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Issues of the Environment: Butterfly Species in Washtenaw County

Aug 21, 2013

Today, we'll  focuses on butterfly species in Washtenaw County. There are more than you might think, and these delicate creatures can be a good indicator as to the health of the local environment.

This week's guest is John Swales,  the North American Butterfly Association  population survey coordinator for Washtenaw County and U-M Professor Emeritus of Linguistics. We'll be discussing his long-standing participation with the NABA butterfly counts, and what the surveys tell us about the status of different butterfly species in Washtenaw County. 

Butterflies as Environmental Indicators in Washtenaw County

Butterflies have short life cycles and thus react quickly to environmental changes. Their limited dispersal ability, larval foodplant specialization and close-reliance on the weather and climate make many butterfly species sensitive to fine-scale changes. Recent research has shown that butterflies have declined more rapidly than birds and plants emphasising their potential role as indicators.

Butterflies occur in all main terrestrial habitat types, so they have the potential to act as indicators for a wide range of species and habitats. Unlike most other groups of insects, butterflies are well-documented, their taxonomy is understood and they are easy to recognise.

Because insects make up the largest proportion of terrestrial wildlife (more than 50% of species), it is crucial that we assess the fate of insect groups in order to monitor the overall state of biodiversity. Being typical insects, the responses seen in butterflies are more likely to reflect changes amongst other insect groups, and thus the majority of biodiversity, than established indicators such as those based on birds.

(Source: http://www.ukbms.org/indicators.aspx)

Examples of Butterflies as Indicators of Environmental Change Washtenaw County

While I can’t say for sure which species John Swales will discuss on IOE, I have spent a lot of time monitoring butterflies with him for NABA. Here are a few species I have heard him mention lately.

Long-term Increases and Decreases in Biodiversity
Increases and decreases in the populations of certain butterfly species in Washtenaw County are likely to be the result of permanent environmental changes, such as climate change.

The “Inornate” Common Ringlet: A small burnt orange butterfly that until just a few years ago reportedly occurred in Washtenaw County less that 10 times in the past century. Recently, this butterfly has been found to be regular in several county locations. 

Reliable accounts of "Inornate" Common Ringlet<http://wisconsinbutterflies.org/butterfly/species/106-common-ringlet> (Coenonympha tullia inornata) began occurring around 2003, and were thought to represent a substantial southern range expansion in the state (PDF of a note in the newsletter of the Michigan Entomological Society here<http://www.rrbo.org/pdf/ringlet.pdf%20>). This species now shows up all over the place in southern Michigan, moving south here as it has in New England and Ontario in the last decade or so.

 The Common Checkered Skipper: The Common Checkered-Skipper

<http://wisconsinbutterflies.org/butterfly/species/126-common-checkered-skipper> (Pyrgus communis), is a southern species that has been seen in southern Michigan with increasing frequency the last few years. Not only is this an uncommon species, it has been seen earlier and earlier in the year. It's unlikely that the larvae (the stage in which they hibernate) can survive overwinter in Washtenaw County, so the individuals we typically see are migrants from the south, or perhaps some progeny from these migrants which appear later in the season.

Short-term/Weather Related Changes in Biodiversity and Abundance

Some butterflies occur with greater or lesser frequency depending on the weather conditions from year to year. Washtenaw County was incredibly dry in 2012, resulting in fewer flowering plants and nectaring sites for butterflies. In 2013, both the number of species observed and the number of individuals of certain species found in Washtenaw County have been down, likely due to a cool spring and cooler and wetter than average summer.

For example, in 2013 observations of every species of butterfly in the skipper family have been down. Skippers are butterflies in the Hesperiidae<http://eol.org/pages/836/overview/> family that are hairy bodied, and tend to be small in size. They closely resemble moths. Skippers have a fast, darting flight, and favor open, sunny areas. The caterpillars of most species feed on grasses or sedges in prairies or wetlands.  Since Washtenaw County has patches of patches of high-quality prairie and wetland habitats (some within protected lands such as NAP/NAPP properties or the Greenbelt and some on private land), skippers are usually seen frequently where they occur.

There are about 40 species of skipper in Washtenaw County, but this year very few have been seen. Both the number and species and the number of individuals of each species has been drastically lower than in previous years.  In fact, during some surveys no skippers were seen at all!

It is likely that the cool spring delayed the emergence of some species, and that relatively cool, wet summer weather has decreased skipper populations in our area.  As long as the availability of habitats with the proper host plants in our county remains the same, these skipper populations will likely return to normal if precipitation and temperature are near to normal in the next few years.

Indicators of environmental destruction outside of Washtenaw County

Even when the proper habitat and environmental conditions exist in Washtenaw County, some butterflies are occurring less frequently here. This type of change is primarily seen in migratory butterfly species. When we don’t see them here it likely means there has been severe environmental damage in a critical part of their range.

By the end of the summer we should be seeing a great influx of Monarch butterflies migrating through our county as they make their remarkable fall pilgrimage to from Canada to Mexico. Tragically, very few numbers of Monarch butterflies have been observed in Washtenaw County this year. Monarchs have declined across their entire range, including in Washtenaw County, because agricultural fields now almost entirely suppress their host plant, milkweed. Not only has this butterflies natural habitat been plowed under to make way for more agricultural fields, today’s fields are now almost entirely planted with genetically modified soybeans and corn and use herbicides that destroy milkweed. In addition, not enough has been done to protect the Mexican forests where Monarchs winter, and today little is left. Private landowners are encouraged to grow milkweed for Monarchs.

The drop off in Monarch numbers that we observed this year is evidence that annual changes like the historic drought of 2012 can be devastating to butterfly species that are already in decline from long-term changes like habitat loss or pesticide use.

(Source: http://www.nytimes.com/2013/03/14/science/earth/monarch-migration-plunges-to-lowest-level-in-decades.html)