Individual study: Effect of mowing and sheep-grazing on insect communities dwelling in the flower heads of dwarf thistle Cirsium acaule and greater knapweed Centaurea scabiosa in the Franconian Jura, Bayern, Germany
Volkl W., Zwolfer H., Romstock-Volkl M. & Schmelzer C. (1993) Habitat management in calcareous grasslands: effects on the insect community developing in flower heads of Cynarea. Journal of Applied Ecology, 30, 307-315
Most dry semi-natural calcareous grasslands in Germany are managed traditionally by light sheep-grazing or mowing and haymaking. These influence the occurrence and abundance of invertebrates through maintenance or changes in floral communities and micro-climate attributable to management. Thistles and knapweeds (Compositae: tribe Cynarea) support phytophage communities within their flower heads, and this study looked at the effect of management (sheep-grazing versus mowing) and unmanaged grasslands on these invertebrate communities.
Study site: The study was undertaken in the Franconian Jura, Northern Bavaria, southern Germany. The phytophagous insect communities dwelling in the flower heads of dwarf thistle Cirsium acaule (a spiny, prostrate perennial with stemless flowers) and greater knapweed Centaurea scabiosa (a non-spiny, upright perennial with flowers on long stems) were studied in three different meadow types (0.5-2 ha in area) were investigated:
Sheep-grazed – lightly grazed in early autumn characterized by sparse low vegetation (average height <10 cm).
Mown – cut once a year (usually mid-summer) characterized by dense vegetation (average height c.25 cm before cutting).
Abandoned – no management over 5 years, tall dense vegetation (>25 cm tall) with invading woody shrubs, mostly sloe Prunus spinosa and hawthorn Crataegus spp. and rose Rosa spp.
Greater knapweed sampling: Paired samples of greater knapweed flowers (100-350 heads, amounting to <20% of all flowers taken from a site) were collected in mid-September-early October 1990 and 1991 from mown (n=7), sheep-grazed (n=7) and abandoned (n=14) grasslands within about 100 m of each other. Additional unpaired samples from the abandoned grassland were also taken in both years. Samples collected between 1980 and 1983 were used to compare long-term fluctuations in two tephritid flies: Urophora cuspidata and Terellia colon.
Dwarf thistle sampling: Paired samples of dwarf thistle flowers (50-150 heads, amounting to <20% of all flowers taken from a site) were collected in mid-September-early October 1990 from mown (n=4), sheep-grazed (n=4) and abandoned (n=4) grasslands. Additional samples from sheep-grazed and abandoned grasslands were also taken in 1983 to 1985. Samples collected between 1980 and 1983 were used to compare long-term fluctuations in two tephritid flies: Urophora cuspidata and Terellia colon.
Flower heads were dissected in the laboratory, and phytophagous insects were counted, assigned a development stage, and identified to species where possible.
The responses of the particular insect species to management were not uniform and depended mainly on life-style and habitat preferences. Most species were most abundant in abandoned sites, but two species had their focus in mown and sheep-grazed sites. However at abandoned sites successional processes would eventually lead to extirpation of both thistle and knapweed, and their associated phytophagous insect communities.
Greater knapweed: The community in greater knapweed was affected both directly by the removal of flower heads and indirectly by changes in microclimate. Mowing reduced insect species packing and mean densities for a number of univoltine species (those which take one year to complete their life-cycle) i.e. two flies:U.cuspidata, T.colon; the weevil Larinus sturnus and the moth Metzneria metzneriella, compared with abandoned or sheep-grazed sites. These insect species were drastically reduced in density and could hardly produce any progeny in mown sites. Bivoltine species (two generations per year) were only slightly affected.
Dwarf thistle: Phytophagous species developing in the stemless dwarf thistle were not affected directly by mowing or grazing, but by changes in microclimate due to habitat management. No differences in species packing were found between managed and unmanaged sites. The densities of the most common species were affected inversely, with the fly Tephritis conura being more abundant in abandoned sites and two weevil species (Larinus spp.) more abundant in managed areas.
Conclusions: Management effects differed between greater knapweed and dwarf thistle. Flower heads of the stemless dwarf thistle were only slightly affected by mowing or grazing, and phytophgous insect species packing did not differ between treatments. In contrast mowing unselectively removed all of the, tall-stemmed, knapweed flower heads and therefore phytophage densities after cutting and removal for hay were effectively zero.
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