Remove or control predators using lethal controls: Snakes & lizards

Supporting evidence from individual studies

1. A before-and-after study in 1986–1993 on Mana Island, New Zealand (Newman 1994) found that following eradication of an invasive house mouse Mus musculus, the abundance of two of four lizard species increased, and two remained stable. Before-and-after comparisons were not statistically tested. In the four years following mouse eradication, the number of McGregor's skinks Cyclodina macgregori increased from one capture/100 trap nights in the year after eradication to 10 captures/100 trap nights three years after eradication. Numbers prior to eradication had been six captures (four years before), 8 captures (2–3 years before) and one capture/100 trap nights (one years before). Common gecko Hoplodactylus maculatus captures increased following eradication (after: 35–70 captures/100 trap nights; before: 5–15). Numbers captured remained similar for common skinks Leiolopisma nigriplantare polychroma (after: 6–10 captures/100 trap nights; before: 9–21) and copper skinks Cyclodina aenea (after: 2–4 captures/100 trap nights; before: 1–9). In 1989–1990, mouse eradication was carried out by distributing poison baits (Storm, Talon 20P, Talon 50W) via two aerial drops and ground baiting (over 5,000 stations in 25 m). In 1986–1987, cattle were also removed from the island. In 1985–1993, lizards were trapped annually (3–8 sessions/year; 2–4 days trapping/session) using pitfall traps (582–4,066 trap nights/session) that were deployed across 27 trapping stations around the island.  

   Study and other actions tested

   Referenced paper

   Newman D. (1994) Effects of a mouse, Mus musculus, eradication programme and habitat change on lizard populations of Mana Island, New Zealand, with special reference to McGregor's skink, Cyclodina macgregori. New Zealand Journal of Zoology, 21, 443-456.

2. A before-and-after, site comparison study in 1986–1993 on two islands near North Island, New Zealand (Towns 1994) found that removal of Pacific rats Rattus exulans and European rabbits Oryctolagus cuniculus resulted in an increase in the abundance of lizards, and when compared to a predator free island, abundance was initially lower but after two years was similar or higher on the removal island. The effects of predator and herbivore control cannot be separated. In forest sites, lizard numbers remained stable for five years following eradication (1986–1991: 2 lizards/100 trap days) before increasing suddenly (1992–1993: 16 lizards/100 trap days), and in coastal sites there was a gradual increase from the year of eradication (3 lizards/100 trap nights) to six years after eradication (70 lizards/100 trap nights). In 1986–1987, lizard abundance was lower on the removal island compared to a predator free island (coastal areas only: removal: 2–5 lizards/100 trap days; predator free: 16–49), but in 1988–1992, abundance was similar in two (1988: 14–15; 1992: 60–69) and higher in two years (1990–1991: removal: 61–67; predator free: 20–36; no data collected in 1989 on predator free island). Rats and rabbits were eradicated in 1986–1987 from one island (rodenticide and shooting) and a nearby island was historically free of invasive mammals. Lizards were counted using pitfall traps along four transects (20 traps/transect, two each in coastal and forested areas) on the removal island (March and November 1986–1993), and on the predator free island (November 1986–1992).

   Study and other actions tested

   Referenced paper

   Towns D.R. (1994) The role of ecological restoration in the conservation of whitaker’s skink (Cyclodina whitaken), a rare new zealand lizard (lacertilia: Scincidae). New Zealand Journal of Zoology, 21, 457-471.

