What was done
The authors analyzed 35 years of butterfly presence-absence data that had been collected by a single observer -- at ten sites approximately every two weeks -- along an elevation gradient stretching from sea level to an altitude of 2,775 meters in the Sierra Nevada Mountains of Northern California (USA), during which period (1) both maximum and minimum temperatures rose, (2) low-altitude habitat was negatively impacted by encroaching land development, but (3) there was no systematic variation in precipitation.

What was learned
Forister et al. report that over the period of their study, species richness "declined at half of the sites, with the most severe reductions at the lowest elevations," where they indicate that "habitat destruction is greatest." At intermediate elevations, they say there were "clear upward shifts in the elevational ranges of species, consistent with the influence of global warming." And at the highest site, they found that species richness actually increased, stating that "in addition to an increase in richness, abundance has also generally increased at the highest-elevation site."

What it means
Once again we are presented with a simultaneous real-world demonstration of the detrimental land-use conversion effects of man on native butterfly species richness, as well as the positive effects of global warming, the latter of which enabled species that were threatened with local extinction to successfully migrate up-and-out of the low-elevation areas that were usurped by local land developers for a number of other purposes, which latter phenomenon did indeed lead to the destruction of pristine butterfly low-elevation habitat and a reduction in the number of species that ultimately were able to cope with man's presence there.