Nearly a year ago, we told our readers about a study of 371 publications testing the hypothetical assumptions of invasion biology, aptly entitled “Support for major hypotheses in invasion biology is uneven and declining.” The hypothesis least studied was “island susceptibility” which predicts that invasive species are more likely to become established and have major ecological impacts on islands than on continents. This hypothesis has considerable intuitive appeal because species that evolve in isolation are more likely to diverge from their ancestors in response to specific conditions which predicts a large number of endemic species unique to that island. In fact, the results of 9 studies of the island susceptibility hypothesis found that only 11% of those studies supported the hypothesis.
Subsequently, we were able to tell our readers about two specific examples of studies of islands where introduced species significantly increased biodiversity and benefited native species. In Puerto Rico, Ariel Lugo found that non-native trees were restoring depleted agricultural soils and providing shelter to native trees. In Hawaii, Joe Mascaro found that non-native trees were performing the same ecological functions as native trees and that non-native trees capable of nitrogen-fixing were creating soil on barren lava flows, greatly accelerating revegetation.
Ascension is another example of an island on which biodiversity has been increased significantly by the introduction of non-native species. Ascension is in the Atlantic Ocean, roughly midway between South America and Africa. It has therefore been crucial for hundreds of years to travelers to resupply their long voyages. The British claimed it as a naval base in 1812. It was vital during World War II as a base for both American and British warships and submarines. It now has a huge air strip used by American and British military.
Ascension emerged from the ocean as a volcano about a million years ago. The volcano was active until about 700 years ago. There was therefore little vegetation on the island when it was first occupied by the British 200 years ago. They introduced domestic animals as well as agricultural crops needed to supply their ships making the voyage across the Atlantic Ocean. Many unintended hitchhikers also made the voyage to Ascension, such as cats and rats.
By the time of the visit of Charles Darwin to Ascension in 1836, little of the sparse natural vegetation remained. Darwin complained of the “naked hideousness” of the island. Returning to England, Darwin convinced his friend and fellow botanist Sir Joseph Hooker that Ascension would benefit from the introduction of plants and trees: “The idea was that the new vegetation on the mountaintop would scavenge moisture from the passing clouds. Further down the slopes, planting would encourage soil growth” on the barren lava. (1)
Drawing from the collection of Kew Gardens in London, Joseph Hooker shipped trees from all over the world to Ascension: “By the late 1870s Norfolk pines, eucalyptus, bamboo, and banana trees grew in profusion at the highest point of the island, Green Mountain, creating a tropical cloud forest.” (2)
Yale Environment 360 recently published the report of a visitor to the man-made tropical forest on Ascension, Fred Pearce. (1) He is a British freelance writer. His guide on Ascension was the island’s conservation development officer, Stedson Stroud. Stroud told him that except for a few native ferns, everything on the island today has been introduced. He acknowledged that as a conservationist he might be expected to be engaged in an effort to eradicate all the non-native species. “But if he did, there would be almost nothing left. And in any case, he mused, he is presiding over something profoundly interesting—a functioning ecosystem to which a ragbag of species shipped in from all over the world thrive as if they had been together for millennia.” (1)
There are 300 introduced plant species on Ascension, adding to the 25 native plant species of which 10 are endemic (unique to Ascension). Three native plant species are said to be extinct. However, “…many of the endemics seem to get on remarkably well with the motley collection of invaders, says Stroud. The ferns that once clung to the bare mountainside now prosper on the branches of introduced trees like bamboo. Stroud showed [Pearce] ferns that he believes now thrive only on the mosses that grow on such branches…And Stroud says the vegetation captures more cloud moisture just as Hooker had hoped, even though rainfall has declined in the lowlands around.” (1)
The message that some scientists don’t want to hear
The conservation officer interviewed by Pearce expressed his frustration that scientists visiting Ascension are not interested in the thriving ecosystem on Green Mountain. Conservation research is still hung up on native ecosystems because the “…standard theory [is that] complexity emerges only through co-evolution…According to mainstream ecological theory, this cloud forest really should not exist. Certainly it should not thrive. Complex forest ecosystems are believed to take millions of years to develop…” But the fact is, the cloud forest on Green Mountain “strongly suggests that even highly biodiverse ecosystems may often be accidental, temporary, and versatile.” (1)
Pearce concludes that there are practical implications for conservationists who are trying to reassemble the complex ecosystems that have been lost. We might have better results from simply letting nature take its course. Most visitors to our cloud forest in San Francisco, Mount Sutro, would probably agree with that strategy. How could destroying 90% of the forest and its understory on 75% of the acres be an improvement over what we have now?
Update: Professors Daniel Simberloff and Donald Strong have posted a “counterpoint” to Fred Pearce’s article about Ascension Island. It is available here.
Professor Simberloff is one of the most notable academic proponents of invasion biology. Professor Strong is the creator of the project to eradicate all non-native Spartina marsh grass from the entire West Coast of the US.
Their counterpoint looks like rhetorical flailing to us. Although littered with detail, its argument seems convoluted and ultimately leaves one wondering if they are making any point other than their deep desire to defend their academic turf.
(1) Fred Pearce, “On a Remote Island, Lessons in How Ecosystems Function,” Yale Environment 360, August 26, 2013