, SW Qld 18.0% (49) NG NG Stomachs Mainland, natural Lapidge and Henshall (2001)
Mt Field and Tasman
Peninsula, Tas.
0% (27) NG NG Stomachs Mainland, natural Lazenby (2012)
Piccaninny Plains, Qld 72.2% (18) NG 1.77 Stomachs Mainland, natural McGregor et al.(2016)
northern Simpson Desert, Qld 57.3% (377) NG NG Scats Mainland, natural Mahon (1999)
Pastoral(mostly Pilbara
and Murchison), WA
44.0% (50) 100% NG Stomachs Mainland, natural Martin et al.(1996)
Rural(mostly wheatbelt), WA 0% (31) NG NG Stomachs Mainland, natural Martin et al.(1996)
Mitchell grass downs, Qld 50.3% (187) 100% 1.96 Stomachs Mainland, natural Mifsud and Woolley (2012)
Lake Burrendong, NSW 3.4% (600) NG NG Scats Mainland, natural Molsher et al.(1999)
Fitzgerald NP, WA 19.5% (41) NG NG Stomachs Mainland, natural OConnell (2010)
Blackall, Qld 46.7% (30) 100% NG Stomachs Mainland, natural R. Palmer (unpubl.)
Davenport Downs, Qld 42.9% (184) 100% 1.93 Scats and
stomachs
Mainland, natural R. Palmer (unpubl.)
Denham Dump, WA 45.3% (53) NG NG Scats Mainland, modied R. Palmer (unpubl.)
Diamantina Lakes, Qld 56.0% (257) 100% NG Scats and
stomachs
Mainland, natural R. Palmer (unpubl.)
Great Western Woodlands, WA 36.4% (11) 100% NG Scats Mainland, natural R. Palmer (unpubl.)
Inglewood, Qld 4.5% (22) 100% NG Scats Mainland, natural R. Palmer (unpubl.)
Monkey Mia, WA 42.1% (19) 100% NG Scats Mainland, modied R. Palmer (unpubl.)
Muncoonie Lakes,
Birdsville, Qld
18.5% (27) 100% NG Scats Mainland, natural R. Palmer (unpubl.)
Mulyungarie, SA 35.0% (40) NG 2.21 Stomachs Mainland, natural R. Palmer (unpubl.)
North Kimberley, WA 21.1% (19) 100% NG Scats Mainland, natural R. Palmer (unpubl.)
Offham, SW Qld 43.5% (23) 100% 2.00 Stomachs Mainland, natural R. Palmer (unpubl.)
Pannawonica, WA 38.5% (13) 100% NG Scats Mainland, natural R. Palmer (unpubl.)
Kintore, NT 62.9% (70) NG NG Scats Mainland, natural Paltridge (2002)
Tennant Creek
(Tanami Desert), NT
72.4% (76) NG NG Scats Mainland, natural Paltridge (2002)
Barkly Tablelands, NT 17.0% (c
A
) NG NG Stomachs Mainland, natural Paltridge et al.(1997)
Tanami, NT 20.0% (b
A
) NG NG Stomachs Mainland, natural Paltridge et al.(1997)
Watarrka, NT 23.0% (a
A
) NG NG Stomachs Mainland, natural Paltridge et al.(1997)
West (Pellew) Island, NT 54.5% (11) NG 3.50 Scats Island, natural Paltridge et al.(2016)
Simpson Desert, NT 9.1% (44) NG NG Scats Mainland, natural Pavey et al.(2008)
Roxby Downs, SA 32.0 (127) NG NG Stomachs Mainland, natural Pedler and Lynch (2016)
Roxby Downs, SA 29.4% (360) 100% 2.62 Stomachs Mainland, natural Read and Bowen (2001)
Heirisson Prong, WA 13.8% (109) NG NG Stomachs Mainland, natural Risbey et al.(1999)
Sandford, Tas. 8.5% (47) NG NG Scats Mainland, natural Schwarz (1995)
Simpson Desert, Qld 33.0% (42) NG NG Scats Mainland, natural Spencer et al.(2014)
Kakadu, NT 4.8% (84) NG NG Scats Mainland, natural Stokeld et al.(2016)
Darwin, NT (urban/rural) 5.6% (18) 100% 1.00 Stomachs Mainland, modied D. Stokeld (unpubl.)
Kakadu/Wardekken, NT 34.8% (23) 100% 1.63 Stomachs Mainland, natural D. Stokeld (unpubl.)
Top End, NT 40.0% (10) 100% 2.50 Stomachs Mainland, natural D. Stokeld (unpubl.)
Southern NT 54.5% (22) 100% 1.42 Stomachs Mainland, natural Strong and Low (1983)
Christmas Island 31.0% (93) NG NG Scats and
stomachs
Island, natural Tidemann et al.(1994)
Croajingalong, Vic. 23.0% (48) NG NG Scats Mainland, natural Triggs et al.(1984)
Armidale, NSW 42.3% (26) 100% 4.09 Scats Mainland, natural van Herk (1980)
Kosciuszko, NSW 5.9% (17) NG NG Scats and
stomachs
Mainland, natural Watson (2006)
Witchelina, SA 63.1% (404) 100% 3.05 Stomachs Mainland, natural Woinarski et al. (in press)
Matuwa (Lorna Glen), WA 26.4% (337) NG NG Scats Mainland, natural Wysong (2016)
Western Qld (boomperiod) 23.0% (152) 100% 1.46 Stomachs Mainland, natural Yip et al.(2015)
Western Qld (bustperiod) 60.0% (35) 100% 2.90 Stomachs Mainland, natural Yip et al.(2015)
A
Sample sizes not given for separate areas in text, but a + b + c = 390.
DWildlife Research J. C. Z. Woinarski
constituent studies collated here spanned several seasons, or
the time of year covered by the sampling was not specied. The
studies occurred over the period 19692017, but we do not
include year in analyses, as any directional trend in diet over
decadal scales is unlikely, and Legge et al.(2017) found no
evidence of trends in cat densities over this period. Inter-annual
variation in rainfall is a key driver of the abundance of many
species in arid and semiarid Australia, including feral cats
(Legge et al.2017), and we include separate analyses (for the
number of reptiles killed by cats) based on modelled densities
of cats in years of good rainfall in arid and semiarid Australia,
and in years of poor or average rainfall there (Legge et al.2017).
Inter-annual rainfall patterns in arid and semiarid Australia
may also inuence the abundance of reptile species, but these
relationships are more complex and vary among reptile species
(Read et al.2012; Dickman et al.2014; Greenville et al.2016).
Collectively, the collated cat dietary studies include 10 744
samples of scats or stomachs. Most studies reported only
frequency of occurrence (i.e. the proportion of stomachs or
scats that contained reptiles) rather than a record of the number
of individual reptiles in those samples. However, in a subset of
the studies (Table 1), tallies were given for the number of
individual reptiles in those samples that contained reptiles.
We assessed whether there was a relationship, across studies,
between the number of reptile individuals in those samples
that contained reptiles and the frequency of occurrence of
reptiles in those diet samples. We modelled this relationship
using a linear least-squares regression model of the form:
log ðindividuals 1Þfrequency:
Here, we assume that that one stomach or scat sample
represents 24 h worth of prey eaten by an individual cat. This
is likely to be a conservative under-estimate of the number of
prey killed per day because: (1) prey are largely digested after
12 h; (2) cats typically produce more than one scat per day;
(3) cats may kill some reptiles but not necessarily consume
them (surplus kill); (4) cats may injure hunted animals
but not directly kill and consume them, and many of those
injured animals will subsequently die; and (5) reptile eggs
and hatchlings may be rapidly digested and leave little trace
(Hubbs 1951; Jackson 1951; George 1978; Davies and Prentice
1980; Read and Bowen 2001; Jessup 2004; Loss et al.2013).
Conversely, cats may also scavenge, so some reptiles included
in cat dietary studies are not necessarily killed by the cat that
consumed them (Hayde 1992; Molsher et al.2017).
For analysis of the environmental (and other) factors that
may have contributed to variation in the frequency of occurrence
and number of reptiles in cat samples, we noted whether the
study was from an island or the mainlands of Australia and
2500 mm
100 mm
Mean annual rainfall
Fig. 1. Occurrence of cat dietary studies collated in this study. There were 89 studies in natural vegetation
(76 on the Australian mainland, three in Tasmania and 10 on smaller islands, including Macquarie and
Christmas Islands, not shown on map). There were another six studies in highly modied environments
(such as rubbish dumps). The map background shows mean annual rainfall (Australian Bureau of
Meteorology 2016b). The dashed line indicates the Tropic of Capricorn.
