Overall, the team analyzed more than 4,000 specimens of ants collected over the past decade or so. And gradually, a turbulent history of Fijian ants started to emerge from the data.
The first colonists
The art of reconstructing the history of entire populations from individual genetic sequences relies on comparing them to each other thoroughly and running a whole lot of computer simulations. “We had multiple individuals per population,” Mikheyev explains. “Let’s say we look at this population and find it has essentially no diversity. It suggests that it very recently descended from a small number of individuals.” When the contrary was true and the diversity was high, the team assumed it indicated the population had been stable for a long time.
With the DNA data in hand, the team simulated how populations of ants would evolve over thousands of years under various conditions, and picked scenarios that best matched the genetic diversity results it obtained from real ants. “We identified multiple instances of colonization—broadscale evolutionary events that gave rise to the Fijian fauna that happened in different timeframes,” Mikheyev says. There was a total of at least 65 colonization events.
The first ants, according to Mikheyev, arrived at Fiji millions of years ago and gave rise to 88 endemic Fijian ant species we have today. These ants most likely evolved from a single ancestor and then diverged from their mainland relatives. Then, a further 23 colonization events introduced ants that were native to a broader Pacific region. These ants, the team found, were a mixture of species that colonized Fiji naturally and ones that were brought by the first human settlers, the Lapita people, who arrived around 3,000 years ago.
The arrival of humans also matched the first declines in endemic Fijian ant species.
Slash and burn
“In retrospect, these declines are not really surprising,” Mikheyev says. The first Fijian human colonists didn’t have the same population density as we have now, but they did practice things like slash-and-burn agriculture, where forests were cut down, left to dry, and burned to make space for farms and fertilize the soil. “And you know, not every ant likes to live in a field, especially the ones that evolved to live in a forest,” Mikheyev adds. But the declines in Fijian endemic ant species really accelerated after the first contact with the Europeans.
