Butterfly Abundances

Population Densities of Butterflies in the Pikes Peak Region

The following records, drawn from my field logs of the last 59 years, sift out the average and the greatest observed densities of many of the common butterflies in the Pikes Peak Region. This work has been done so that future workers in this field can compare observations to see if populations are becoming more or less dense with time.

The “average number per trip” is calculated as the total number of individuals observed divided by the number of trips afield during which the species was encountered. Thus, if I found a total of 320 individuals of a species on 70 trips afield, the average density would be recorded as 320/70 = 4.6 / trip (not per hour). The “maximum density” is derived from the single trip afield during which the observed number of individuals of a species divided by the time afield equals the largest quotient. For example, if a three-hour trip found 29 individuals, the recorded density would be 29/3 = 9.7/hour. If on another trip, two and a half hours turned up 27 individuals, then 27/2.5 = 10.8/hour, a greater observed density. Thus, the greatest density observed is reported. “TNTC” means too numerous to count, usually reflecting population explosions.

The Pikes Peak Region is herein defined as including all of El Paso and Teller Counties, and portions of adjacent Park, Lincoln, and Kiowa Counties. Sites north of the Monument Divide are excluded, as they were poorly sampled, and in many cases include clear ecological differences from the Pikes Peak area itself. The Rampart Range is included north only to Mount Herman and Carroll Lakes. A radius of roughly thirty-five miles from Pikes Peak is roughly 3850 square miles or 2.46 million acres. The life zones include the Sonoran short grass prairie, the pinyon-juniper and oak-pine transition woodlands, the Canadian (Taiga), a thin, poorly resolved Hudsonian, and the Arctic-Alpine (Tundra) on Pikes Peak itself. At least 206 butterfly species fly in El Paso County alone, although many of them are occasional or stray species from the south and east.

Hesperiidae

Papilionidae

Pieridae

Lycaenidae (Lycaeninae)

Lycaenidae (Theclinae)

Lycaenidae (Polyomatinae)

Riodinidae

Libytheidae

Nymphalidae

Danaidae

Ontogeny Suite

The following pieces track the ontogeny of a butterfly, from the magnificent egg through the busy larval and anticipatory pupal stages, to the flying and mating adult, and finally the memory of a life well lived. These pieces come from my archives, from 2008 through 2020. The last (Theme 1) was the first piece of instrumental music I ever wrote on GarageBand. Enjoy.

The magnificent Egg (Theme 101)
Anticipation: The Pupa (Theme 157)
The Single-minded Larva (Theme 7)
The Butterfly Takes Wing (Theme 44)
Remembering a Life Well Lived (Theme 1)

Abundance Trends in Butterflies of the Pikes Peak Region

(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.

HESPERIIDAE:

Mean slope : y = – 0.0292x

PAPILIONIDAE:

Mean slope : y = – 0.0124x

PIERIDAE:

Mean slope: y = – 0.046x

LYCAENIDAE:

Mean slope: y = – 0.077x

NYMPHALIDAE:

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.

Update on Butterfly Numbers

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:Regional Trend.png

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:BCC Trenda negative slope (from 87/h to 80/h). And when graphed according to the average number per hour for each year, we get:mean BCC Trend.pnga 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:AB Trendeven when plotted according to annual averages:mean AB Trend.pngI also created box and whisker plots for the numbers of butterflies per hour for each summer month in Bear Creek!box plot databox plots.png

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.