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Rapid Recolonisation of Feral Cats Following Intensive Culling in a Semi-Isolated Context


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Invasive feral cats threaten biodiversity at a global scale. Mitigating feral cat impacts and reducing their populations has therefore become a global conservation priority, especially on islands housing high endemic biodiversity. The New Caledonian archipelago is a biodiversity hotspot showing outstanding terrestrial species richness and endemism. Feral cats prey upon at least 44 of its native vertebrate species, 20 of which are IUCN Red-listed threatened species. To test the feasibility and efficiency of culling, intensive culling was conducted in a peninsula of New Caledonia (25.6 km²) identified as a priority site for feral cat management. Live-trapping over 38 days on a 10.6 km² area extirpated 36 adult cats, an estimated 44% of the population. However, three months after culling, all indicators derived from camera-trapping (e.g., abundance, minimum number of individuals and densities) suggest a return to pre-culling levels. Compensatory immigration appears to explain this unexpectedly rapid population recovery in a semi-isolated context. Since culling success does not guarantee a long-term effect, complementary methods like fencing and innovative automated traps need to be used, in accordance with predation thresholds identified through modelling, to preserve island biodiversity. Testing general assumptions on cat management, this article contributes important insights into a challenging conservation issue for islands and biodiversity hotspots worldwide.
Content may be subject to copyright.
Rapid recolonisation of feral cats following intensive
culling in a semi-isolated context
Pauline Palmas1,2,3, Raphaël Gouyet1, Malik Oedin1,4, Alexandre Millon5,
Jean-Jérôme Cassan6, Jenny Kowi6, Elsa Bonnaud2, Eric Vidal1,7
1Institut Méditerranéen de Biodiversité et d’Ecologie marine et continentale (IMBE), Aix Marseille Université,
CNRS, IRD, Avignon Université, Centre IRD de Nouméa, BPA5, 98848, Nouméa cedex, New Caledonia,
France 2Ecologie Systématique Evolution, Univ. Paris-Sud, CNRS, AgroParisTech, Université Paris-Saclay,
91400, Orsay, France 3Univ. Polynesie Francaise, Ifremer, Ilm, Ird, Eio Umr 241, Tahiti, French Polynesia
4Institut Agronomique Néo-Calédonien (IAC), Equipe ARBOREAL (AgricultuRe BiOdiveRsité Et vALori-
sation) BP73, Païta, New Caledonia, France 5Institut Méditerranéen de Biodiversité et d’Écologie marine
et continentale (IMBE), Aix Marseille Université, CNRS, IRD, Avignon Université, Europôle de l’Arbois,
BP 80, 13545, Aix-en-Provence, France 6Direction du Développement Economique et de l’Environnement
(DDEE), Koohnê (Koné), Province Nord, New Caledonia, France 7UMR ENTROPIE (IRD-Université de la
Réunion-CNRS), Laboratoire d’Excellence Labex-CORAIL, Institut de Recherche pour le Développement, BP
A5, 98848, Nouméa Cedex, New Caledonia, France
Corresponding author: Pauline Palmas (,
Academic editor: J. Jeschke |Received 26 August 2020|Accepted 26 November 2020|Published 29 December 2020
Citation: Palmas P, Gouyet R, Oedin M, Millon A, Cassan J-J, Kowi J, Bonnaud E, Vidal E (2020) Rapid
recolonisation of feral cats following intensive culling in a semi-isolated context. NeoBiota 63: 177–200. https://doi.
Invasive feral cats threaten biodiversity at a global scale. Mitigating feral cat impacts and reducing their
populations has therefore become a global conservation priority, especially on islands housing high en-
demic biodiversity. e New Caledonian archipelago is a biodiversity hotspot showing outstanding ter-
restrial species richness and endemism. Feral cats prey upon at least 44 of its native vertebrate species, 20
of which are IUCN Red-listed threatened species. To test the feasibility and eciency of culling, intensive
culling was conducted in a peninsula of New Caledonia (25.6 km²) identied as a priority site for feral cat
management. Live-trapping over 38 days on a 10.6 km² area extirpated 36 adult cats, an estimated 44%
of the population. However, three months after culling, all indicators derived from camera-trapping (e.g.,
NeoBiota 63: 177–200 (2020)
doi: 10.3897/neobiota.63.58005
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Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
abundance, minimum number of individuals and densities) suggest a return to pre-culling levels. Com-
pensatory immigration appears to explain this unexpectedly rapid population recovery in a semi-isolated
context. Since culling success does not guarantee a long-term eect, complementary methods like fenc-
ing and innovative automated traps need to be used, in accordance with predation thresholds identied
through modelling, to preserve island biodiversity. Testing general assumptions on cat management, this
article contributes important insights into a challenging conservation issue for islands and biodiversity
hotspots worldwide.
Camera trap monitoring, invasive predator, invasive species control, live-trapping, SECR analysis
Feral cats are among the most harmful invasive predators for insular native fauna (Bon-
naud et al. 2011; Medina et al. 2011; Bellard et al. 2016; Doherty et al. 2016). ey
threaten more than 430 vertebrate species, including mammals, birds and reptiles, and
are implicated in the recent extinction of 63 species (40 bird, 21 mammal and 2 rep-
tile species), i.e. 26% of recent terrestrial vertebrate extinctions since AD 1500 (Do-
herty et al. 2016; Palmas et al. 2017). Mitigating feral cat impacts and reducing their
populations has therefore become a global conservation priority (Doherty et al. 2017),
especially on islands housing high endemic biodiversity (Nogales et al. 2013). Feral cat
eradications have been successfully conducted on islands worldwide, generally resulting
in clear conservation benets for many island mammals, birds and reptiles (e.g. Camp-
bell et al. 2011; Jones et al. 2016). However, although recent management actions
succeeded in eradicating cats from small and medium-sized islands (up to 29,000ha
– Marion, Bester et al. 2002 and up to 63,000 ha – Dirk Hartog – Algar et al. 2020)
including fenced enclosures, to date feral cat eradications remain largely unfeasible on
the largest islands, particularly when inhabited (Nogales et al. 2004; Campbell et al.
2011; Oppel et al. 2011; DIISE 2020), and even harder to achieve in mainland areas.
If eradication is not feasible, population control – i.e. local limitation of predator
abundance by culling or other measures – could constitute an alternative management
strategy (Doherty et al. 2017). As for any “open” populations though, cats present a
high risk of re-invasion since they can move rapidly and over long distances (Schmidt
et al. 2007; Moseby and Hill 2011; Leo et al. 2016; McGregor et al. 2017): a typical
response to spatially restricted culling is compensatory immigration from surrounding
source populations (e.g. Lieury et al. 2015; Millon et al. 2019). Population control
may thus entail a continuous removal of individuals (Lazenby et al. 2015). is is
generally not a sustainable management strategy given the usually limited resources
and time available for such conservation programmes (e.g. Doherty and Ritchie 2017;
Venning et al. 2020). Most studies that found feral cat culling to be eective and with
a lasting impact on the cat population were examining either intensive and sustained
Eect of control on feral cat population 179
management eorts (Algar and Burrows 2004) or situations where populations are
relatively closed (e.g. peninsulas and fenced areas, Short et al. 1997; Moseby and Read
2006). Our study area, a peninsula, was chosen for its potential to act as a population
lter and limit immigration from surrounding populations (like Heirisson Prong in
Short et al. 2002, and the Tasman Peninsula in Lazenby et al. 2015).
Camera trapping and a spatially explicit capture-recapture approach (hereafter,
SECR) are novel and eective tools that are increasingly used to estimate occupancy
rates, abundances and densities for feral cats in natural areas. ey provide relevant
information for conservation practitioners (such as recolonisation rate, spatial distribu-
tion of cats) and allow for testing the eciency of culling as a management technique
(Robley et al. 2010; Bengsen et al. 2012; Lazenby et al. 2015; McGregor et al. 2015).
Surprisingly little is known about the speed with which a treated area is recolonised by
cats. is is a crucial parameter for managers to estimate how long the positive eect of
their control operations is lasting, so as to determine how frequently these have to be
repeated in order to maintain invasive predators at a low density (Denny and Dickman
2010; Leo et al. 2018). e rate of re-invasion probably depends on the abundance
of cats outside the treated area, the degree of connectivity of the treated area with the
untreated peripheral areas and the intensity of removal of individuals during culling.
Nor is there adequate data on the magnitude of control (i.e. the number of individuals
or percentage of a population to remove) required to successfully reduce the invasive
predators’ population and impacts (e.g. Reddiex et al. 2006; Kapos et al. 2009; Denny
and Dickman 2010; Walsh et al. 2012). Modelling studies can estimate optimal re-
moval rates (e.g. Lohr et al. 2013), but proper modelling requires information on
numerous parameters like the biology and distribution of both managed and sympa-
tric species, or population sizes (Leo et al. 2018). is would enable to determine the
viability of prey populations in the face of predation under dierent conditions and
management programmes (e.g. King and Powell 2011).
We report herein a short but intensive feral cat culling operation conducted at
Pindaï peninsula (New Caledonia), which is a priority conservation area for seabirds
(it hosts a large colony of Wedge-tailed shearwaters, Ardenna pacica) (Spaggiari et al.
2007). It is a case study of how ecient and durable the eects of such short intensive
operations are, taking advantage of the peninsulas setting and simulating the typical
resources currently available to local managers of natural areas (DDEE – Province
Nord, New Caledonia).
