Not so widespread butterflies

Honey bees, birds, bats and insects play a vital role in the pollination of the majority of fruits and vegetables. According to the U.S. Fish and Wildlife Service, more than 75% of flowering plants rely on animals for pollination. Over the last 10 years the U.S. Department of Agriculture has referred to the declining populations of pollinators as an “impending pollination crisis” and some estimates put the honeybee decline as high as 50%. These dramatic declines are believed to be the result of pesticides, reduced availability of nectar, parasites, diseases, destruction of habitats along migratory routes, modern agricultural practices, grazing, and invasive species.

Besides the role they fill in the success of our flowering plants, the USFWS reports that honeybees are responsible for pollinating $15 billion worth of U.S. crops per year, and 15% of the global common food crops.  Thus, when a new publication surfaced describing a 30% decline of common butterflies over 16 years in lands altered by agriculture and urbanization in the Netherlands it was definitely worth sharing some of the results.

The authors checked the annual relative abundance of 20 common species that have also been identified as of no conservation concern.  Data from the Butterfly Monitoring Scheme of The Netherlands and weekly counts of transect routes spread throughout the Netherlands yielded a decline in butterfly abundance that was prominent in farmland, urban areas, woodlands, and other regions marked by a change in vegetation that deviates from naturally occurring types (i.e. outside nature reserves). In contrast, those regions preserving natural vegetation did not decline.

Distribution and Range

• 9 of the 20 widespread butterfly species showed a significant decrease in distribution
• 5 species expanded significantly in range
• 6 species showed no significant change in distribution

Population Abundance

• A significant decrease in relative abundance was detected in 8 of the 20 species (40%)
• 7 species showed a modest to clear significant increase in abundance
• The remaining 5 species showed no significant change in increasing or decreasing abundance

Of the 7 increasing species, 6 are bivoltine (number of broods or generations of an organisms in a year), whereas only 3 of the 8 declining species have more than 1 generation a year. Species with multiple generations have evolved plastic life histories that allow them to respond to seasonally changing conditions. We hypothesize that insect life-history plasticity provides an advantage in fast-changing anthropogenic landscapes.

Although the results showed that 12 individual species either increased or had no significant change in abundance, the data did highlight the fact that:

• “Since 1992 overall abundance of all 20 species declined continuously.”
• Over 16 years, “overall cumulative butterfly abundance declined by around 30%.”
• 55% of the 20 species showed at least some degree of decline in abundance (no significant to severe declines)
• 2 species qualified as endangered, 2 as vulnerable, and 3 as near threatened
• “Some of the species in decline used to be omnipresent in gardens and parks, and 2 of the species were previously considered agricultural pests.”

Based on these results the distribution and abundance of butterflies is affected by the availability of native plants and lands that are free from development and alteration to ensure continued biodiversity. Additionally, out of the originally 20 species a total of 5 had been found to indeed be of conservation concern.

Reference:
VAN DYCK, H., VAN STRIEN, A., MAES, D., & VAN SWAAY, C. (2009). Declines in Common, Widespread Butterflies in a Landscape under Intense Human Use Conservation Biology DOI: 10.1111/j.1523-1739.2009.01175.x