Humans, and the invasive species we bring with us, are frequently viewed as destroying ecosystems. But we alter them just as often, inadvertently picking winners and losers from among the species as we transform their environment. A paper out in today’s describes a case where our actions made a butterfly species a winner, but then changed the game so fast that the local population went extinct.
All of this because one man died and his cattle ranch shut down.
The cast
This is a story with a lot of moving parts, so we’ll take some time to introduce them. The protagonist is Edith’s checkerspot butterfly, or . A native to the west coast of the United States, one of its populations lived on a site called Schneider’s Meadow, named after the rancher whose cattle grazed there. These and other populations of the butterfly lay eggs on a plant called . That plant provides a good growth environment for the butterflies’ offspring, but the plant is a bit fickle, prone to sudden die-backs that lead to a reasonably high rate of caterpillar mortality.
The cattle ranch altered the local surroundings, with the grazers lowering large areas of vegetation to nearly the ground. This allowed the final character to introduce itself to the story: , an invasive species that grows well in heavily grazed environments, and has spread widely throughout the west.
Two European researchers, Michael Singer and Camille Parmesan, were studying the butterflies in Schneider’s Meadow when the invasive plant arrived there. They found it quickly created what’s called an “ecological trap.” About 20 percent of the butterflies at Schneider’s would lay some eggs on the invasive plant as well as their normal target. And, while the caterpillars grew more slowly on the unfamiliar plant, more of them survived because it doesn’t die back frequently. And from there, evolution took over.
By 1982, about seven percent of the butterfly population laid eggs exclusively on the invasive plant. In 1990, the number was up to half, and by 2005, the entire population in this area had converted to laying eggs exclusively on the newly arrived species. The ecological trap had been set, as the butterflies had become dependent upon a species that was only there because of human habitat disruptions.
A surprise ending
In 2005, those disruptions experienced a disruption of their own when Harry Schneider died, and the land was sold. The new owners didn’t graze cattle there, which led to an explosion in the growth of grasses at the site. The invasive plants were quickly buried by fast-growing grasses; Singer and Parmesan estimate that the temperatures around the plants dropped by about 7°C. The caterpillars, which already grew slowly on the invasive plant, couldn’t cope with the lower temperatures. Within three years, the butterfly population was gone and remained absent through 2012.
But that wasn’t the end of the story. After a few years of flourishing, the grasses burned through the fertilizer left behind during the ranching period, and the area began to thin out. And in 2012, a fire went through a nearby area that was home to a different population of checkerspot butterflies. In the ensuing years, a population boom at the site of the fire allowed some of these butterflies to recolonize Schneider’s Meadow.
All these butterflies are currently laying eggs on native plants. But the invader is still in the meadow, and there’s a chance for a repeat of the dietary shift here.
The story has a happy ending, but if this were a completely isolated population—or the only population left on the planet—that wouldn’t be the case. The story also has a lot of lessons for conservation efforts, as it makes clear the human influence doesn’t have a clean beginning and ending. We caused just as much ecological disruption when we left as when we arrived, some of the effects took several years to resolve, and at least one (the invasive plant species) is still there. While the authors use the metaphor of a trap to describe the butterfly’s situation, a minefield might be more accurate.
, 2018. DOI: 10.1038/s41586-018-0074-6 (About DOIs).