Our specic aims were to (i) assess feral cat abundance and density, (ii) test a live-
trapping protocol and its success in controlling feral cats, (iii) test the durability of the
culling eect on feral cat abundance and densities, and (iv) derive guidance for adap-
tive and eective management.
While a compensatory eect from immigration was expected, we hypothesised
that the lower connectivity between treated and untreated areas at this peninsular tip
would limit cat re-colonisation as observed in dierent studies conducted in peninsulas
or fenced areas (Short et al. 1997; Read and Bowen 2001).
Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
Materials and methods
Study site
e New Caledonia main island (“Grande Terre”) is an old continental island located
in the Pacic Ocean (Grandcolas et al. 2008). With an area of 16,372 km2, it houses
three main natural habitats: Dry forest, Humid forest and Maquis mosaic. e New
Caledonian biodiversity hotspot shows outstanding terrestrial species richness and en-
demism rates (Myers et al. 2000; Mittermeier et al. 2011).
Since their introduction around 1860 (Beauvais et al. 2006), cats have invaded
the New Caledonian archipelago, from seashore habitats to the highest altitude for-
est (1,628 m). A recent study showed that feral cats preyed upon at least 44 native
vertebrate species, 20 of which are IUCN Red-listed threatened species (Palmas et al.
2017). As a result, the feral cat has been listed among the ve priority invasive species
for future management in New Caledonia. e Pindaï peninsula (Northern Province)
has been identied as a priority site for feral cat management, part of a move to address
conservation issues in natural areas through expert management.
e Pindaï Peninsula (21°19.40'S, 164°57.50'E; Fig. 1), with an area of 25.6 km²,
is between 2.45 km and 3.24 km wide and a maximum 7 km long. It has a low
(<15m) canopy and mean annual rainfall of less than 1,100 mm (Jaré et al. 1993).
It is covered in dry forest composed of a mosaic of sclerophyllous and mesic forests on
Figure 1. Location of the Pindaï Peninsula and sampling design; camera trap stations (cross, n = 77),
live-trap positions (circle, n = 32), seabird colony (grey area), roads and trails (grey lines).
Eect of control on feral cat population 181
sedimentary and metamorphic rocks (Gillespie and Jare 2003; Isnard et al. 2016).
Secondary successional sclerophyllous forests dominate this peninsula with Acacia
spirorbis and Leucana leucocephala formations, and there is a large remnant of closed
sclerophyllous forest to the East and South. To implement our culling campaign, we
specically chose the southern part of the peninsula because (i) it houses the largest
Wedge-tailed shearwater colony of Grande Terre, the mainland of New Caledonia,
with about 10,000–15,000 breeding pairs present from mid-October (adult arrival) to
the end of May (juvenile edging) (Table 1; Fig. 1) (Spaggiari and Barre 2003; BirdLife
International 2016); (ii) the peninsula narrows (2.45 km) in the middle, providing
lower connectivity between treated and untreated areas ; and (iii) it aords an area of
10.6 km2 for intensive treatment, using the available human and material resources
(i.e. local managers).
Camera trapping design
40 camera traps (three were stolen during the study period) were deployed along
paths and unsealed roads according to a systematic grid covering the study area
(10.6km2). is grid was constructed on GIS (QGis 2.2.0), and was overlaid on
an aerial photograph of the Peninsula to maximise homogeneity of camera trap dis-
tribution. Automated digital cameras with ash (7), infrared ash (2), black light
(31) (CuddebackAmbush 1170, Cuddeback Attack IR 1156, Moultrie M1100i, re-
spectively) were used. To ensure homogeneous detection probabilities throughout a
camera trapping session, no baits or lures were used. Cameras were set up at a height
of between 30 and 100 cm (to cover cat body height), directed towards the track
preferentially used by cats (Turner and Bateson 2014; Recio et al. 2015), and were
checked to conrm that the camera’s shutter was triggered (Wang and Macdonald
2009; Nichols et al. 2017). ere was an interval of ten seconds between trigger
events, with three images captured in each of them, to maximise cat identication
and to reduce the risk of fuzzy pictures.
Camera trapping was conducted for 30 successive days in both sessions (Table2).
A capture event was dened as all photographs of unique individuals within a 30-
min time period (Di Bitetti et al. 2006; Farris et al. 2015). A sampling occasion was
considered as one day (24 h) (Otis et al. 1978; Wang and MacDonald 2009). Camera
traps were inspected at least once every two weeks to check battery system charge and
Table 1. Control schedule using live-traps and camera trapping according to Wedge-tailed shearwater
breeding periods. Dash indicate inter-periods.
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Wedge-tailed shearwater
presence (P.) and breeding
P. P.
P. P. Juv.
P. P.
Camera trapping 908 trap-days 1181 trap-days
Feral cat control by
1200 trap-days
Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
to download data from memory cards. At the end of each trapping period, the cameras
were retrieved and the images downloaded. e trapping eort was obtained by mul-
tiplying the number of traps by the number of active capture days over the considered
periods (Table 1). Capture per unit eort (camera trapping sampling occasion) was
calculated by dividing the numbers of trapped cats per 100 trap-days.
Feral cat trapping and culling
Cat trapping and culling were carried out for 38 days over 3.5 months (2–3 working
days per week) during the dry cold season (between mid-May and July 2015, austral
winter) in collaboration with wildlife rangers. In predator trapping, food availability
in the targeted site may be decisive for control eciency (i.e., baited traps may be
more attractive when few alternative food resources are available) (Algar et al. 2013;
Rocamora and Henriette 2015). erefore, feral cat trapping and culling were car-
ried out during the dry cold season, when resources are scarcer (i.e., before seabird
arrival, a low activity period for squamates and invertebrates and probably the lowest
rodent abundance).
Table 2. Model selection results for density estimation (SECR) using four habitat masks (ZE; study
area, ZE_AV; using MDMM pre-culling, ZE_AP; using MDMM post-culling and ZE_moy; using mean
MDMM pre- and post-culling). Models are based on Akaike’s information criterion corrected for small
sample sizes (AICc). Delta AICc is the dierence in AIC values between each model and the model with
the lowest AIC. AICcwt is the model weight.
Model name Model Detection function No.
LogLik AICc delta
M1 #secr_dfn15_ZE_Buer_AP λ(0)~1 σ~1 z~1 hazard hazard rate 3 -1853.106 3712.798 0 0.5325
M2 #secr_dfn1_ZE_Buer_AP g0~1 σ ~1 z~1 hazard rate 3 -1853.236 3713.058 0.26 0.4675
M3 #secr_dfn15_ZE_Buer_Moy λ(0)~1 σ ~1 z~1 hazard hazard rate 3 -1864.527 3735.64 22.842 0
M4 #secr_dfn1_ZE_Buer_Moy g0~1 σ ~1 z~1 hazard rate 3 -1864.62 3735.826 23.028 0
M5 #secr_dfn15_ZE_Buer_AV λ(0)~1 σ ~1 z~1 hazard hazard rate 3 -1874.757 3756.1 43.302 0
M6 #secr_dfn1_ZE_Buer_AV g0~1 σ ~1 z~1 hazard rate 3 -1874.792 3756.169 43.371 0
M7 #secr_dfn1_ZE g0~1 σ ~1 z~1 hazard rate 3 -1884.633 3775.851 63.053 0
M8 #secr_dfn15_ZE λ(0)~1 σ ~1 z~1 hazard hazard rate 3 -1884.694 3775.973 63.175 0
M9 #secr_dfn2_ZE_Buer_AP g0~1 σ ~1 exponential 2 -1887.627 3779.54 66.742 0
M10 #secr_dfn16_ZE_Buer_AP λ(0)~1 σ ~1 hazard exponential 2 -1889.41 3783.105 70.307 0
M11 #secr_dfn2_ZE_Buer_Moy g0~1 σ ~1 exponential 2 -1897.213 3798.711 85.913 0
M12 #secr_dfn16_ZE_Buer_Moy λ(0)~1 σ ~1 hazard exponential 2 -1898.902 3802.091 89.293 0
M13 #secr_dfn2_ZE_Buer_AV g0~1 σ ~1 exponential 2 -1906.91 3818.106 105.308 0
M14 #secr_dfn16_ZE_Buer_AV λ(0)~1 σ ~1 hazard exponential 2 -1908.556 3821.397 108.599 0
M15 #secr_dfn2_ZE g0~1 σ ~1 exponential 2 -1920.357 3844.999 132.201 0
M16 #secr_dfn16_ZE λ(0)~1 σ ~1 hazard exponential 2 -1921.938 3848.162 135.364 0
M17 #secr_dfn0_ZE_Buer_AP g0~1 σ ~1 halfnormal 2 -1942.385 3889.055 176.257 0
M18 #secr_dfn14_ZE_Buer_AP λ(0)~1 σ ~1 hazard halfnormal 2 -1942.945 3890.175 177.377 0
M19 #secr_dfn0_ZE_Buer_Moy g0~1 σ ~1 halfnormal 2 -1946.147 3896.58 183.782 0
M20 #secr_dfn14_ZE_Buer_Moy λ(0)~1 σ ~1 hazard halfnormal 2 -1946.684 3897.653 184.855 0
M21 #secr_dfn0_ZE_Buer_AV g0~1 σ ~1 halfnormal 2 -1952.44 3909.165 196.367 0
M22 #secr_dfn14_ZE_Buer_AV λ(0)~1 σ ~1 hazard halfnormal 2 -1952.966 3910.217 197.419 0
M23 #secr_dfn0_ZE g0~1 σ ~1 halfnormal 2 -1963.612 3931.509 218.711 0
M24 #secr_dfn14_ZE λ(0)~1 σ ~1 hazard halfnormal 2 -1964.072 3932.429 219.631 0
Eect of control on feral cat population 183
Live traps (2 WIRETAINERS models, CatTrap and PossumTrap; 32 traps in to-
tal, 17 and 15 respectively of each model) were deployed across the 10.6 km2 covered
(Fig.1). e trapping density rate (3 traps per km2) was comparable to that of similar
studies (e.g. Algar et al. 2010; Lazenby et al. 2015). Traps were deployed near paths
and unsealed roads used by cats (Turner and Bateson 2014; Recio et al. 2015; Palmas
et al. 2017). ey were hidden in vegetation and out of direct public sight. Feral cats
were live-trapped during both day and night, since our study site does not house non-
target native species liable to be caught by this type of trap (Desmoulins and Barré
2005). Traps were checked and baited with oiled sh (tinned sardines) twice a day
(Peters et al. 2011).
