The Road To Extinction Is Paved With Good Intentions

It’s no secret that our species disproportionately inflicts heavy selection pressures on other species around the world. Unsurprisingly, that anthropogenic pressure occasionally creates problems. One need look no further than the way in which human activity has driven climate change and ocean acidification for evidence that the evolution of plants and animals is affected by our own behavior. Other times, our intervention is intentional and meant to be beneficial, such as with the recovery of the American bison or California condor. But it isn’t always necessarily clear, from the outset, whether a particular action will lead to a positive or negative outcome for any given species, as new research with the black robin makes clear.

The black robin (Petroica traversi) is a small, endangered passerine bird native to the Chatham Islands, a small archipelago several hundred kilometers east of New Zealand. After mammalian predators were introduced to the islands several bird species, including the robin, became extinct or severely endangered. By 1938, the species was reduced to some 20 to 35 robins, all on just one island. By 1980, the population was down to five individuals, including only a solitary breeding pair. That’s when conservationists like Don Merton, from New Zealand’s Department of Conservation stepped in. After eighteen years of hard work, the species rebounded to approximately two hundred birds, all descended from that single, original breeding pair.

But that success came with a cost.

During the early years of their intervention, the conservationists noticed that some females laid part or all of their eggs along the rim of their nests, rather than in the center. The so-called “rim eggs” were therefore not incubated and failed to hatch. As a result, female “rim-layers” had smaller clutches than their counterparts who incubated all of their eggs. The New Zealand Wildlife Services staff realized that if they simply repositioned the rim eggs, the chicks would hatch just fine. That resulted in larger clutches, which consequently increased the size of the breeding population of the species. That, after all, was the entire point.

That first single breeding female nicknamed “Old Blue” laid her eggs normally in 1980. Just nine years later, thanks to human intervention, more than half of the females in the population laid at least one rim egg. The problem was that by repositioning those rim eggs, the conservationists relaxed the existing natural selection pressures against rim-laying, creating an environment that allowed for the “survival of the not-so-fit,” as Charles Sturt University researcher Melanie Massaro and colleagues put it in a paper in PLOS ONE. The increase in rim eggs over time suggests a genetic component to rim-laying, but it doesn’t prove it.

Massaro created a family tree for every black robin on the island from 1980 to 1989. That allowed her to determine whether rim-laying was a response to some environmental cue, or whether it was the result of genetics, as she suspected. Given the pattern that emerged, the researchers determined that the behavior was probably caused by a single dominant allele. That means that a female could become a rim-layer if she received just one copy of that allele from either parent, rather than requiring both parents to pass the gene variant on to her.

The complete family tree for all black robins, 1980-1989, descended from a single breeding pair. Males are squares, females rim-layers are red circles, and females that do not lay rim eggs are blue circles.

“We surmise that the spread” of rim-laying behavior, she writes, “was facilitated by the egg repositioning that rendered this otherwise maladaptive trait effectively neutral.” And since the allele can be carried by male robins without any negative consequences, it has persisted at a higher-than-optimal frequency within the population, even long after human intervention ceased in 2007. Indeed, for the years 2007 through 2011, an average of 9% of females still laid rim eggs.

This raises an important dilemma as researchers and activists continue to create and implement wildlife conservation strategies. One one hand, rapid population increases often via controlled or assisted breeding are necessary to avoid extinction. On the other hand, management programs that prioritize fecundity over genetic diversity may result in the promotion of maladaptive traits and the genes that underlie them. “Genetic dependency of the endangered population on continued management is a genuine risk,” Massaro cautions, and it’s a potentially expensive risk at that.

History is rife with examples of well-meaning human intervention having unintended consequences. Extreme examples include plants or animals like the domesticated silkworm or the domestic turkey which, thanks to thousands of years of artificial selection, are no longer able even to reproduce without human intervention. Luckily, the black robins of the Chatham Islands appear to be doing okay on their own. Still, that nearly one in ten females still lays rim eggs after more than two decades left to their own devices presents a stark reminder that human interference can leave deep scars in the genetic legacy of a species. – Jason G. Goldman | 8 January 2014

Sources: Massaro M., Sainudiin R., Merton D., Briskie J.V., Poole A.M., Hale M.L. & Rutherford S. (2013). Human-Assisted Spread of a Maladaptive Behavior in a Critically Endangered Bird, PLoS ONE, 8 (12) e79066. DOI: 10.1371/journal.pone.0079066.s002

Photo: Wikimedia Commons/frances schmechel.

Figure: Massaro et al. (2013)