Monitoring, modeling and harvest management of non-native invasive green iguanas on Grand Cayman, Cayman Islands

The green iguana ( Iguana iguana ) was most likely introduced as a pet and became overabundant during the last 20 years on Grand Cayman. Because negative impacts were unmanageable (e.g., damage to buildings and other infrastructure), a harvest management strategy was developed and implemented, and over 874,252 green iguanas were removed between October 2018 and August 2019. Distance sampling surveys were conducted to estimate abundance in February 2019 and annually in August 2014–2019. Abundance estimates were used to develop a Bayesian state-space logistic model, generate the posterior distributions of population and harvest management parameters, and make future predictions of abundance for August 2020–2030. Abundance increased over fivefold between August 2014 and 2018, from an average of 254,162 to 1,319,939 green iguanas. However, after harvesting for 5 months, abundance declined to an average of 600,113 green iguanas in February 2019; and after 11 months, abundance declined to an average of 103,020 green iguanas in August 2019. Maximum population growth rate averaged 1.552, carrying capacity averaged 1,611,013, equilibrium abundance averaged 805,506, maximum sustainable total harvest averaged 628,491, and maximum sustainable harvest rate averaged 0.776. With harvest rates between 0.600 and 0.800, predicted abundance averaged 28,751 green iguanas for August 2020–2030. However, harvest mortality may have unforeseen outcomes due to the release from density dependence and overcompensation through high survival and fecundity rates. Because natural resource managers have partial control over harvesting and incomplete understanding of green iguana population dynamics, monitoring and modeling are essential to assess population response and guide harvest management decisions.