Trapped cats were euthanised by an accredited veterinarian using rst a light anaes-
thetic via intramuscular injection of Tiletamine/Zolazepam (10 mg kg-1 body-weight),
followed by an intracardiac injection of Pentobarbital 500 mg/cat. e cats were han-
dled in compliance with the directives of the Department of Conservation’s Animal
Ethics Committee, and the traps were used in accordance with New Caledonian regu-
lations (Northern Province Environmental Code, New Caledonia).
Data analyses
Camera trapping was used to calculate three complementary indicators of population
abundance and density pre- and post-culling: (i) a general index of feral cat activity
(GI), (ii) the minimum number of feral cats present in the study area (MKTBA), and
(iii) feral cat absolute density (SECR).
e general index (GI) allowed us to estimate feral cat activity over the study area
by measuring the mean of virtual camera capture events per station and per sampling
occasion. is index follows the equation of Engeman (2005):
GI x
d sj
=∑∑ ,
with d = the day, s = the station, and xij the number of captures at the ith station on
occasion jth.
To compare the GI calculated before and after culling, we used bilateral mean
comparison: t-test with Welch approximation for unequal variance.
Camera-trapped cats were identied based on distinct natural markings (Karanth
and Nichols 1998; Bengsen et al. 2012). First, adult cats were classied by coat colour
and patterns on left or right anks. en morphological criteria were used: number,
shape, dimension and position of stripes, bands and spots on the trunk and limbs;
number and shape of rings on the tail; body signs such as scars or other distinctive
traits; and sex (observation of the genital area or female with cubs). Pictures from each
session were sorted into folders, one for each potential individual (McGregor et al.
Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
2015). All identication folders were checked twice, by two dierent operators, for any
inconsistencies requiring the pictures to be reassigned. e folders were then reviewed
by another operator for validation.
Culled cats were identied using the same morphological criteria from the pictures
of both anks to (i) identify cats camera trapped during the pre-culling session and (ii)
match right- and left-ank pictures of the same individual from the pre-culling pictures.
e minimum number of feral cats known to be alive (MKTBA, Lazenby et al.
2015) was calculated as the total number of individuals identied from one side (left
or right side of all cat pictures). is ensured the identication of a maximum of in-
dividual cats. Since uniformly black cats are very dicult to identify individually, we
assumed that our number of dierent black individuals was an underestimation.
Spatially explicit capture-recapture models were applied to capture-mark-recapture
data to provide population density estimations (Eord et al. 2015). is allows not
to use the study area calculation as a density reference (a major bias) and gives greater
exibility in study design (Eord et al. 2009). SECR models require that: (i) every
animal has a non-zero probability of encountering a camera trap station during the
sampling period (Karanth and Nichols 1998), (ii) the location and density of stations
ensure that any feral cats (adult) can be photographed from at least two camera trap
stations (Foster and Harmsen 2012; McGregor et al. 2015), and (iii) sampling design
maximises capture probabilities (Burnham et al 1987). SECR estimations also require
encounter histories for density calculations (Eord et al. 2015; McGregor et al. 2015).
Here, such histories were built separately for pre- and post-culling sessions by divid-
ing each of them into a series of 25 and 35 days, respectively (one sampling occasion
corresponding to 24 h). is involved identifying each cat as observed or not, with
the location of the camera trap. Cat density was estimated using the ‘secr’ library in R
(Eord 2020). To avoid bias linked to low condence in identication of black cats,
the latter were excluded from the analyses (McGregor et al. 2015). Excluding black
cats from SECR analyses reduced photo capture events by 13.05%, while black cats
accounted for 11.1% of total culled cats.
e sampled population was assumed to be demographically closed during each
camera trap session, based on the fact that (i) kittens were not considered in the analyses
(Otis et al. 1978; McGregor et al. 2015 who used a 3–6 week survey period and SECR
analysis for closed populations), (ii) there was a very low probability of mortality over
the period considered, as this site houses no cat predators and is infrequently used by hu-
mans. e spatial-history capture matrix for camera trapping data was then constructed
by linking each capture of each individual with the respective coordinates of the camera
station and j-occasion, which covered 24 h. Trap detector type ‘count’ was chosen for the
SECR analysis (allowing for multiple detections of the same individual within the same
occasion, and including the two camera trapping sessions within the same analysis).
We evaluated six dierent spatial detection functions (half-normal, hazard half-
normal, hazard rate, hazard hazardrate, hazard exponential, exponential), using two
dierent functions for the distribution of home range centres: (i) a Poisson point
Eect of control on feral cat population 185
process (Borchers and Eord 2008) and (ii) a binomial point process (Royle et al.
2009). We created four habitat masks using (i) the Mean Maximum Distance Moved
(MDMM), the average maximum distance between detections of each individual
(Otis et al. 1978), and (ii) the function SECR which excludes areas inaccessible to cats
(open water) (Oppel et al. 2012). is yielded twenty-four dierent candidate models
using all combinations of detection functions and masks. Root Pooled Spatial Variance
(RPSV) was used to measure the dispersion of the sites where individual animals were
detected, pooled over individuals (Calhoun and Casby 1958; Slade and Swihart 1983;
Eord 2011). Mean home ranges pre- and post-culling were calculated using MDMM
estimations (O’Connell et al. 2010).
SECR models were compared using delta-corrected Akaike Information Criterion
(AICc) values and selected using the weighted AIC (AICwt) of each model (Burnham
and Anderson 2002).
We then compared home range at individual level between the two sessions. Home
range was calculated per individual using a Minimum Convex Polygon estimator
(MCP 95%) and the “sf” package (Pebesma 2018), and compared using mean com-
parison analysis after checking that variance is homogeneous. Individuals with more
than three dots from three dierent detectors out of alignment were kept. Generalized
Linear Models (GLM) were run to test the eect of period on home range size. A
Gaussian distribution and ‘weights’ option were used.
Residual homoscedasticity and normality were assessed via Q-Q plots and Shapiro-
Wilk tests. All statistical analyses were conducted with R 3.0.3 software (R Core Team
2014), using ‘‘ade4’’ (Chessel et al. 2004), “pROC” (Fawcett 2006) “plyr” (Wickham
2011), ‘‘varComp’’ (Qu et al. 2013), ‘‘maptools’’ (Bivand and Lewin-Koh 2013) and
‘‘GISTools’’ (Brunsdon and Chen 2014) packages. For all analyses, signicant relation-
ships were inferred at α = 0.05.
Camera trapping
ere were 908 camera trap-days in the pre-culling session and 1181 camera trap-days
in the post-culling session. ese yielded 473 feral cat detections from 51 of the 77
stations for pre-culling and 514 feral cat detections from 35 of the 40 stations for post-
culling (Fig. 2). e camera trapping rates for the pre- and post-culling sessions were
50 and 43 detections/100 trap-days, respectively. Feral cat camera trapping rates varied
spatially between pre- and post-culling sessions (Fig. 2).
Camera trapping yielded 416 feral cat pictures showing identiable cats (209 left-
anked and 207 right-anked). Pictures of cats’ left ank, matched with the corre-
sponding right ank, were used for the pre- and post-culling camera trap analyses
Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
ere was at least one uniformly black individual in the pre-culling session and two
in the post-culling session, one of which was distinguished by distinctive damage to
its tail. Uniformly coloured (here black) cats’ pictures were not included in the SECR.
A total of 36 cats were trapped and culled during the campaign (26 females, 10 males),
with a trapping eort of 1200 trap-days representing a capture per unit eort of 3
trapped cats / 100 trap-days. Females comprised 72.2% of all captured cats. e trap-
ping campaign culled 44% of the feral cats previously identied by the pre-culling
camera trap survey.
Culling effect on cat indices and density
e General Index (GI ± S. E) did not dier signicantly between pre- and post-cull-
ing sessions (t = 1.28, df = 37, p-value = 0.21), with respectively 0.50 ± 0.24 and 0.43
± 0.15 virtual capture per sampling occasion per station (Suppl. material 3: Fig. S3).
A total of 40 dierent cats (MKTBA) were identied over the whole study pe-
riod, with 25 and 23 dierent individuals from pre- and post-culling camera trap
Figure 2. Variation in number of camera trapping events (black circles) and number of cats individually
identied at camera trap stations pre- (a) and post- (b) culling. e sizes of black circles are proportional
to the number of camera-trap capture events per sampling occasion. Camera trap stations; temporary
locations (white stars), permanent locations (white points).
