by Isle Bastille
figures by Allie Elchert

In an episode of the BBC show Planet Earth there is a harrowing scene depicting thousands of freshly hatched baby sea turtles scuttling away from the sea towards a busy highway. A somber voice-over relays that the hatchlings are misguided by the nearby city lights. Paradoxically, while this was a human-driven habitat disruption, the turtles will not survive without human intervention. Human-related disruptions in natural habitats and rapid climate change are driving many species towards extinction, causing a drastic decline in biodiversity across the world. What if there was a way to replenish endangered populations?

A conservation technique known as assisted colonization involves releasing animals raised in captivity or relocating animals from other geographical areas to replenish endangered populations (Figure 1). This technique has brought several species back from the brink of extinction. For example, in the early 1980s, the Cayman Island green sea turtle (Chelonia mydas) was considered nearly extinct due to overfishing. In 1983, a reintroduction program rescued the species from extinction by releasing 25,000 captive-bred turtles in the Cayman Islands. However, this approach has stirred debate within the conservation community since the long-term consequences of assisted colonization are unknown. Is it possible to successfully and ethically implement assisted colonization to rescue certain endangered populations? For the sake of our misguided turtles, let’s hope so.

What are the risks of assisted colonization?

Several experts argue that assisted colonization is too risky to be a viable long-term conservation technique. A chief concern is the potential threats to ecosystem stability. Food chains have evolved over thousands of years to maintain balanced populations of predators and prey, and introduced animals could cause unintended disturbances to this balance. One way this could happen is if an introduced species becomes invasive, multiplying out of control and expanding outside of the intended habitat. For example, Cane toads were introduced to Australia in the 1930s. This release was not intended as an assisted colonization, but rather as a conservation effort to control beetle populations that were destroying sugar cane crops. But it didn’t work: the Cane toads do not eat adult beetles and couldn’t access beetle larvae underground. Meanwhile, the lack of viable natural predators allowed the initial 101 toads to multiply out of control into millions of toads and spread across Australia. Toad poisoning caused native predatory bird populations to decrease by fifty percent which resulted in the unregulated expansion of natural prey populations, such as the crimson finch. In an unexpected turn of events, the introduction of the Cane toad had long-lasting ripple effects in the local ecosystem that eventually pushed several species to the brink of extinction.

While no known cases of invasiveness have been observed during assisted colonization efforts, the Cane toad release highlights the need for careful design of animal relocations since similar unintended consequences could occur in assisted colonization releases. Without sufficient monitoring, invasiveness or pathogens carried on released animals could cause swift, destabilizing effects that affect the whole ecosystem (Figure 2).

Assisted colonizations can also negatively impact genetic diversity since released populations can be small and genetically homogeneous. Genetic diversity in a population is critical for the long-term viability of a species, granting two distinct advantages over a genetically homogenous population. Firstly, the capacity for adaptive evolution is higher; a genetically diverse population has more gene variants, and therefore more traits to “choose” from, for adapting to environmental pressures. Secondly, deleterious mutations are less likely to stick around in a diverse population. This is due to the availability of alternative gene variants that can be selected over the harmful mutations (Figure 2).

In the 1970s, the Mauritius Kestrel population plummeted to only four individuals, causing a drastic decrease in the population’s genetic diversity. As a response, a conservation program released a small number of captive bred kestrels in 1987. Although the population has recovered into the hundreds, the low genetic diversity has surpassed a critical threshold by which natural selection will not remove harmful mutations from the gene pool. This result makes it unlikely that the population can persist without continued human intervention.

Finally, the ability of the introduced animals to acclimate to the wild is also a concern since released populations are often captive-bred (Figure 2). In 2010 and 2011, over four thousand captive-bred and relocated Boreal Toad tadpoles were released in Rocky Mountain National Park in Colorado. Of these, only 4-8% survived to become toads. This low survivability raises animal welfare concerns since it causes preventable suffering of the released tadpoles. It was soon discovered that tadpoles had a higher chance of surviving if they were slightly older when released.

