Island Eradications

New Zealand’s Dept of Conservation dropping 1080 poison over Tongariro Forest. Photo courtesy of Clyde Graf.

FAQs (Frequently Asked Questions)

What is an Island Eradication?

To “eradicate” an island usually means an attempt to eliminate an entire species of plant, animal or insect from an island ecosystem. Typically, it refers to removing animals such as pigs, goats, cats, rabbits, etc. that were brought to an island to provide a food supply or pest management for humans and have increased to the point where they are dominating and destroying the ecosystem. It also refers to animals such as rats and mice brought to the island by accident.

What methods are typically used?

Depending on the species to be removed, a variety of methodologies can be used. They include the use of chemicals on plants and insects and shooting, trapping, and poisoning for animals. One eradication practice gaining wide-spread use around the world in attempts to remove rats and mice is the use of helicopters to aerially disperse high volumes of pelletized rodenticides in an effort to kill every targeted animal.

If poisons are used, are there unintended consequences?

Yes, depending on the poison. The two types of poisons currently in use are anticoagulants which kill slowly (commonly sold as rat and mouse poisons like D-Con) and Compound 1080 (used in New Zealand for rats, mice and possums and on one Caribbean island to kill cats) cyanide and strychnine, unavailable to the general public, which kill immediately. The anticoagulants are the most commonly used around the world.  Some of them, such as a compound called brodifacoum, is capable of killing any animal and can remain a long time in the ecosystem.  This occurs because many invertebrates (crabs, insects) can eat brodifacoum and live but can kill animals eating them.  Other animals dying from the poison retain it in the carcass and will poison scavengers.  Compound 1080 is used extensively in New Zealand as an aerially-disbursed rodenticide but with questionable success.  It also remains in the ecosystem, killing many beneficial microorganisms, aquatic organisms, birds, and deer.

What typically are the results of attempts to eradicate an entire species?

If the species is large enough to shoot or trap and the island is small enough to thoroughly cover, the chances of success are good but it amazing difficult to find every single individual and destroy it, particularly if the terrain is steep and the cover is dense. If the island is large and/or inhabited, the failure rate increases exponentially.  If the island is large, it is more difficult to assure that every inch is covered.  If there are inhabitants and the targets are rats and mice, it is very difficult to place poison wherever  they are hiding and not expose people and pets to poisoning risk. There is also the fact that even if a species is completely removed, the island may be re-invaded unless strict measures are in place to prevent it.  Such “biosecurity” measures are difficult and expensive to implement and usually fail.

Are island eradications necessary?

This is the most difficult question to answer and every situation is different.  To make this decision, every island must be carefully studied to determine whether a specific animal, plant or insect is truly dominating the ecosystem in a negative way.  While the answer may be yes from the human point of view, it is also true that plants, animals and insects have been migrating around the world for as long as the world has existed.  In short, if a creature is populating an island to the point that no habitat remains for anything else, then, yes, some form of control may be justified but the method must be one that does not destroy the very ecosystem we are trying to preserve.

How many have been done around the world?

Eradications of invasive vertebrate species have a long history.  According to Keitt, et al. (2010) the first eradications were attempted some 250 years ago with the first apparently successful attempt achieved 150 years ago in Australia.  They have compiled a database that lists approximately 950 eradication attempts.  It is certain there have been a number attempted since then.

How successful are they?

“Success” is very difficult to achieve.  It implies that every targeted animal has been killed and no re-invasion has or will occur.  It is impossible to determine that every animal has been killed on a rugged and/or heavily vegetated island since comprehensive monitoring is expensive, and, if performed by the same contractor doing the eradication, may be biased toward claiming success when it has not been achieved.  Also, the possibility of re-invasion always exists, meaning that millions of dollars may be spent for nothing.  In addition, we believe that “success” should mean that the ecosystem is intact and functioning with no or very little impact to non-target animals. That is very difficult or impossible when using poisons which will, to some extent, enter the food chain. It is also true, as New Zealand has discovered, that removing one predator species may mean an explosion in the population of other predator species, further unbalancing the ecosystem.  The law of unintended consequences tends to prevail.

What is the failure rate for island eradication projects?

Given the difficulty, if not complete impossibility, of determining whether every animal of a target species has been removed and the tendency for monitoring to be short or not performed at all, it is very difficult to determine how many island eradications are truly successful.  Information available on the internet claims a range of successful projects but no one can be sure if this is true unless very thorough and ongoing monitoring is performed.  To our knowledge, this has never happened.


  1. Keitt, K. Campbell, A. Saunders, M. Clout, Y. Wang, R. Heinz, K. Newton, and B. Tershy. 2011. The Global Islands Invasive Vertebrate Eradication Database: A tool to improve and facilitate restoration of island ecosystems. Pages 74-77. In: Veitch, C.R., Clout, M.N. and Towns, D.R. (eds). Island Invasives: eradication and management. IUCN. Gland, Switzerland.