Egging Around West Sussex

I spent last Sunday and Tuesday (19th and 21st January) on the Knepp Castle Estate ‘Wildland’ project area, where I’m involved in the long-term monitoring of butterfly populations as they react to the process of re-wilding. Some species, such as Purple Emperor, are undergoing a ‘population explosion’ at Knepp, but, unsurprisingly, there are both winners and losers.

It appears that one of those species which doesn’t seem to enjoy re-wilding, at least under the current herbivore stocking densities, is the Brown Hairstreak. From a distance, the Knepp Estate appears to offer a fantastic home for this species, with many miles of blackthorn-rich hedges punctuated by potentially suitable master trees. Short, suckering growth can also be seen widely around the Estate, which appears ideal for the butterfly, until examined more closely.

These short suckers are actually composed of tough, old wood, now invariably caked in lichens. Younger shoots of fresh-looking, purplish-grey blackthorn on which female hairstreaks prefer to lay their eggs are all-but-absent. It appears that the free-ranging herds of Longhorn cattle, Fallow, Red and Roe deer, Exmoor ponies and Tamworth pigs are browsing off almost all the young wood suitable for oviposition.

This innovative re-wilding scheme often causes confusion amongst the public, as it is a project without any defined management outcomes or goals. The project seeks only to observe and measure the effects of a more naturalistic grazing regime, as the land is allowed to largely manage itself. This sort of re-wilding might provide an alternate approach to the use of land which is at best only marginally profitable for more traditional farming. Re-wilding does, however, bring the sort of benefits (Ecosystem Services) which we have traditionally undervalued, such as carbon sequestration and water purification.

The degree to which the blackthorn at Knepp is currently being browsed may reflect pressures more complex than livestock densities alone. One of the more spectacular (although almost certainly short-term) effects of ceasing to plough and fertilise arable land is the periodic dominance of some invasive plant species. Currently, large areas at Knepp appear to be under a monoculture of fleabane. This will reduce the available grass for herbivores, probably driving a greater reliance on the browsing of shrubs. Although heavier browsing of the blackthorn won’t be suiting the Brown Hairstreak, the development of sallow jungles elsewhere is certainly putting a smile on the Emperor’s face.

Having discovered last year that Brown Hairstreak egg densities over much of the ‘Wildland’ area were much lower than initially expected (from e.g. satellite imagery of blackthorn distribution), this winter I am looking at comparable areas of food-plant just outside the fenced boundaries. One stretch of hedgerow within the project area which did yield a good number of eggs (23 per hour) last year, running along the edge of a public ‘green lane’, will now be surveyed annually as a ‘control’; this area produced 19 eggs per hour on Sunday. It would appear that browsing pressure here is much reduced, probably due to the regular passage of walkers and riders nearby. I even managed to see adult hairstreaks along this field margin last August.

Although more research is required to make a water-tight case, some of my results this year have fallen into a clearly developing pattern. A two hour search around the margins of four fields within the project area produced only two eggs, both being on new growth tight against the boundary fence. I subsequently found 20 eggs per hour on the other side of the fence, where the hedge-line runs alongside a road ditch.

I also surveyed another area of the Knepp Castle Estate lying outside the ‘Wildland’ project area, finding 16 eggs per hour along the picturesque banks of the upper Adur, where it runs close to Shipley Church and the famous windmill. Sadly, this former home of literary genius and Sussex-lover Hilaire Belloc is no longer open to the public, although its great frame remains an integral part of the very beautiful local landscape.

I fully expect future surveys to confirm this emerging pattern. I shall also be surveying local roadside hedges which have been flailed in an all-too-familiar fashion, to see how surviving egg densities compare with the heavily browsed ‘Wildland’ area.


Post a Comment