Reptiles killed by cats in Australia Wildlife Research E
Tasmania (64 519 km
2
), and if on an island the size of the
island. We derived a composite variable expressing whether
the site was an island, and the size of the island:
island size index ¼log10 minimum 1;
area
10 000
no
;
where area is land mass or island area in km
2
. Therefore, any land
mass or island with area 10 000 km
2
(i.e. the Tasmanian and
Australian mainlands) has an index of 0. Islands <10 000 km
2
have negative values, which become increasingly negative
with decreasing island area. From the reported location of the
study, we also determined mean annual rainfall (Australian
Bureau of Meteorology 2016b), mean annual temperature
(Australian Bureau of Meteorology 2016a), mean tree cover
within a 5-km radius (Hansen et al.2003) and topographic
ruggedness (standard deviation of elevation within a 5-km
radius) (Jarvis et al.2008).
We used generalised linear modelling to examine geographic
variation in the frequency of occurrence of reptiles in the diet of
feral cats. The response variable was a proportion (number of
samples containing reptiles out of the total number of samples),
and thus was analysed using the binomial error family. We
examined a set of 40 candidate models representing all
combinations of the ve explanatory variables described above
(island size index, rainfall, temperature, tree cover, ruggedness),
including an interaction between rainfall and temperature (to
account for a possible negative effect of temperature on water
availability). Models were evaluated using a second-order form
of Akaikes Information Criterion (QAIC
c
), which is appropriate
for small sample sizes and overdispersed data (Burnham and
Anderson 2003). There was evidence of strong overdispersion,
so we used the quasibinomialerror structure to estimate
coefcient standard errors and condence intervals.
The nal model was based on multi-model averaging of the
entire candidate set, with each model weighted according to w
i
,
the Akaike weight, equivalent to the probability of a particular
model being the best in the candidate set (Burnham and Anderson
2003). The nal model was used to predict the frequency of
occurrence of reptiles in cat diet across Australias natural
environments (i.e. excluding areas of intensive land use, such
as urban areas and rubbish dumps, following Legge et al.(2017)).
The predicted frequency of occurrence of reptiles in cat
diets was used to estimate the number of individual reptiles in
those cat samples that contained reptiles, using the linear least-
squares regression model described earlier (log [individuals 1]
~frequency). We multiplied the predicted frequency of
occurrence of reptiles in cat samples across Australia by the
predicted number of individual reptiles in those cat samples
with reptiles, which provided a spatial representation of the
estimated number of reptiles killed per feral cat per day. This
result was multiplied by the modelled density of cats in natural
environments across Australia (Legge et al.2017), and then by
365.25 (days in a year), to provide a spatial representation of
the estimated number of reptiles killed by cats km
2
year
1
.We
summed this rate across the natural environments of Australia
to derive the total number of reptiles killed by feral cats.
We followed the approach of Loss et al.(2013) and Legge
et al.(2017) and characterised the uncertainty of the estimated
total number of reptiles killed by feral cats using bootstrapping.
Bootstrapping is an appropriate approach because we needed
to propagate errors through several analytical steps (e.g. the
estimate of the total feral cat population, the number of reptiles
eaten per cat per year). We simultaneously bootstrapped (20 000
times) the three underlying datasets: (1) cat density; (2) frequency
of reptiles in cat diet samples; and (3) the number of individual
reptiles in cat diet samples containing reptiles. For each random
selection of these underlying data, we recalculated the total number
of reptiles killed. We report the 2.5% and 97.5% quantiles for
the 20 000 values of the total number of reptiles killed.
Based on studies that included identication of reptile
species in cat dietary items, we also calculated the mean
percentage of reptile items consumed that were native species.
From all studies that reported the total number of individual
reptiles in cat samples, and the taxonomic identity (to family,
genus or species), we also calculated the percentage of cat-killed
reptiles by broad reptile group: crocodiles (family Crocodylidae),
marine turtles (Cheloniidae and Dermochelyidae), freshwater
turtles (Carettochelydidae and Chelidae), geckoes
(Carphodactylidae, Diplodactylidae, Gekkonidae), pygopodids
(Pygopodidae), agamids (Agamidae), skinks (Scincidae),
monitors (Varanidae), blind snakes (Typhlopidae), pythons
(Boidae), le snakes (Acrochordidae), colubrid snakes
(Colubridae) or elapid snakes (Elapidae).
Feral cats in highly modied landscapes
Legge et al.(2017) estimated that there are 0.72 million feral
cats occurring in the ~57 000 km
2
of Australia that comprise
highly modied landscapes (such as rubbish dumps, intensive
piggeries, urban areas) where food supplementation for feral
cats is unintentionally provided by humans. There were only six
Australian studies (with >10 samples) that reported frequency of
reptiles in the diet of feral cats occurring in highly modied
environments (Table 1). This small number provides little scope
for assessing variability, so we simply use the average frequency
of occurrence of reptiles in samples across these six studies and
multiply this mean by the expected number of individual reptiles
in cat samples with that frequency, and then by the density (and
hence population size) of feral cats in these environments as
estimated by Legge et al.(2017). We also compare the frequency
of reptiles in these samples with those from feral cats in natural
environments, using MannWhitney U-tests, but note that the
small sample size of dietary studies for cats in highly modied
environments constrains the reliability of such comparisons.
Pet cats
From national surveys of pet ownership, the population of pet
cats in Australia has been previously estimated at 3.88 million
(Animal Medicines Australia 2016). The average number of
reptiles killed by pet cats in Australia has been estimated in
several studies that have involved cat owners tallying the number
of prey items brought in by pet cats over xed time periods
(Paton 1990; Paton 1991; Paton 1993; Trueman 1991; Barratt
1997; Barratt 1998). There is substantial variation in such tallies
according to the amount of time the pet cat is allowed to roam
outside (Trueman 1991).
The actual number of kills by pet cats is likely to be
appreciably higher than these owner-reported tallies, given
FWildlife Research J. C. Z. Woinarski
that studies elsewhere indicate that pet cats typically return
home with a relatively small proportion of prey actually taken
(Blancher 2013), with estimates of this proportion from studies
on other continents being 12.5% (Maclean 2007), 23% (Loyd
et al.2013), and 30% (Kays and DeWan 2004). Here, we average
across Australian studies the number of individual reptiles
reported by pet owners to be killed by their pet cats per year,
and scale this up to account for the number of reptiles killed but
not returned to the cats home, using the mean (22%) from the
three studies that provide estimates of this proportion.
Comparison of frequency of reptiles in the diet of feral cats
with that of other co-occurring mammalian predators
Australian reptiles face many introduced and native predators in
addition to cats. A subset of the feral cat diet studies collated here
also included comparable sampling of the diet of other co-
occurring mammalian predators, notably the introduced red
fox and dingo (including wild dog) (Canis dingo/familiaris).
For studies that included at least 10 samples of feral cats and at
least 10 samples of one other mammalian predator species, we
compared the frequency of reptiles in samples, using Wilcoxon
matched-pairs tests.
List of reptile species reported to be killed by feral cats
From a collation of individual cat dietary studies, Doherty et al.
(2015) derived a list of Australian reptile species known to be
killed by feral cats. We add to that list by incorporating
information from additional cat dietary studies, autecological
studies of reptiles and specimens reported as cat-killed from all
main museums in Australia. There are some notable caveats and
biases in constructing such a list. First, the Australian reptile
fauna has been subjected to major taxonomic overhaul in recent
decades, resulting inter alia in a rapid increase in the number of
described species (Oliver et al.2009; Cogger 2014; Meiri 2016),
therefore specic names given for reptiles in older studies may
now be difcult to reconcile unambiguously with the currently
recognised taxonomy. Second, diagnostic morphological
characteristics for reptile species in some groups (e.g. many
small skinks, blind snakes) are difcult enough to resolve in the
eld with intact specimens, but such ne species-level resolution
will be impossible in many circumstances for the partly digested
and fragmented material within cat stomachs or even more
challenging from reptile scales and skeletal residue in cat scats;
thus many reptiles in cat dietary samples have been listed only to
family or genus level. Third, many reptile species have highly
localised ranges (Rosauer et al.2016; Oliver et al.2017), and
there may have been no sampling of cat diet in the small areas that
such species occupy.
We also report the conservation status of reptile species
recorded to be killed by cats, although we note that conservation
status has not yet (as at February 2018) been completed for most
Australian species (Böhm et al.2013; Meiri and Chapple 2016).
Results
Feral cats in natural environments
Based on 89 estimates from natural environments in Australia
(Table 1; Fig. 1), the overall frequency of occurrence of reptiles in
cat scat and stomach samples was 25.6% (95% CI; 21.030.7%),
with frequency ranging widely across individual studies, from
0 to 100%.