Eect of control on feral cat population 187
sessions, respectively. Eight individuals (29%) were identied during both pre- and
post-culling periods.
Of the twenty-four models tested (Table 2), model M1 (parameters: “hazard haz-
ard rate” function, a probability function of λ(d) and mask « ZE+Buer S2 ») and
model M2 (parameters: “hazard rate” function, a probability function of g(d) and
mask « ZE+Buer S2 ») gave the best estimation of cat densities. Model M1 showed a
ΔAICc = 0 and AICwt = 0.53, and Model M2 showed a ΔAICc = 0.26 and AICwt =
0.47 (Table 2). ese two models yielded very similar parameter values (λ(0), g(0), σ,
z) and densities (Table 3).
Estimated feral cat densities (D ± S. E.) were 1.60 ± 0.33 adult cats/ km2 pre-
culling and 1.38 ± 0.30 adult cats/ km2 post-culling. e movements and home range
of feral cat populations did change following culling. Root Pooled Spatial Variance
(RPSV) was higher post-culling, with 752.2 m pre-culling and 878.9 m post-culling.
e mean home range estimation using MDMM was more than twice as high post-
culling (0.95 km² pre-culling and 2.21 km² post-culling). Mean home range (95%
MCP) did not dier signicantly between sessions, but appeared slightly higher post-
culling (0.784 ± 0.338 km² pre-culling and 0.827 ± 0.351 km² post-culling). Before
culling, the highest numbers both of detections and of identications of individual
cats were in the South of the Peninsula, around the seabird colony. After culling, the
highest numbers of detections were in the North-West of the study area and the high-
est number of individually identied cats in the North-West and North-East (Fig. 2).
Discussion and conclusion
e camera trapping method provided adequate cat detection, enabling us to estimate,
for the rst time, accurate cat densities in New Caledonia. It also provided an eective
way to monitor variations in feral cat abundance, as in previous studies (e.g. Comer et
al. 2018). Moreover, this trapping design enabled us to live-trap cats with a success rate
within, or even slightly above, the range of other studies using wire cage traps (Algar
et al. 2010; McGregor et al. 2015; Lazenby et al. 2015). is short but intense culling
of resident feral cats proved to be eective in rapidly reducing the target population.
Table 3. Mean Maximum Distance Moved (MDMM), the average maximum distance between detec-
tions of each individual (km2) and feral cat density estimations (number of individuals per km2) pre- and
post-culling of feral cat populations. Results are given for the best SECR models; Model 1 (M1) and
Model 2 (M2) according to AIC criteria.
Model Session MDMM (km²) Density ± S. E (cat.
Inf. limit 95% Sup. limit 95%
M1 Pre-culling 11.00 1.601 ± 0.327 1.077 2.380
Post-culling 16.68 1.379 ± 0.301 0.903 2.105
M2 Pre-culling 11.00 1.600 ± 0.327 1.077 2.379
Post-culling 16.68 1.378 ± 0.300 0.903 2.104
Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
However, three months later, the dierent cat population indicators calculated post-
culling showed little dierence from those calculated pre-culling. Our culling cam-
paign simulating the resource eort that might currently be expected from local natu-
ral site managers failed to reduce the feral cat population over the mid-term. Despite
the favourable peninsula setting, this cat population recovered through recolonisation
faster than expected. e natural geography of the site, a semi-isolated peninsula, did
not limit connectivity between the treated and untreated feral cat sub-populations.
Camera trap monitoring: advantages and consistency of the three indicators
Camera trapping at our study site resulted in a high level of feral cat detection, simi-
lar to or even higher than in studies using either un-baited or baited camera trap-
ping methods. e high level of detection, and the high number of individual cats
identied from at least two dierent stations, met the two requirements for accurate
SECR calculations (Eord et al. 2015; McGregor et al. 2015). In addition, camera
trap capture probabilities were optimised in this study by positioning camera trap
stations close to open roads and tracks. us, we were able to almost systematically
observe pictures of the stripe patterns on cat legs, which are considered to be suitable
for individual identication (Bengsen et al. 2012). However, more pictures of cats’
two anks could be obtained by using paired cameras at each camera station (Karanth
and Nichols 1998; McGregor et al. 2015), which would further improve cat iden-
tication. Moreover, all undistinguishable black cats were excluded from MKTBA
and SECR analysis. Future studies could usefully attempt to incorporate uniformly
coloured cats in analysis when they represent a signicant proportion of the popula-
tion, for example by using robust home range data based on a sample of GPS-tracked
animals (e.g. Bengsen et al. 2011). Our camera trapping method provided an eective
way to monitor variations in feral cat abundance, and the consistency of its estima-
tion calculated with GI, MKTBA and densities via SECR should prove widely useful.
e GI could be used to monitor changes in the feral cat population as an alternative
to SECR estimations, which require more time and can be used to respond to more
specic research questions (Bengsen et al. 2012; Legge et al. 2017). However, conclu-
sions are often based on relative abundance indices, and this kind of index does not
consider important parameters such as variable detection (Sollman et al. 2013). Since
relative abundance indices do not systematically reect dierences in density (Sollman
et al. 2013), a valuable avenue for future research would be to compare these dierent
indices. In particular, we recommend that in areas of interest to managers, the rst
step should be to calculate all of the dierent indices (GI, MKTBA, densities). Sec-
ond, the relationship between GI and the other indices should be determined; if GI is
suciently reliable and in line with the densities estimated by SECR, only GI should
be used. For this reason, we advocate hand-in-hand collaboration between researchers
and managers from project set-up to evaluation of management results, especially in
such remote areas (Meyer et al. 2018).
Eect of control on feral cat population 189
Effect of culling on cat abundance/density over time
ree months after the end of the culling campaign that eliminated 36 cats over
10.6km2, no meaningful dierences in the relative abundance and density of feral cats
were observed in response to culling, whatever the indicator of population size consid-
ered. e abundance index (GI) indicated a similar cat presence in the peninsula, the
minimum number of individuals (MKTBA) decreased by only 8%, and estimated feral
cat densities (SECR) were similar between the two sessions. No lasting eect of culling
eort was therefore observed, despite the intensity of trapping and of traps deployed.
e recovery of the feral cat population is probably attributable to the immigra-
tion of new individuals rather than to a demographically-dependent process, as cat
detections were mainly recorded in the North of the peninsula during the post-culling
session. Culling operations could have removed dominant individuals whose extirpa-
tion enhanced the permeability of the population to young individuals. In fact, the
abundance and distribution of feral cats are partly controlled by territorial behaviour
and social interactions (Goltz et al. 2008). Removing dominant individuals could in-
crease numbers, particularly of sub-adults (e.g. Lazenby et al. 2015) presenting lower
home-range delity than adults and still seeking and delimiting their home ranges
(McGregor et al. 2014). e probable attractiveness of the tip of this peninsula, with
its large shearwater colony, could explain the rapid recolonisation of the culled area and
the changes observed in activity patterns.
Post-culling, estimated home range and RPSV (Root Pooled Spatial Variance)
increased by approximately 132% and 16.8% respectively. We also observed a
trend towards a higher home-range Minimum Complex Polygon (MCP). Taken
together, these ndings may indicate that the cats recolonising the peninsula are
largely young males travelling long distances in search of a territory (Algar et al.
2013; McGregor et al. 2014). ese results could also support the hypothesis that
the remaining cats may increase their range post-culling, having to move farther
to access mates. Male territories are primarily determined by access to females,
whereas female territories are primarily determined by prey availability and dis-
tribution of other females (Liberg et al. 2000; Turner and Bateson 2014). For this
reason, the cats increasing their range in our study are more likely to be males, since
we removed more females. e female-biased sex ratio of culled feral cats prob-
ably reects a trapping bias due to dierences between male and female behaviour
(females may seek food resources more actively due to reproductive costs, “sex-
bias” on trap attractiveness may also be linked to trapping method), rather than a
disproportionate number of females (Molsher 2001; Short and Turner 2005; Algar
et al. 2014). If future studies show a female-biased sex ratio, however, this would
suggest faster population growth than with a non- or male-biased sex ratio (Short
and Turner 2005). In any case, trapping more females could signicantly contrib-
ute to controlling cat population dynamics, which suggests that trap attractiveness
to females might be worth investigating.
Pauline Palmas et al. / NeoBiota 63: 177–200 (2020)
Culling may provide a greater access to resources for the remaining local cats, thus
promoting juvenile survival, although this would probably be more pronounced at a
larger temporal scale. Since we only measured density across one season, we are unable
to identify possible season-related or breeding-related changes in cat density.
While recovery or even increases in populations due to compensatory demograph-
ic response have been documented for numerous species, in contrast to our study, these
were observed following low-level culling (Sinclair et al. 2006; Lazenby et al. 2015).
Fortunately, most studies report a post-culling reduction in feral cat numbers, al-
though often after an intensive and sustained control eort (Algar and Burrows 2004)
or in situations where populations show limited population ows (e.g. peninsulas and
fenced areas, Short et al. 1997; Moseby and Read 2006).
