As insect populations decrease worldwide in what some have called an "insect apocalypse," biologists are desperate to determine how the six-legged creatures are responding to a warming world and to predict the long-term winners and losers.
A new study of Colorado grasshoppers 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 findings, published Jan. 30 in the journal PLOS Biology, come thanks to the serendipitous discovery of 13,000 grasshoppers all collected from the same Colorado mountain site between 1958 and 1960 by a biologist at the University of Colorado Boulder (CU Boulder). After that scientist's untimely death in 1973, the collection was rescued by his son and donated to the CU Museum, where it languished until 2005, when César Nufio, then a postdoctoral fellow, rediscovered it. Nufio set about curating the collection and initiated a resurvey of the same sites to collect more grasshoppers.
The newly collected insects allowed Nufio and his colleagues — Caroline Williams of the University of California, Berkeley, Lauren Buckley of the University of Washington in Seattle and postdoctoral fellow Monica Sheffer, who has an appointment at both institutions — to assess the impact of climate change over the past 65 years 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 biologists found that some of the grasshopper species actually got larger over the decades, taking advantage of an earlier spring to fatten up on greenery. This worked only for species that overwinter as juveniles — a stage called nymphal diapause — and thus can get a head start on chowing down in the spring. Species that hatch in the spring from eggs laid in the fall — the egg diapausers — did not have this advantage and became smaller over the years, likely as a result of vegetation drying up earlier.
"This research emphasizes that there will certainly be species that are winners and losers, but subgroups within those species populations, depending on their ecological or environmental context, will have different responses," Sheffer said.
The authors of the new study predicted much of this based on the life cycles 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 Williams, the John L. and Margaret B. Gompertz Chair in Integrative Biology at UC Berkeley.
"Understanding what species are likely to be winners and losers with climate change has been really challenging so far," Buckley said. "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."
Rescued grasshoppers
The 65-year-old grasshopper collection was assembled by entomologist Gordon Alexander of CU Boulder over three summers. 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 until Nufio came across them in 2005 and recognized their value if they could be compared to grasshoppers today.
Museum collections have become invaluable for long-term studies of the effects of climate change, as exemplified by a survey of mammals, birds, reptiles and amphibians conducted between 1904 and 1940 by Joseph Grinnell of UC Berkeley's Museum of Vertebrate Zoology. Recent resurveys of the same areas 100 years later that Grinnell visited helped biologists document the effects of climate change on California wildlife.
Nufio and many others eventually collected about 17,000 new grasshopper specimens from the same or similar sites around Boulder. While the new paper is the first to report the grasshopper size changes between 1960 and 2015, the authors leveraged previous studies in the lab and from experimental plots to understand why they found the patterns they did.
The insects were from a large group of non-descript grasshoppers in the Acrididae family that are so-called short-horned grasshoppers. Most were generalized grazers, though some specialized in grasses. Two species (Eritettix simplex and Xanthippus corallipes) were nymphal diapausers, achieving adulthood as early as May; two (Aeropedellus clavatus and Melanoplus boulderensis) were early season egg diapausers, maturing in mid-June; and two (Camnula pellucida and Melanoplus sanguinipes) were late season egg diapausers, maturing in late July.
The researchers found that the nymphal diapausers increased in size at lower elevations, around 6,000 feet, while the early and late emergers from overwintering eggs decreased in size over the decades at these elevations.
"For those that come out in late August, when it's very crispy and dry and we get very hot temperatures, we saw the most negative impacts of climate change," Williams said.
One thing that surprised the researchers, however, was that none of the species increased in size at higher elevations, up to about 13,000 feet, despite the fact that summer warming due to climate change is greater at higher elevations. This may be because, at higher elevations, snow inhibits early season greening up, reducing the food supply. The results confirm what the team found when it caged grasshoppers at various elevations to see how they adapted to elevational changes in heat and dryness.
"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.
Other experiments performed by Buckley on butterflies 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," she said.
The team is continuing its collaboration to understand the metabolic, biochemical and genetic changes that underlie the size changes.
"Using those museum collections allowed us to go back in time to compare exactly the same sites — there hadn't been any changes in the land use over this 60-year period of warming — using exactly the same methodology," Williams said. "Having those unique historical specimens enabled us to look at the changes through time."
Other co-authors of the study are Julia Smith of the University of Washington; 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 (DEB-1951356, DEB-1951588, DEB-1951364).
