Perseus wrote:
Virtually all the possible answers have been ruled out. It is optimum condition for the species. What is the maximum per acre hour count on the other sites please in the last two years.
Hi Andy,
That's not the sort of data that I have to hand. Besides, it could be inconsistent/misleading, i.e. max counts per acre of site (eg. reserve, down, hillside etc.) or max counts per acre of suitable breeding areas within the site, or max counts per acre of suitable breeding habitat+roosting areas+nectaring areas?.
My observations around this come from transect data as well as personal observations. The factors that unite the sites on which the CHB numbers are increasing are (amongst others) a profusion (and I mean real profusion) of nectar sources as well as a considerable area of vegetation that is suitable for roosting imagines (which of course has to be topographically correctly situated as well). A super abundance of larval food plant alone does not seem to equate to a super abundance of butterflies; although clearly there is a relationship between the quantity of larval food plant in the correct topographical situation+relative sward height to gradient etc. and the size of the population that the site is able to support.
It is interesting that you state that the habitat at Mill Hill is optimal and yet also suggest that the population is in decline. Hence my line of enquiry on your thoughts as to possible causes of decline at Mill Hill when upon sites where the conditions (by definition) are excellent to optimal the butterfly is recovering from a decade or there abouts of decline. There is as much to be learnt from sites where the population is in decline as well as from sites where the butterfly is increasing.
Some fairly small sites (a few acres) are able to support a population of massive relative density (several thousand at peak) when conditions allow.
Felix.