Report on Dorset's Butterflies 2001
by Bill Shreeves
Each new butterfly season is to a large extent governed by the weather in the proceeding year which would, for example, have determined the number of eggs laid. As the table below shows, this means that 2001 was partially doomed before it even began! Only the month of March 2000 had scored a ‘bull’s eye’ with higher than average mean temperatures and sunshine and lower than average rainfall. When a hugely above average winter rainfall between October 2000 and March 2001 is added to the tally sheet the prospects were not at all good. Fortunately as the table makes clear, the summer 2001 weather turned out quite well.
Once the disastrous wet, cold and sunless April was out of the way, May and June turned out better than average and, while July-September may have lacked warmth, they were at least relatively dry. Thanks to this, a year which suffered the additional handicap of Foot and Mouth effectively closing down the countryside until June, did have its silver lining. This report has been written with the aid of the 48 Transect Walks which collected enough data to make comparable totals possible for most species. Four further walks assembled enough information for abundance codes to be allocated to some species. Regrettably no data was received for the walks at Corfe, Kinson and Lulworth Lakes. A new walk at Mude Valley organised by Christchurch Borough Council has been welcomed into the system. In addition to the usual walks and km square bashing, this year has seen the start of the new project run by the local regional coordinators in which key sites are surveyed on a more regular basis via timed counts. Largely thanks to this & despite the restrictions imposed by Foot and Mouth , 49 new km squares were recorded for 10 of the key ‘rarer’ species. No less than 36 of these came from the Central and Southern Chalkdownland Regions to the south of the River Stour. My grateful thanks to all the walkers, recorders, surveyors, coordinators and data inputers who have made this report possible.
Only two species qualified in 2001 for excellent status on the grounds of showing above 50% increases on both 2000 & their averages on over 50 % of the walks (see table1). Of these the Essex Skipper is more a case of better identification skills. It was recorded for the first time on the walks at Clubmens’Down, Lydlinch, & Avon North & 16 new km squares were added to the distribution map. This makes the Small Blue a definite contender for the coveted award of Dorset butterfly of the year. Since 1998 most of the walks have registered a ‘boom’ in Small Blue numbers though table 2 does suggest that this may be coming to an end.
TABLE 1: EXCELLENT SPECIES.
Although the number of sites recording at least one Small Blue in 2001 reached a record high of 10, the actual total count on walks dropped from its best ever of 299 in 2000 to 234 in 2001.While Durlston East and Melbury Down both attained their all time records in 2001,Broadcroft Quarry dropped back below average and Cashmoor fell to its lowest ever total. Giving the award to the Small Blue might be a bit like awarding the Booker prize for a novel, which although good, was inferior to earlier books the author had written!
TABLE 2.
Five species come into the good performers category because they show increases of over 50% on either 2000 or their moving averages on over 50% of the walks .Of these Brown Hairstreak and Small Pearl-Bordered Fritillary can hardly qualify for awards as they are only recorded on a few sites. The Marsh Fritillary, however, has had an exceptionally good year.
TABLE 3: GOOD PERFORMERS.
Up to now the walks have not shown any evidence of a common trend across the county. However 2001 must definitely rank as a Marsh Fritillary year. If we ignore Kingcombe and Brackets where the species has been more or less unrecorded since 1998, most of the walks, Deadmoor, Lydlinch, Cerne Giant & Powerstock, showed above average totals. On just one day’s count at Hogcliffe astronomical numbers were recorded. Better still new records turned up all over Dorset. Eight of these came from the Central Chalk Region at Forston (Phil Sterling),Burlston Down & Lankham Bottom (De Whalley), Blakeley Hill, Piddlewood Down, & Norden Hill (Harris) and Weam & Rowden Hills (Ian Hounsell). Another calcareous record came from ,of all places, the Purbeck Region on Ballard Down near Studland Wood ,9th June (Dave Howard).Since the last record on Purbeck was in 1900-39 this poses problems! Checking a field at Bugden’s Copse near Verwood in the Northern Heath Region on 5th June ,Phil Baarda found a colony flourishing not far from where larvae were released when a site was destroyed by the construction of a Safeways supermarket in 1982.Naturally many of these records may turn out to have been wanderers rather than representatives of stable colonies and two sites ,Hod Hill and Melbury Down, bucked the trend by recording below average totals. Nevertheless this must go down as an excellent year for the Marsh Fritillary and perhaps this species should be made Butterfly of the Year in Dorset?.
