Report on Dorset's Butterflies 2000

by Bill Shreeves

The weather in 1999 did not give much confidence that 2000 would be the sort of year to gladden the hearts of butterfly walkers. April, May, June, August and September were all wetter than average according to the weather data collected at Fontmell Magna. Only excellent weather in 2000 could neutralise the probable egg bank deficit which must have resulted. Hopes were raised by yet another dry, warm and sunny March but dashed by an April with record rainfall and a May which also had well above its fair ration. With June and July afflicted by well below average daily temperatures, and August and September warm but not especially sunny, there was no way that we could expect much from the 2000 walks.

At first sight the table of best and worst performing butterflies seems quite cheering. Three species attained 'Premier' league status with over 50% of walks achieving more than 50% improvements on both 1999 and their average counts since the walks started. This compares favourably with only 2 species ranked in the 'Premier' league in 1999 and 1 in 1998. We have to go back to the 5 of 1997 to find a better result. A closer analysis of the 3 species, however, gives less cause for optimism. The Wood White results come from just one site, Powerstock Railway, because for one reason or another the other Powerstock walks which normally record it were not valid in 2000. The Essex Skipper admittedly was recorded on 8 sites, with Melbury Down, Stour Valley and Cashmoor counting it for the first time ever. But with numbers very low, the results probably tell us more about improving recording skills than real fluctuations in the fortunes of the species. This leaves the Clouded Yellow as our butterfly of the year - despite recent proof of its ability to overwinter on the Bournemouth undercliffs, definitely not a native! The first records of Clouded Yellow on Dorset walks came with a count of 3 in late April at Sopley Common. The combined count on all walks reached a small peak in the week June 24th-30th, went into decline, and then reached a climax of 207 between September 9th and 15th. The season reached its conclusion with a freshly emerged butterfly on Southbourne Undercliff on 20th November, accompanied by about 80 caterpillars still feeding. Was the second peak in September the offspring of the earlier migrants, or due to a second wave of invaders? A count of around 300 on Portland Bill on 4th September would tend to suggest that further migration was responsible. How did 2000 rate in the historical ranking of great Clouded Yellow years? The total transect walk count was 1,035 compared to 164 in 1998 and 734 in 1996. Four of our walks were running at the time of the notorious 1983 invasion and together amassed a total of 251; the same walks totalled 228 in 2000 so we may tentatively conclude that 2000 was the best Clouded Yellow year since 1983.

Referring again to the table, it can be seen that, despite the unfavourable weather, 6 species managed to improve substantially on their performances in 1999. Most of the improvement for Adonis Blue came from the walks in the South, especially Durlston and Portland, where a first record was made at our own reserve at Broadcroft. In the north better 2nd broods helped the species to outperform 1999. For the first time Brimstones showed a major improvement across the whole County. The reasons for this are by no means clear; there was certainly a very large Spring count which may reflect a warmer than average winter securing better hibernation survival rates. On top of this, the late Summer count was also good, which might tentatively be put down to the unusually warm March enabling plentiful egg-laying. Results from the Monks Wood National Monitoring Scheme and from Hampshire Butterfly Walks show that the Brimstone did well over most of the country and not just in Dorset. The Comma was another species which had an exceptionally good year everywhere. North Dorset walks did not have a single decline. The Green Hairstreak improved considerably on its 1999 performance on most sites, with a record count being notched up at Fontmell Down.

The performance of the other two species, Marsh Fritillary and White Admiral, which appear among those which improved on 1999, was much less convincing. While it is true that the Marsh Fritillaries on Deadmoor, Lydlinch and Powerstock did well, this was counter-balanced by low counts on Hod Hill and Melbury Down and a zero count from Brackett's Coppice. The showing of the White Admiral was misleadingly boosted by new records from Pamphill and Sopley Common, and improved counts from a low base for Stubhampton Bottom, Duncliffe Wood and Garston Wood. The big colonies at Piddles Wood and Fifehead did poorly. There are 6 species in the table which did exceptionally badly (part 4) and which, in football league terms, would qualify for relegation. Somewhat surprisingly, this is no worse than in 1999 when there were also 6 and better than 1998 when there were 7. The plight of the two smaller Fritillaries, Pearl-bordered and Small Pearl-bordered, now looks very serious indeed. The single Pearl-bordered counted on the Stubhampton Bottom transect was the only one seen in Dorset and we fear that the colony there may have collapsed. Only two walks recorded Small Pearls and there are only at the most two known colonies not covered by walks. As the table below shows, the Dark Green Fritillary still stagnates and it must be noted that 52 of those counted in 2000 were at Melbury Down.

The Chalkhill Blue is experiencing a violent slump on nearly all its Dorset walks. Only Badbury Rings, Durlston West and Portland Tout are bucking the trend. Spectacular declines since 1998 include Melbury Down from 656 to 2, Fontmell Down from 3,533 to 272 and Clubmen's Down from 223 to 25. The weather data from Fontmell Magna strongly suggests that heavy above-average rainfall in May-June for 3 or more years is the cause of these slumps. When this happened between 1981-87 Fontmell Down's Chalkhill Blues dropped to an all time low of 40 and took 7 years to recover to their average count. It is possible that the heavy rainfall in May-June increases grass heights and leads to heavy mortality in the night feeding caterpillars. Alan Holden, who runs the walk on English Nature's Castle Hill, has also been experiencing a similar slump in numbers and has written an interesting paper pulling together other sites in Hampshire and Wiltshire. The severity of the slumps depends on local factors such as aspect, rabbit numbers and grazing. It is possible that recovery could be aided by increasing grazing to compensate for the heavier rainfall. This could be why Badbury Rings, where grazing has been hard, has so far escaped the slump.