3. A controlled, before-and-after study in 1990–1994 in mixed heath and dune habitat in Western Australia, Australia (Risbey et al. 2000) found that where cats Felis catus and foxes Vulpes vulpes, or just foxes were controlled, captures of reptiles did not increase. The number of geckos and skinks were similar in areas with cat and fox control, fox control only or no control (geckos: 1–2 individuals/trap grid; skinks: 2 individuals/trap grid). Dragon lizard numbers were lower in areas with greater predator control (cat and fox control: 2 individuals/trap grid; fox control only: 5 individuals/trap grid; no predator control: 7 individuals/trap grid). In areas with predator control, there was no clear change in reptile numbers from before control began (0–24 individuals/group/year) compared to the three years after control began (0–12 individuals/group/year). In 1991, a mainland peninsula was divided into three areas: one area (12 km²) where cats and foxes were controlled (using electrified fencing, poison baiting, or secondary poisoning by poisoning European rabbits Oryctolagus cuniculus, trapping or shooting); one area (120–200 km²) where foxes were controlled by baiting but cats were not targeted; and one area where no control occurred. Reptiles were monitored with six pitfall-trap and drift fence grids in each area (18 in total). Each grid had eight pitfall traps, 30–50 m apart. Sampling was conducted over three consecutive days in March–April and June–July in 1990–1994 in predator control areas and 1992–1994 in the area without predator control. Reptiles captured included dragon lizards, skinks, geckos, snakes, and a species of monitor lizard and blind snake but only species that could be toe clipped (dragon lizards, skinks and geckos) were included in analysis.

   Study and other actions tested

   Referenced paper

   Risbey D.A., Calver M.C., Short J., Bradley J.S. & Wright I.W. (2000) The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. II. A field experiment. Wildlife Research, 27, 223-235.

4. A study in 2003 on an island offshore of East-Kalimantan, Indonesia (Meier et al. 2003) found that carrying out lethal control of black rats Rattus rattus using poison baits did not have detrimental effects on monitor lizards Varanus salvator. No illness or mortality was recorded in any monitor lizards. The last living rat was observed 5–6 months following the deployment of poison baits. In February 2003, a 25 x 25 m grid was established across the whole island and a bait station placed in each grid square. An additional 23 bait stations were established around the perimeter of the island. Blocks of rodenticide (Klerat®) were deployed at each bait station from the 7th April 2003 and replaced as needed. All non-target species were monitored throughout the baiting period. 

   Study and other actions tested

   Referenced paper

   Meier G. (2003) Eradication of invasive rats on Sangalaki-Island, East-Kalimantan: Part of a project for marine turtle conservation. InGrip-Consulting & Animal Control report, number 1.

5. A replicated, site comparison study in 1999–2000 in two sites of semi-arid shrubland, grasses and sparse woody plants in New South Wales, Australia (Olsson et al.) found that an area with long term poison baiting for foxes Canis vulpes had more sand goannas Varanus gouldii compared to an un-baited area, but effects on small lizards were mixed. More goannas were found in the area with fox baiting (52 individuals) compared to the un-baited area (9 individuals). Overall, small lizard abundance was similar between the baited area (0.4 lizards/trap) and un-baited area (0.5 lizards/trap), but in one of three habitat types lizards were less abundant in the baited (0.2/trap) compared to un-baited area (0.4/trap). Skinks were more abundant in the baited areas in one of three habitat types (baited: 0.6/trap; un-baited: 0.2/trap) and geckos were less abundant in baited areas in one of three habitat types (baited: 0.1/trap; un-baited: 0.5/trap), but in all other comparisons abundances were similar. At one site, poison baiting (1080-bait) started in 1995 along roads, and from 1997 three aerial baitings/year were also carried out. An additional site (75 km away) received no baits. In 2000, two areas in each site (baited area: >20 km apart; un-baited area: 15 km apart) were surveyed for sand goannas from a vehicle (580–590 km/site). In November–December 1999 (11 days), at each area in both sites, small reptiles were trapped across three habitat types (grassland, mallee/woodland and spinifex), with three traps lines/trapping location (15 m drift fence, with 5 pit-fall traps).

   Study and other actions tested

   Referenced paper

   Olsson M., Wapstra E., Swan G., Snaith E., Clarke R. & Madsen T. (2005) Effects of long-term fox baiting on species composition and abundance in an Australian lizard community. Austral Ecology, 30, 899-905.

6. A before-and-after study in 1992–1996 on a Pacific island off the east coast of North Island, New Zealand (Parrish et al. 2005) found that following Pacific rat Rattus exulans eradication, reptile abundances increased. After Pacific rats were eradicated, abundances of skinks, geckos (including the Duvaucel’s gecko Hoplodactylus duvaucelii) increased (no data are provided). Monitoring after the poison-bait was deployed revealed no signs of rats on the island. Rats were eradicated using aerial-deployed rodenticide bait on Lady Alice Island (145 ha) in 1994 (8 kg brodifacoum-impregnated bait/ha). Reptile monitoring started in 1992, two years before rat eradication and continued for at least two years afterwards. Skinks and geckos were surveyed using pitfall traps.