Of the two other Fritillaries in the ‘good performers’ division the Silver Washed did well at most sites. Record counts were registered at Lydlinch & Wimborne St Giles & the former recorded the highest count of Valezina in Dorset in 2001 with 5.Table 4 shows that despite the marginal improvement which took it into the ‘good’ category, the Small Pearl-bordered Fritillary is in serious trouble. With the number of walks covering the species having dropped to two only, Deadmoor and Powerstock North, it would be nice to say that there are other colonies still not covered by transect walks but alas this is not the case. Only the colonies at Stonebarrow Hill &,just possibly, Povington Heath seem to be ok. The Small-Pearl may be following in the foosteps of the Pearl Bordered Fritillary which was not seen at all in Dorset in 2001 and is probably now extinct.
TABLE 4: SMALL PEARL-BORDERED FRITILLARY ON DORSET WALKS.
As Table 5 shows the Holly Blue is slowly starting to begin another of its upward swings in its parasite driven cycle. However ,a more detailed analysis of the data made possible by dividing the walks into rough Dorset regions ,indicates a more complex pattern as shown in Table 6. Back in 1997-8 the East region numbers were falling when the total count of the other three regions was still rising. When all four regions fell in 1999-2000 the numbers in the East dropped more reluctantly so that in 2000 it held 45% of the Dorset count compared to only 10% in 1998. This year, in 2001, Eastern Holly Blues have recovered rapidly to reach 58% of the count while the North has only made slow progress & the West and South none at all! Are the parasites less active in the East?
TABLE 5: HOLLY BLUE ON ALL DORSET WALKS..
TABLE 6: DISTRIBUTION OF HOLLY BLUE ON DORSET WALKS
The bad performers with over 50% decreases on both 2000 and on their averages on over 50% of the walks are displayed in table 7.
TABLE 7: BAD PERFORMERS.
The White Admiral is experiencing a slump on most of its best sites, especially in the Blackmoor Vale. Although the count on the walks at two of the Blackmoor Vale strongholds for this species ,Piddles Wood & Duncliffe Hill, dropped to zero this year ,three new km records still surfaced. One was in the Blackmoor Vale at Ryewater Nursery, 26th July (Roger Smith).At Highwood, in the South Heath Region it turned up in a new km square ,11th July (Fox). Several were also recorded in that region on Brownsea Island (Durran & Long) where it had not been officially notified since the 1980-94 Dorset Atlas. The crash of the Green Hairstreak was only to be expected after it had appeared in the ‘good division’ both in 1999 and 2000 - success in butterfly league tables is even more fleeting than in football! The other five species did not do too badly compared with 2000 but are still languishing well below their average performances. The Silver Spotted Skipper at Fontmell Down fell to one of its lowest counts ever ,largely as a result of the diminution in hot bare ground brought about by a decline in rabbits. The introduction of more cattle grazing to counteract this development is not at present succeeding. Despite languishing well below its average counts on most walks ,the Grizzled Skipper had 10 new km squares added. Six of these were in the Central Chalk region (De Whalley & Harris),and two others were in the Southern Chalk at Combe Bottom and West Compton (Harris). Most important, in view of the decline of this species outside the chalk downs ,were records from the Heathland Regions at Broadstone Golf Course (Hull) and Povington Heath (Fox).It is doubtful whether the walks tell us much about the cycles of Purple Hairstreak. Too much depends on whether the walk takes place on a day when they have decided to come down to viewing range. The walks at Iford Landfill & Badbury Rings recorded their first ever sightings this year. The Grayling is in something of a slump at present but the big totals at West Moors continue (only 1438 this year!) & Higher Hyde went against the prevailing trend with its second highest total ever. As usual the non heathland sites continue to cause concern with low counts on Portland & the Durlstons and zero counts from all four sites visited in the West Dorset Coast Region. It was pleasing however to see this species recorded again for the first time at the Five Marys in Purbeck since the 1970-1985 Atlas (Elliott,27.8).