Apart from the Brown Hairstreak, (Deadmoor recorded the only one seen on the walks), the other species in the 'Relegation' group was the Holly Blue. Here Dorset is out of step with both the National survey run from Monk's Wood and the Hampshire results; these report the start of a recovery from the latest downswing. It is interesting to note that if the East Dorset walks had been taken separately they would agree with the National and Hampshire assessment. Could it be that Holly Blues in North, South and West Dorset are finding it more difficult to escape the attentions of their parasitic wasps?

Another 5 species performed poorly in Dorset (See sections 5 & 6 of the table). In the case of the Wall, 65% of the walks recorded numbers over 50% down on the counts for the previous year. In addition there has been no improvement on the alarming scarcity of this species away from the coast. Out of the 234 counted only 6 came from North Dorset walks and 3 from West Dorset inland. Of the 3 counted in East Dorset, only one (from Avon Heath CP) was genuinely non-coastal as the other two, Ham Common and Holton Lee, were from the shores of Poole Harbour. The remaining 222 Walls counted all came from Purbeck and Portland. Both Hampshire walks and the National Monitoring Scheme also showed 2000 to be a poor year for the species. Even in the stronghold of Purbeck/Portland, 83% of the walks showed numbers significantly down on 1999.

The other 4 performers (Section 6 of table) did not do especially badly compared to 1999 but were all significantly below their average counts since walks began. The Silver-spotted Skipper had a low count (though by no means the lowest ever) on Fontmell Down where a new vegetation monitoring scheme showed an alarming shortage of the bare ground surrounding Sheep's Fescue grass which the females choose for egg laying. The Silver-studded Blue in Dorset is in the midst of a similar slump to the Chalkhill Blue. Since 1998 nearly all sites, both on heathland and on Portland lime, have been recording declines. Jean Turner from the Devon Branch of Butterfly Conservation has observed a similar trend on the commons of East Devon. Only two Dorset sites, Higher Hyde and Holton Lee, are at present bucking the trend. There must be a weather-related factor but in the absence of a good run of data from a heathland weather station comparable to Fontmell Magna for chalk downland, we can only guess. Followers of the fortunes of the Small Tortoiseshell in these reports may remember that back in 1997 it achieved excellent, 'Premier' league status only to crash into disgraceful relegation in 1998. It now languishes well below average for the second year, which follows the National trend fairly closely. The Small Heath is one of the worrying species in the Monks Wood National Monitoring system, which has declined steadily since 1976. Its performance in Dorset was very bad in North, South and West but better on some of the East Dorset heathland. Worryingly there were 8 Dorset sites, which used to record this species, down to 0 in 2000: Stubhampton Bottom and Oaker's Wood are woodland sites where it is understandable that it might be shaded out. But what are we to make of its dismal showing on some of the Dorset Wildlife Trust's grassland sites? On Sovell Down the count in 1985 was 112; by 1998 it was down to 0 and has not been recorded on transect since. In Kingcombe, where one would hope conservation-friendly management would befriend such a species, the Redholm walk has only recorded 1 since 1995 and the Stones transect has witnessed a catastrophic decline from 264 in 1990 to 0 in 2000. As in the case of many species of previously common farmland birds this must cause serious concern.

It only remains to mention the 2 species in the 'improving but could still do much better' category (section 7 in the table). As has been said constantly in these reports, the Painted Lady is likely to be in this category forever because of its astronomical migration in 1996. The first record on a walk was not on Purbeck or any coastal location but miles inland at Duncliffe Hill on 5th May. The combined walk totals trickled up to a small peak of 17 in the week June 24th-30th, fell back, and then built up to a major peak of 153 in the week September 2nd-8th. As with the Clouded Yellow, a count of over 100 on Portland Bill between September 4-5th would suggest that this was comprised mainly of new migrants. How did the 2000 Painted Lady year rank? The total count on all walks of 798 would make it the best year since the 18,311 of 1996! Readers may remember that the other butterfly in this 'could do better' category, the Small Copper, distinguished itself by falling into the disastrous 'Relegation' group in both 1998 and 1999. Even now its slow recovery is very tenuous and confined mainly to sites in East Dorset.

This report has been based on the 43 walks for which adequate data was received for both 1999 and 2000. Altogether 5O walks contributed viable data for 2000. Five new walks were Kinson Common (Bournemouth Borough Council) with a good Grayling colony, our own Reserve at Perryfields on Portland with small numbers of Adonis, Chalkhill, Small and Silver-studded Blues, Hethfelton Wood (Forestry Commission), which recorded a White Admiral and Silver-washed Fritillaries, Lorton Meadows (Dorset Wildlife Trust) with grassland species like Marbled White and Small Heath, and Corfe Castle Mound (National Trust)which counted Lulworth Skipper. Data was also received from Milldown (North Dorset District Council), Corfe West Hill (National Trust), and Hog Cliff(English Nature) but not enough weeks were covered to enable an index to be made. Thanks are due to all the volunteers who gave generously of their time and also to those who slaved over their computers entering the data. More volunteers are still needed especially to help with the walks mentioned above which find it hard to cover enough of the 26 weeks, and also for Powerstock Bridleway and Poorwood which have lain dormant for several years, but which we would like to restart. At the time of writing Foot and Mouth disease seems likely to prevent walking in 2001 for at least part of the year. It goes without saying that we should not do amy of the walks unless the appropriate bans have clearly been lifted but start 'normal service' as soon as this is possible.