   Study and other actions tested

   Referenced paper

   Parrish R. (2005) Pacific rat Rattus exulans eradication by poison-baiting from the Chickens Islands, New Zealand. Conservation Evidence, 2, 74-75.

7. A before-and-after study in 1999–2003 on a tropical island in the Galápagos, Ecuador (Phillips et al. 2005) found that during an ongoing iguana reintroduction, more offspring of Galapagos land iguanas Conolophus subcristatus were captured following a successful cat Felis catus eradication program. Results were not statistically tested. The number of offspring of reintroduced iguanas captured was higher after most cats were eradicated (1–14 adults and 6–14 sub-adults and juveniles/year) than before eradication began (0–1 adults and 4–6 sub-adults and juveniles/year). The number of reintroduced iguanas that were recaptured varied throughout the study (after most cats eradicated: 21–32 individuals/year; before eradication: 17–30 individuals/year). Reintroduction efforts were ongoing through the study, with six releases totalling 183 individuals during 1991–2003. In 2001–2003, cat eradication was carried out with poison baits (1080 poison), trapping and shooting, and cats were considered eradicated by 2003. Iguanas were surveyed (6 days in June–July) before (1999–2000) and after (2002–2003) the majority of cat eradication had been completed.

   Study and other actions tested

   Referenced paper

   Phillips R.B., Cooke B.D., Campbell K., Carrion V., Marouez C. & Snell H.L. (2005) Eradicating feral cats to protect Galapagos land iguanas: methods and strategies. Pacific Conservation Biology, 11, 257-267.

8. A before-and-after study in 1995–2004 in coastal forest on Great Bird Island, Antigua (Daltry 2006) found that eradicating black rats Rattus rattus increased the abundance of Antiguan racer snakes Alsophis antiguae. No statistical tests were carried out. The snake population doubled in 2 years after rat eradication compared to before eradication (pre-eradication population estimate: 51 snakes; 2 years post-eradication estimate: 115 snakes) and, although there were between year fluctuations, the snake population remained greater than pre-eradication (population estimates 2–9 years post-eradication: 78–161 snakes). The author reported rat eradication took place on Great Bird Island (10 ha) over three weeks in late 1995. The island was checked monthly for signs of rats after the eradication program ended. In total, 730 bait stations with the rodenticide brodifacoum dispensed in wax blocks were placed in a 10 x 10 m grid across the whole island. Rats were also eradicated from two neighbouring islands. Snakes were surveyed over six weeks in 1995 before rat eradication and annually after rat eradication in 1997–2004. Snake population estimates were calculated using mark-recapture of individual snakes. Rat eradication programme details were sourced from an associated article (Daltry 2006).

   ^{Daltry J. (2006) Control of the black rat Rattus rattus for the conservation of the Antiguan racer Alsophis antiguae on Great Bird Island, Antigua. Conservation Evidence, 3, 28–29.}

   Study and other actions tested

   Referenced paper

   Daltry J.C. (2006) The effect of black rat Rattus rattus control on the population of the Antiguan racer snake Alsophis antiguae on Great Bird Island, Antigua. Conservation Evidence, 3, 30-32.

9. A before-and-after study in 1995–1998 on a tropical island, western Mexico (Rodríguez et al. 2006) found that following eradication of cats Felis catus, the abundance of black iguana Ctenosaura pectinata and Clark’s spiny lizard Sceloporus clarkii increased. The authors reported that after the start of the cat eradication programme, black iguana abundance doubled or quadrupled and that the Clark’s spiny lizard was more frequently observed. In 1995–1998, cats (113 individuals/km²) were eradicated from Isla Isabel (194 ha), by trapping, poisoning (with 1080 sodium monofluoroacetate) and shooting. An attempted eradication of black rats Rattus rattus at the same time, using brodifacoum poisoning failed. No details on reptile monitoring were provided.

