Individual study: Responses of phytophagous insects to cessation of grazing at Barton Hills, Bedfordshire, England
Morris M.G. (1967) Differences between the invertebrate faunas of grazed and ungrazed chalk grassland. I. Responses of some phytophagous insects to cessation of grazing. Journal of Applied Ecology, 4, 459-474
In the UK, chalk grassland has been maintained in the past by grazing, mostly by sheep. Where land is still farmed for sheep the traditional methods of 'drifting' flocks over large areas with a shepherd are no longer used. Myxomatosis has greatly reduced European rabbit Oryctolagus cuniculus populations, these heavily grazing some grasslandin the past. The removal of both sheep and rabbits from areas of chalk downland has resulted in invasion of the formerly short closely-cropped turf by coarser grasses and scrub. To redress this, conservationists may reintroduce grazing regimes in some areas, and experiment with other treatments which partly simulate the effects of grazing, e.g. mowing. This paper is the first of a series dealing with various aspects of the invertebrate faunas of grazed and ungrazed chalk grassland in relation to conservation management.
Study site: The study was undertaken at Barton Hills (National Grid ref. TL 090297), Bedfordshire, southeast England. Exclosures and control plots measured 40 x 25 m and one of each were established in March 1965 on two adjacent sites about 50 m apart, on the upright brome Zerna (= Bromopsis/Bromus) erecta-dominated grassland. The exclosures were allowed to grow up until July 1965 when sampling began.
Grazing regime: The area had been and still was intensively grazed, with sheep grazing in all seasons (but not continuously) of the year. Rabbits are infrequently seen.
Sampling: Flowers within plots were counted in sampling strips and the follwing were counted:
1) Insect larvae in fruits - bird’s-foot trefoil Lotus corniculatus seed pods and hare-bell Campanula rotundifolia seed capsules were collected from two 1 x 25 m strips chosen at random per plot, dissected in the laboratory and insects identifield;
2) Graminivorous bugs (Hemiptera) - indices of occurrence were obtained by sweeping 200 strokes of a sweep net on each plot;
3) Bumblebees (Bombus spp.) - each plot was observed for 30 min. Bombus visiting flowers (making visits to four flowers, or 1 min after being first observed) were caught for identification, and flower species visited were recorded;
4) Grasshoppers (Orthoptera: Acrididae ) - were collected during 20 min sampling periods on each plot, supplemented by catches within the sweep net samples.
Main findings were:
Populations of two weevils Apion loti and Miarus campanulae (estimated at 5-8 weevils/m² of grassland in 1966) increased markedly in the ungrazed exclosures in response to increased numbers of fruits of L.corniculatus and C.rotundifolia, their respective larval foodplants.
Adult Neophilaenus exclamationis (a froghopper: Homoptera) taken in sweep net samples were more abundant on ungrazed plots, whislt N.campestris, was more frequent in the exclosures at the beginning of its season, but present in larger numbers on the grazed plots later in the year. Greater numbers of both larvae and adults of Leptopterna ferrugata (a plant bug) were taken in the exclosures by sweeping in 1966.
More bumblebees visited flowers in the exclosures than on the grazed plots.
Regards grasshoppers, more Chorthippus parallelus were taken in the exclosures than on the grazed plots, and vice versa for C.brunneus.