So, after some waffling I forked out the six bucks for temporary access, and I now have thoughts. I should be hesitant to pass judgment as a total amateur, so qualify this accordingly, but the main issue I'm fixating on is that they examined a single watershed catchment of a small, steeply-sloped river (1700 square km, elev. 45m to 1700m) in rural China. Your classic watershed is pie-shaped, with more land as you move uphill, even though our planet's land surface isn't similarly shaped. I think their finding, that in their case climate change benefits biodiversity when it moves habitat ranges uphill, is an artifact of their study area's topography. I looked for any discussion of this disparity between their study area and the terrestrial world in general, and didn't see it.
- It studies stream macroinvertebrates, just one specialized aspect of biodiversity, and is very dependent on projected changes in hydrology. I'm surprised to read the claim that one could do meaningful, quantitative predictions for future hydrology at a small scale. Maybe I shouldn't be.
- Speaking of predictions, the paper ends its predictions in 2050. I'd guess wildly that 2050 is about when land use impacts would've hit their maximum for a rural part of China, but climate change impacts are just beginning. Extending the analysis to a full century might give a different sense about the negative aspects of climate change.
- The paper discusses the issue of a "summit trap" where the species habitat hits the maximum altitude and then disappears, but apparently it just wasn't an issue in this study.
In summary, I have no sense that paper is denialist or the authors were skewing it in that direction, but the catchment area and 2050 timeline IMHO exclude applying this analysis to saying climate change is secondary to land use in impact, let alone that climate change has a positive impact on biodiversity.