Generalised linear modelling suggested that two variables
were clear predictors of the frequency of reptiles in feral cat diet
samples: mean annual rainfall (with higher frequency of reptiles
in cat samples in areas of lower rainfall) and mean annual
temperature (with higher frequency of occurrence in areas of
higher temperature) (Fig. 2). These variables were included in
the 17 most highly ranked models, all with a very high level of
support (QAIC
c
<14.9; Table 2). The best models had R
2
of
0.56. Other variables (whether the sample was from an island
0
20
40
60
80
100
0 5 10 15 20 25 30
Mean annual temperature (°C)
0
20
40
60
80
100
0 1000 2000 3000
Frequency of reptiles in cat diets (%)
Mean annual rainfall (mm)
(a)
(b)
Fig. 2. Variation in the frequency of reptiles in cat samples in relation
to (a) mean annual rainfall and (b) mean annual temperature in Australia.
Observations from the mainland, comprising Tasmania and greater
Australian mainland, are indicated by lled circles, while those from
islands smaller than Tasmania (64 519 km
2
) are indicated by unlled
circles. Regression lines represent the predictions of generalised linear
models (quasibinomial errors), with 95% condence intervals.
Reptiles killed by cats in Australia Wildlife Research G
or mainland, topographic ruggedness or tree cover) had little
inuence on the frequency of reptiles in cat diet. These modelled
relationships were used to project the frequency of reptiles in cat
diets across Australia (Fig. 3a).
The number of individual reptiles in cat dietary samples that
contained reptiles was correlated with the frequency of reptiles
in cat samples (Fig. 4): i.e. when a high proportion of the cat
samples in a study contained reptiles, each of those samples
with reptiles was likely to include many individual reptiles.
There were many notable cases of high numbers of individual
reptiles in single cat stomachs, including 24 individual reptiles
in a single cat stomach (Muir 1982), 40 (including 34
Tympanocryptis lineata) and 21 individual reptiles (including
15 T. lineata) (Brooker 1977), at least 19 skinks (Jones and
Coman 1981), 32 (including 24 Ctenophorus pictus), 22, and
19 reptile individuals (Read and Bowen 2001), 27 individual
skinks (all Pseudomoia pagenstecheri) (Cahill 2005), 20
individual skinks (all Ctenotus regius) (Bayly 1976) and 33,
18 and 17 individual reptiles (Woinarski et al. in press).
Spatial analyses revealed a clear contrast in the relative
numbers of reptiles killed per km
2
between mesic coastal
Australia (with relatively low numbers of reptiles killed, with
a minimum of 0.1 km
2
year
1
) and arid and semiarid areas
of the Australian interior (with relatively high kill rates, to
a maximum of 219 km
2
year
1
) (Fig. 3b).
Summing these rates provides an estimate of 466 million
reptiles (95% CI; 2711006 million) killed by feral cats across
the natural environments of Australia per year (varying from
250 million (95% CI; 168501 million) in dry or average years
to 1.49 billion in wet years (95% CI; 0.583.56 billion))
(Fig. 5a). On average, a feral cat kills 225 reptiles per year
(95% CI; 157344) (Fig. 5b). The average number of reptiles
killed by feral cats in natural environments is 61.1 km
2
year
1
(95% CI; 35.5131.8), varying from 32.7 km
2
year
1
(95% CI;
22.065.6) in dry and average years to 194.7 km
2
year
1
(95%
CI; 76.2466.0) in wet years.
For the 41 studies where all the species of reptiles present in
cat dietary samples were reported, all reptile species were native.
However, two studies from Christmas Island (Corbett et al.2003;
Tidemann et al.1994) reported some occurrence of introduced
reptile species in cat diet, but information presented in those
studies did not allow the calculation of a proportion of all cat-
killed reptiles that was native. Given that the two Christmas
Island studies are atypical in sampling an area with a relatively
high proportion of introduced reptiles, and that introduced reptile
species are absent from most sites in studies where reptiles in cat
diet samples were not identied to species, it is highly likely that
native reptile species in the diet of feral cats comprise close to
100% of all reptiles consumed across most of Australia.
Reptiles killed by cats are taxonomically diverse. Across
cat dietary studies that reported the identity and number of all
reptiles killed (Table 3), skinks (35.0%), agamids (30.3%) and
geckoes (21.5%) comprised the largest proportions of cat-
killed reptile individuals. Pygopodids (1.6%), goannas (3.5%),
blind snakes (1.8%), pythons (0.1%), colubrid snakes (0.1%) and
elapid snakes (6.2%) comprised smaller proportions, and these
studies reported no crocodiles, marine turtles, freshwater turtles
or le snakes in the cat dietary samples (Table 3)although there
are records of cats consuming freshwater turtles and hatchling
marine turtles in other studies that provided less quantitative
descriptions of cat diet (Supplementary Table S1). There was
substantial variation among studies in the relative proportions of
different reptile families killed by cats (Table 3).
Feral cats in highly modied landscapes
Of the six studies that reported the frequency of occurrence of
reptiles in samples from feral cats in highly modied
environments (Table 1), the mean frequency of occurrence of
reptiles was 24.7%, similar to, and not signicantly different
from, that for cats in largely natural environments (mean 25.6%;
MannWhitney U-test, z= 0.47, P= 0.64). Multiplying the
reptile frequency in the six studies by the expected number of
reptile individuals in samples with reptiles (relationship shown in
Fig. 4), by 365.25 (days in a year) and then by the total population
size of feral cats in highly modied landscapes (0.72 million;
Legge et al.(2017)) produces an estimate of 130.0 million
reptiles killed per year by feral cats in modied environments.
Pet cats
Pet owners reported an average of 7.8 reptiles observed to be
taken home as prey per cat per year in Adelaide from a sample of
166 cats (with the highest rates of predation of reptiles by pet cats
in rural areas, followed by country towns and then suburbs)
(Paton 1991). In comparable studies, pet owners in Canberra
reported an average of 0.6 reptiles taken home as prey per cat
per year from a sample of 138 cats (Barratt 1998), and those in
Hobart reported an average of 0.6 reptiles per year from a sample
of 166 cats (Trueman 1991). Using the average of 3.0 reptiles
observed to be taken per cat per year across these studies, and
scaling this mean by the average proportion of all kills that are
returned home (i.e. 22%), the average number of reptiles killed
Table 2. Models explaining variation in frequency of reptiles in cat
diets in natural environments throughout Australia, and the results of
the model selection procedure
The models are shown ranked in ascending order of the model selection
criterion, DQAIC
c
, which is the difference between the models QAIC
c
value and the minimum AIC
c
value in the candidate set. w
i
is the Akaike
weight, or the probability of the model being the best in the candidate set.
Models that are well supported relative to the null model (within 2 QAIC
c
units) are shaded grey; models with limited support (DQAIC
c
>5), or lower
support than the null model, are not included in the table
Model DQAIC
c
w
i
R
2
~log
10
(rainfall) + temperature 0.0 0.29 0.56
~log
10
(rainfall) * temperature 1.5 0.14 0.57
~log
10
(rainfall) + temperature + tree cover 2.1 0.10 0.56
~log
10
(rainfall) + temperature + ruggedness 2.3 0.09 0.56
~island size index + log
10
(rainfall) + temperature 2.3 0.09 0.56
~log
10
(rainfall) * temperature + tree cover 3.7 0.05 0.57
~log
10
(rainfall) * temperature + ruggedness 3.8 0.04 0.57
~island size index + log
10
(rainfall) * temperature 3.8 0.04 0.57
~island size index + log
10
(rainfall) + t
emperature + tree cover
4.4 0.03 0.56
~log
10
(rainfall) + temperature + tree
cover + ruggedness
4.4 0.03 0.56
~island size index + log
10
(rainfall) +
temperature + ruggedness
4.6 0.03 0.56
HWildlife Research J. C. Z. Woinarski
60%
10%
150 reptiles km–2 year–1
0 reptiles km–2 year–1
(a)Frequency of reptiles in
feral cat diets
(b)Number of reptiles eaten
by feral cats
Fig. 3. Model projections of (a) the frequency of reptiles in cat diets, and (b) the number of reptiles killed by
cats each year, in natural environments throughout Australia. For (a), predictor variables in the regression
model are: mean annual rainfall; mean annual temperature; tree cover; and ruggedness, weighted according to
Akaike weights (w
i
) for the candidate models (Table 2). The dashed lines indicate the Tropic of Capricorn.
Reptiles killed by cats in Australia Wildlife Research I
by individual pet cats is 13.6 year
1
. Hence, with a total
Australian population of 3.88 million pet cats, the estimated
annual tally of reptiles killed by pet cats is 52.9 million. We
note the marked disparity in estimated rates of predation by pet
cats on reptiles between the studies by Paton (1991) and those by
Barratt (1998) and Trueman (1991), and thus attach low
condence to our collated tally.
