The analyses showed that plant species richness tended to decline with increasing management intensity. These findings conform to several studies, which have found pronounced decreases in species diversity after nutrient enrichment (Gough et al. 2000). There is evidence that the combination of above-ground and below-ground competition in highly fertilized meadows reduces species richness because of an interspecific competitive exclusion (Rajaniemi 2002). Intensive management caused a vegetation structure dominated by tall grasses and forbs with competitor or ruderal functional traits (Table 1), which reduce the light availability for smaller plant species (Grime 2001). In contrast, extensively fertilized meadows, where resources such as nutrients and soil moisture are generally limiting, promoted diversity by enabling the coexistence of many stress-tolerant species as a result of niche overlaps. Site conditions such as steep slope may also contribute to high plant species richness.
Besides fertilization, the cutting regime of the hay meadows is likely to affect plant diversity patterns. Principally, hay cutting has a short-term effect by removing above-ground biomass periodically, and so influencing plant dispersal, competition and germination conditions. In observational studies, the effect of cutting frequency per se remains controversial, because cutting frequency is highly confounded by the history of fertilizer applications and length of growing season (Zechmeister et al. 2003). However, experimental long-term monitoring studies have confirmed the positive effect of hay-making once a year (September or alternating July and September), while hay-making twice a year has shown a unimodal relationship on plant species richness after fertilization cessation (Bakker, Elzinga & de Vries 2002).
Analogously, Orthoptera species richness declined across the gradient of management intensity. This effect was even more pronounced if species composition (abundance) was considered. The species composition in the intensive meadows comprised only a few Caelifera species, such as Chorthippus parallelus and Glyptobothrus sp., which were the only species able to develop substantial populations in the intensive meadows. The Ensifera species were all found at very low densities, and they were probably foraging individuals from surrounding habitats (e.g. hedgerow edges). With a decrease in management intensity, the diversity of the Orthoptera assemblages increased, probably because of higher species turnover, as indicated by the high proportion of exclusive species in the extensively managed areas (> 20% of the overall species).
Although interspecific competition among phytophagous insects is recognized as an important factor controlling local population dynamics in grassland environments (Denno, McClure & Ott 1995; Chase 1996), density-independent factors might better explain the Orthoptera diversity distribution among different grassland types (Stoner & Joern 2004). Fertilization can affect Orthoptera populations in two contrasting ways: (i) highly fertilized meadows may provide a better food resource for Orthoptera because of greater N tissue concentration (Ritchie 2000); (ii) high long-term nutrient inputs create tall, dense and relatively ‘cold’ swards (Schwab et al. 2002), with strong consequences on the Orthoptera communities (Van Wingerden, Van Kreveld & Bongers 1992; Willott & Hassall 1998; Gardiner et al. 2002; Gardiner 2006).
Even though no data on food quality were available to assess this important factor in this study, the trophic resources seemed to be important at least for Chorthippus parallelus, which appeared to be highly specialized for feeding on intensively managed swards dominated by grasses. This result conforms to studies on the feeding behaviour of this species, which show they prefer to feed on tall and coarse grasses such as Dactylis glomerata and Lolium perenne (Gardiner & Hill 2004), which were common and abundant species in our intensive meadows. Regarding the sward structure, which was not directly measured because of time constraints, the functional traits of the plant species found and the vegetation cover in the three management regimes clearly differed (Table 1). Orthoptera diversity may benefit from low-input fertilization as the less dense and tall structure created by stress-tolerant plant species provides more suitable microhabitats. These results support the hypothesis that most temperate Orthoptera species require open, thin and relatively dry and warm meadows (Craig et al. 1999; Vickery et al. 2001; Gardiner et al. 2002; Knop et al. 2006). A low temperature within the sward strongly limits the above-ground stages of the life cycle, controlling all the key physiological processes (e.g. feeding and reproduction) and thus determining the Orthoptera performance and fitness (Willott & Hassall 1998). Additionally, the tall and dense sward may have led to lower temperatures in the egg environment, which possibly has negative effects on more thermophilous species (Van Wingerden, Musters & Maaskamp 1991; Gardiner 2006). In this context, we suggest that future studies aiming to develop new insights into the relations between grassland management and Orthoptera communities should include accurate measurements of food quality, vegetation structure and within-sward microclimate.