Local and general implications for feral cat management
Camera trapping yields data on pre-culling population density, key information for
scientists and managers who aim to control invasive predators. We provide here the
rst feral cat density estimates from New Caledonia. At our study site, feral cat density
was estimated to be relatively high compared to many places in Australia (Bengsen
et al. 2012; McGregor et al. 2015; Hohnen et al. 2020) and on two Salomon islands
(Lavery et al. 2020). However, it is lower than at other locations: one Salomon island
(Lavery et al. 2020), Great Britain (Langham and Porter 1991), Europe (Liberg 1980),
New Zealand (Macdonald et al. 1987), United States (Warner 1985), and highly mod-
ied landscapes in Australia (Legge et al. 2017). According to the model by Legge et al.
(2017), the feral cat density at Pindaï Peninsula (1.6 cats/ km²) is higher than expected
(0.5–1 cat/ km²). is unexpected density illustrates the importance of specically
evaluating animal densities at each site before management actions start, especially
given that New Caledonia tends to use base data from Australia. e higher density
found here and the rapid return to initial densities argue for increasing the intensity
and/or duration of trapping, which we calculated based on mean densities found in
the literature.
As we co-conducted an intense but short culling eort, our trapping success is
similar to that reported in comparable studies using wire cage traps (Algar et al. 2010;
Lazenby et al. 2015; McGregor et al. 2015). e culling of 44% of camera-trapped
feral cats is within or slightly below the range of other studies (e.g. 65% for Kangaroo
Island in Bengsen et al. (2012), 44% and 56% for the Mount Field and Tasman Pen-
insula sites in Lazenby et al. (2015)). is culling eort can therefore be concluded to
have been eective, but should be implemented longer (i.e. continuously) if possible,
using more cage traps and at peninsula scale. Our ndings support the view that lethal
control in unfenced areas needs to be intense and continuous to reduce populations
of resident animals, and immigration from the perimeter of core conservation areas
needs to be limited (Veitch 1985; Norbury et al. 1998; Short et al. 1997; Edwards et al.
2001; Campbell et al. 2011; McCarthy et al. 2013). is applies even when recolonisa-
tion seems low due to the natural geography of the site, like a peninsula. Intense lethal
control could be implemented during the presence of Wedge-tailed shearwaters in the
Eect of control on feral cat population 191
Pindaï peninsula colony, but their long breeding cycle (from October to May) makes
this type of annual control costly and labour-intensive. Moreover, it is likely to result
in large numbers of trap-shy feral cats (Parkes et al. 2014). We also recommend acting
on a larger spatial scale, i.e. on the scale of the whole peninsula, which is rather wide
and short compared to other peninsulas (e.g. Heirisson Prong in Short et al. 2002 and
Tasman Peninsula in Lazenby et al. 2015).
For several years, innovative technical solutions have been sought to optimise
the management of feral cats. ese include both baiting and trapping strategies, as
well as the development of ecient baits (e.g. Eradicat and Curiosity baits) and of
automated traps that specically recognise and poison feral cats (Algar et al. 2011;
Johnston et al. 2011; Fisher et al. 2015; Fancourt et al. 2019; Read et al. 2019; Mo-
seby et al. 2020). Other highly innovative genetic, cellular or behavioural methods
are also being developed and oer promise for controlling feral cats in the future
(Kinnear 2018; Moro et al. 2018). An interesting physiological and behavioural
method called “Toxic Trojan prey”, based on making the prey of feral cats speci-
cally toxic to them, could be considered for feral cat control on our study site (Read
et al. 2016).
Guard dogs could also be trained to protect wildlife and to prevent predation by
feral cats on the Wedge-tailed shearwaters’ breeding colony, as reported in two cases
in South-West Victoria involving little penguins Eudyptula minor and gannets Morus
serrator (van Bommel et al. 2010; Doherty et al. 2016). Exclusion fencing, widely
used in Australia and New Zealand to protect biodiversity (Long and Robley 2004;
Woinarski et al. 2014), might be another eective way to limit the recolonisation
process that is particularly protable and ecient in the peninsular context (Young et
al. 2018; Tanentzap and Lloyd 2017). Last but not least, modelling approaches can
provide numerical estimates of parameter values (e.g. predation rate) beyond which
the prey population will decrease and/or cannot be sustained (Keitt et al. 2002; Peck
et al. 2008; Bonnaud et al. 2009). Knowing such threshold values would support and
greatly improve future management decisions.
is study was funded by Province Nord (Contracts N°14C330, 15C331). We are very
grateful to Corentin Chaillon, Agathe Gerard, Mathieu Mathivet, Edouard Bourguet and
Province Nord landowners for providing support in eldwork. We are also very grateful
to the veterinary sta of Koné, particularly Henri Lamaignère and Yann Charpentier, for
handling the feral cats. We thank Marjorie Sweetko for English language editing.
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Supplementary material 1
Figure S1
Authors: Pauline Palmas, Raphaël Gouyet, Malik Oedin, Alexandre Millon, Jean-Jé-
rôme Cassan, Jenny Kowi, Elsa Bonnaud, Eric Vidal
Data type: gure
Explanation note: Box plot home range MCP pre post.
Copyright notice: is dataset is made available under the Open Database License
( e Open Database License
(ODbL) is a license agreement intended to allow users to freely share, modify, and
use this Dataset while maintaining this same freedom for others, provided that the
original source and author(s) are credited.
Supplementary material 2
Figure S2
Authors: Pauline Palmas, Raphaël Gouyet, Malik Oedin, Alexandre Millon, Jean-Jé-
rôme Cassan, Jenny Kowi, Elsa Bonnaud, Eric Vidal
Data type: gure
Explanation note: Accu curve preculling.
Copyright notice: is dataset is made available under the Open Database License
( e Open Database License
(ODbL) is a license agreement intended to allow users to freely share, modify, and
use this Dataset while maintaining this same freedom for others, provided that the
original source and author(s) are credited.
Supplementary material 3
Figure S3
Authors: Pauline Palmas, Raphaël Gouyet, Malik Oedin, Alexandre Millon, Jean-Jé-
rôme Cassan, Jenny Kowi, Elsa Bonnaud, Eric Vidal
Data type: gure
Explanation note: Accu curve livetrapping.
Copyright notice: is dataset is made available under the Open Database License
( e Open Database License
(ODbL) is a license agreement intended to allow users to freely share, modify, and
use this Dataset while maintaining this same freedom for others, provided that the
original source and author(s) are credited.
... The removal of 35 cats in the area greatly reduced cat activity and abundance at the seabird colony, as shown by camera-trap monitoring. The problem for cat control on large islands is that the area may be reinvaded rapidly (Moseby and Hill 2011; Palmas et al. 2020). ...
... Notre étude confirme l'efficacité d'un suivi par camera-traps pour suivre la dynamique d'une population de chats et quantifier les effets des opérations de contrôle (Bengsen et al. 2011;Comer et al. 2018;Palmas et al. 2020 (Bengsen et al. 2011;Lazenby et al. 2015;Palmas et al. 2020) (Scott et al. 2002;Nutter et al. 2004) qui est une source supplémentaire de nouveaux individus. Cet afflux de chats a inévitablement mené à une forte augmentation des densités trois mois après l'arrêt du contrôle. ...
... Notre étude confirme l'efficacité d'un suivi par camera-traps pour suivre la dynamique d'une population de chats et quantifier les effets des opérations de contrôle (Bengsen et al. 2011;Comer et al. 2018;Palmas et al. 2020 (Bengsen et al. 2011;Lazenby et al. 2015;Palmas et al. 2020) (Scott et al. 2002;Nutter et al. 2004) qui est une source supplémentaire de nouveaux individus. Cet afflux de chats a inévitablement mené à une forte augmentation des densités trois mois après l'arrêt du contrôle. ...
The southern coastal cliffs of Reunion Island (tropical island in the Western Indian Ocean) host a unique flora and fauna: the last populations of Manapany day gecko (Phelsuma inexpectata, an endemic reptile in critically endangered), relics of indigenous vegetation including endemic and/or threatened species (e.g.: Euphorbia viridula, Psiadia retusa, Latania lontaroides) and breeding colonies of three native seabirds (white-tailed tropicbirds, Phaethon lepturus ; brown noddies, Anous stolidus and wedge-tailed shearwaters, Ardenna pacifica). This biodiversity is threatened by habitat transformations due to invasive plants, human activities (urbanization and culture) and invasive mammals (especially cats, felis catus, and rodents). Moreover, little is known about biology and ecology of the native species, which does not allow the implementation of effective conservation strategy. Based on hand-in-hand collaboration between researchers (UMR ENTROPIE) and managers (CDL, NOI, AVE2M) working on different taxa, the aim of this thesis was to provide multispecies conservation prescriptions on cliffs study for the Manapany day gecko and the wedge-tailed shearwater. We undertook a progressive approach from describing of species conservation states through understanding threatening processes to the prescription and monitoring of management actions. Three research topics were targeted: (i) demography and reproductive biology, (ii) terrestrial habitat requirements, and (iii) impacts and management of invasive mammals (especially cats). Our results highlighted the critical conservation state of Manapany day geckos and wedge-tailed shearwaters populations. Invasive plants and mammals (especially cats) are threats to the conservation of native biodiversity. We provide several local and general conservation prescriptions, including management of invasive species, multispecies terrestrial habitat restoration and captive head-start program of Manapany day geckos. Several of these prescriptions were implemented during this thesis (invasive species management and captive breeding program) and monitored as part of active adaptive management approach. This multispecies study at the interface between research and management must be continued and supported by a strong federating regulatory tool as a National Nature Reserve (NNR). Keywords: Ardenna pacifica, biological invasions, captive head-start program, Capture-Mark-Recapture, cat control, cat tracking, conservation biology, Felis catus, habitat selection, multispecies management, Phelsuma inexpectata, population dynamics, Population Viability Analyses, Reunion Island, Spatial Mark-Resight, tropical island
... The removal of 35 cats in the area greatly reduced cat activity and abundance at the seabird colony, as shown by camera-trap monitoring. The problem for cat control on large islands is that the area may be reinvaded rapidly (Moseby and Hill 2011;Palmas et al. 2020). Cat density may increase after cat removal because new individuals may move into the area after dominant cats are removed (Lazenby et al. 2015). ...
