Hawaiian biodiversity has been declining for millions of years
Hawaii's unique animal and plant diversity has been declining on all but the Big Island for millions of years, long before humans arrived, according to a new analysis of species diversity on the islands by University of California, Berkeley, evolutionary biologists.
They reached this conclusion with a new method for analyzing the species diversity on the different islands in the multiple-island chain, deducing the history of diversification on each island with their new approach for 14 different groups, or clades, of birds, insects, spiders and plants.
"On the older islands, Kauai, Oahu and the four islands that were once parts of a bigger island called Maui Nui, it looks like most groups are now in long-term evolutionary decline," said senior author Charles Marshall, a UC Berkeley professor of integrative biology and director of the University of California Museum of Paleontology. "The older islands were all much larger than they are now, and it looks like the the flora and fauna filled up the ecological space fast enough that once the islands began to contract the crowding generated drove species to extinction."
|Ariamnes uwepa, one of a group of spiders that has radiated throughout the Hawaiian Islands |
[Credit: Rosemary Gillespie and George Roderick, UC Berkeley]
"Biologists don't often think about the evolutionary trajectory of their group because without a fossil record they have no data that bear on whether diversity is increasing or decreasing," Marshall said. "This study adds weight to the argument that there might be a lot of groups living today that are actually in long-term evolutionary decline. So this paper also serves as a consciousness-raising exercise -- how might we identify living groups that are in decline in the absence of a fossil record?"
He is currently developing ways to extend the new approach developed to analyze Hawaii to other parts of the globe.
Marshall and Lim published their findings online in the journal Nature.
The volcanic Hawaiian islands we see today emerged from the waves over a period of about 6 million years, carried northwestward as the ocean crust moved over from the hot spot that brought the magma from inside Earth to the sea floor to build the islands. Kauai emerged slightly more than 6 million years ago, the newest, the Big Island of Hawaii, only about 1.3 million years ago.
|A silversword, one of Hawaii’s unique, endemic plants on the slopes of Haleakalā on East Maui |
[Credit: Jun Ying Lim]
Marshall realized that "the progression of islands of the Hawaiian archipelago can be viewed as an evolutionary time machine," revealing "rates of species-richness change for endemic species of the archipelago," which has virtually no fossil record.
"It is increasingly appreciated that the biota of any particular place is a dynamic, ever-changing association of species," Lim said. "The beauty of islands like Hawaii is that their geologic setting provides multiple temporal snapshots, and in so doing provides us a window to understanding the processes that have shaped its assembly though time."
|A beach on the island of Hawaii [Credit: Jun Ying Lim]|
The Hawaiian archipelago proved a good place to test that hypothesis, since the islands, once active volcanism ceases, steadily shrink. Maui Nui is less than one-third its original size 2-3 million years ago.
"With a quantitative measure of changing carrying capacity from the geologic record, Jun and I have been able to invert the process and infer the diversity trajectories for Hawaii, discovering on the way that none of the Hawaiian groups are at dynamic equilibrium," Marshall wrote in a blog post on the "Behind the Paper" portion of the Nature Ecology and Evolution website. "Very satisfying!"
Author: Robert Sanders | Source: University of California - Berkeley [March 16, 2017]