A decades-long program monitoring the butterfly abundance in Ohio has shown that the general abundance of butterflies in the region has declined 33% in the past 21 years.
Although the study was limited to one geographic area and one group of butterflies, the findings show the broader effects of climate change and other human-caused disturbances on insect populations.
This study is published in PLOS ONE.
“These declines in abundance are happening in common species,” said study leader Tyson Wepprich, an Oregon State University researcher. “Declines in common species concern me because it shows that there are widespread environmental causes for the declines affecting species we thought were well adapted to share a landscape with humans. Common species are also the ones that contribute the bulk of the pollination or bird food to the ecosystem, so their slow, consistent decline is likely having ripple effects beyond butterfly numbers.”
Wepprich used more than 24,000 butterfly surveys from trained citizen scientists in Ohio, completed between 1996 and 2016, to come to his conclusion. The data collected was then used to estimate population trends for 81 butterfly species.
“Because it’s easier to monitor butterflies than other insects — lots of people like butterflies and enjoy keeping track of them — butterflies tend to be the best source of abundance data for tracking insect population declines and increases,” Wepprich said. “Environmental assessments use them as an indicator for the general trajectory of biodiversity since they experience the same types of pressures from land-use changes, climate change and habitat degradation as other insect groups.”
Wepprich found that three times as many butterfly species were trending downward as upward. In fact, three out of every four species with a positive or negative trend grew less abundant during the course of the study. Forty of the species looked at had no significant trend up or down.
“Species with more northern distributions and fewer annual generations declined the most rapidly,” Wepprich said. He explained that these species are adapted to cooler regions and therefore could do worse in Ohio with warming temperatures. And some invasive species linked to human-dominated landscapes are on the decline, which suggests that trends are rooted in broader environmental causes.
“Analyses of insect declines are dominated by European studies about butterflies, but our study is showing that the rate of change in Ohio butterfly abundance is very similar to that found in monitoring programs in the UK, the Netherlands and Spain,” he said. “The rate of total decline and the proportion of species in decline mirror those documented in comparable monitoring programs. What’s common among all of the monitoring programs is that they are in areas with a high human impact and made possible by dedicated volunteer recorders.”
Common butterfly species are not close to extinction, however, there is still a shocking negative impact on their populations, which trickles down to larger ecosystems. Furthermore, Weppich noted that within the past twenty years, the population of the migratory eastern North American monarch has declined by more than 85% and the western North American monarch by more than 95%.
“Monarchs and rare species were monitored because people are worried about them going extinct,” Wepprich said. “In Ohio, they monitored every species they could and found declines in species previously not on the radar for conservation.”
This study shows that the butterfly decline rate in Ohio is greater than the global rate of 35% over four decades, but it’s closer to the estimated rate for insects in general — a 45% decline over 40 years.
“Our study adds another example of declines in common butterfly species thought to be well suited to human-modified habitat,” Wepprich said.
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