Local weather change is a stressor of insect populations around the globe

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Climate change is an important contributor to the global decline in insects, according to a new study published in the journal PNAS that examined continuous long-term monitoring of butterfly and other insect populations.

“In particular, we examined long-term data sets from relatively protected areas – areas where the effects of other stressors are weaker, for example in the mountains,” says study director Chris Halsch, PhD student in the fields of ecology, evolution and conservation. Reno said. “We didn’t specifically look at agriculture or urban areas in this study. Most of our locations are in undisturbed natural areas. “

Few insect monitoring programs include extensive elevation changes, but the exception is the Shapiro Transect in Northern California, which has 10 locations and 163 species of butterflies from sea level to 8,200 feet. The sites have a wide variety of land uses, from the heavily altered Central Valley in California to above the tree line in the Sierra Nevada mountains, which this study includes. The observations at these locations are carried out every two weeks during the butterfly season between 32 and 48 years, depending on the location.

“The problem is that there are multiple reasons for this decline: agriculture, pesticides, urban growth that leads to habitat loss, pollution, forest fires and extreme weather events – and in most cases this is not just one thing,” said Halsch . “And none of these factors is as geographically ubiquitous or as likely to interact with all other factors as climate change.”

In the Donner Pass and Castle Peak areas of Northern California’s Sierra Nevada, researchers (including Halsch) found butterfly populations that were relatively stable until the 2000s. Then the extreme drought from 2011 to 2015 set in and the population did not recover.

University of California Davis research team members James Thorne and David Waetjen provided climate models that match population data for the Northern California transect. They found warming minimum temperatures and nights that do not cool down as much, which all affects biological processes.

The butterflies’ nectar sources are becoming increasingly difficult to find as the plants wither in the heat, especially towards the end of summer and into early autumn. Heat is responsible for the dry vegetation, which was a catalyst for last year’s devastating forest fires that devoured butterflies’ habitat in the west.

“Extreme events are an important aspect of climate change in insects,” said Halsch. “About six of the studies we examined looked at extreme events (flood, drought, etc.) and those events were more likely to be consistently negative. The negative effects of extreme events can be seen in the drought in the mountains – the butterflies in the mountains have done badly; and the butterflies in California’s Central Valley did relatively well during the drought, but since the end of the drought the declines have come back and the numbers have come down again. “

Art Shapiro, who began butterfly research in Northern California in the 1970s, said that generalists – butterflies that can survive in a variety of environments – in many cases do poorly compared to those that only thrive in a narrow range of environmental conditions .

“Generalists tend to be colonizers in the mountains,” said Shapiro, co-author of the study. “Their decline in the mountains probably reflects the fact that the main population is down here [in the Central Valley] it is bad. “

Halsch and his co-authors checked studies from around the world for relevant information on butterflies, moths, ants and flies. Butterflies are by far the most monitored of all insects, with the bulk of the studies coming from North America and Northern Europe.

In total, the team used 60 studies from around the world for their research, 11 studies using the Shapiro Transect data, and 12 studies based on monitoring butterflies in the UK – monitoring programs known as some of the best in the world .

“Butterflies are more likely to have worse problems in Europe than they are here in North America,” Shapiro said. “The decreases themselves are similar, but the reasons for the decrease are different.”

Resilient insect populations are critical for a variety of reasons, from supporting global food supplies to supporting backyard flowers through pollination. Biologists are particularly interested in seeing how insects are responding to current climate change as they represent the most diverse line of multicellular organisms on the planet and are fundamental to the functioning of freshwater and terrestrial ecosystems.

Colleague Lee Dyer of the Biology Department of the College of Science and the EECB program at the University of Nevada, Reno, recently completed one of the few studies of insect populations in tropical forests and found strong climate signals related to population declines.

Matt Forister, Halsch’s school advisor and co-author of the study, which is part of the PNAS published special on insect declines, is an expert on insect and butterfly populations. He has taken the lead in maintaining and expanding Shapiro Transect surveillance in the Sierra Nevada. He said that butterflies, like the rest of the natural world, are in decline, but they can recover from their dire situation.

“We’re probably on the verge of losing some butterflies locally or regionally,” he said. “While these endangered butterflies will not become extinct globally, that could change in the next 30 to 50 years. Insects really are survivors. We’re going to lose more, but if we can improve our farming practices and curb climate change a little, there is much hope. “