Is assisted colonization a viable conservation method?

These risks have all caused several scientists to wonder about the conservation potential of assisted colonization. While assisted colonization does carry risks, a majority of experts agree that it is a critical tool for preventing extinction. Historically, very few cases have had uncontrollable effects on biodiversity. Moreover, ongoing genetic analyses and population monitoring is showing that assisted colonization can be quite successful. However, experts also emphasize that there is a critical need to establish standardized international policy for this technique to be effective and safe.

Policies aimed at regulating assisted colonization have been gradually emerging over the last decade. In 2013, the International Union for the Conservation of Nature (IUCN) published a set of guidelines for the translocations of species. These guidelines included general suggestions for planning, feasibility, and design of a translocation. While the IUCN guidelines were a good start, they lacked actionable details such as quantifiable criteria for selecting candidate populations for this approach and specific data requirements for planning, risk mitigation, and longitudinal monitoring. In 2021 at the 15th Convention on Biological Diversity, an annual conference that convenes representatives from 196 countries, several experts called for the development of more specific policies on assisted colonization. The convention highlighted the need for specific selection criteria for using assisted colonization versus other conservation solutions such as assisted reproduction, habitat restoration, and habitat protection. The convention also asked for better techniques to continuously assess outcomes of assisted colonization programs.

Several scientists have started empirically establishing selection and monitoring criteria by analyzing the long-term consequences of past assisted colonization efforts. For example, the Cayman Turtle Farm reintroduction program that rescued the green sea turtle from extinction has been controversial since it was started without any tangible policy and plans for monitoring risks to the dwindling wild and released turtles. Retrospective analyses on this reintroduction program have revealed the reasons for its success and the need for risk assessment protocols.

Researchers conducted genetic analysis and monitoring on wild and released turtles as well as their offspring. The results were encouraging. They concluded that the released turtles acclimated well to their new environment, were not displaying any signs of invasiveness, and that there was still significant genetic variation in the population. Moreover, they identified specific characteristics of these sea turtles that made them optimal candidates for assisted colonization. For example, these sea turtles return each year to nest in the place they were born. This behavior means that the turtles are unlikely to colonize new environments, making monitoring released turtles much easier. It also means the turtles are unlikely to move away from inhospitable environments on their own, underscoring the necessity for human aided relocation. The researchers emphasized that continued monitoring and regulation will ensure the longevity of this effort (read more about these analyses here and here).

Assisted colonization saved the Cayman Island green sea turtle. But just like any tool, assisted colonization is only powerful when used safely and in the right situations. Illegal harvesting, climate change, and artificial lights still pose a great threat to the continuation of the sea turtle population, and many other species are similarly threatened by human-related disruptions. These complex threats call for a diverse toolbox of conservation approaches for saving our planet’s animals.

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Isle Bastille is a graduate student in the Ph.D. Program in Neuroscience at Harvard Medical School. She studies the genetic mechanisms that regulate neuronal diversity in the inner ear. You can find her science photography blog on Instagram @mindful_photon

Allie Elchert is a Ph.D. candidate in the Biological and Biomedical Sciences program at Harvard Medical School, where she is studying transcription regulatory processes in yeast.

Cover image by Pexels from pixabay

For More Information:

• For a general overview on assisted colonization, see this review. See this review to read more about the risks and opportunities of assisted colonization. 
• To read about the original analysis done on the Cayman Island Turtle project, check out the paper here. To read the more recent analysis, check out this paper. 
• Here are the IUCN’s 2013 guidelines for reintroductions and other conservation translocations. To read more about recent policy updates and plans, check out this summary from IUCN on the 15th Convention on Biological Diversity (CoP15) and this call for global policy at CoP15.

This article is part of our special edition on diversity. To read more, check out our special edition homepage!