The worst performers featured in table 8 managed to drop by over 50% on both 2000 and their average counts on over half their walks .By definition the main migrants escalate wildly between the two ends of the list every year. Table 9 shows that while the total Red Admiral count held up reasonably well Painted Lady and Clouded Yellow registered their worst performances since 1997
TABLE 8: THE WORST PERFORMERS
TABLE 9:MIGRANTS ON DORSET WALKS. .
The first record of Clouded Yellow in the year was seen by Michael Skelton on the beach at Boscombe on April 19th. This & further records from the area from May 2nd onwards have been shown by Michael to be decended from larvae which overwintered on the Boscombe cliffs (See his article in Atropos no.14.Aug.2001).At roughly the same period between 29th April and 24th May Clouded Yellows were recorded on Portland (Martin Cade) & it is possible that they also had descended from over-wintering larvae. A third area of interest was West Durlston where 3 were recorded on the transect walk in week 8(May 20th-26th). No records were received from any other walk in Purbeck or indeed anywhere else in Dorset in the April-May period but Martin Cade saw one on the West Cliffs at Portland Bill on June 5th and singletons were also counted on the Ham Common Walk on June 5th and again on 19th (Ian Rabjohn).Was this evidence of a new migration? A second brood emerged at Boscombe on July 1st & the species continued to be seen right up to November when ovipositing was still taking place on the cliffs. Could the other July-August sightings in East Dorset (Badbury Walk & Broadstone Golf Course on 16.7,Holton Lea Walk on 28.7 & Ham Common Walk (10.8) have their origins on the Boscombe Cliffs or were they part of a ’foreign’ migration? More observations were coming from the Purbeck region in late summer with 2 more on the West Durlston Walk, Chapman’s Pool on 31st Aug .(Hull) and one on the Townsend Quarry Walk, 16th Sept. Portland was meanwhile continuing to log records up to 24th Sept. when 2 were seen on the Bill (Whitby).Finally a clutch of late summer records came from further West at Muckleford Reserve on 15th Aug.(Allen),2 on the Stones Common Walk at Kingcombe (28th Aug), behind the Fleet at Herbury on 22nd Sept.(Harris) and at Cogden on 23rd Sept (Durrant & Long).As on the Boscombe cliffs late survivors were still around on Portland until 14th November (Cade) and Winspit, Purbeck, until 21st October (Gibbons). Is it possible that ova may result from these and survive the winter?
Very early post hibernation (?) records for the Painted Lady came from Weymouth on 14-15th February (B,Young)and near Lyme Regis on February 20th (M.Oates).Individuals began to be seen from the 15th April on Portland and small numbers were building up there by 24th May (Cade).As in 2000 the first Painted Lady record on a transect walk did not come from the coast but well inland, this time on the Hod Hill Walk on 11th May .Amazingly none were seen at any of the south coast walks until the 2nd week in July on Ballard Down and Durlston East. Numbers built up a little in the late summer to reach a high point of 60 counted on all walks in the week August 19th-25th.In the end,in spite of their late start, the Southern walks recorded the highest total (87) followed by the North (56) the East (49) & the West (37).Townsend Quarry recorded the highest walk total with 46. The last sighting for the year came from Easton, Portland (Cade) on 26th November..
