All over the world, islands are suffering catastrophic declines in animal and plant species, directly or indirectly from humanity's influence. Strangely, however, in Hawaii, the islands were experiencing local extinctions long before humans turned up, according to a study in Nature that attributes the decline to the older islands' loss of land area.
Hawaii is famous for being created by a volcanic hotspot. Lava built up the islands directly above the spot, but they started to subside and erode once the spot had moved relative to the Earth's crust. Today only the Big Island is still growing.
"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,” said Professor Charles Marshall of the University of California, Berkeley in a statement.
The oldest Hawaiian islands are only a little over 6 million years old, and inevitably it took time for many species to reach such a remote location. Once birds, plants, and insects colonized Kauai, the first island, life jumped quickly across narrow channels to newer bits of land as they poked out of the sea.
Species lucky enough to reach Kauai found a paradise, free from their predators and competitors elsewhere. Colonizing species quickly diversified to fill available niches.
Eventually, however, these Edens became less idyllic as the inhabitants literally lost the ground from beneath their feet, with the loss of land forcing neighbors to compete for territory.
Estimates of the growth and decline of each of Hawaii's islands. Lim and Marshall/Nature
Marshall chose to study Hawaii's biological history to test the theory that declines in landmass can have more severe impacts on biological diversity than previously thought. The discovery that some mammal groups had declined over periods of millions of years puzzled biologists, since diversity is expected to increase in the absence of catastrophic events, and overcrowding was raised as a possible explanation.
Although Hawaii has no native mammals, the islands' diverse ages and shrinking sizes – Maui Nui is now two-thirds smaller than it was at its peak – provided a good opportunity to test the idea across 14 other groups of plants and animals.
The challenge was to estimate the past diversity since Hawaii is largely bereft of fossils. Marshall used models of how the species' richness would grow and decline as the islands' area waxed and waned, and compared them with what we know of the current richness of each island using the differences in island ages to see which models described reality best.
The implications may extend beyond islands. The paper refers to moist forests and the Eastern Pacific marine tropics as ecosystems that have contracted over millions of years, and may have lost diversity in the process.
I'iwi, or scarlet honeycreepers, have survived despite the diversity of their genus declining as the Hawaiian islands have shrunk, contradicting the expectation that biological diversity grows with time. Nate Yuen