The latest update from Michael Walter:
The demographics of the brimstone butterfly population at Blean Woods make for interesting reading and viewing. Back in the 1980s this large, yellow butterfly was relatively scarce here, as the graph below shows, but in the 1990s conditions for the gorgeous insect evidently improved, and numbers increased strongly, peaking in 1996, to be followed immediately by a remarkable collapse. From a magnificent 127 that year, numbers over the following three years declined to 74, then 34, and to an ignominious one in 1999. Since then the index has bumped along the bottom of the graph in single figures until 2013 when there were 11. Hopes for an immediate recovery were dashed the following year, when only 12 were recorded, but then in the last three years numbers have roughly doubled each season, the figure so far this year standing at 79, with the prospect of a further increase in the final weeks of the survey. Why the brimstone disappeared in the first place isn’t known, although during the lean period they still seemed to be fairly common in west Kent: equally mysterious is its mercurial recovery since 2014. Parasites, predators or some subtle change in its habitat due to the vagaries of our weather are all possible suspects, but the truth is that we shall probably never know.
Interestingly, the Blean nuthatch population slumped just as the brimstone slid towards oblivion. The graph below shows that, like the butterfly, nuthatches were doing well until the fateful year of 1996, after which their population plummeted to near-extinction on the reserve in 1999. Since then there has been a hesitant recovery but, unlike the brimstone, it has failed to demonstrate a consistent increase. Each year I hope for better news, but the dearth of calls and sightings this spring gave me a broad hint that 2017 was not to be the year that finally saw the nuthatch on the road to recovery, and the figure of 12 pairs on my monitored area confirmed that success has again eluded it. As with the brimstone, I have no very convincing theory as to why the nuthatch disappeared in the first place, nor why it is enjoying a partial recovery now, but it may be significant that both species remained commoner in west Kent during the lean years. Our part of the county has a slightly more continental climate than the west, with hotter, drier summers, and it seems quite plausible that this difference has become more marked and is now having an impact on the survival of nuthatch, brimstone, and many other species of wildlife.
Michael Waltermichaelwalter434@gmail.com 01227 462491
Brimstone by Glynn Crocker
Nuthatch by Dave Smith