As you read this, the 2001 butterfly count season is well along.  Wherever and whenever you counted butterflies, you learned more about the species in your area. 

But how are butterflies doing overall in North America?  To answer that, we need scientists to analyze all our count results, using advanced statistical techniques to tease out the order and sense lurking in the jumble of results that is our collective butterfly count.  Scott D. Kocher and Ernest H. Williams took up this challenge, as reported in their article, "The Diversity and Abundance of North American Butterflies Vary with Habitat Disturbance and Geography" in the Journal of Biogeography (volume 27, pages 785-794, published in 2000).

As we counters know, what we find on count day depends on many factors.  Sure enough, Mr. Kocher and Dr. Williams verified geographic expectations, especially that "species richness" (number of butterfly species) increases as we go southward but abundance increases as we go northward.  This is, to my relief, similar to my much less sophisticated graphs in the Fall and Winter 2000 American Butterflies count columns!  Species richness also increased with greater topographic relief.  In other words, you counters find relatively fewer species in level than mountainous terrain, which offers more kinds of habitats and seasonal timings on a count day than flatland does.

As for weather, relatively more butterflies were found when the lowest temperature on count day was higher, rather than lower.  No surprise there.  Who wouldn't want to start the count day at 70°F instead of 55°F?  But beyond that, the butterflies had less clear relationships to weather factors such as clouds and wind, which may surprise butterfliers until they, too, experience "perfect" weather with few butterflies but amazing butterflies in mediocre weather.

Scientists used count results to show conclusively that the number of butterfly species decreases significantly where habitats are more modified (altered and disturbed) by humans.  But butterfly abundance did not decrease there because of the increased abundance of non-native butterflies, the European Skipper and especially the Cabbage White.

We counters also know that habitat affects what we can find.  We worry about how "our" butterflies are faring in the modern landscape.  These scientists confirm our fears.  They classified counts by whether they occurred in areas with more or less than half the landscape modified by agricultural, suburban, and/or urban development.  Butterfly species richness significantly declined when the landscape tipped over half developed.  But butterfly abundance did not, because of increased numbers of one or two non-native butterflies, the European Skipper and especially the Cabbage White.  This shows the importance of counting species, and not just total numbers, of butterflies.

While we counters are having fun, we are also providing the data necessary for scientists to make the case for habitat conservation, so that we will continue to have fun counts in the future.

Copyright © 2001 by the North American Butterfly Association, Inc. All rights reserved.