Comparison of frequency of occurrence of reptiles in the diet
of feral cats with that of other co-occurring mammalian
predators
Comparative data on the frequency of occurrence of reptiles in
samples of feral cats and other co-occurring mammalian
predators are summarised in Table 4. Across 24 studies where
the diet of co-occurring cats and foxes was reported, the
frequency of occurrence of reptiles was appreciably higher in
the diet of cats (mean = 31.1%) than of foxes (mean = 20.0%)
(Wilcoxon-matched-pairs test z= 3.46, P= 0.0005). Across 18
studies in which the diet of co-occurring cats and wild dogs
(including dingoes) was reported, the frequency of occurrence of
reptiles in the diet of cats (26.3%) was higher than that of dogs
(16.1%), but not signicantly so (z= 1.50, P= 0.14). Only two
studies with sample sizes of >10 samples per predator species
have considered the diet of cats and a co-occurring native
marsupial predator, in both cases, the spotted-tailed quoll
Dasyurus maculatus (Burnett 2001; Glen et al.2011). In these
studies, the frequency of reptiles was low in both species (mean
of 1.0% for cats and 3.9% for quolls).
Reptile species reported to be killed by feral cats
We collated records of 258 Australian reptile species known to be
killed by cats (Supplementary Table S1), a substantial increase
from the 157 species previously reported by Doherty et al.
(2015). This tally represents about a quarter of the described
Australian terrestrial reptile fauna (997 species are listed in
Supplementary Table S1, which excludes sea snakes), and
includes representation of all families with primarily
terrestrial species. Varanidae had the highest proportion
(47%) of species reported in cat diets, possibly because these
are relatively large lizards and are readily identiable to species
in cat dietary samples. Species known to be killed by cats
comprised 1030% of the species complement for most other
terrestrial families.
Although there is insufcient information in the collated
sources to report on species-level impacts of cat predation,
some reptile species have been reported as cat prey from
many studies: the tree dtella (Gehyra variegata), Bynoes
prickly gecko (Heteronotia binoei), central bearded dragon
(Pogona vitticeps), robust ctenotus (Ctenotus robustus),
broad-banded sand-swimmer (Eremiascincus richardsonii)
and curl snake (Suta suta) have all been reported as cat prey
in at least 10 studies describing cat diet. This may be because
these species are targeted by cats, they are common and
widespread, and/or relatively many cat dietary studies have
been undertaken within their range.
The list of cat-killed reptile species includes four of the 31
Australian terrestrial squamate species listed as threatened by the
IUCN (as at February 2018) (1) the Christmas Island forest
skink, (2) dwarf copperhead (Austrelaps labialis), (3) Lord Howe
0
1
2
3
4
5
6
0 20406080
Individual reptiles per diet sample
Frequency of reptiles in cat diets (%)
Fig. 4. The modelled relationship between the number of individual
reptiles in those samples containing reptiles and the frequency (incidence)
of reptiles in cat dietary samples, according to a linear least-squares
regression model of the form: log (individuals 1) ~frequency.
0
5
10
15
20
25
30
35
0 800 1600 2400 3200 4000
Frequency (%)
Total number of reptiles eaten by feral cats (M reptiles year–1)
0
3
6
9
12
0 100 200 300 400
Frequency (%)
Reptiles eaten by each feral cat (reptiles cat–1 year–1)
Dry–average periods only
All observations
Wet periods only
(a) Total number of reptiles eaten
(b) Number of reptiles eaten by each feral cat
Fig. 5. Uncertainty in (a) the total number of reptiles eaten, and (b) the
number of reptiles eaten by each feral cat, based on bootstrapping of the
dataset 20 000 times. At the top of each panel is the mean (lled circle)
and 95% condence bounds (lines). In (a), this is shown separately for
analyses with cat density observations from wet periods, dryaverage
periods, and including all observations (wet and dryaverage) (as dened
in Legge et al.2017).
JWildlife Research J. C. Z. Woinarski
Table 3. Percentage of all reptiles identied in cat dietary samples, by reptile family, for studies reporting the identity and numbers of all reptiles consumed by feral cats
Freq.Occ.is the % of cat samples that contained reptiles. No. reptilesis the total number of reptile individuals reported in cat dietary samples in the study. Reptile families: Geckoes
(Carphodactylidae, Diplodactylidae, Gekkonidae); Pygop. (Pygopodidae); Agam. (Agamidae); Scinc. (Scincidae); Varan. (Varanidae); Typhl. (Typhlopidae); Colub. (Colubridae); Boid. (Boidae);
Elap. (Elapidae). Note that none of the listed studies included crocodiles, marine or freshwater turtles, or le snakes as cat dietary items
Location Source Freq. Occ. No. reptiles Geckoes Pygop. Agam. Scinc. Varan. Typhl. Colub. Boid. Elapid.
Wedge Island, Tas. Beh (1995) 1.5 8 0.0 0.0 0.0 100.0 0.0 0.0 0.0 0.0 0.0
Ningaloo/Cape Range, WA Delanzy, Thompson and
Vanderkift (unpubl.)
40.0 12 66.7 8.3 0.0 8.3 16.7 0.0 0.0 0.0 0.0
Mt Isa, Qld Dickman (unpubl) 46.2 26 11.5 3.8 30.8 34.6 0.0 3.8 0.0 0.0 15.4
Katherine VRD, NT Dickman (unpubl) 41.4 33 39.4 6.1 3.0 45.5 0.0 3.0 0.0 0.0 3.0
Rottnest Island, WA Dickman (unpubl) 21.9 10 10.0 0.0 0.0 70.0 0.0 0.0 0.0 0.0 20
Kellerberrin Durokoppin, WA Dickman (unpubl) 10.4 6 0.0 0.0 33.3 66.7 0.0 0.0 0.0 0.0 0.0
Dwellingup, WA Dickman (unpubl) 7.1 1 0.0 0.0 0.0 100.0 0.0 0.0 0.0 0.0 0.0
Great Dog Island, Tas. Hayde (1992) 51.6 47 0.0 0.0 0.0 93.6 0.0 0.0 0.0 0.0 6.4
Flinders Ranges, SA Johnston et al.(2012) 29.2 9 0.0 0.0 0.0 100.0 0.0 0.0 0.0 0.0 0.0
Phillip Island, Vic. Kirkwood et al.(2005) 5.0 20 0.0 0.0 0.0 100.0 0.0 0.0 0.0 0.0 0.0
Inland NE Qld Kutt (2011) 63.0 277 35.7 2.2 34.3 15.5 5.1 0.0 0.0 0.4 6.9
Picanniny Plains, Qld McGregor et al.(2016) 72.2 23 8.7 0.0 21.7 73.9 0.0 0.0 0.0 0.0 0.0
Mitchell grass downs, Qld Mifsud and Woolley (2012) 50.3 131 9.2 2.3 51.9 20.6 0.0 2.3 0.0 0.0 13.7
Offham, Qld Palmer (unpubl.) 43.5 20 30.0 0.0 15.0 45.0 5.0 0.0 0.0 0.0 5.0
Blackall, Qld Palmer (unpubl.) 46.7 27 63.0 0.0 18.5 3.7 0.0 0.0 0.0 0.0 14.8
Mulyungarie, Qld Palmer (unpubl.) 35.0 31 54.8 0.0 16.1 3.2 0.0 0.0 0.0 0.0 12.9
West (Pellew) Island, NT Paltridge et al.(2016) 54.5 21 9.5 0.0 33.3 47.6 0.0 0.0 4.8 0.0 4.8
Roxby Downs, SA Read and Bowen (2001) 29.4 305 21.6 0.7 24.9 45.6 3.3 2.3 0.0 0.0 1.6
Heirisson Prong, WA Risbey et al.(1999) 13.8 15 33.3 0.0 33.3 33.3 0.0 0.0 0.0 0.0 0.0
Armidale, NSW van Herk (1980) 42.3 25 12.0 0.0 4.0 84.0 0.0 0.0 0.0 0.0 0.0
Witchelina, SA Woinarski et al. (in press) 63.1 442 17.4 1.6 34.8 30.5 5.7 3.8 0.0 0.2 5.9
Western Qld (boom period) Yip et al.(2015) 23.0 50 8.0 2.0 60.0 22.0 2.0 0.0 0.0 0.0 6.0
Western Qld (bust period) Yip et al.(2015) 60.0 54 13.0 3.7 31.5 33.3 3.7 0.0 0.0 0.0 14.8
Totals 1588 342 25 481 555 5 28 1 2 99
Reptiles killed by cats in Australia Wildlife Research K
Island skink (Oligosoma lichenigera) and (4) great desert
skink (Liopholis kintorei)as well as eight of the 45
terrestrial squamate species listed as threatened nationally
under AustraliasEnvironment Protection and Biodiversity
Conservation Act 1999 (as at February 2018) and one marine
turtle and one freshwater turtle listed as threatened nationally
(Supplementary Table S1).
