A grasshopper, Melanoplus boulderensis, typical of the Rocky Mountains in Colorado. This species matures later in the year, in mid-June. Research shows that its body size has decreased over the decades as greenery emerges earlier in the spring and dries up earlier in summer.Thomas Naef
As insect populations decrease worldwide - in what some have called an "insect apocalypse" - biologists seek to understand how the six-legged creatures are responding to a warming world and to predict the long-term winners and losers.
A new study of grasshoppers in the mountains of Colorado shows that, while the answers are complicated, biologists have much of the knowledge they need to make these predictions and prepare for the consequences. The study, published Jan. 30 in PLOS Biology, compares thousands of grasshoppers collected in Colorado between 1958 and 1960 with modern-day specimens.
"Understanding what species are likely to be winners and losers with climate change has been really challenging so far," said corresponding author Lauren Buckley, a professor of biology at the University of Washington. "Hopefully this work starts to demonstrate some principles by which we can improve predictions and figure out how to appropriately respond to ecosystem changes stemming from climate change."
Comparing museum specimens and newly collected insects allowed the research team to assess the impact of 65 years of climate change on the sizes of six species of grasshopper. Because insects are cold-blooded and don't generate their own heat, their body temperatures and rates of development and growth are more sensitive to warming in the environment.
Despite much speculation that animals will decrease in size to lessen heat stress as the climate warms, the study found that some of the grasshopper species actually grew larger over the decades, taking advantage of an earlier spring to fatten up on greenery.
Growth was seen only in species that overwintered as juveniles and thus could get a head start on chowing down in the spring. Species that hatched in the spring from eggs laid in the fall did not have this advantage and became smaller over the years, likely as a result of vegetation drying up earlier in the summer.
"This research emphasizes that there will certainly be species that are winners and losers, but sub-groups within those species' populations, depending on their ecological or environmental context, will have different responses," said co-author Monica Sheffer, a postdoctoral researcher at both the UW and the University of California, Berkeley.
The authors of the new study predicted much of this based on the lifecycles of the grasshoppers and the environmental conditions at the site.
"We sat down and looked at all that was known about the system, such as elevational gradients and how that should modify responses and how different grasshoppers might respond, with all the wealth of information we knew about their natural history. And while not all our predictions were supported, many of them actually were," said co-author Caroline Williams of the University of California, Berkeley.
The 65-year-old grasshopper collection was first assembled over three summers by the late entomologist Gordon Alexander of the University of Colorado Boulder. He not only collected and mounted the specimens from plots in the Rocky Mountains near Boulder, but also documented the timing of six different life stages of the grasshoppers. His death in a plane crash in 1973 left the specimens, pinned in neat rows in 250 wooden boxes, in limbo.
The collection languished until 2005, when César Nufio, then a postdoctoral fellow at CU Boulder, set about curating the collection and initiated a re-survey of the same sites to collect more grasshoppers.
Nufio and many others eventually collected about 17,000 new grasshopper specimens from the same or similar sites. While the new paper is the first to report the grasshopper size changes between 1960 and 2015, the authors conducted other studies to help explain the patterns.
The insects were from a large group of nondescript grasshoppers in the Acrididae family that are known as short-horned grasshoppers. Most graze on many types of plants, though some specialize in grasses. Two species (Eritettix simplex and Xanthippus corallipes) achieve adulthood as early as May; two (Aeropedellus clavatus and Melanoplus boulderensis) mature in mid-June; and two (Camnula pellucida and Melanoplus sanguinipes) mature in late July.
The researchers found that the grasshopper species that achieve adulthood in May increased in size at lower elevations, around 6,000 feet, while the early and late emergers in these locations decreased in size over the decades.
"The data are consistent with grasshoppers either being able to take advantage of warming by getting bigger and coming out earlier, or for grasshoppers to experience stress and get smaller," Buckley said.
"We would expect similar trends for grasshoppers in mountains in Washington, but some later snow melt in Washington state might alter the importance of seasonal timing," she added.
Other experiments Buckley performed on butterflies in Colorado show some of the same trends.
"We find a pretty similar message with butterflies, which is hopeful to me, in that if we can consider some basic biological principles, we really increase our ability to predict climate change responses," Buckley said.
At the UW, Buckley's research group is repeating surveys begun in 1995 of cabbage white butterflies in Seattle and western white butterflies in central Washington to see how those insects might have changed over the past 30 years.
Buckley also recently became an adjunct curator of entomology at the Burke Museum, where she hopes to continue leveraging museum collections for ecological research.
Other co-authors on the new study are Julia Smith, a graduate student in biology at the UW; Simran Bawa of UC Berkeley; and Ebony Taylor, Michael Troutman and Sean Schoville of the University of Wisconsin, Madison. The work was supported by the National Science Foundation.
