The results are drawn from a 35-year database of butterfly observations made twice monthly at 10 sites in north-central California from sea level to tree line. The butterfly database is unique in science for its combination of attributes: one observer (which reduces errors), very long-term, multiple sites surveyed often, a large number of species (more than 150), and attendant climatological data.
The study's most significant findings include:
- Butterfly diversity (the number of different species present) is falling fast at all the sites near sea level. It is declining more slowly or holding roughly constant in the mountains, except at tree line.
- At tree line, butterfly diversity is actually going up, as lower-elevation species react to the warming climate by moving upslope to higher, cooler elevations.
- Diversity among high-elevation butterflies is beginning to fall as temperatures become uncomfortably warm for them and, Shapiro says, "There is nowhere to go except heaven."
Using a battery of statistical approaches, researchers concluded that climate change alone cannot account in full for the deteriorating low-elevation numbers. Land-use data show that the butterfly losses have been greatest where habitat has been converted from rural to urban and suburban types.
California butterflies are reeling from a one-two punch of climate change and land development, says an unprecedented analysis led by UC Davis butterfly expert Arthur Shapiro. In one example, this butterfly, the Clodius Parnassian (Parnassius clodius), is more common at the top of its elevation range in California mountains than in the past.
(Photo Credit: Heather Dwyer / UC Davis)
Researchers added that one of the most surprising findings was that ruderal ("weedy") butterfly species that breed on "weedy" plants in disturbed habitats and are highly mobile are actually declining faster than "non-weedy" species -- those that specialize in one habitat type.
This is especially true in the mountains, where such species do not persist over winter but must recolonize every year from lower altitudes. As their numbers drop in the valleys, fewer are available to disperse uphill, and the rate of colonization drops.
"Butterfly folks generally consider these ruderal species to be 'junk species,' sort of the way bird watchers think of pigeons and starlings," said lead author Arthur Shapiro. "So it came as a shock to discover that they were being hit even harder than the species that conservationists are used to thinking about.
"Some of the 'weedy' species have been touted as great success stories, in which native butterflies had successfully adapted to the changed conditions created by European colonization of California. That was the case for many decades, but habitat loss has apparently caught up with them now."
Citation: A. Shapiro et al. 'Compounded effects of climate change and habitat alteration shift patterns of butterfly diversity', PNAS, January 11, 2010