Discussion
We provide the most robust estimate to date on the extent
of, and geographic variation in, continental-scale predation of
reptiles by an introduced species. That take is substantial: feral
cats in Australias largely natural environments kill more than
one million reptiles per day, and individual cats take, on
average, more than 225 individual reptiles year
1
, with almost
all of these killed reptiles being native species (consistent with
the generally low proportion of non-native to native reptiles
in most parts of Australia).
The incidence of predation on reptiles by feral cats shows
marked geographic variation, being signicantly higher in hotter
and drier regions, consistent with results reported by Doherty
et al.(2015). Given that cat density also tends to be higher in
arid Australia, at least in wetter seasons (Legge et al.2017), the
total number of reptiles killed by cats per unit area increases
by at least an order of magnitude from Australias higher
rainfall coastal fringe to inland deserts. Although there is little
comparable information on geographic variation in the density
of reptiles, variation in numbers of reptiles killed by cats is
broadly consistent with patterns in the species richness of the
Australian reptile fauna, with species richness highest for
many groups in more arid areas (Pianka 1969,1981;1989;
Morton and James 1988; Powney et al.2010; Cogger 2014),
with some indication that density of reptiles is also highest in
arid areas of Australia (Read et al.2012).
The only previous estimate of the numbers of reptiles killed
by cats per area or per year for any site in Australia is that of
Read and Bowen (2001) for a site (Roxby Downs) in arid South
Table 4. Australian studies reporting the frequency (%) of reptiles in the diet of cats and co-occurring mammalian predators
Studies are included only where at least 10 samples were reported for cats and at least one other predator. The sample size is given in parentheses
Co-occurring predators Location Source
Feral cat Fox Dog/dingo Spotted-tailed quoll
55.0 (21) 10.7 (29) Farina, SA Bayly (1978)
0.0 (123) 1.5 (282) 2.2 (1252) Wet Tropics, Qld Burnett (2001)
42.1 (19) 41.0 (22) Gibson Desert, WA Burrows et al.(2003)
30.1 (112) 23.3 (288) Yathong, NSW Catling (1988)
55.3 (38) 11.4 (44) 11.9 (285) Erldunda, NT Corbett (1995)
46.3 (123) 5.4 (37) Charles Darwin reserve, WA Doherty (2015)
11.4 (35) 0.0 (26) 5.5 (310) Irving Creek and Hale River, NT Foulkes (2002)
2.0 (49) 12.6 (95) 2.9 (68) 5.6 (168) Barrington Tops, NSW Glen et al.(2011)
14.0 (50) 1.7 (105) Flinders Ranges, SA Hart (1994)
13.1 (288) 9.5 (774) Flinders Ranges, SA
(post rabbit control)
Holden and Mutze (2002)
17.0 (88) 46.6 (73) Karijini NP, WA Johnston et al.(2013)
3.0 (117) 6.0 (166) Eastern Highlands, Vic. Coman (1972); Jones and Coman (1981)
5.0 (277) 0.0 (147) Phillip Island, Vic. Kirkwood et al.(2005)
18.0 (49) 2.5 (38) SW Qld Lapidge and Henshall (2001)
57.3 (377) 36.6 (382) Simpson Desert, Qld Mahon (1999)
50.3 (187) 25.0 (52) Mitchell Grass Downs, Qld Mifsud and Woolley (2012)
3.4 (600) 9.2 (261) Lake Burrendong, NSW Molsher et al.(1999);
Molsher et al.(2000)
10.1 (217) 33.6 (756) Astrebla Downs, Qld R. Palmer (unpubl.)
42.9 (184) 17.9 (28) Davenport Downs, Qld R. Palmer (unpubl.)
36.4 (11) 27.8 (18) 2.9 (105) Great Western Woodlands, WA R. Palmer (unpubl.)
4.5 (22) 3.8 (53) Inglewood, Qld R. Palmer (unpubl.)
18.5 (27) 23.4 (64) Muncoonie Lakes, Birdsville, Qld R. Palmer (unpubl.)
43.5 (23) 36.5 (74) Offham, SW Qld R. Palmer (unpubl.); Palmer (1995)
38.5 (13) 0.0 (50) Pannawonica, WA R. Palmer (unpubl.)
62.9 (70) 65.7 (70) Kintore Paltridge (2002)
72.4 (76) 49.1 (53) 76.6 (77) Tennant Creek Paltridge (2002)
9.1 (44) 3.2 (63) 22.8 (316) Simpson Desert, NT Pavey et al.(2008)
29.4 (360) 19.0 (105) Roxby Downs, SA Read and Bowen (2001)
13.8 (109) 2.1 (47) Heirisson Prong, WA Risbey et al.(1999)
31.1 (254) 18.2 (572) 26.3 (236) Simpson Desert, Qld Spencer et al.(2017)
4.8 (84) 0.1 (1100) Kapalga, NT Stokeld et al.(2016)
23.0 (48) 3.0 (937) 3.0 (412) Croajingalong, Vic. Triggs et al.(1984)
63.1 (404) 68.6 (51) 0.0 (11) Witchelina, SA Woinarski et al. (in press)
26.4 (337) 2.5 (353) Matuwa (Lorna Glen) Wysong (2016)
LWildlife Research J. C. Z. Woinarski
Australia. They estimated a feral cat density there of 2 km
2
,
and average kill rate by individual cats of 350 reptile
individuals year
1
, thus concluding that feral cats in that area
consumed 700 individual reptiles km
2
year
1
. These estimates
are somewhat higher than our modelled estimates for arid
Australia, with cat density in that study notably higher than
typical values reported in Legge et al.(2017).
Our focus was primarily on the numbers of reptiles killed
by feral cats in natural environments, and most of the evidence
that we collated refers to this component of the Australian cat
population. With much less evidence, we also estimated the
numbers of reptiles killed by pet cats at 53 million year
1
and by
feral cats in highly modied environments at 130 million year
1
,
hence summing to 649 million reptiles year
1
(i.e. ~1.8 million
reptiles day
1
) killed by all components of the Australian cat
population.
It is difcult to contextualise and interpret our estimates
of the annual take of Australian reptiles by cats in terms of
conservation impact, consequences to the population viability
of any reptile species or relative to other causes of reptile
mortality, because very few studies have assessed such
parameters (Braysher 1993). There are remarkably few studies
that provide robust information on population size or density
for any Australian reptile species, or the overall density of
reptiles. At an arid site in South Australia, Read et al. (2013)
used markrecapture analyses to estimate densities for six
common reptile species (of skinks and geckoes), with these
varying from 85 to nearly 400 individuals ha
1
. Ehmann and
Cogger (1985) collated the few available density estimates
for individual Australian reptile species across a broader
environmental range, and Cogger et al.(2003) used these to
estimate that the average density (across all reptile species) in
Australia was 200 individual reptiles ha
1
, giving a national tally
of 154 000 million reptiles. Theseare clearly bold extrapolations,
but they are the only available estimates of density and the total
numbers for Australian reptile assemblages. Albeit recognising
the meagre evidence base, these tallies indicate that cats kill
~0.4% of Australian reptiles per year.
Cogger et al.(2003) also provided an estimate for one other
major source of reptile mortality in Australia, the number of
reptile individuals killed by land clearing. Based on their
estimates of average reptile density, and a then-annual rate of
vegetation clearance of ~4500 km
2
in Queensland (the
Australian state with highest rate of loss of native vegetation),
they concluded that this clearing resulted in the loss of 89 million
individual reptiles per year. The national rate of deforestation
has declined, unevenly, since then, and in 201314 was
~2000 km
2
year
1
(Evans 2016). However, deforestation in
Queensland increased again in 201516, with 40 million
reptiles estimated to be killed there in each of those years
(Cogger et al.2017). These annual rates of loss of reptiles
due to habitat clearance are appreciably less than our
estimates of the numbers of reptiles lost annually to cat
predation (~649 million). However, we recognise that such a
comparison has interpretational constraints: clearing results in
permanent loss of habitat suitability and reduction in reptile
density, whereas a larger annual tally of reptiles killed due to cat
predation may have far less acute or longer-term impact on
individual reptile species or communities than habitat loss.
Our collation includes records of more than 250 Australian
reptile species killed by feral cats, including 10 species nationally
listed as threatened. The actual number of reptile species killed
by cats is likely to be appreciably higher than this tally, given that
many cat dietary studies have not reported reptile prey to species
level, many Australian reptile species are highly localised and
there have been no cat dietary studies in many parts of Australia.
