Shrinking of land linked to isles’ biodiversity loss

It is not news that animal and plant diversity has been declining on every major Hawaiian island except for Hawaii island.

But a recent study conducted by evolutionary bio- logists from the University of California, Berkeley, has uncovered some newsworthy context: It’s nothing new.

In fact, the researchers report, shrinking land areas have been contributing to declines in biodiversity for millions of years, far predating human influence.

“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,” senior author Charles Marshall, a UC Berkeley professor of integrative biology and director of the University of California Museum of Paleontology, said in a release issued earlier this month. “The older islands were all much larger than they are now, and it looks like 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.”

Findings from the research, which Marshall conducted with UC Berkeley graduate student Jun Ying Lim, will be included in a forthcoming edition of the journal Nature.

Marshall and Lim employed a new method for analyzing the species diversity on the different islands, ultimately determining the history of diversification on each island for 14 different groups of birds, insects, spiders and plants.

Some biologists had previously questioned whether these species had radiated fully throughout the relatively young island chain and whether, in fact, evolutionary diversity has not yet peaked.

“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 in the release. “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?”

As Marshall and Lim explained, each new island was colonized by plants and animals from the older islands, leading to a wealth of new species that filled each island’s ecological niches.

“It is increasingly appreciated that the biota of any particular place is a dynamic, ever-changing association of species,” Lim said in the release. “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.”

The researchers said that previous work suggested that some mammal groups declined over millions of years because the carrying capacity of the ecosystem had crashed.

Across the Hawaiian archipelago, declining biodiversity can be charted in relation to shrinking land areas since islands steadily shrink once removed from active volcanism. Maui Nui, the land mass from which Maui, Molokai, Lanai and Kahoolawe were formed, is less than one-third its original size dating 2 million to 3 million years ago.