The order Squamata, which includes lizards, is one of the most successful and diverse groups of vertebrates on Earth, encompassing over 7,000 known species globally. Lizards display an immense variety of forms, behaviors, and ecological roles. While many lizard populations remain stable, a significant and growing number are facing severe threats to their survival. The conservation status varies dramatically from species to species, but a large percentage of these reptiles are now categorized as facing a high risk of extinction, driven by human activities.
How Conservation Status is Determined
The global standard for assessing a species’ risk of extinction is the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. This system provides an objective framework for classifying species based on factors like population decline rate, geographic range size, and the probability of extinction. The assessment is conducted on an individual species level, meaning a single conservation status cannot be applied to the entire group of lizards.
The Red List uses a spectrum of categories ranging from Least Concern (LC) to Extinct (EX). The categories of most concern are Vulnerable (VU), Endangered (EN), and Critically Endangered (CR), which are collectively known as “threatened” categories. Critically Endangered species face an extremely high risk of extinction in the wild. Species for which there is insufficient data to make an assessment are classified as Data Deficient (DD), which is a common issue for many smaller, less-studied lizard species.
Primary Drivers of Lizard Population Decline
Habitat destruction and fragmentation represent the largest threat to lizard populations worldwide. The conversion of natural landscapes for agriculture, urban expansion, and resource extraction eliminates the shelter, food sources, and breeding grounds lizards require to survive. This destruction often isolates remaining populations into small, disconnected patches of habitat, preventing genetic exchange and making them more vulnerable to localized extinction events.
A second major mechanism of decline is the impact of global climate change, particularly for lizards that use temperature-dependent sex determination (TSD). For species with TSD, the sex of the offspring is determined by the temperature experienced during egg incubation. Research indicates that an increase in mean temperature can severely skew the sex ratio of hatchlings, potentially leading to a population composed almost entirely of one sex. This sex ratio imbalance can cause a rapid decline in reproductive success and result in population collapse.
Invasive species pose a third serious threat, especially to island-dwelling lizards that evolved without large predators. Introduced predators, such as feral cats, dogs, and rats, are highly effective hunters that prey on native lizards, their eggs, and their young. Invasive competitors, such as other lizard species or non-native herbivores, can also negatively affect native populations by outcompeting them for limited resources.
Critically Threatened Species Spotlights
The Grand Cayman Blue Iguana (Cyclura lewisi) is a classic example of a species that faced near extinction and is currently listed as Endangered by the IUCN. Endemic to Grand Cayman, its wild population was reduced to as few as 10 to 25 individuals in 2002, making it functionally extinct. Habitat destruction from land development and predation by introduced feral cats and dogs were the primary drivers of this collapse.
A dedicated conservation program, the Blue Iguana Recovery Programme, successfully implemented a captive breeding and “head-starting” strategy. This involves raising hatchlings in a protected environment until they are large enough to avoid most feral predators before being released into protected reserves. Due to these intensive efforts, the species was downlisted from Critically Endangered to Endangered in 2012, and the wild population has successfully rebounded to over 1,000 individuals.
Another species facing peril is the Fiji Crested Iguana (Brachylophus vitiensis), which is listed as Critically Endangered. Found only on a few islands in Fiji, this lizard is threatened primarily by habitat fragmentation and the presence of the Small Indian Mongoose, an invasive predator introduced to the islands. The mongoose preys heavily on the iguanas and their eggs, devastating populations.
Conservation efforts for the Fiji Crested Iguana focus on eradication programs for the mongoose on key islands and the establishment of protected sanctuaries with strict biosecurity protocols. The species is also vulnerable to the impacts of extreme weather events, such as cyclones, which can wipe out entire small island populations. These case studies highlight the need for tailored intervention to secure a future for the world’s most threatened lizard species.