The Current Moment

The island marble butterfly (Euchloe ausonides insulanus) has a narrow, endemic range belonging currently to only the San Juan Islands. Through geographic isolation of populations, also known as allopatry, the IMB formed into a subspecies of the large marble butterfly (Euchloe ausonides). The island marble butterfly's range once included Vancouver and Gabriola Islands in British Columbia. After 90 years without any sitings, the butterfly was rediscovered in 1998. At this time, it was known to occur on San Juan and Lopez islands. Since 2005–2006, the island marble butterfly's range has contracted to a small area on the southern end of San Juan Island with nearly the entirety of the single known island marble butterfly population occurring inside at American Camp. By definition, the island marble butterfly's endangered status indicates that to the best of our knowledge, without action, this organism, which lives nowhere else in the world will become extinct.

Much work has been completed in the last 20 years by many biologists and conservationists to prevent extinction. Captive rearing and recent habitat enhancement efforts are believed to be keeping this butterfly alive. The island marble butterfly needs stewards to write a new chapter in the species's conservation history to help the butterfly survive and increase beyond its current range.

​It has become necessary to expand the island marble butterfly's habitat across public and private lands in the San Juan Islands to support stable and growing populations. This will increase the odds of survival in the face of small population size, habitat fragmentation, climate change, and other challenges. This can also promote within-species diversity.

We must act now to prevent extinction so that this rare, endemic butterfly can thrive in its habitat. ​

---

A Narrow, Endemic Range

812 acres of critical habitat were designated on San Juan Island at the time the IMB was listed as an endangered species in 2020 (red area). The island marble butterfly was made up of approximately five populations in 2005 when surveys were conducted to determine the extent of their range. Over the course of five or six years from 2006–2012, the species's range contracted to American Camp and Cattle Point. Surveys have been conducted in many areas in nearby counties where butterflies might occur, but no island marble butterflies in any life stage have been found elsewhere.​

Dispersal, A Key Process for Recovery 

IMBs are known to usually disperse less than 0.4 miles or about 2,000 feet but can fly further distances that are less than 1.2 miles. Dispersal into new habitats is important for recovery efforts.​

Draft Recovery Plan

The criteria to downlist and delist the island marble butterfly include requirements for the number of core occurrence complexes and the islands that these groups of island marble butterflies would have to inhabit. For more information on the downlisting and delisting processes, please refer to United States Fish and Wildlife Service's Island Marble Butterfly Recovery Plan. Recovery is expected to take thirty years or more. 

DOWNLISTING FROM ENDANGERED TO THREATENED
The island marble butterfly was listed as an endangered species in 2020 and currently is only known to occur on the south end of San Juan Island (see the maps above). There are conditions set to track recovery efforts that can trigger downlisting from federally endangered to threatened. There are a number of considerations in this decision, one of which is the number of core occurrence complexes (central groups of butterflies) on San Juan and Lopez Islands. To move the island marble butterfly off of the endangered species list and onto the threatened species list, there must be at least two core occurrence complexes on San Juan Island, one on Lopez, and one on San Juan, Lopez, or somewhere within the historical distribution of the species. 

DELISTING OFF OF THE THREATENED LIST
In terms of core occurrence complexes, for the island marble butterfly to be removed from both the endangered and threatened lists, there would have to be at least three core occurrence complexes on San Juan Island, two on Lopez, and one that is either on San Juan, Lopez, or within the historical distribution.  

​Life History     Threats     Conservation      More Information    

​Home     About Us     Events     Add Habitat     Donate