Cats consume a broad taxonomic spectrum of Australian reptiles,
but may prey selectively on some species or species groups. In a
comparison of actual abundance and frequency of prey in cat
dietary samples at a site in inland Queensland, Kutt (2012)
concluded that cats preyed selectively on reptile species in the
size range 1050 g, and less so in the size ranges 50100 g and
1003500 g, and selected against reptile species <10 g. However,
such a size preference is challenging to relate to impacts on
individual species because many reptile species exhibit marked
size changes over their lifetime. At a site in arid South Australia,
Read and Bowen (2001) found that cats ate fewer individuals
of the reptile species (relative to their actual abundance)
associated with stony plains than species associated with sand
dunes, and concluded that this was because, at the local scale,
cats were more abundant in the latter habitat. They also noted
some differences in cat predation rates between similarly
sized co-occurring reptiles, and considered that this was
possibly due to different defence responses, with reptile
species that responded vigorously to potential attack (such as
the gecko Underwoodisaurus milii and some large elapid snakes)
experiencing lower rates of predation.
Given cat hunting behaviour (and geographic variation in
their density), reptile groups likely to be most affected by cat
predation are those that: (1) are relatively long-lived and have
low rates of reproduction (because these may be most affected
by any factor causing increase in mortality rates); (2) have high
predictability in activity, such as those with permanent burrows
or latrine sites (because the ambush strategy typically employed
by cats will be most effective with such prey types: Moore et al.
(2018)); (3) occur in habitats with relatively open ground
vegetation and/or in sites subject to frequent and extensive
re (because cats may occur more commonly in such areas
and hunt most effectively in them: Leahy et al.(2015);
McGregor et al.(2015)); (4) are colonial or semi-colonial
(because cats may develop effective search images for such
species and target them selectively); (5) are predominantly
terrestrial, rather than arboreal or fossorial (although cats can
hunt in trees and can dig up prey: Saunders (1991)); (6) do not
occur in rugged rocky areas (because cat density and hunting
efciency may be lowest in such habitats: Hohnen et al.(2016));
and (7) occur mainly in arid or semiarid areas. Several genera of
large Australian skinks (Bellatorias,Cyclodomorphus,Egernia,
Eulamprus,Liopholis,Nangura,Tiliqua) and agamids (e.g.
Pogona) exhibit many of these characteristics (Chapple 2003;
Moore et al.2018), and therefore can be expected to show the
most pronounced impacts from cat predation. Impacts may also
be severe for some reptile species that occur on islands where
breeding seabirds occur in part of the year, allowing for high cat
densities, but where few other prey items are available at other
times of the year.
When averaged across Australia, the rate of predation by cats
on reptiles may indicate (albeit with very low condence) that a
Reptiles killed by cats in Australia Wildlife Research M
small proportion of the national reptile population is killed by
cats each year; however, cases of very high (and selective)
predation pressure are evident in some reported instances of
large numbers of individual reptiles (often mostly of one species)
in stomachs of single cats (Woinarski et al. in press). Such high
predation rates may well lead to local depletion of populations
of some cat-targeted reptile species. There are few relevant
studies that have assessed rates of mortality, and their causes,
for individual reptile species in Australia. Sweet (2007)
radio-tracked 50 individuals of Varanus tristis and V. scalaris
in Kakadu National Park, northern Australia, over a 10-month
period, and recorded predation by cats for six of these
individuals, by far the largest source of mortality for those
marked individuals. A radio-tracking study of two large
elapid snakes in south-eastern Australia also found that cats
were a major source of mortality (Whitaker and Shine 2000),
and a recent study of the threatened great desert skink also
concluded that mortality due to feral cats was higher than that
from any other predator (Moore et al.2018). In a less quantitative
study, Read and Bedford (1991) considered threats to the highly
localised snake Austrelaps labialis, recorded its occurrence
in samples from pet cats and consequently suggested cat
predation rates on threatened pygmy copperheads may be
signicant due to the large domestic and feral cat population
in the Mt Lofty Ranges(p. 3). While such studies suggest that a
relatively high proportion of individuals may be killed by cats,
few studies have demonstrated population-level impacts, partly
because such evidence may require long-term research and
intensive monitoring.
Introduced predators may have direct detrimental impacts
on some reptile species, and they may also have more diffuse
community-level impacts. A small number of Australian
studies have considered reptile assemblages in areas where
contrasting management has led to marked differences in the
abundance of foxes (Olsson et al.2005; Sutherland et al.2011),
cats (Stokeld et al.2016) or both foxes and cats (Moseby et al.
2009; Read and Scoleri 2015). These studies indicate that many
components of these reptile assemblages change markedly due
to the impacts of introduced predators, most likely through
their suppression of previously apex reptilian predators
(Jessop et al.2016). Given that the rate of predation by cats is
higher than that by foxes (the present study; Catling (1988); Read
and Bowen (2001)) cats occur over much more of Australia
than do foxes, and may often also be at higher abundance than
foxes (Read and Bowen 2001)it is plausible that feral cats have
had major impacts on reptile communities across much of
Australia. However, these impacts may be complex and
highly interactive, as predation by cats may also have resulted
in marked decreases in some native bird, mammal and reptile
species that also prey on reptiles, therefore the net impact of cats
on individual reptile species and reptile assemblages may be very
difcult to assess.
The results reported here for the extent of predation by cats on
Australian reptiles can be compared with a recent study of the
extent of predation by cats on Australian birds (Woinarski et al.
2017a), and also of the extent of predation on reptiles by cats in
other continents. Although mammals are typically the main
component of cat diet in most areas of Australia, as elsewhere
in the world (Doherty et al.2015,2017), feral cats in largely
natural environments in Australia include a high and broadly
similar proportion of birds and reptiles in their diet (overall
frequency of 31.6% and 25.6%, respectively; Woinarski et al.
(2017a)). Unlike for reptiles, cat predation on Australian birds is
highest on islands, especially smaller islands. This is largely
because cats on many Australian islands prey heavily on
dense populations of breeding seabirds, and may also be
because many islands do not support mammalian prey. As
with reptiles, more birds are killed by cats in hot and dry
regions (at least in good rainfall years), although this pattern
is notably more pronounced for reptiles than for birds.
Nationally, the total number of reptiles killed by all cats in
Australia is substantially higher than the number of birds
killed by cats (649 million versus 377 million).
There are no comparable robust assessments of the extent of
cat predation on reptiles for other continents. In a recent review
of the impacts of cats on wildlife in the contiguous USA, based
on analysis of predation rates reported in a series of collated
studies, Loss et al.(2013) located only one such study of un-
owned cats that reported on reptile predation, with that per
capita rate (59 reptile individuals cat
1
year
1
:Parmalee
(1953)) appreciably lower than that reported here (225
reptile individual cat
1
year
1
). However, estimates of the
numbers of feral (or free-roaming) cats are far higher for
the US (3080 million) than for Australia (2.16.3 million:
Legge et al.(2017)), so the overall take of reptiles by cats
for the US (median 478 million, with a range between 258 and
822 million; Loss et al.(2013)) is comparable to that reported
here for Australia. In their collation, Loss et al.(2013)also
noted only one comparable study from Europe, which reported
a rate of predation by unowned cats of 4.15 reptile individuals
cat
1
year
1
:Biróet al.(2005)). Although this is a very sparse
base for comparison, it suggests that the per capita rate of
predation on reptiles by feral cats in Australia is likely to be
substantially higher than in North America and Europe.
This possibly reects the strong association between high
frequency of reptiles in cat diets (and high densities of
reptiles) and hot and dry climate zones (Fig. 2;Doherty
et al.(2015)), given that most of North America and Europe
is cooler and wetter than Australia.
Our results suggest that cat predation may be a major
source of mortality for Australian reptiles. However,
much of the interpretation of this result is constrained by
shortcomings in evidence: our study did not seek to assess
whether predation by cats is leading to chronic ongoing
depletion in the standing cropof Australian reptiles. There
are some priority areas of research that could most effectively
address those shortcomings. One priority is more autecological
research (including assessments of demographic factors such
as the rate of predation and other mortality factors),
particularly for some reptile genera with traits that render
them likely to be susceptible to cat predation. Such an
approach would allow for an assessment of the extent to
which mortality rates due to cat predation affect population
viability and size. Another priority is targeted assessments
of responses of reptile species at sites with contrasting
management of feral cats (such as predator-proof exclosures
or broad-scale baiting). Our study is based on collations of
cat dietary studies spaced widely across Australia, but we
NWildlife Research J. C. Z. Woinarski
recognise that some environments (notably rainforests) and
regions were relatively under-sampled, and additional studies
in such sites would increase the representativeness of our
analysis. Another priority for research is to contextualise the
threat posed by cats relative to mortality due to other threat
factors, and to better understand the interactions between cat
predation and those factors. A nal priority is to substantially
increase the extent and coverage of population monitoring
in Australian reptile species, to provide more information on
reptile population trends and to provide timely warning of
declines that may have conservation signicance (Woinarski
2018).
