Action

Remove or control predators using lethal controls: Tortoises, terrapins, side-necked & softshell turtles

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Seven studies evaluated the effects of removing or controlling predators using lethal controls on tortoise, terrapin, side-necked and softshell turtle populations. Four studies were in the USA and three were in Australia.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (7 STUDIES)

  • Reproductive success (7 studies): Six of seven studies (including four replicated, controlled studies) in Australia and the USA found that in areas with mammal or fire ant control, and in two cases with fencing, fewer tortoise, turtle and terrapin nests were predated compared to areas with no control, or before control began. Two studies also found that predation increased again a year after control or in the second year of control. The other study found that following short-term fox control, a similar number of artificial eastern long-necked turtle nests were predated by foxes compared to before control began.
  • Survival (3 studies): Two of three replicated, controlled studies (including one before-and-after study and one randomized study) in Australia and the USA found that in a fenced area with mammal or fire ant control, more gopher tortoise hatchlings survived for one year or at least 150 days compared to fenced areas with no control. The other study found mixed effects of fox control on survival of Murray short-necked turtles and broad-shelled turtles depending on turtle species, age and sex.

BEHAVIOUR (1 STUDY)

  • Behaviour change (1 study): One replicated, controlled, before-and-after study in Australia found that in areas with fox control, freshwater turtles nested further from the water and nests were more spread out compared to areas with no control, or before control began.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, controlled, before-and-after study in 1996–2000 in an area with four lagoons in south-eastern Australia (Spencer 2002, same experimental set-up as Spencer & Thompson 2005) found that removing foxes Vulpes vulpes resulted in lower predation rates of Murray short-necked river turtle Emydura macquarii nests and changes in nesting behaviour compared to when foxes were present. Nest predation was lower after fox removal (<50% nests predated) compared to areas with no fox removal (>85%) and before fox removal started (85–93% of 12–29 nests predated). Following fox removal, turtles nested further from the water (25–26 m) compared to before removal and no-removal sites (14–19 m), and nests were more spread out (removal: 12–16 m between nests; no removal: 8–11 m). In May 1997 to January 1999, fox control was carried out at two lagoons by burying poison baits (35 g FOXOFF baits) along fence lines, hill ridges and access roads (150–200 m apart, 48 baits/site; laid every 1–2 months) and shooting foxes. A further two lagoons had no fox removal. Searches for turtle nests were conducted in November 1996–1998.

    Study and other actions tested
  2. A replicated, controlled, before-and-after study in 1996–2000 in an area with four lagoons in south-eastern Australia (Spencer & Thompson 2005, same experimental set-up as Spencer 2002) found that removing foxes Vulpes vulpes resulted in higher nesting success of Murray short-necked river turtles Emydura macquarii and broad-shelled turtles Chelodina expansa and higher survival of female short-necks compared to areas with no removal. Nest predation was lower after fox removal (short-necked turtles: <50% nests predated; broad-shelled: 18–38% predated) compared to areas with no fox removal (short-necked: >85% predated; broad-shelled: 57 and 50% of 7 and 10 nests) and before fox removal started (short-necked: 85–93% of 12–29 nests predated; 55–70% of 8–11 nests) (results were not statistically tested). Survival of female short-necks was higher following fox removal (97–98% survival) compared to areas with no removal (93–95%) and before fox removal (94 and 95%), though no effects of fox removal were found for short-neck males (95–99% survival), juveniles (69%), or any group of broad-shelled turtles (84–92%). In May 1997 to January 1999, fox control was carried out at two lagoons by burying poison baits (35 g FOXOFF baits; 150–200 m apart; 48 baits/site, laid every 1–2 months) and shooting foxes. A further two lagoons had no fox removal. In 1996–1998, searches for turtle nests were conducted in late autumn and trapping was conducted every 14–18 days in September–March.

    Study and other actions tested
  3. A before-and-after study in 1997–2000, and 2005–2006 on an intracoastal island in Florida, USA (Munscher et al. 2012) found that removing raccoons Procyon lotor resulted in reduced predation of Carolina diamondback terrapin Malaclemys terrapin centrata nests compared to when no removal was carried out. Nest predation was lower in the year with racoon removal (7 of 93, 8% of nests predated) than in years prior to removal (1997: 61 of 114, 54%; 2000: 57 of 112, 51%). Predation increased again in the year after removal (39 of 45, 87%). Racoons were trapped daily from February to April 2005, and then intermittently until September 2005 using live traps (23 raccoons removed). Raccoons were anesthetised and euthanised by lethal injection. Beaches were searched daily from April–October for signs of nesting turtles, and these nests were then monitored daily for signs of predation and emerging young.  