Full-text available
Cats (Felis catus) introduced on islands have strong impacts on biodiversity, and the main conservation actions to protect native fauna is cat eradication or control (i.e., regular culling). The situation is more complicated on inhabited islands because unowned cats coexist with owned cats. The social acceptance of cat control implies separating the impacts of unowned and owned cats. We investigated the spatial ecology and impacts of owned and unowned cats at a seabird colony in a periurban area on Reunion Island (Indian Ocean). We used multiple methodologies to investigate this question: GPS-tracking of cats, camera-traps at seabird nests, cat scat analysis and cat control. Owned cats had small home ranges and did not forage at the seabird colony. Unowned cats had larger home ranges and foraged at seabird colony. We identified two kinds of unowned cats, stray cats and semi-feral cats. Stray cats relied on food waste and rarely foraged at seabird colony. Semi-feral cats foraged mostly in natural habitats, including the seabird colony and rarely used food waste. Semi-feral cats were very active at the seabird colony and several preyed upon seabirds. Restaurants are an abundant source of food for cats and help sustain populations of unowned cats. Control of unowned cats during this study resulted in reduced cat activity at the seabird colony. To minimize negative impacts of cats on seabirds, our results suggest that the most effective strategy includes the permanent control of unowned cats, efficient management of food waste and sterilization of owned cats.
... These ''costs'' are the unforeseen consequences of the management of NIS, usually derived from poorly understood facilitation-competition relationships. Management initiatives involving costly programs can result in failures (Palmas et al. 2020), or even unexpected and unwanted outcomes whereby eradication of the target NIS ends up endangering native species and communities (Bergstrom et al. 2009;Courchamp et al. 2011;Vince 2011;Bonanno 2016;Kopf et al. 2017;Lurgi et al. 2018;Ward et al. 2019;Ortega et al. 2021;Travers et al. 2021). They can also facilitate a different invader, sometimes as damaging as-or even worse-than the one targeted. ...
The economic costs of non-indigenous species (NIS) are a key factor for the allocation of efforts and resources to eradicate or control baneful invasions. Their assessments are challenging, but most suffer from major flaws. Among the most important are the following: (1) the inclusion of actual damage costs together with various ancillary expenditures which may or may not be indicative of the real economic damage due to NIS; (2) the inclusion of the costs of unnecessary or counterproductive control initiatives; (3) the inclusion of controversial NIS-related costs whose economic impacts are questionable; (4) the assessment of negative impacts only, ignoring the positive ones that most NIS have on the economy, either directly or through their ecosystem services. Such estimates necessarily arrive at negative and often highly inflated values, do not reflect the net damage and economic losses due to NIS, and can significantly misguide management and resource allocation decisions. We recommend an approach based on holistic costs and benefits that are assessed using likely scenarios and their counter-factual.
... The broadscale control of feral cats and their impacts is notoriously challenging. Traditional control tools such as trapping, shooting and exclusion fencing can be effective at reducing the impacts of feral cats in small areas, but recolonisation can occur rapidly (Palmas et al. 2020), and these approaches are neither cost-effective nor logistically feasible at larger scales (Bengsen 2015;Short et al. 1997). In Australia, poison baiting is considered the most cost-effective tool for controlling introduced foxes (Vulpes vulpes) and wild dogs (including dingoes, feral dogs and hybrids; Canis familiaris) across the landscape Fleming et al. 2014), but traditional predator baits are typically less effective at controlling feral cats Fancourt et al. 2021a). ...
Full-text available
Reducing the damage caused by feral cats (Felis catus) to wildlife, livestock and human health is a key objective for many land managers and human health agencies globally. The lack of safe and efficacious lethal control tools in many regions, however, makes the control of feral cats and their impacts challenging. We performed a baiting trial in central Queensland to measure the efficacy and safety of Eradicat®, a feral cat bait currently approved for use only in the state of Western Australia, as a potential tool for the broadscale control of feral cats in eastern Australian environments. We used camera traps, cat-borne GPS collars and chemical residue analysis to monitor mortality and changes in feral cat populations following baiting. We also used camera traps and bird count surveys to monitor the response of key at-risk non-target species, specifically wild dogs (Canis familiaris), common brush-tailed possums (Trichosurus vulpecula) and 10 bird species at risk of consuming baits. Feral cat abundance reduced significantly (29–40%) following baiting, with reductions observed across 83% of the site. There were no significant changes in wild dog, possum or potentially bait-consuming bird populations following baiting. Our findings suggest that Eradicat® could potentially be a safe and efficacious tool for the landscape control of feral cats at some sites in eastern Australia. Future research is required to test the safety and efficacy of Eradicat® at other sites in eastern Australia, as suites of non-target species will vary among sites in different environments.
... Whilst the translocation of individuals of conservation concern (e.g., Irish hare) to suitable, low-density areas could help to control airside population size whilst aiding national conservation efforts, removal programs on their own may not always be an effective long-term solution due to rapid recolinisation by a species (e.g. Palmas et al. 2020). Therefore, the presence and activity patterns of animals at airfields can be determined through the use of modern, remote monitoring equipment (e.g., camera traps and GPS collars) allowing for the collection of high-quality data in sensitive, airport environments (e.g. ...
Full-text available
Collisions between wildlife and aircraft are a serious and growing threat to aviation safety. Understanding the frequency of these collisions, the identity of species involved, and the potential damage that can be inflicted on to aircraft aid mitigation efforts by airfield managers. A record of all animal carcasses recovered from Dublin International Airport, Ireland’s largest civil aviation airport, has been maintained since 1990 where strikes with the endemic Irish hare (Lepus timidus hibernicus), a protected subspecies of mountain hare, are of particular concern despite substantial management efforts from the airfield authority. The first strike event with a hare was recorded in 1997, and strike events have substantially increased since then, with a sharp increase recorded in 2011. Over a 30-year period, a total of 320 strike events with the Irish hare have been recorded at the airfield. To date, no strike event with a hare has resulted in damage to an aircraft. However, carcasses can present as a major attraction to avian scavenger species in addition to posing as a risk of causing foreign object damage in the event of an undetected carcass. Hare strikes are discussed in the context of the rate of civil aircraft movements, possible direct and indirect damage to aircraft, and airfield wildlife hazard management. Here, we demonstrate that not only are strike events increasing by 14% on an annual basis, but that the kinetic energy of such an event has the potential to cause significant damage to an aircraft.
... All three islands have permanent human residents (French Island: 110; Bruny Island: 800; Christmas Island: 1840) and are considered large (>1,000 ha; Nogales et al., 2004). Our model is also applicable to mainland density control and eradications but it is only recommended for eradication in exclusion zones because cats can rapidly recolonize areas that have undergone density reduction (Moseby & Hill, 2011;Palmas et al., 2020). Our model also has applications for other species, including European red foxes (Edwards, Pople, Saalfeld, & Caley, 2004) in Australia (particularly mainland exclusion zones), brush-tailed possums (Trichosurus vulpecula) and stoats (Mustela erminea) in New Zealand (Brown, Elliott, Innes, & Kemp, 2015), and mainland application for species such as racoons (Procyon lotor) in central Europe (Beltr an-Beck, García, & Gort azar, 2012). ...
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Feral cats are some of the most destructive invasive predators worldwide, particularly in insular environments; hence, density-reduction campaigns are often applied to alleviate the predation mortality they add to native fauna. Density-reduction and eradication efforts are costly procedures with important outcomes for native fauna recovery, so they require adequate planning to be successful. These plans should include empirical density-reduction models that can guide yearly culling quotas, and resource roll-out for the duration of the culling period. This ensures densities are reduced over the long term and that resources are not wasted. We constructed a stochastic population model with cost estimates to test the relative effectiveness and cost-efficiency of two main culling scenarios for a 10-year eradication campaign of cats on Kangaroo Island, Australia: (a) constant proportional annual cull (one-phase), and (b) high initial culling followed by a constant proportional maintenance cull (two-phase). A one-phase cull of at least 0.35 of the annual population size would reduce the final population to 0.1 of its original size, while a two-phase cull with an initial cull of minimum 0.6 and minimum 0.5 maintenance cull would reduce the final population to 0.01 of its initial size within the 10-year time frame. Cost estimates varied widely depending on the methods applied (shooting, trapping, aerial poison baits, Felixer™ poison-delivery system), but using baiting, trapping and Felixers with additional shooting to meet culling quotas was the most cost-effective combination (minimum cost: AU$19.56 million; range: AU$16.87 million–AU$20.69 million). Our model provides an adaptable and general assessment tool for cat reductions in Australia and potentially elsewhere, and provides relative culling costs for the Kangaroo Island campaign specifically.