Cat predation may also subvert the assumed conservation
security provided to native reptiles by the conservation reserve
system, given that feral cats occur in similar density within and
outside Australias reserve system (Legge et al.2017). Therefore
conservation of reptiles likely to be susceptible at population
scale to predation by cats will require targeted and effective
control of cats, rather than simply inclusion of those susceptible
species within the reserve system.
The loss of ~1.8 million native reptiles per day due to
predation by cats provides further evidence of the potential
conservation impact of this introduced predator on Australian
biodiversity, and underscores the value of efforts now being
made to manage feral cat populations (e.g. through local-scale
exclosures, enhanced island biosecurity, broad-scale predator
control programs) and the predation pressure they exert (e.g.
management of re and grazing pressure), especially targeting
conservation management for species whose population
viability is most vulnerable to cat predation (Commonwealth
of Australia 2015; Department of the Environment 2015).
Conicts of interest
The authors declare no conicts of interest.
Supplementary material
Supplementary Table S1 provides a list of all Australian
terrestrial reptile species, showing sources of records of
predation by cats for each species.
Acknowledgements
The data collation, analysis and preparation of this paper was supported by
the Australian Governments National Environmental Science Program
(Threatened Species Recovery Hub). We thank the Museum and Art
Gallery of the Northern Territory (and curator Gavin Dally), Museum of
Victoria (Laura Cook), Tasmanian Museum and Art Gallery (Belinda Bauer),
Western Australian Museum (Rebecca Bray), Australian National Wildlife
Collection (CSIRO: Leo Joseph), Queensland Museum (Heather Janetzki,
Andrew Amey), South Australian Museum (David Stemmer, Philippa
Horton) and Australian Museum (Cameron Slatyer, Mark Eldridge) for
records of reptiles in their collection reported as cat-killed. We thank the
Australian Research Council for grant funding (project DP 140104621) to
CRD. We thank Hal Cogger for information and advice, and Katherine
Moseby and an anonymous referee for helpful comments on an earlier draft.
We thank Flavie Delanzy, Taaryn Thomson and Mat Vanderklift for access
to some unpublished information on reptiles from cat samples. This paper
rests on data arising from the inglorious labours of many people who have
searched for and through cat faeces and the internal organs of dead cats that
effort is much appreciated, and we hope this collation contributes towards
a demonstration of the value of such dedicated effort.
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TWildlife Research J. C. Z. Woinarski
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... A parallel set of analyses evaluated the total numbers of individual vertebrates killed annually by cats in Australia Woinarski et al., 2018), including by pet cats . Here, we build upon these recent papers by addressing the following research questions: ...
... The approach taken to estimate the number of individual animals killed by cats in Australia has been described previously Woinarski et al., 2018). ...
... The recent analyses of the numbers of animals killed by cats in Australia Woinarski et al., 2018) considered three components of the overall Australian cat population: pet cats, feral cats in highly modified environments (e.g., urban areas) and feral cats in largely natural environments [as defined by Legge et al. (2017)]. A comparable subdivision is not used in our assessment of fox diet because foxes are not kept as pets, and because we included parameters relating to habitat modification in our modelling. ...
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Aim Introduced predators negatively impact biodiversity globally, with insular fauna often most severely affected. Here, we assess spatial variation in the number of terrestrial vertebrates (excluding amphibians) killed by two mammalian mesopredators introduced to Australia, the red fox (Vulpes vulpes) and feral cat (Felis catus). We aim to identify prey groups that suffer especially high rates of predation, and regions where losses to foxes and/or cats are most substantial. Location Australia. Methods We draw information on the spatial variation in tallies of reptiles, birds and mammals killed by cats in Australia from published studies. We derive tallies for fox predation by (i) modelling continental‐scale spatial variation in fox density, (ii) modelling spatial variation in the frequency of occurrence of prey groups in fox diet, (iii) analysing the number of prey individuals within dietary samples and (iv) discounting animals taken as carrion. We derive point estimates of the numbers of individuals killed annually by foxes and by cats and map spatial variation in these tallies. Results Foxes kill more reptiles, birds and mammals (peaking at 1071 km⁻² year⁻¹) than cats (55 km⁻² year⁻¹) across most of the unmodified temperate and forested areas of mainland Australia, reflecting the generally higher density of foxes than cats in these environments. However, across most of the continent – mainly the arid central and tropical northern regions (and on most Australian islands) – cats kill more animals than foxes. We estimate that foxes and cats together kill 697 million reptiles annually in Australia, 510 million birds and 1435 million mammals. Main conclusions This continental‐scale analysis demonstrates that predation by two introduced species takes a substantial and ongoing toll on Australian reptiles, birds and mammals. Continuing population declines and potential extinctions of some of these species threatens to further compound Australia's poor contemporary conservation record.
... There are estimated to be somewhere around 1.4 to 3.4 million feral cats across Australia, around one feral cat per four square kilometers [160]. There is no doubt that feral cats eat large numbers of native fauna [161][162][163] but the studies are not conclusive on the actual effects on prey populations generally [162][163][164], and observed effects do not necessarily equate to causation [160]. It has been argued that feral cats have led to the demise of mammalian fauna [23,29,43,44,156,165], and an exclusion study in the Kimberley region suggested that feral cats can extirpate local populations of native mammals [35]. ...
... There are estimated to be somewhere around 1.4 to 3.4 million feral cats across Australia, around one feral cat per four square kilometers [160]. There is no doubt that feral cats eat large numbers of native fauna [161][162][163] but the studies are not conclusive on the actual effects on prey populations generally [162][163][164], and observed effects do not necessarily equate to causation [160]. It has been argued that feral cats have led to the demise of mammalian fauna [23,29,43,44,156,165], and an exclusion study in the Kimberley region suggested that feral cats can extirpate local populations of native mammals [35]. ...
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Northern Australian biomes hold high biodiversity values within largely intact vegetation complexes, yet many species of mammals, and some other taxa, are endangered. Recently, six mammal species were added to the 20 or so already listed in the Australian endangered category. Current predictions suggest that nine species of mammal in northern Australia are in imminent danger of extinction within 20 years. We examine the robustness of the assumptions of status and trends in light of the low levels of monitoring of species and ecosystems across northern Australia, including monitoring the effects of management actions. The causes of the declines include a warming climate, pest species, changed fire regimes, grazing by introduced herbivores, and diseases, and work to help species and ecosystems recover is being conducted across the region. Indigenous custodians who work on the land have the potential and capacity to provide a significant human resource to tackle the challenge of species recovery. By working with non-Indigenous researchers and conservation managers, and with adequate support and incentives, many improvements in species' downward trajectories could be made. We propose a strategy to establish a network of monitoring sites based on a pragmatic approach by prioritizing particular bioregions. The policies that determine research and monitoring investment need to be re-set and new and modified approaches need to be implemented urgently. The funding needs to be returned to levels that are adequate for the task. At present resourcing levels, species are likely to become extinct through an avoidable attrition process .
... Feral Cats have been suggested as a potential threat to E. s. badia populations in the past (Desmond & Chant 2001), particularly to juveniles (Pearson 2012), and a range of anecdotal evidence indicates they may be one of the skinks' main predators (Lee-Steere 2008). Reptiles in general are a significant proportion of the Feral Cat diet (Dickman 1996;Paltridge et al. 1997;Doherty et al. 2015;Woinarski et al. 2018), particularly medium-sized reptiles (Stobo-Wilson et al. 2021). Although Feral Cats were not found to actively target CWD, their capacity to hunt adult (as well as the smaller juvenile) skinks was confirmed through camera trap imaging. ...
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Patchy resource distribution can cluster predator activity around areas of the highest productivity in ecosystems. For the endangered Western Spiny‐tailed Skink (Egernia stokesii badia) in Western Australia, the log piles that they permanently inhabit in an otherwise patchy, arid landscape, represent a potentially reliable, high abundance food source for predators. Not only are encounter rates by potential predators of E. s. badia likely to be influenced by vegetation structure at the microhabitat scale but also E. s. badia occurs in a region where mine sites and associated infrastructure, such as landfill sites, likely concentrate generalist predators (e.g. Feral Cats and corvids). We assessed the influence of the presence of coarse woody debris (CWD) and distance to the landfill on predator behaviour towards E. s. badia through plasticine model experiments, unbounded point count bird surveys and camera trapping. We found that CWD inhabited by E. s. badia attracted a greater relative activity of corvids compared with uninhabited CWD, or control sites without CWD. The relative activity of corvids and predatory birds combined increased with decreasing distance from the landfill. Preferential hunting by corvids at CWD inhabited by E. s. badia compared to both uninhabited CWD and open sites suggests that inhabited CWD may be targeted by generalist predators in the region, and that adaptive management may be required for species conservation around active mining areas.
