Gently fluttering its wings, an orange, yellow, and black butterfly makes its way to an elongated stalk of pink flowers, stopping on a blossom to sip nectar, before flying away to the next flower, a few grains of pollen in tow.
When the Edith’s checkerspot butterfly (Euphydryas editha) reaches its next flower, a few of the tiny flecks of pollen stuck to it will be left behind, pollinating the flower. However, Edith’s checkerspot is one of many butterfly species in decline, and some of its subspecies are currently listed as threatened or endangered under the Endangered Species Act.
Matt Forister, a professor of biology at the University of Nevada, Reno, is the lead author of a Science study analyzing butterfly populations. He and his co-authors found butterfly observations have declined an estimated 1.6% annually over the past four decades in the western U.S., a decline they note as being “associated in particular with warming during fall months.”
Climate among several factors having detrimental impacts
“We already know land use, habitat loss, pesticide, and contamination are all very important for butterflies and other insects,” Forister says. “We already knew that based on studies close to [agricultural] and urban centers. So our study is not saying those things aren’t important. We already know they’re important. We turned to these other places, these natural ecosystems, undeveloped areas, to ask about the influence of climate.”
The researchers used three data sets, including the “Shapiro transect” – which includes 10 sites that have been studied since the 1970s – and data from the North American Butterfly Association and the iNaturalist web platform (which vets each image with an algorithm and two human experts). Forister and his colleagues analyzed a number of factors, including temperature and precipitation, in areas ranging from major cities to national parks, analyzing “elevational and latitudinal gradients that contain great habitat and climatic diversity,” according to the paper.
Their findings were not good news for butterfly lovers, or for people who rely on pollination to fill their grocery stores and stomachs. There was a significant decline in butterfly numbers.
“It’s impressively uniform, sort of a reduction in overall numbers being felt across a really large number of species,” Forister says.
While the results were largely uniform, Forister says he and his colleagues did notice a few other patterns, including that species with multiple generations per year seem to be faring better than others. He also points to findings from Western Europe that found specialist species that only eat one or two types of plants were often faring worse than their counterparts that ate a greater variety of food items.
Butterflies, ‘exquisitely adapted’ to their environment, are on ‘a knife’s edge’
Jeffrey Glassberg, president of the North American Butterfly Association, emphasizes how vulnerable butterflies are, susceptible to even minute changes. “Butterflies are really on a knife’s edge,” Glassberg says. “They are exquisitely adapted to their environment and [affected] if it just gets a little too wet or a little too dry. Almost all butterflies die before reproducing. The great majority of individuals are killed off by diseases, by predators … and very few survive. And so, if there’s a small shift in the favorability, unfavorability of the environment, it can have a really outsized effect on the population.” He notes other species, such as birds, mammals, and vertebrates, generally see more gradual effects from environmental changes.
While many animals, such as mammals, typically live for multiple years – or even decades – butterflies have a much shorter lifespan. As a result, there can be a great deal of variability in terms of population, with large fluctuations from year to year, leaving them vulnerable to sharp declines in years with smaller populations.
“In a year, it’s not uncommon to find 100-fold differences in population size for individual species depending on what’s happening that year,” Glassberg says.
One factor that may be contributing to overall declines is fall temperatures, which are slowly warming. Forister says he plans to conduct future research on the impact of fall warming to learn more. He says he hopes to examine a variety of possibilities, such as whether nectar resources may be drying out more rapidly at the end of summer and into fall, or if warming fall weather is affecting the cues insects need to become dormant and survive winter. “[It] raises all kinds of really interesting biological questions,” Forister says, noting he plans also to study the geography of the declines.
Rising temperatures are problematic also because they can turn once perfectly suitable habitat for a particular species into far less suitable habitat. “The problem is that where butterflies are right now … might not be where they can survive anymore, a particular species,” Glassberg says, noting some researchers have found southern butterflies are moving northward and some parts of butterfly ranges – such as parts of South Florida – are slipping underwater as seas rise.
Pollination services worth $34 billion in 2012 alone
Butterflies and other pollinators provide humans with huge economic benefits, which scientists from the University of Pittsburgh and Penn State tabulated in a recent paper in Environmental Science & Technology. Using data from an agricultural census conducted every five years, Vikas Khanna, associate professor of civil and environmental engineering at the University of Pittsburgh, and colleagues, quantified the economic value of pollinators – including butterflies, bees, beetles, moths, and other species. They found pollinators’ work in 2012 alone was worth $34 billion. They are currently analyzing data from the 2017 agricultural census.
“The challenge is that engineering typically takes ecosystem goods and services for free, because we don’t pay for these ecosystem goods and services and benefits that we derive from ecosystems,” Khanna says. “As a result what has happened is the way we design our technological engineering systems, we place this huge stress pressure on the very ecosystems that we depend on for extracting all our resources as well as all our needs, whether it’s food, fuel, fiber,” he continued. “And pollinators happen to be one of those, especially pollination provided by wild insects, and they’ve been in decline.”
He notes that people often don’t think about the value of these services or assign a value to them. “The challenge with us as a society is that anything that we don’t or we cannot measure, it does not get valued and that is the case with insect pollinators,” Khanna says. “[With] honey bees, yes there’s a market, they are managed pollinators. But wild insects, we don’t pay for them. It’s a free service, but at the same time they enable agricultural productivity.”
Plants can still be pollinated without pollinators, but using other methods of pollination can become very complicated and expensive. Wind is a natural method of pollination, though farmers can obviously not control when and how the wind blows. Manual pollination – where people physically brush pollen on individual flowers – is possible, though it is incredibly labor intensive and expensive. While these methods can be used, they can’t compare to the natural method.
“[There is] something to be said about the fact that ecosystems and ecosystem species, insects, have optimized the way they do it over decades and hundreds of years, so these plant and pollinator communities, they have co-evolved [and become] highly optimized,” Khanna says. He notes insects also provide many other important ecosystem services – such as biodiversity and decomposition – in addition to pollination. “Those are things that are hard to put monetary value on,” Khanna says.
Conserving pollinators like butterflies requires a multi-pronged approach, according to Forister. He notes it is important to focus on a number of factors, including climate change, land use decisions, and protecting ecosystems.
“Finding that climate change is important for insects doesn’t mean that other things are not really important,” Forister notes. “Land loss to development and pesticides – we already know those things are really important. We’re just sort of elevating climate change to be part of that suite of things that are important.”
Also see: What will happen to pollinators?