Introduced species dilute the effects of evolution on diversity

Understanding how biodiversity is shaped by multiple forces is essential to protect rare species and unique ecosystems. Today, an international research team led by the University of Göttingen, the German Centre for Integrative Biodiversity Research (iDiv) and the Helmholtz Centre for Environmental Research (UFZ), as well as the University of Hawaii at Mānoa, found that biodiversity is higher on ancient islands than on young people. In addition, they found that introduced species dilute the effects of island age on local biodiversity models. The results were published in the PNAS (Proceedings of the National Academy of Sciences).

Oceanic islands, like the Hawaiian archipelago, have long been a natural laboratory where scientists analyze evolutionary and ecological processes. In these archipelagos, islands formed by underwater volcanoes often differ by several million years, allowing scientists to study the long-term impacts of geology and evolution on biodiversity. In this study, researchers used data from more than 500 forest plots in the archipelago to explore how historical and recent ecological processes influence the number of species that coexist, whether on an island scale or in a much smaller area such as a typical backyard. Their analysis showed that even within small plots, the older islands had a higher number of rare and native species than the more recently formed islands.

Researchers were able to compare data from older islands such as Kau'i (which is about 5 million years old) with islands such as the Big Island of Hawaii (which is only about 500,000 years old and still growing). "To be honest, I was a little surprised by the results. I expected that ecological mechanisms would outweigh macroevolutionary forces at the scale of these small plots and that there would be no difference in local diversity between the islands," explains Jonathan Chase (iDiv and Martin Luther University Halle-Wittenberg), lead author of this study. "So, for me, this is the coolest discovery, the one that challenges your assumptions."

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They also showed that large-scale introduced species weakened the effect of island age on biodiversity, making Hawaiian forests more similar to each other. Dylan Craven, University of Göttingen and lead author of the study, says: "We see that human activity - such as the planting of introduced species in our gardens and parks - is beginning to erase millions of years of history, of interactions between plants and animals and their environment.