Remove or control predators using lethal controls: Sea turtles

Overall effectiveness category Awaiting assessment
Number of studies: 4
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Background information and definitions

Predators can drive declines or local extinctions of vulnerable reptile species. Non-native predators may be a particular problem for native reptiles that lack sufficient predator avoidance behaviours. Native predators can also threaten populations of reptiles that persist in low numbers. Removing or controlling predators, especially native predators, for the benefit of their wild prey species can be a controversial management strategy. Nonetheless, there is potential for such management to lead to increases in the abundance, survival or reproductive success for species of conservation concern.

Due to the number of studies found, this action has been split by species group, though no studies were found for amphisbaenians.

Study locations

Key messages

Four studies evaluated the effects of removing or controlling predators using lethal controls on sea turtle populations. All four studies were in the USA.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (4 STUDIES)

Reproductive success (4 studies): Two before-and-after studies (including one controlled study) in the USA found that on islands where raccoons and feral pigs or only feral pigs were eradicated, fewer loggerhead and loggerhead and green turtle nests were predated than before predator control began. One replicated, randomized, controlled study in the USA found that controlling raccoons on short sections of a beach resulted in similar predation of loggerhead turtle nests compared to in sections of the beach with no control. One before-and-after study in the USA found that disruptions to a programme controlling raccoons and armadillos resulted in more predation of loggerhead, leatherback and green turtle nests.

BEHAVIOUR (0 STUDIES)

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

A replicated, randomized, controlled study in 1993–1994 on a long sandy beach in Florida, USA (Ratnaswamy et al. 1997) found that raccoon Procyon lotor control on sections of beach did not reduce predation of loggerhead turtle Caretta caretta nests compared to sections with no control. Predation of turtle nests was similar in sections of the beach with raccoon control (1993: 489 of 1,359 nests, 36% predation; 1994: 460 of 1,686, 27%) and those with no control (1993: 72 of 231 nests, 31%; 1994: 92 of 379, 24%). In 1993–1994, a long stretch of barrier beach (37 km) was broken down into four experimental blocks, and around half of each block (4 km) was selected for raccoon removal. Raccoons were captured with live traps baited with sardines, anesthetised and lethally injected (215 individuals; estimated as 50% of raccoon population). Nests in the remainder of the block were either received no treatment or were part of further tests of the effect of nest screening and taste aversion on predation. Nests were monitored 2–4 times/month in 1993 and 2–3 times/week in 1994.
Study and other actions tested
Referenced paper
Ratnaswamy M.J., Warren R.J., Kramer M.T. & Adam M.D. (1997) Comparisons of Lethal and Nonlethal Techniques to Reduce Raccoon Depredation of Sea Turtle Nests. Journal of Wildlife Management, 61, 368-376.
A before-and-after study in 2002–2004 on a sandy beach in Florida, USA (Engeman et al. 2006) found that disruptions to the control of raccoons Procyon lotor and invasive armadillos Dasypus novemcinctus resulted in reduced survival of loggerhead Caretta caretta, leatherback Dermochelys coriacea and green turtle Chelonia mydas nests due to predation. In 2002–2004, months when predator control was consistent across all years (May), nest survival was similar (>80% after 80 days). However, disruptions to control in June 2004 resulted in lower survival for nests laid in June–July 2004 (60–70% after 60–80 days) compared to June–July 2002–2003 (>80% after 60–80 days). In 2002–2004, raccoons were live trapped and killed, and both raccoons and armadillos were shot (0.22 calibre rifle with a noise suppressor and night-vision equipment). In 2004, predator control ceased for 2 weeks in June, re-started in July, and then ended completely in August. The beach was monitored daily starting in March 2002–2004, and all leatherback and green turtle nests were marked, but only every eighth loggerhead nest marked and monitored. Marked nests were monitored daily for predation and excavated after hatchling emergence to assess hatching success.
Study and other actions tested
Referenced paper
Engeman R.M., Martin R.E., Smith H.T., Woolard J., Crady C.K., Constantin B., Stahl M. & Groninger N.P. (2006) Impact on predation of sea turtle nests when predator control was removed midway through the nesting season. Wildlife Research, 33, 187-192.
A controlled, before-and-after study in 2006–2008 on two barrier islands in Florida, USA (Engeman et al. 2010) found that eradication of racoons Procyon lotor and feral pigs Sus scrofa resulted in lower predation rates of loggerhead turtle Caretta caretta nests compared to sites with no predator control. Predation of sea turtle nests was lower following predator removal (0–16% of 2–143 nests) compared to before predator removal began (60–84% of 20–76 nests). Two study islands were established with no predator control taking place in 2006. Control of raccoons and feral swine began in 2007 on one island and in 2008 on the other. Raccoons and pigs were captured in baited traps, and free-roaming pigs were also shot with a noise suppressed rifle. Sea turtle nests were monitored by patrolling beaches in the morning during the turtle nesting season noting new nests and evidence of predation.
Study and other actions tested
Referenced paper
Engeman R.M., Duffiney A., Braem S., Olsen C., Constantin B., Small P., Dunlap J. & Griffin J.C. (2010) Dramatic and immediate improvements in insular nesting success for threatened sea turtles and shorebirds following predator management. Journal of Experimental Marine Biology and Ecology, 395, 147-152.
A before-and-after study in 2007–2010 on a sandy beach on an island off the coast of Florida, USA (Engeman et al. 2016) found that eradicating feral pigs Sus scrofa ended pig predation of unhatched loggerhead Caretta caretta and green turtle Chelonia mydas nests. In the nesting season after feral pigs were eradicated, no marine turtle nests were lost to pig predation, compared to 50 of 50 nests predated by pigs in the nesting season prior to their eradication, including 36 nests covered with nest cages. In 2005–2010, turtle nests were monitored daily throughout the nesting season on a 13.4 km long stretch of beach on an island (526 ha). In 2007, nest predation by pigs was observed for the first time (pigs were present on the island from 2001). In May–June 2008, a total of 39 feral pigs were eradicated from the island by trapping and shooting over bait. Following eradication, no spoor or other signs of feral pigs were found. The authors reported that pigs reinvaded the island in 2014. Thirty-six of the 50 nests monitored had been covered by partially buried cages (91 cm long x 91 cm wide x 76 cm tall, 5 x 10 cm wire mesh) to protect them from raccoon predation. After hatching, all nests were excavated to record hatching success and predation levels. See ‘Use nest cages’ for more details on their effectiveness.
Study and other actions tested
Referenced paper
Engeman R.M., Addison D. & Griffin J.C. (2016) Defending against disparate marine turtle nest predators: Nesting success benefits from eradicating invasive feral swine and caging nests from raccoons. Oryx, 50, 289-295.

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?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Reptile Conservation

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