And for those of you who appreciate history and religion…
(This is the last in a series on this subject. Earlier graphs are replaced herein).
Modern research shows a precipitous decline in insect numbers worldwide in response to climate change and agricultural practices. For the frightening details, see the Guardian’s overview of the major reports (https://www.theguardian.com/environment/2019/feb/10/plummeting-insect-numbers-threaten-collapse-of-nature). The current study adds data to this general question, and includes 232 visits to the site (Bear Creek) across 21 sampling years from 1998 through 2020.
Observations were made along a haphazard transect (a road) about 1 km long from the caretaker’s house at the parking lot to the Specimen Rock trailhead and about 150 m beyond. The average hike spanned between 1½ and 2 hours, and the sampling time was recorded in every case.
Forty-six species were chosen from the 125 that have been found at the site. These species are generally widespread, found almost every year, and do not tend to produce population explosions. Formerly rare species such as Amblyscirtes eos, Brephidium exile, Junonia coenia, and Adelpha eulalia were excluded because they are moving into the region in numbers as of the last five to seven years. These, of course, show increasing numbers.
Butterfly abundances are recorded in terms of numbers observed per hour, and the average for each year is recorded for graphing purposes. Thus, the graphs show the annual average abundances as a function of years. Slopes are plotted for each data set, and although none of the effects approach linear, the slopes are shown to address the general trends across the years. The broad scatter is due to many factors, including climatic and weather conditions, experimental error, and the natural fluctuations of populations.
Most butterfly species in the American West are highly adaptable to wide swings in weather conditions, making them resilient against changes in climate. Regional land use is generally innocuous in terms of butterfly dynamics, neither encouraging nor discouraging population development. In comparison with agricultural regions, Pikes Peak has generally escaped the deleterious affects of pesticides, herbicides, and monocultural practices. Nevertheless, populations are beginning to show a slow decline across all butterfly families, the overall average slope being y = – 0.028x, suggesting a very slight decline in numbers across the 20 years.
Disturbance at the site has been minimal and unchanged until the spring of 2020, when a mitigation effort was initiated to assist in the management of the protected green-backed cutthroat trout. The changes affected mainly the upper south slope, destroying a significant stand of Monarda fistulosa, a mint, the flowers of which are highly attractive to butterflies, especially some members of the family Hesperiidae and the Nymphalid genus Speyeria. The numbers of these groups were not affected in 2020, probably due to the numerous other nectar sources available. Graphic results follow.
Mean slope : y = – 0.0292x
Mean slope : y = – 0.0124x
Mean slope: y = – 0.046x
Mean slope: y = – 0.077x
Mean slope: y = – o.oo64x
The interpretation of these data is difficult, of course, but the general trend is that butterfly populations are stable across this period. Out of the 46 species studied, 32 (70%) show negative trends. The very slight downward slope is suggestive and holds for every group. Other studies (posts below) show a loss of total numbers of butterflies, and a loss of species diversity across the same time period.
A piece of Deep Thoughts music (Theme 63) to read by, and two Poems.
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.
This is a summary of my 37 years of data on the butterflies of the Pikes Peak Region. The following graphs show trends in abundances as measured by butterflies observed per hour afield. Each data point is a record of the number of butterflies observed per hour of observation time. First, the whole data set for the Pikes Peak Region, with tons of scatter:
This shows an almost flat line, but slightly increasing (from 69/h to 78/h). But when I restrict the data to only Bear Creek, where I have spent most of my time, we find:a negative slope (from 87/h to 80/h). And when graphed according to the average number per hour for each year, we get:a more negative slope (from 102/h down to 79/h). This is surprising to me, as 11 years at an Austin Bluffs site shows no such decline:even when plotted according to annual averages:I also created box and whisker plots for the numbers of butterflies per hour for each summer month in Bear Creek!
In sum, butterfly numbers appear generally stable in this region, but further study is always needed as we enter a hotter earth with more fires.
A couple of new poems and songs from June and July 2020, courtesy of Covid-19.
Good old Theme 122:
And Theme 123: This one actually makes me laugh in the middle.
And Theme 119, with drums.