Here is a case in which a generally “obvious” fact is mathematically demonstrable. The premise is that if a butterfly requires nectar from only one particular flower species, it is an extreme specialist, whereas if a butterfly will accept nectar from any flower species, it is an extreme generalist. But it is also possible to distinguish the degree of specialization by extended observation of flower visits. I have made records of about 11,400 visits by 135 butterfly species to 110 species of flowers. From these data I have produced a graph that relates the number of flower species visited frequently by a butterfly (i.e. 10+ recorded visits) to the total number of flower species that that butterfly species visits. In case it’s not obvious, let me further clarify this.
This is a picture of specialization versus generalization in nectaring. Predictably and happily, the correlation, R2 = 0.936, suggests linearity. On average, butterflies that regularly visit only one flower species can be expected to ever visit only about 5 flower species, while butterfly species that regularly visit seven different species of flowers will be found to visit about 24 species of flowers in total. Remember, these are averages.
As examples, Oarisma edwardsi (Edward’s skipperling) feeds mainly from alfalfa, but may be found on three other flower species. In the middle ground, Satyriumcalanus, (the banded hairstreak) commonly visits four flower species, but has been found on 13 total flower species. A still more extreme generalist, Phyciodes cocyta (the northern crescent) is frequently encountered on nine flower species, but also visits at least 18 other flowers.
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.