Responses of the bug community
The data presented here show that management effects are clearly important (Figs 2 and 3) for the species composition of the heteropteran bug community. They account for 30% of species variance and the ordination diagrams indicate that the extensive and intensive meadows have differing species compositions (Fig. 3a,b). The extensively managed meadows show a high variation in species composition, especially between areas, indicating that apart from management several other factors, such as aspect, light conditions and isolation, affect the species composition of these sites. The intensive sites, however, have very similar communities, suggesting that management is the most important factor influencing the species composition of these meadows. Further, species diversity tends to be greater in extensively managed meadows than in intensive ones. In three areas the extensive sites have more individuals and show a more even rank abundance distribution (Fig. 1a–c). One area (Hinterranden), however, is different. The rank abundance distributions (Fig. 1d) and the ordination diagrams (Fig. 3a,b) for Hinterranden show that the bug communities are very similar in both management types. As already mentioned, the intensive grasslands have been managed less intensively in the last few years. Interestingly, the vegetation does not reflect the recent changes in management so clearly and the species composition remains typical of intensive and extensive sites elsewhere (S. Studer, unpublished data); this suggests that the bug communities react more quickly than plants to management changes (Mortimer, Hollier & Brown 1998).
The environmental variables included in the CCA model explain most of the species variance (72·3%). The interaction term is very small (7·2%), indicating that the spatial and management variables are clearly separable in their effects (Borcard, Legendre & Drapeau 1992). Spatial effects account for an important part of the total variation (35·4%) and have a strong effect on the structure and assemblage of bug communities (Figs 2 and 3). It is not yet clear whether this is due to historical factors (e.g. differences in former land use) or to present day differences in landscape structure. However, the observation is consistent with various studies that show that geographical distributions of terrestrial invertebrates tend to be patchier, with more species having restricted ranges than is the case for terrestrial vertebrates and plants (Colwell & Coddington 1994; Otto 1996). The possibility that the structure of the landscape surrounding a site interacts with management effects on the insect community will be addressed in a more detailed spatial analysis of the data using Geographical Information Systems (GIS).
Interestingly, there is no correlation between the species number of plants and heteropteran bugs (Fig. 4) and only a weak relationship between plant species number and the species composition of the bug fauna as represented in the CCA (Table 2 and Fig. 3a,b). Our findings are supported by the study of Otto (1996), who showed that the heteropteran fauna is significantly correlated with the structure of the plant community but not with species richness. Apparently the bug fauna is affected more strongly by the structure and the microclimatic conditions in the grasslands than by plant species richness per se. As many bug species are zoophagous or polyphagous herbivores the plant species richness might play a less important role than the structural diversity of the habitat.
Responses of individual species
Although the trends associated with management are clear, species do differ in their responses to management, and not all are adversely affected. For example, Notostira erratica is most abundant in intensive meadows (Fig. 5). This mirid bug has at least two generations per year. It feeds on grass leaves and can find its food even in freshly cut meadows. It is not yet clear how this species survives the frequent disturbances by cutting. It may be that it recolonizes meadows soon after the cut (Remane 1958; Morris 1969, 1979; Gibson 1980; Otto 1996). Bockwinkel (1990) has shown that at least a proportion of the adults takes shelter in adjacent meadows. If this is the case, then it is rather surprising because the females are very weak fliers due to their reduced flight muscles. The males, in contrast, are good fliers and seem to be quite mobile (Kullenberg 1944; Dolling 1991).
Nabis pseudoferus is a macropterous nabid with two to three generations per year that lives in various habitats, e.g. grasslands, cereals fields and field margins (Remane 1958; Southwood & Leston 1959; Péricart 1987). It is a predatory species that feeds mostly on smaller insects (Péricart 1987). Little is known about its biology, although it appears to benefit from intensive management and is more abundant in the intensive grassland sites (Figs 5 and 6). Macropterous nabids are often associated with ephemeral habitats and seem to be fast and strong fliers (Southwood 1960). In general, insect species with several generations per year have a better chance of persisting in highly disturbed habitats than those with only one (Southwood 1960; Morris 1979). This appears to be the case for both Notostira erratica and Nabis pseudoferus. In contrast to these species, Megaloceraea recticornis is severely reduced by cutting and is more abundant in extensive meadows (Fig. 5) or fallow land with a high proportion of grasses (Morris 1979; Otto 1996). Larval development takes place in spring, when the intensively managed grasslands are cut for the first time. This phase is the most sensitive of the whole life cycle and so susceptible to disturbances. The life cycle may therefore explain the sensitivity of the species to intensive management. Moreover, this species is a poor flier and therefore may be slow to colonize new habitats (Southwood 1960).
