It turns out a 2014 paper that found a surprising pattern of plant migration in response to global warming was not so surprising after all — it’s been retracted by the authors due to a mistake in the statistical analysis.

Most studies on migrating populations have found that species around the globe move north to escape the rising temperatures. But the authors of the 2014 paper in Global Change Biology found the opposite — according to their analysis, many plant species in Western North America had been migrating south, toward warmer climates.

The lead author Melanie Harsch told Science in 2014 that the team had been suspicious of the analysis, and so ran it twice:

“Initially, we thought there was something wrong with our analysis—species distributions are expected to shift upward, not downward,” says team leader and plant ecologist Melanie Harsch. “But we redid the analysis and we got the same results.”

In the end, the authors reasoned the plants were moving south to where there was more water available. That explanation, however, ultimately didn’t hold water — according to the notice, a “coding error” had invalidated the results.

Here’s the notice for “Species distributions shift downward across western North America”:

The above article, published online on 18 August 2014 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, Dr Melanie Harsch and Associate Professor Janneke Hille Ris Lambers, journal Editor-in-Chief, Professor Stephen Long, and John Wiley & Sons Ltd. The retraction has been agreed for the following reasons: a coding error affected the results and therefore invalidated the broad-scale conclusions presented in the article. The article presented broad-scale patterns of species distribution shifts in response to recent climate change. Unfortunately, it has since been found that one approach used to account for sampling bias, the null model approach, was affected by the coding error. Following the identification of the coding error, we are therefore retracting the article. We thank Drs Adam Wolf and William Anderegg for bringing this issue to our attention.

Adam Wolf, a biologist at Princeton University, sent us some background on what happened:

The short story is that we had done some research on a fairly similar topic, and were rejected from the same journal earlier in the year even though we had put the data through alot of careful checks to make sure of our analysis. We were annoyed to see they had submitted something basically the same, but with a more dramatic finding, and had gotten published in that journal. Looking at the supplementary material I realized there is no way they could have gotten their results without some flaw in the code. Basically: they found that climate forces species to shift uphill, but human sampling of plants in their data had also shifted uphill, so the two are confounded. They claimed that they removed this source of bias, but it was evident from their supplementary figures that they hadn’t. The editors brokered a deal where they shared their data with us, and I re-did their analysis and found a different result. Next thing I found out is they decided to retract completely. They said it was the first time that journal had ever issued a retraction!

We asked him if we thought the original conclusions, that species move downhill in response to global warming, made sense:

It’s possible for species to move all sorts of directions, based on all sorts of drivers. Invasions, acid rain and ozone, land use change (forestry and ag, urbanization), and others. Our conclusion is there is some evidence for some species to move uphill, plenty of species not moving uphill (especially endemics with restricted range), many species moving up hill but probably not because of climate change, and generally so much ambiguity in the data that it’s hard to detect or attribute specific observations to specific mechanisms.

We’ve reached out to Harsch, Anderegg, and the journal, and will update if we hear back.

The news article in Science about the research now includes a note saying the study has been retracted.

Update 11:30 a.m. EST 4/13/15: Melanie Harsch emailed us the following statement, and asked that we attribute it to both her and co-author Janneke Hille Ris Lambers:

We retracted the paper after identifying a coding error in the null model analysis that invalidated one of the major conclusions of the paper. We first became aware that there might be a problem with the analysis when we received a draft of the response letter by Wolf and Anderegg (to whom we had sent the data to analyze upon request). We then tried to replicate their methodology to determine why there was a discrepancy. We were dismayed to find an error in our code and to discover that this error was responsible for our counterintuitive prediction that the majority of flora across western North America are moving to lower elevations. At this point, we immediately contacted the journal and worked with the editors to arrange retraction. We are currently reanalyzing the data and will resubmit a revised manuscript. We will make the data and code publicly available.

Hat tip Rolf Degen

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