... The population of snakes has witnessed a decline globally due to habitat destruction, fragmentation, and environmental degradation (Doherty et al., 2020), road mortalities (Wagner et al., 2021), climate change (Lourenço-de-Moraes et al., 2019), chemical pollutants and toxins (Hoang et al., 2021), diseases and pathogens (Walker et al., 2019), invasive alien species (Woinarksi et al., 2018), and trade and exploitation (Böhm et al., 2013;Schmidt, 2018). Snakes play important roles in natural systems and make easier the movement of energy and contaminants across ecosystems as both predators and prey (Haskins et al., 2019;Onyishi et al., 2021). ...
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Microplastics are plastic fragments with a size less than 5 mm in length. In addition to a threat to the marine environments where these are abundantly present, these have also started polluting freshwater ecosystems. However, the uptake of microplastics by living organisms differs depending on their habitats and feeding behaviors. We investigated the presence, size, type, and color of microplastics in the gastrointestinal tract contents of two water snakes, namely Natrix natrix and Natrix tessellata. The snakes were collected from different regions of Turkey and preserved as museum materials. Our results showed that fibers constituted the predominant polymer type in both snake species (94.7% for N. natrix and 87.9% for N. tessellata), whose dimensions ranged from 250 to 3750 µm. We did not find any significant difference in the number of microplastics ingested between the two species. In addition, the uptake of microplastics did not relate to the size and weight of snakes. Microplastics were not consistently present all years, and similarly, these were not detected in all samples within the distribution area. These results could be attributed to the environment and diet of two snake species.
... Since their domestication approximately 9,000 years ago (Vigne et al. 2009), cats (Felis catus) have accompanied humans in their movements, reaching a large number of territories, disturbing the environments in which they settle and preying on native species. This has occurred and has been observed in both continental (Woods et al. 2003;Loss et al. 2013;Woinarski et al. 2017Woinarski et al. , 2018Mori et al. 2019) and island territories (Nogales et al. 2006;Bonnaud et al. 2011;Medina et al. 2011Medina et al. , 2014. Island environments are often more vulnerable to biological invasions, as the species that inhabit them are often more vulnerable to alien predators, competitors, pathogens, and parasites. ...
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Cats have accompanied humans since they were domesticated, spreading and settling in a large number of islands where they modify environments and prey on native species. In the Canary Islands, several studies have shown that cat’s prey on native species, including Critically Endangered species such as the giant lizards of Tenerife (Gallotia intermedia), La Gomera (G. bravoana) and El Hierro (G. simonyi). This study focuses on the trophic ecology of the cat in Montaña de Guaza, where a population of the giant lizard of Tenerife is located. The current results show important changes in the diet of this alien species at this area in relation to the obtained by others author in 2004. The main changes detected are the low level of consumption of mammals and the high levels of birds and lizards. These changes seem to be a direct consequence of a low density of rabbit (Oryctolagus cuniculus), the main prey at the different habitats of Canary Islands, probably due to the rabbit hemorrhagic disease virus (RHDV) incidence. The high consumption of reptiles detected in this study could explain the reduction of both distribution area and number of giant lizards at this locality.
... Cats are opportunistic hunters and their predatory behavior is innate, and can persist even if the individuals are sufficiently fed (Adamec 1976;Spotte 2014). Their prey commonly includes wildlife, such as birds (Dauphiné and Cooper 2009;Woods et al. 2003;Woinarski et al. 2017), mammals (Loss et al. 2013;Krauze-Gryz et al. 2017;Murphy et al. 2019), reptiles (Arnaud et al. 1993;Barratt 1998;Woinarski et al. 2018), amphibians (Woods et al. 2003;Baker et al. 2005;Woinarski et al. 2020) and invertebrates (Gillies and Clout 2003;Medina and García 2007;Woolley et al. 2020), many of which are native. However, most cat predation studies have focused on the birds and mammals (Blancher 2013;Mella-Méndez et al. 2019;Murphy et al. 2019), with little attention paid to other groups like amphibians, reptiles and invertebrates (Seymour et al. 2020;Woinarski et al. 2020). ...
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Domestic cats are a potential risk for native fauna in the Neotropics. Intrinsic (age, weight, sex, color) and extrinsic (nocturnal confinement, time spent outside the home, distance to green areas, etc.) factors can influence the type and quantity of prey that cats take to their homes. The study goal was to evaluate domestic cat predation in a Neotropical city. We intend to answer the following questions: (1) Which is the richness, dominance and abundance of prey captured by domestic cats? (2) Which are the extrinsic or intrinsic factors that most influence prey capture by domestic cats? We predict that: (a) cats will capture a large diversity of native wildlife and (b) extrinsic factors will have a greater effect compared intrinsic factors due to the innate predatory cat’s behavior. We chose 120 cats from 44 households in the city of Xalapa, Veracruz, Mexico. We documented the richness, dominance and abundance of wildlife species captured and brought home by cats during March to August 2019 and those intrinsic and extrinsic factors that modulate their predatory behavior. The cats captured 246 prey items, 35.8% were reptiles, 23.2% invertebrates, 17.9% amphibians, 15.4% birds and 7.7% mammals. The prey items belonged to 64 taxa (17 birds, 17 invertebrates, 15 reptiles, nine mammals and six amphibians). The lizard Sceloporus variabilis was the most captured prey. Of the prey items, 93.5% were native and 6.5% non-native. Five intrinsic and 5 extrinsic factors contributed most to the predation events, of which cat stripe color, time spent by the cats outside of the home and nocturnal confinement were the most important. These results allow us to understand the harmful effect of cats on wildlife in a Neotropical city.
... Feral cats (1.5-5 kg) are generalist and adaptable predators. Across Australia, at least 778 vertebrate species have been recorded in their diet, comprising 151 mammal (Woolley et al. 2019), 339 bird , 258 reptile (Woinarski et al. 2018) and 30 frog species. The number of invertebrate species consumed is likely to be far higher, with estimates of 1086 million invertebrate individuals consumed by feral cats in Australia every year . ...
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A vast global literature documents that free‐roaming domestic cats (Felis catus) have substantial negative effects on wildlife, including through predation, fear, disease, and competition‐related impacts that have contributed to numerous wildlife extinctions and population declines worldwide. However, no study has synthesized this literature on cat impacts on wildlife to evaluate its overarching biases and major gaps. To direct future research and conservation related to cat impacts on wildlife, we conducted a global literature review that entailed evaluation and synthesis of patterns and gaps in the literature related to the geographic context, methods, and types of impacts studied. Our systematic literature search compiled 2,245 publications. We extracted information from 332 of these meeting inclusion criteria designed to ensure the relevance of studies analyzed. This synthesis of research on cat impacts on wildlife highlights a focus on oceanic islands, Australia, Europe, and North America, and on rural areas, predation, impacts of unowned cats, and impacts at population and species levels. Key research advances needed to better understand and manage cat impacts include more studies in underrepresented, highly biodiverse regions (Africa, Asia, South America), on cat impacts other than predation, and on methods designed to reduce impacts on wildlife. The identified areas of needed research into cat impacts on wildlife will be critical to further clarifying the role of cats in global wildlife declines and to implementing science‐driven policy and management that benefit conservation efforts.
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An important but understudied modality for eavesdropping between predators and prey is olfaction, especially between non-mammalian vertebrate predators and their prey. Here we test three olfactory eavesdropping predictions involving an apex reptilian predator, the sand goanna Varanus gouldii, and several species of its small mammalian prey in arid central Australia: 1) small mammals will recognize and avoid the odour of V. gouldii; 2) V. gouldii will be attracted to the odour of small mammals, especially of species that maximize its energetic returns; and 3) small mammals will be less mobile and will show higher burrow fidelity where V. gouldii is absent compared with where it is present. As expected, we found that small mammals recognized and avoided faecal odour of this goanna, feeding less intensively at food patches where the odour of V. gouldii was present than at patches with no odour or a pungency control odour. Varanus gouldii also was attracted to the odour of small mammals in artificial burrows and dug more frequently at burrows containing the odour of species that were energetically profitable than at those of species likely to yield diminishing returns. Our third prediction received mixed support. Rates of movement of three species of small mammals were no different where V. gouldii was present or absent, but burrow fidelity in two of these species increased as expected where V. gouldii had been removed. We conclude that olfaction plays a key role in the dynamic interaction between V. gouldii and its mammalian prey, with the interactants using olfaction to balance their respective costs of foraging and reducing predation risk. We speculate that the risk of predation from this apex reptilian predator drives the highly unusual burrow-shifting behaviour that characterizes many of Australia's small desert mammals.