   Study and other actions tested

   Referenced paper

   Rodríguez C., Torres R. & Drummond H. (2006) Eradicating introduced mammals from a forested tropical island. Biological Conservation, 130, 98-105.

10. A study in 2009–2012 in an area of mixed shrub and grassland in Otago, New Zealand (Norbury et al. 2014) found that survival of captive-bred Otago skinks Oligosoma otagense released into an enclosure was higher for those released when house mice Mus musculus had been eradicated compared to when skinks were released in the presence of mice. Authors reported that post-release survival was higher for skinks released with no mice present (44%) compared to survival of skinks released just prior to reinvasion by mice (15%). Survival of established skinks (2 years after their release) after the mouse reinvasion was higher (91%) than for newly released skinks in the presence of mice (17%). In 2009, a 0.3 ha area was enclosed within a mammal resistant fence (1.9 m high) and over a six-month period, all mammals inside the enclosure were eradicated using a range of baited traps. After eradication, 12 captive-bred adult skinks were released in the enclosure following eight weeks in quarantine. In 2011, an additional 16 skinks were quarantined and released. House mice reinvaded during 2012 and were again eradicated using live capture traps and poison bait stations. In 2009–2012, starting 7–10 days after release, skinks were monitored every 15 days by a walking survey of the enclosure.

    Study and other actions tested

    Referenced paper

    Norbury G., Van Den Munckhof M., Neitzel S., Hutcheon A., Reardon J. & Ludwig K. (2014) Impacts of invasive house mice on post-release survival of translocated lizards. New Zealand Journal of Ecology, 38, 322-327.

11. A before-and-after study in 1994–2013 on an offshore cay in San Salvador, Bahamas (Hayes et al. 2016) found that using rodenticides to control invasive black rats Rattus rattus did not increase the abundance of San Salvador rock iguana Cyclura rileyi rileyi. Results were not statistically tested. The authors reported that following the eradication of black rats, abundance of San Salvador rock iguanas did not increase (population estimate after rat eradication: 28–159 iguana; population estimate before 36–144 iguanas). Black rats were controlled using rodenticide (brodifacoum) administered in wax blocks in covered bait stations in 1999 and 2000 (see original paper for detailed methods) on an island (25 acres). In summer 1999, the eradication attempt failed due to bait station design. Eradication was considered successful in summer 2000. San Salvador rock iguana were surveyed across the whole island in 1994, 1998–2007 and 2012–2013 (3 years before eradication and 9 years after eradication) using visual encounter surveys.

    Study and other actions tested

    Referenced paper

    Hayes W.K., Cyril Jr S., Crutchfield T., Wasilewski J.A., Rothfus T.A. & Carter R.L. (2016) Conservation of the endangered San Salvador rock iguanas (Cyclura rileyi rileyi): population estimation, invasive species control, translocation, and headstarting. Herpetological Conservation and Biology, 11, 90-105.

12. A study in 2012–2014 on an island in the Galápagos, Ecuador (Rueda et al. 2016) found that controlling black rats Rattus rattus using anticoagulant rodenticides lead to widespread secondary exposure to rodenticides in endemic lava lizards Microlophus duncanensis, although no population level impacts were observed. Rodenticide was detected in livers of 270 lizards (brodifacoum concentrations: 10 ppb–2000 ppb in individual livers) and was still being detected up to 850 days after the baiting took place. The authors noted that the secondary exposure of lizards to rodenticides was implicated in the exposure and mortality of 22 Galapagos hawks Buteo galapagoensis. Black rat eradication commenced on Pinzon Island (1,815 ha, tropical forest and savanna) in 2012 using aerial deployment of brodifacoum bait (25 ppm). Lizards were trapped for rodenticide testing. The authors reported that Pinzon giant tortoise Chelonoidis ephippium hatchling survival increased after rat eradication (see original paper for details).

    Study and other actions tested

    Referenced paper

    Rueda D., Campbell K.J., Fisher P., Cunninghame F. & Ponder J.B. (2016) Biologically significant residual persistence of brodifacoum in reptiles following invasive rodent eradication, Galapagos Islands, Ecuador. Conservation Evidence, 13, 38-38.