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- Materials and methods
1. The patterns of arthropod diversity were investigated in 24 montane wetlands in Switzerland. These differed in altitude, management regime (cattle-grazing vs. mowing), vegetation structure (index combining vegetation height and density) and degree of habitat fragmentation.
2. The general arthropod diversity was determined by net sampling at 10 sampling points per site. The diversity of grasshoppers and butterflies was measured by counting species richness at the site and species density (species richness per unit area) on transects. The species richness of grasshoppers and butterflies was found to be more sensitive to the geographical attributes of the site whereas species density was more affected by the habitat quality.
3. Grasshopper diversity decreased within the observed altitudinal range (800–1400 m) and was higher at grazed sites, whereas butterfly diversity was higher at mown sites. Arthropod diversity but not abundance of arthropods was positively related to the vegetation structure.
4. The species richness of butterflies was negatively influenced by the degree of habitat fragmentation: both the size of habitat as well as the area of wetland habitats within 4 km were related positively to the number of specialist wetland butterflies.
5. Late mowing as well as low-density cattle-grazing are appropriate management actions to maintain arthropod diversity in montane wetlands. In order to establish site-specific management plans, the biology of the present target species as well as the historical context should be considered.
6. We suggest that the best protection for the species examined in this study would be a network of wetland sites managed using a variety of traditional, non-intensive methods. This can only be achieved by coordinated planning of conservation measures among sites.
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- Materials and methods
Wetlands continue to suffer from the severe modification and fragmentation that has occurred over the past decade, with only 10% of the original area left in lowland Switzerland (Broggi 1990). The legal protection of nationally important wetlands called for by the Swiss nation and accepted in 1989 (the ‘Rothenturm-Initiative’) demands that conservation biologists develop the tools to prevent further habitat and species loss.
The general factors that influence species diversity in terrestrial ecosystems include climate (Currie 1991), habitat structure and productivity (Pianka 1966; Tilman 1982; Greenstone 1984; Morris 1990; Rosenzweig & Abramsky 1993), and biogeographical factors such as habitat area and isolation (MacArthur & Wilson 1967; Connor & McCoy 1979). All these factors are increasingly influenced by human activities. In addition, it is expected that global warming will have significant consequences for the distribution of species and the composition of communities along gradients of temperature (Root 1993; Schneider 1993; Guisan et al. 1995). Furthermore, changes in agricultural practices are directly modifying habitat quality at the sites themselves. In addition to these continuous (and therefore less spectacular) modifications, catastrophic habitat destruction such as drainage, afforestation, building activity or road construction cause further reductions and fragmentation of entire habitat systems. Together with the decrease in the area of the remaining habitat islands, their isolation from each other increases together with the risk of local and regional extinction (Levins 1970; Saunders, Hobbs & Margules 1991; Groom & Schumaker 1993; Hanski 1994).
Arthropods are useful indicators of environmental change because of their diverse ecological characteristics and requirements. However, while the negative effects of environmental changes have been demonstrated on species or communities, only a few quantitative studies have tried to combine taxa or groups with different ecological characteristics (Gibson et al. 1992). In this study, we investigated grasshoppers and butterflies as two contrasting groups of arthropods inhabiting the same wetland areas. Grasshoppers live their whole ecopteric life cycle within vegetation but generally do not specialize on specific host plants, whereas butterflies are much more mobile as adults but their caterpillars often specialize on one or a few food plant species.
In previous studies, little attention has been paid to the importance of considering the scale of measurements of diversity. Measurement of species richness at the site level considers very rare and non-resident species as equivalent to common species. In contrast, by measuring species richness at the level of a smaller transect in a defined area (i.e. species density), the common species will be observed most frequently and species occurring rarely may not be recorded. It is therefore conceivable that species richness at the site level and species density at transect level will show different responses to environmental change.
The northern part of the Swiss Alps, where patches of wetland of different ecological characteristics and area are still relatively widespread, offer an ideal opportunity for comparative studies of the effects of environmental factors on species distributions and community composition. The aim of the present study was to analyse the influence of altitude, different farming practices (habitat quality), and habitat fragmentation (area and isolation) on arthropod communities across a large area in Switzerland. We asked the following questions. (1) Does arthropod diversity increase or decrease with increasing altitude? (2) Do grazed or mown sites show higher diversity of arthropods; and (3) what influence has the vegetation structure (which is related to the productivity) on arthropod diversity? (4) Does habitat fragmentation have a negative influence on arthropod diversity? (5) Do different arthropod taxa respond in different ways to the factors mentioned in (1)–(4)? (6) What conclusions can be drawn for the conservation of arthropods in montane wetlands?