It is known that Leptopterna dolobrata does not occur on very poor soils (Kullenberg 1944) and that nitrogen is a factor that limits its larval development (McNeill 1973; McNeill & Southwood 1978). We might expect that intensively managed meadows with a high nitrogen availability would be a favourable habitat for this species. However, frequent and early cutting reduces this species severely (Figs 5 and 6) and the life cycle may be the critical factor. Because the females lay their eggs on the bottom part of grass stems (Kullenberg 1944), damage to the eggs is probably not the reason that it is restricted to extensive sites. Larval development, however, may be the critical phase because this takes place in June when the intensive meadows are cut (Kullenberg 1944; Wagner 1952; Southwood & Leston 1959).
Peritrechus gracilicornis is restricted to extensively managed meadows (Fig. 5). It is a lygaeid bug that occurs in warm, dry places (Wagner 1966), although little more is known about its biology. The heterogeneous and open vegetation of extensive sites provides a warm and dry microclimate on the ground and therefore ideal conditions for xerothermophilic species (Otto 1996). Adomerus biguttatus is also a xerothermophilic species and therefore favoured by extensive management (Fig. 5). Moreover, it lives on Melampyrum where it feeds mostly on the roots (Southwood & Leston 1959; Wagner 1966). It is therefore restricted to the habitats of its host-plant, which is found in extensive but not in intensive meadows (Lauber & Wagner 1996).
Hadrodemus m-flavum is a monophagous species that lives on Salvia pratensis (Wagner 1952; Southwood & Leston 1959). Like its host-plant, it is restricted to extensively managed meadows (Figs 5 and 6). This species, too, overwinters as an egg and develops in early summer (Southwood & Leston 1959), therefore early cutting may damage the larval stages. Adelphocoris seticornis lives on Vicia species and is reduced by frequent and early cutting (Figs 5 and 6). The females lay their eggs in the upper part of the host-plant stem and larval development begins in spring (Kullenberg 1944; Wagner 1952; Southwood & Leston 1959). Cutting in May or June therefore may destroy its eggs or damage the larval stages.
For the species that do not appear to respond to management, two main groups can be identified. (i) Widespread species, which occur in various types of habitats: Lygus pratensis, Lygus rugulipennis (Southwood & Leston 1959; Morris 1979), Carpocoris fuscispinus (Wagner 1966), Dolycoris baccarum (Wagner 1966), Trigonotylus caelestialum (Remane 1958; Otto 1996), Adelphocoris lineolatus (Kullenberg 1944; Southwood & Leston 1959), Stenodema laevigatum (Wagner 1952; Remane 1958; Otto 1996), Polymerus unifasciatus (Kullenberg 1944; Wagner 1952), Eurygaster maura (Southwood 1960) and Nabis brevis (Southwood 1960; Péricart 1987). (ii) Polyphagous species, which live in a wide range of grasslands and which show a certain affinity to managed meadows: Chlamydatus pulicarius (Otto 1996), Halticus apterus (Kullenberg 1944; Otto 1996), Plagiognathus chrysanthemi (Kullenberg 1944; Morris 1979; Otto 1996), Catoplatus fabricii (Péricart 1983; Otto 1996) and Berytinus minor (Péricart 1984; Otto 1996).
In conclusion, this study shows that extensively managed meadows can be species-rich habitats and valuable for specialized insect species. In contrast, the bug community of intensive meadows is dominated by more widespread and less specialist species. Frequent and early cutting reduces the abundance of many species, and especially certain specialized or rare species like the xerothermophilic element of the Swiss bug fauna. The more widespread species, however, do not show a consistent response to management and some may even benefit from intensive management. Our results indicate that the extensive management of grasslands, as supported by agricultural schemes in Switzerland, is effective in maintaining insect diversity in agricultural landscapes. In our study we have mainly investigated management techniques that are used in practice, and we therefore cannot say whether there are other management techniques that would support even more species. Moreover, our study reveals that the extensively managed meadows can vary greatly in respect to their species composition, suggesting that to maintain species richness on a landscape level it is crucial to preserve a range of sites in various regional locations.