Significance Although popular companion animals, domestic cats pose numerous problems when free-roaming, including predation of wildlife, hazards to humans, impaired sanitation, and a decrease in their welfare. Thus, managing their populations is essential. The trap–neuter–return method (TNR; capturing, sterilizing, returning/releasing) is widely employed for managing cat populations. However, there is a lack of long-term controlled evidence for its effectiveness. We examined the outcomes of high-intensity TNR by performing a 12-y controlled field experiment. Neutering over 70% of the cats caused population decline when applied over contiguous areas. However, it was limited by a rebound increase in reproduction and survival. These findings provide a robust quantification of the limitations and the long-term effectiveness of TNR.
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Substantial societal investment is made in the management of free-roaming cats by various methods, with goals of such programs commonly including wildlife conservation, public health protection, nuisance abatement, and/or promotion of cat health and welfare. While there has been a degree of controversy over some of the tactics employed, there is widespread agreement that any method must be scientifically based and sufficiently focused, intensive and sustained in order to succeed. The vast majority of free-roaming cat management in communities takes place through local animal shelters. Throughout the 20th century and into the 21st, this consisted primarily of ad hoc admission of cats captured by members of the public, with euthanasia being the most common outcome. In North America alone, hundreds of millions of cats have been impounded and euthanized and billions of dollars invested in such programs. Given the reliance on this model to achieve important societal goals, it is surprising that there has been an almost complete lack of published research evaluating its success. Wildlife conservation and public health protection will be better served when debate about the merits and pitfalls of methods such as Trap-Neuter-Return is grounded in the context of realistically achievable alternatives. Where no perfect answer exists, an understanding of the potential strengths and shortcomings of each available strategy will support the greatest possible mitigation of harm—the best, if still imperfect, solution. Animal shelter function will also benefit by discontinuing investment in methods that are ineffective as well as potentially ethically problematic. This will allow the redirection of resources to more promising strategies for management of cats as well as investment in other important animal shelter functions. To this end, this article reviews evidence regarding the potential effectiveness of the three possible shelter-based strategies for free-roaming cat management: the traditional approach of ad hoc removal by admission to the shelter; admission to the shelter followed by sterilization and return to the location found; and leaving cats in place with or without referral to mitigation strategies or services provided by other agencies.
Feral Cat (Felis catus) (cat) is a predator of the Bridled Nail‐Tailed Wallaby (BNTW) (Onychogalea frenata) living at Taunton National Park (Scientific) (Taunton). The aim of this study was to determine if traps and poison baits could be used to control feral cats without impacting non‐target species at Taunton. The techniques trialled included poison fresh meat baits and several types of traps presented in different ways and with various lures. Thirty‐one percent of fresh meat baits was taken during bait uptake trials; corvids removed 40% of these and dogs removed 16%. Cats were not detected, on camera traps, taking a bait. The elevated soft‐jaw traps (81 trap nights/cat) and single‐entry cage traps (98 trap nights/cat) were found to be the most successful of all the trap types trialled and had low amounts of by‐catch. Other trap types trialled took more than 166 trap nights to catch a cat. The elevated soft‐jaw trap configurations had the lowest amount of by‐catch (avg. 0.33%), and the log trap had the highest amount of by‐catch (1%). Ground‐set traps successfully trapped cats (305 trap nights/cat) but caught more by‐catch (0.9%) compared to the elevated soft‐jaw trap types and most wallabies caught in these traps had to be euthanised.
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Habitat degradation, invasive species and overexploitation are currently the three main threats to biodiversity. Here we present a study on the population status of two sympatric flying fox species, Pteropus ornatus (endemic) and P. tonganus (native), and the impact of hunting and predation by the feral cat Felis catus in New Caledonia. The study of flying fox roost occupancy in the North Province shows a 33% disapearance in 40 years. The flying fox population on Grande Terre is estimated at about 735,000 individuals (of both species) and the annual hunting rate at 7%. Integrated stochastic modelling of this population suggests that current harvesting levels could lead to a decline of up to 80% in the next 30 years. Temporary hunting ban and/or protected areas appear, in addition to being combinable, to be the most acceptable and effective management options for hunters. An analysis of the data available worldwide shows that all forms of cats prey on bats in all habitats and that this threat is probably largely underestimated. Finally, initial results suggest that flying fox predation by feral cats in New Caledonia is of the same order of magnitude as hunting. This study proposes a framework for assessing the sustainability of hunting game species in an integrated adaptive management approach, taking into account other threat factors such as invasive species.
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Context. Introduced predators, especially cats, are a major cause of extinction globally. Accordingly, an extensive body of literature has focussed on the ecology and management of feral cats in continental and island systems alike. However, geographic and climatic gaps remain, with few studies focusing on rainforests or tropical islands of the south-western Pacific. Aims. We aimed to estimate cat densities and elucidate activity patterns of cats and sympatric birds and mammals in tropical island rainforests. We hypothesised that cat activity would be most influenced by the activity of introduced rodents and ground-dwelling birds that are predominant prey on islands. Methods. We used camera traps to detect feral cats, pigs, rodents and birds on four tropical islands in the south-western Pacific. We used spatial capture–recapture models to estimate the abundance and density of feral cats. Relative abundance indices, and temporal overlaps in activity were calculated for feral cats, pigs, rodents, and birds. We used a generalised linear model to test for the influence of pig, rodent, and bird abundance on feral cat abundance. Key results. The species most commonly detected by our camera traps was feral cat, with estimated densities between 0.31 and 2.65 individuals km�2. Pigs and introduced rodents were the second- and third-most commonly detected fauna respectively. Cat activity was bimodal, with peaks in the hours before dawn and after dusk. Cat abundance varied with site and the abundance of rodents. Conclusions. Feral cats are abundant in the tropical rainforests of our study islands, where one bird and two mammal species are now presumed extinct. Introduced rodents possibly amplify the abundance and impacts of feral cats at our sites. Peak cat activity following dusk did not clearly overlap with other species detected by our camera traps. We postulate cats may be partly focussed on hunting frogs during this period. Implications. Cats are likely to be a major threat to the highly endemic fauna of our study region. Management of feral cats will benefit from further consideration of introduced prey such as rodents, and their role in hyperpredation. Island archipelagos offer suitable opportunities to experimentally test predator–prey dynamics involving feral cats.
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Feral cats are one of the most destructive invasive predators worldwide. Due to the high risk of pushing native species to extinction in Australia, density-reduction or eradication campaigns can greatly improve the persistence probability and recovery of native fauna. Kangaroo Island — Australia’s third-largest island — was recently nominated as a complete cat-eradication site by the federal government. Because most population density-reduction campaigns are costly and not effective in the long term, mathematical models predicting optimal culling regimes can guide management plans, especially if they include estimates of costs under different policy decisions. We constructed a stochastic population model with cost estimates to test the relative effectiveness and cost-efficiency of two main culling scenarios for Kangaroo Island to 2030: (1) constant proportional annual cull, and (2) high initial culling followed by a constant proportional maintenance cull. We also examined the effectiveness of a trap-neuter-return scenario to compare with the culling outcomes. We found that an average culling proportion of ≥ 0.3 would reduce the population to ≤ 10% of the founding population, while a two-phase cull where an initial cull of ≥ 0.6 was followed by a maintenance cull of ≥ 0.45 would reduce the final population to 1% of its initial size by 2030. Costs estimates varied widely depending on capture techniques used, but a combination of Felixer™️ cat-eradication units, conventional traps, and hunting was the most cost-effective (minimum costs estimated at AU$46.5 million–AU$51.6 million). Our model results provided direction for the efficient eradication of feral cats on Kangaroo Island and can be transferred to feral-cat management elsewhere.
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Feral cats are known to drive numerous extinctions of endemic species on islands. Predation by feral cats also currently threatens many species listed as critically endangered. Island faunas that have evolved in the absence of mammalian predators are particularly susceptible to cat predation. Dirk Hartog Island is no exception as cats have caused the local extinction of its once high vertebrate diversity. A programme to reconstruct the native fauna on the island necessitated feral cat eradication. In this paper we outline the strategy, removal techniques and monitoring methods used in this successful eradication of feral cats. Globally, the Dirk Hartog project has become the largest successful island feral cat eradication campaign attempted to date.
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The demography of a population is often reduced to the apparent (or local) survival of individuals and their realised fecundity within a study area defined according to logistical constraints rather than landscape features. Such demographics are then used to infer whether a local population contributes positively to population dynamics across a wider landscape context. Such a simplistic approach ignores a fundamental process underpinning population dynamics: dispersal. Indeed, it has long been accepted that immigration contributed by dispersers that emigrated from neighbouring populations may strongly influence the net growth of a local population. To date however, we lack a clear picture of how widely immigration rate varies both among and within populations, in relation to extrinsic and intrinsic ecological conditions, even for the best‐studied avian and mammalian populations. This empirical knowledge gap precludes the emergence of a sound conceptual framework that ought to inform conservation and population ecology. This review, conducted on both birds and mammals, has thus three complementary objectives. First, we describe and evaluate the relative merits of methods used to quantify immigration and how they relate to widely applicable metrics. We identify two simple and unifying metrics to measure immigration: the immigration rate it defined as the ratio of the number of immigrants present in the population at time t + 1 and the total breeding population in year t, and πt, the proportion of immigrants among new recruits (i.e. new breeders). Two recently developed methods are likely to provide the most valuable data on immigration in the near future: individual parentage (rather than population) assignments based on genetic sampling, and spatially explicit integrated population models combining multiple sources of demographic data (survival, fecundity and population counts). Second, we report on a systematic literature review of studies providing a quantitative measure of immigration. Although the diversity of methods employed precludes detailed analyses, it appears that the number of immigrants exceeds locally born individuals in recruitment for most avian populations (median πt = 0.57, N = 45 estimates from 37 studies), a figure twofold higher than estimated for mammalian populations (median πt = 0.26, N = 33 estimates from 11 studies). Third, recent quantitative studies reveal that immigration can be the main driver of temporal variation in population growth rates, across a wide array of demographic and spatial contexts. To what extent immigration acts as a regulatory process has however been considered only rarely to date and deserves more attention. Overall, it is likely that most populations benefit from immigrants without necessarily being sink populations. Furthermore, we suggest that quantitative estimates of immigration should be core to future demographic studies and plead for more empirical evidence about the ways in which immigration interacts with local demographic processes to shape population dynamics. Finally, we discuss how to tackle spatial population dynamics by exploring, beyond the classical source–sink framework, the extent to which populations exchange individuals according to spatial scale and type of population distribution throughout the landscape.