Other migrant records were scarce. A Geranium Bronze (unverified) was reported for August 28th on Portland and assumed to have originated from on pelargonium from South Africa (J.Lucas). An unverified American Painted Lady was rumoured to have been seen near Wool on October 5th followed by Long-tailed Blue, also unverified, at Winspit on the 20th.By this time the usual invasion of Monarchs was under way. A scout had turned up well ahead of the main party at Littlemoor near Weymouth on 26th August ( Ponting).However the main invasion began on Portland on October 1st with an individual recorded at Culverwell by Martin Cade which stayed around all day and into the next. Purbeck became involved on October 2nd with individuals seen at Durlston (Black), Langton Matravers (Grey) and as a welcome guest at the new BC. HQ at Lulworth. The 4th October included Winspit (Williams) and 6th ,Sandpit,Swanage(Leadbetter). On 9th October one had gone inland to the Fleets Bridge Roundabout, Poole (Davey) , on the 12th to Lulworth Cove (Campbell)and the 13th to Swineham (Grey). Further records continued in Purbeck & Portland throughout October. with the last reports at Winspit on 21st October (Gibbons) and Portland Bill on November 1st (Fletcher & Vickery).Table 10 puts the year’s records into perspective.
TABLE 10:
The three other worst performing species in Table 8 are all experiencing serious downswings which could turn into something worse. Table 11 shows only too well that for the Dark Green Fritillary the heady days of 1997 are well and truly over for this species. The fact that 52 of the 2001 count come from the one site at Melbury Down serves to highlight the serious nature of the decline even more.
TABLE 11: DARK GREEN FRITILLARY ON DORSET WALKS.
The Wall Brown never seems to be out of the bad performing divisions of these reports & we still seem no nearer to sorting out what is going wrong. Table 12 shows a further decline for the Wall. Although the number counted on inland walks has risen slightly since 2000, the percentage of walks recording at least one Wall has fallen to an all time low & even the coastal sites have begun to spiral downwards. The Regional Action Plan hoped that the coastal Walls might be able to recolonise the interior via the belt of heathland which mostly lies to the north. Back in 1999 this seemed a real possibility with high counts on some of the heathland walks; Tadnoll - 38, Higher Hyde 12, Oakers Wood - 29,& Ham Common - 4. But in 2001 the combined counts of these walks only totals 6. Urgent research is needed on the requirements of this species before it dwindles to coastal sites only.
TABLE 12: WALL BROWN ON DORSET WALKS.
The slump which began to afflict most of the sites which regularly record the Chalkhill Blue in 2000 has now gathered momentum. Ten of the 12 sites were below their average counts. The worst cases were in the North Chalk Down Region; Fontmell has fallen from a count of 3,533 in 1998 to only 31 in 2001; Clubmens’ Down from 223 to 10 and Melbury Down from 653 to 1 in the same period. Fontmell, where the walk has been running since 1980,has seen all this before. Back in 1983 the count reached a low of 40 before climbing back up to a high of 3,781 in 1996! The causes of these alarming booms & slumps have not been ascertained but there is at Fontmell a strong correlation between wet May/Junes when the night feeding larvae are struggling to reach maturity and falling numbers in the following July-August. If this combines with diminution of grazing, sometimes a result of falling rabbit populations, and rising sward heights, a downward spiral results. It is perhaps not a coincidence that the two sites which have bucked the trend, Badbury Rings and Cerne Giant, are generally thought to have had much tighter grazing. It is some consolation that amidst the slumps seven new km square records, five in the Central Chalk Region and two in the South Chalk (Gray & Harris), were still discovered and a single male Chalkhill appeared on the Sovell Down walk after an absence of the species since 1993!
In conclusion a reminder to keep all types of records flowing in; we need common species as well as rarities, records from gardens and farmlands as well as high priority sites. If recording is to continue to give us an accurate reflection of real trends we need more recorders, surveyors ,walkers and data inputers .If you can help please contact me. It would be very helpful if all recorders who send data to organisations like Butterfly Line & Insect line could remember to send or phone or e-mail a copy to me as well.