More isolated oceanic islands harbor fewer native species due to the fact that plants and animals are less capable of naturally dispersing to and colonizing those islands. This is known as the species-isolation relationship (SIR), one of the most fundamental concepts in the study of island biogeography.

While the isolation of islands means that they tend to be low in species richness, it also means that they are home to exceptionally high levels of endemism. Because a disproportionately high number of island species are found nowhere else, the estimated 465,000 islands on Earth are critically important to overall global biodiversity, even though they comprise just 5 percent of our planet’s total land area.

But as humans have extended their reach ever-farther across the globe, the introduction of non-native species has come to have an increasingly critical effect on island biodiversity. Alien species can often be invasive, driving native species numbers down or even pushing them towards extinction. A 2015 study found that nearly 40 percent of all species currently listed as critically endangered are confined to islands, and identified invasive species as one of the chief threats to islands’ native flora and fauna.

Now, new research shows that SIR does not hold for non-native species, determining that more remote islands may actually be more susceptible to invasion by alien species, which could have important implications for the preservation of global biodiversity.

“While isolation has shaped natural colonization of islands for millennia, globalization in trade and transport has led to a breakdown of biogeographical barriers and subsequent colonization of islands by alien species,” according to a study to be published this week in the Proceedings of the National Academy of Sciences that explores the relationship between island remoteness and invasive species. “Geographical isolation does not protect islands from alien species, and island species richness may reach a new dynamic equilibrium at some point, likely at the expense of many endemic species.”

The authors of the study, an international research team led by Dietmar Moser, Bernd Lenzner, and Franz Essl of the University of Vienna, used a dataset of 257 subtropical and tropical islands to determine that non-native species richness actually increases as the isolation of islands increases. In other words, the SIR pattern is reversed: the more remote an island, the more non-native species it harbors.

“What really surprised us was to see that the isolation of an island — its distance from the mainland — had opposite effects on native and non-native species richness,” co-lead author Moser said in a statement. “Native species declined whereas non-native species numbers increased with isolation.”

Moser and team found this pattern to be consistent for four of the five taxonomic groups they studied: plants, ants, mammals, and reptiles. (Birds, the fifth taxonomic group studied, were the exception.) And they say that their research shows that this phenomenon cannot be explained by island economics or trade status alone.

If the economy and trade status of remote islands doesn’t explain why more non-native species thrive there, what does? “We argue that the reversal of the SIR for alien species is driven by an increase in island invasibility due to reduced diversity and increased ecological naiveté of native biotas on the more remote islands,” the authors write in the study.

That is, because fewer native species call more remote islands home, there are plenty of ecological niches left open for alien species to exploit. But also, by adapting to island life, many native species become highly specialized, losing behaviors that would help them survive on the mainland. For instance, defense strategies that reduce their risk of being preyed upon are no longer necessary if a species’ natural predators don’t exist on its new island home. This adaptation to life in highly specific island ecosystems over evolutionary timespans is what the researchers refer to as the “ecological naiveté of native biotas.”

“With increasing distance to the mainland, native species become more evolutionarily isolated and ecologically distinct,” Lenzner explains. “This results from the fact that only some species are able to disperse over such long distances, and also from the fact that successful colonizers have become genetically adapted to the specific conditions of the island.”

Some non-native species might even find success on remote islands by preying on the naive natives, or they may be able to avail themselves of resources that more specialized native species are not able to exploit. Because they are so specialized, many native species may not be able to survive in human-impacted habitats, for instance, whereas invasive species might be better equipped to do so.

The effects of introduced species on island ecosystems are only expected to become more pronounced in the future. “Globalization in trade and transport will increasingly decouple geographical distance from isolation,” the researchers warn. “As a consequence, immigration rates will increase even on remote islands, which will become packed with species.”

Florian Hofhansl, a study co-author and researcher with the International Institute for Applied Systems Analysis, added that the rate of invasion is especially high on islands in coastal regions with warmer climates, such as in tropical regions. Thus, he said, the study shows that “native, endemic, and rare plant and animal species could be particularly vulnerable in these regions.”

There are steps that authorities on remote islands can take to lessen the impact of introduced species on natives, however. Moser recommends that “especially on remote islands, strict actions against the introduction of non-native species need to be implemented.”

CITATIONS

• Moser, D., Lenzner, B., Weigelt, P., Dawson, W., Kreft, H., Pergl, J., Pysek, P., van Kleunen, M., et al. (2018). Remoteness promotes biological invasions on islands worldwide. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1804179115

• Tershy, B. R., Shen, K. W., Newton, K. M., Holmes, N. D., & Croll, D. A. (2015). The importance of islands for the protection of biological and linguistic diversity. Bioscience, 65(6), 592-597. doi:10.1093/biosci/biv031