Conference Paper
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Feral cats (Felis catus) are notoriously difficult to control using traditional management approaches such as baiting, reportedly due to their preference for hunting live prey. Many factors, however, can potentially influence the success of feral cat baiting programs. As baiting efficacy is rarely measured, the factors contributing to low baiting success are often assumed, but poorly understood. We used a combination of camera traps and cat-borne GPS collars to measure the efficacy of two feral cat baiting programs at Taunton National Park (Scientific) in central Queensland. We trialled a fresh meat bait (the Queensland 'Curiosity 1080 Cat Bait', ~125 g fresh kangaroo meat, 6 mg 1080) during winter 2016, and a chipolata-style meat bait (Eradicat®, ~20g kangaroo mince, chicken fat and flavour enhancers, 4.5 mg 1080) during winter 2017. Track-based ground baiting using Curiosity baits was ineffective, with only 11% of collared cats killed and no observed reduction in population-level feral cat abundance across the site. Low track use by cats and rapid removal of baits by non-target species contributed to low bait encounter rates by cats. In addition, palatability of baits rapidly declined due to meat-ant infestations and bait desiccation. Aerially deployed Eradicat® baits were more effective, with 40% of collared cats killed, and a similar significant reduction in population-level feral cat abundance across the site. The key factors contributing to the observed differences in efficacy were compared and evaluated. We discuss the implications of our findings and recommend approaches to improve the efficacy of feral cat baiting programs.
Feral cats (Felis catus) are a significant threat to wildlife in Australia and globally. In Australia, densities of feral cats vary across the continent and also between the mainland and offshore islands. Densities on small islands may be at least an order of magnitude higher than those in adjacent mainland areas. To provide cat-free havens for biodiversity, cat-control and eradication programs are increasingly occurring on Australian offshore islands. However, planning such eradications is difficult, particularly on large islands where cat densities could vary considerably. Aims. In the present study, we examined how feral cat densities vary among three habitats on Kangaroo Island, a large Australian offshore island for which feral cat eradication is planned. Densities were compared among the following three broad habitat types: forest, forest-farmland boundaries and farmland. To detect cats, three remote-camera arrays were deployed in each habitat type, and density around each array was calculated using a spatially explicit capture-recapture framework. The average feral cat density on Kangaroo Island (0.37 cats km2) was slightly higher than that on the Australian mainland. Densities varied from 0.06 to 3.27 cats km2 and were inconsistent within broad habitat types. Densities were highest on farms that had a high availability of macropod and sheep carcasses. The relationship between cat density and the proportion of cleared land in the surrounding area was weak. The total feral cat population of Kangaroo Island was estimated at 1629 +- 661 (mean +- s.e.) individuals. Cat densities on Kangaroo Island are highly variable and may be locally affected by factors such as prey and carrion availability. For cat eradication to be successful, resources must be sufficient to control at least the average cat density (0.37 cats km2), with additional effort around areas of high carcass availability (where cats are likely to be at a higher density) potentially also being required.
ContextFeral cats pose a significant threat to wildlife in Australia and internationally. Controlling feral cats can be problematic because of their tendency to hunt live prey rather than be attracted to food-based lures. The Felixer grooming trap was developed as a targeted and automated poisoning device that sprays poison onto the fur of a passing cat, relying on compulsive grooming for ingestion. AimsWe conducted a field trial to test the effectiveness of Felixers in the control of feral cats in northern South Australia where feral cats were present within a 2600-ha predator-proof fenced paddock. Methods Twenty Felixers were set to fire across vehicle tracks and dune crossings for 6 weeks. Cat activity was recorded using track counts and grids of remote camera traps set within the Felixer Paddock and an adjacent 3700-ha Control Paddock where feral cats were not controlled. Radio-collars were placed on six cats and spatial mark–resight models were used to estimate population density before and after Felixer deployment. Key resultsNone of the 1024 non-target objects (bettongs, bilbies, birds, lizards, humans, vehicles) that passed a Felixer during the trial was fired on, confirming high target specificity. Thirty-three Felixer firings were recorded over the 6-week trial, all being triggered by feral cats. The only two radio-collared cats that triggered Felixers during the trial, died. Two other radio-collared cats appeared to avoid Felixer traps possibly as a reaction to previous catching and handling rendering them neophobic. None of the 22 individually distinguishable cats targeted by Felixers was subsequently observed on cameras, suggesting death after firing. Felixer data, activity and density estimates consistently indicated that nearly two-thirds of the cat population was killed by the Felixers during the 6-week trial. Conclusions Results suggest that Felixers are an effective, target-specific method of controlling feral cats, at least in areas in which immigration is prevented. The firing rate of Felixers did not decline significantly over time, suggesting that a longer trial would have resulted in a higher number of kills. ImplicationsFuture studies should aim to determine the trade-off between Felixer density and the efficacy relative to reinvasion.
Felixer grooming “traps” provide a novel technique for controlling invasive red foxes (Vulpes vulpes) and feral cats (Felis catus) by ejecting a dose of poison onto the fur of a target animal, which is subsequently ingested through grooming. The Felixer achieves target specificity through a discriminatory sensor arrangement and algorithm as well as a dosing pathway and toxin, which together make feral cats and foxes more vulnerable than humans and nontarget wildlife. The toxin 1080 used in many pest control projects in Australia is derived from native plants, which renders Australian wildlife, including potential scavengers of poisoned carcasses, that have co-evolved with these toxic plants less sensitive than their nonnative counterparts to 1080 poisoning. We investigated the success of the Felixer sensor system in discriminating target cats and red foxes from nontargets under field conditions. All foxes and 82% of feral cats were correctly identified as targets. No people or medium-sized marsupials—including brush-tailed possums (Trichosurus vulpecula), bettongs (Bettongia spp.), bilbies (Macrotis lagotis), and western quolls (Dasyurus geoffroii)—were incorrectly assigned as targets, suggesting Felixers could provide safe and specific feral-predator control at many conservation sites, albeit not at sites with threatened endemic small felids or canids. A low false-positive detection rate was recorded in larger macropods and poultry that will be addressed with more sophisticated sensor positioning and algorithms in optimized Felixers, along with more careful installation. The low sensitivity of macropods and malleefowl (Leipoa ocellata) to 1080, and their reduced grooming behavior relative to feral cats, suggests these species will not be affected by Felixer deployment.
The Action Plan for Australian Mammals 2012 is the first review to assess the conservation status of all Australian mammals. It complements The Action Plan for Australian Birds 2010 (Garnett et al. 2011, CSIRO Publishing), and although the number of Australian mammal taxa is marginally fewer than for birds, the proportion of endemic, extinct and threatened mammal taxa is far greater. These authoritative reviews represent an important foundation for understanding the current status, fate and future of the nature of Australia. This book considers all species and subspecies of Australian mammals, including those of external territories and territorial seas. For all the mammal taxa (about 300 species and subspecies) considered Extinct, Threatened, Near Threatened or Data Deficient, the size and trend of their population is presented along with information on geographic range and trend, and relevant biological and ecological data. The book also presents the current conservation status of each taxon under Australian legislation, what additional information is needed for managers, and the required management actions. Recovery plans, where they exist, are evaluated. The voluntary participation of more than 200 mammal experts has ensured that the conservation status and information are as accurate as possible, and allowed considerable unpublished data to be included. All accounts include maps based on the latest data from Australian state and territory agencies, from published scientific literature and other sources. The Action Plan concludes that 29 Australian mammal species have become extinct and 63 species are threatened and require urgent conservation action. However, it also shows that, where guided by sound knowledge, management capability and resourcing, and longer-term commitment, there have been some notable conservation success stories, and the conservation status of some species has greatly improved over the past few decades. The Action Plan for Australian Mammals 2012 makes a major contribution to the conservation of a wonderful legacy that is a significant part of Australia’s heritage. For such a legacy to endure, our society must be more aware of and empathetic with our distinctively Australian environment, and particularly its marvellous mammal fauna; relevant information must be readily accessible; environmental policy and law must be based on sound evidence; those with responsibility for environmental management must be aware of what priority actions they should take; the urgency for action (and consequences of inaction) must be clear; and the opportunity for hope and success must be recognised. It is in this spirit that this account is offered. Winner of a 2015 Whitley Awards Certificate of Commendation for Zoological Resource.