    Study and other actions tested
  4. A replicated, controlled study in 2002–2005 in a pine forest in Georgia, USA (Smith et al. 2013) found that removing predators from fenced exclosures resulted in higher survival of gopher tortoise Gopherus polyphemus nests and hatchlings compared to areas with no fencing or predator removal. The effects of predator removal (lethal controls and relocations) and fencing cannot be separated. Survival was higher inside fenced areas with predator removal compared to outside for both nests (fenced: 52 of 78, 66% survived; unfenced: 26 of 73, 35%) and hatchlings (fenced: 74% survived for 1 year; unfenced: 38%). In 2002–2003, four plots (40 ha) were randomly selected and enclosed in 1.1m high mesh fence with electrical wires at the top and bottom. A further four plots were left unfenced.  In 2002–2003, all mammalian predators within the exclosures were live-trapped and relocated, and in 2003–2005, further trapping of predators was conducted. Predators that re-entered exclosures were euthanized. In May–June 2003–2005, all tortoise burrows were searched for nests, and all active nests were monitored 1–2 times/week up to 110 days. In 2004, forty hatchlings from 13 different nests were fitted with radio transmitters and monitored for up to a year.

    Study and other actions tested
  5. A replicated, randomized, controlled study in 2014–2015 in mixed forest and agricultural land in Georgia, USA (Dziadzio et al. 2016) found that when fire ants Solenopsis invicta were controlled with insecticide, gopher tortoise Gopherus polyphemus nests were not predated and hatchling survival rates increased. None of 16 gopher tortoise nests were predated by fire ants in insecticide-treated enclosures, compared to eight of 16 nests in untreated enclosures. Hatchling survival was higher in insecticide-treated enclosures (16 of 16 individuals survived at least 150 days) compared to untreated enclosures (5 of 16 individuals survived; five were killed by fire ants and six by raccoons Procyon lotor). Fire ant abundance were reduced in insecticide-treated enclosures (0.3–10.0 fire ants) compared to unmanaged enclosures (122–537 fire ants). In May–June 2014, wild-laid gopher tortoise nests were relocated to eight fenced 0.2 ha enclosures (four nests/enclosure, two eggs/nest, 64 total eggs). All nests were covered with cloth cages (30 x 30 x 12 cm). Four of eight enclosures were treated with Amdro® insecticide (1.7 kg/ha) to reduce fire ant numbers. Fire ants were monitored using baited traps. Nests were monitored weekly until two weeks before expected emergence, daily thereafter and excavated after 120 days. Hatchlings were radio tracked (16 individuals each from insecticide-treated and untreated enclosures) from August 2014 to March 2015.

    Study and other actions tested
  6. A before-and-after study in 2014–2015 around four lakes in northwest Victoria, Australia (Robley et al. 2016) found that short-term fox Vulpes vulpes control did not reduce predation on artificial eastern long-necked turtle Chelodina longicollis nests. The number of artificial nests predated by foxes was similar following short term fox control (78 of 95, 82% of nests) compared to before control (59 of 70, 84%). In November 2014, twenty-one days of fox control was implemented by burying baits (1080/sodium monofluoroacetate) across 175 bait stations (25,000 ha site). Artificial nests were randomly placed around 14 sites along the shores of four lakes in sandy soil, 5–30 m from the lake's edge (70 nests pre-control; 95 nests post-control). Each nest consisted of a hand-dug boot-shaped chamber 10–15 cm deep with five quail eggs sprayed with water from captive turtle ponds and covered with sand and surface litter. Nests were inspected four times (up to 35–41 days after construction) for signs of predation by foxes.

    Study and other actions tested
  7. A before-and-after study in 2013–2014 in mixed sandy grassland, woodland and marsh habitats along the Illinois-Wisconsin state borders, USA (Urbanek et al. 2016) found that removing raccoons Procyon lotor led to less Blanding’s turtle Emydoidea blandingii nest predation in the first year, but not in the second year, of predator management. Results were not statistically tested. In 2013, one of seven (14%) Blanding’s turtle nests were partially predated and no nests were completely predated. In 2014, nine of 15 (60%) of turtle nests were predated (one partially, eight completely predated). The authors reported that before predator management, 12 of 13 (92%) of turtle nests and 88% of monitored artificial nests were predated (see original paper for details). In April-May 2013 and 2014, a total of 78 raccoons (an estimated 83–89% of the total population) were trapped and euthanized in a designated nature reserve and adjacent areas (338–389 ha). In 2013–2014, twenty-two gravid female turtles were captured and monitored closely until egg laying using radio-telemetry. Nests were marked and monitored daily for evidence of excavation or predation. Turtle nests were not protected.

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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Reptile Conservation

This Action forms part of the Action Synopsis:

Reptile Conservation
Reptile Conservation

Reptile Conservation - Published 2021

Reptile synopsis

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