Regarding the cutting regime, Orthoptera species are likely to be affected by mowing in different ways. The greater disturbance by the two-cutting regimes may have led to higher mortality because of injuries from blades and machine movements (Wagner 2004; Gardiner & Hill 2006b). The first cut probably injures the low-mobile early instars, while the second cut affects the adults negatively (Gardiner 2006). The removal of vegetation shelter could enhance bird predation (Ingrisch & Köhler 1998) and, in some cases, could create overheating conditions (Willott 1997). Furthermore, local communities may be reduced by increased short-term dispersal to adjacent, more favourable habitats because of a lack of resources and sward protection from predation on cut meadows (Guido & Gianelle 2001). Chorthippus parallelus appeared to be able to recolonize the meadow microhabitats from adjacent undisturbed areas used as a shelter during hay-making (Thorens 1993), while the low individual abundance of the Ensifera species confirmed their higher susceptibility to a two-cutting regime (Guido & Gianelle 2001). However, a very early first cut before nymph hatching and a late second cut after reproduction might create more favourable thermic conditions in intensively fertilized meadows and might reduce mortality related to mechanical operations (Van Wingerden, Van Kreveld & Bongers 1992; Chambers & Samways 1998; Gardiner 2006).
The non-significant influence of landscape variables on plant species richness and composition conforms to several studies (Dauber et al. 2003; Krauss et al. 2004) reflecting that the actual state of meadow vegetation largely depends on local factors. Hence we assume that landscape processes such as dispersal and species-pool limitations (Poschlod, Tackenberg & Bonn 2005) are not relevant determinants compared with local resource competition and disturbance caused by grassland management.
However, the landscape composition variables showed a significant effect on Orthoptera diversity. We found a negative effect of the proportion of urban elements in the surrounding landscape on Orthoptera species richness. To our knowledge, no studies have examined the direct effects of surrounding urbanization on Orthoptera diversity in Alpine grasslands. A large amount of urban areas is likely to increase the proportion of inhospitable habitat for most Orthoptera species within the surrounding landscape of hay meadows. We hypothesize that meadows in urban areas are more prone to pollution (dry and wet deposition), recreational use and random disturbance events (e.g. fires and dumping), with detrimental effects on Orthoptera populations. However, direct causation is difficult to establish, and it remains unclear whether this result may be explained by dispersal limitations, population fragmentation or historical disturbances (e.g. differences in former land use near the villages). Thus further studies are necessary to elucidate the effect of this complex factor.
The Orthoptera species richness and composition were also significantly related to the proportion of grassland in the surrounding landscape. Chorthippus parallelus benefited from a large proportion of grassland, while most of the species belonging to Ensifera and Caelifera were affected negatively (Fig. 3). At the landscape scale, an enhanced mortality because of mowing of large areas is suggested to be the main constraint to high diversity of Orthoptera communities (Gardiner 2006). In contrast, in landscapes with a low proportion of grassland, the local diversity could benefit from the presence of ecotonal habitats such as forest edges, hedgerows and bushes. Small grassland areas possibly accumulated more visiting species from these habitats by providing suitable conditions for foraging and reproduction.
implications for conservation
We conclude that the extensive management regime (no or a very low level of fertilization and cut once a year) was most suitable for the conservation and promotion of diversity of both vascular plant and Orthoptera species. Intensive management was highly detrimental for both taxonomic groups. Compared with the intensive management regime, the low intensive regime may combine both adequate forage production and biodiversity improvements. For low intensive or intensive meadows, we suggest, as a first approximation, that the first cut should be at the end of May and the second one should be delayed until the middle of September; this should reduce Orthoptera mortality and create more favourable thermic conditions during the above-ground stages (Gardiner 2006). When hay-making, sectors of tall and dense vegetation should be left as a shelter for Orthoptera during the post-cutting period (Van Wingerden, Musters & Maaskamp 1991). The conservation of extensive hay meadows surrounded by a low proportion of urban elements and intensive meadows seemed to be important for Orthoptera, although it was of secondary importance in comparison with local management. According to recent studies, this extensive management regime is also suitable for the conservation of other invertebrate taxa such as bugs (Di Giulio, Edwards & Meister 2001), spiders (Schwab et al. 2002), butterflies (Vickery et al. 2001) and wild bees (Knop et al. 2006). In conclusion, our results indicate that the implementation of well-targeted agri-environment schemes for compensation payments against intensification and abandonment of extensive hay meadows appears to be a promising tool to protect both plant and Orthoptera diversity. In this context, we suggest that the ecological compensation area (ECA) hay meadow agri-environment scheme, already operating in Switzerland, which requires at least one cut every year and no fertilizer application (Knop et al. 2006), may also be suitable for the Italian Alpine hay meadows. Moreover, the significant influence of landscape composition on Orthoptera diversity indicates that the effectiveness of measures applied at a field scale may be improved by integration of traditional protected area schemes with farms or groups of farms managed extensively (Whittingham 2007).