Recently I posted a summary of my 37 years of butterfly records for the Pikes Peak Region, showing a loss in numbers of butterflies. Mark Wilson raised the question of species diversity, wondering if it has remained stable or changed across these years. My regional data present a false impression of diversity loss because of sampling bias. During the first 27 years I visited many sites in the region, while during the last ten years I focused mainly on Bear Creek. One site won’t normally have as many species as the whole surrounding region. But I have robust data from Bear Creek during the 22 years from 1998 through 2020, during which I recorded both the number of butterflies per hour and the number of species observed. These Bear Creek data taken alone show a slight negative trend. First, the graph of 167 data points across all the years.This shows a negative slope from 21 down to 16 species. When all the data points from each year are averaged, the trend is very similar,showing a decline from 22 to 18 species across these 22 years.
There is little doubt that as time passed I became more practiced at butterfly field identification, and thus might have been expected to find more species, not less. But the data indicate that the average walk in the woods in 2020 will not turn up as many species as it did in 1998. This trend is disturbing. I hope someone can find a flaw in my reasoning.