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Keywords:

  • biodiversity;
  • global change;
  • management strategies;
  • metapopulation structure

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

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.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

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?

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study sites

The study sites were selected out of the fen sites of the Swiss wetland inventory from five cantons of the central and north-eastern part of the country in the alpine foothill zone (Broggi 1990). This area holds a particularly high density of calcareous fens (plant communities of the vegetation alliance Caricion davallianae, Ellenberg 1986). The aim was to select a range of study sites balanced for altitude, farming management (an attribute of habitat quality) and habitat area (an attribute of habitat fragmentation). Only sites situated between 800 and 1400 m a.s.l. were considered and these were split up into three altitudinal levels (‘low’: 800–1000 m; ‘medium’: 1000–1200 m; ‘high’: 1200–1400 m). Furthermore, only predominantly and traditionally cattle-grazed or mown sites were considered and these were classified, respectively. Out of these six categories (309 sites in total), six sites per category were selected at random. In an attempt to balance the selected sites with respect to habitat area, the two sites with the most extreme habitat area were subsequently excluded in each category. The final set of study sites contained 24 fens at three altitudinal levels, two management types and habitat areas ranging from 0·8 to 15·4 ha (Table 1). General arthropod data (order level, see below) and butterfly data were collected during a first visit from 4 June to 18 July 1996 and during a second visit from 19 July to 26 August. Grasshopper data were collected at only one visit per site from 28 July to 17 September 1996. To minimize the effect of observation date on the results, the sites were grouped into four blocks such that within each of four corresponding monitoring sequences during the two time periods mentioned above, one site per treatment combination (altitude × management) was visited once. The sites within a monitoring sequence were arranged to minimize travelling distance. No significant differences between the blocks were observed for any diversity measure recorded and, therefore, blocks are not mentioned further in the text.

Table 1.  List of sites visited during the study, the measured habitat variables and the dates of the visits at the sites. M, mown; G, grazed; l, 800–1000 m a.s.l.; m, 1000–1200 m a.s.l.; h, 1200–1400 m a.s.l.; Veg. str., index of vegetation structure; AWH, additional area of wetland habitats within a radius of 4 km; HCR, proportion of further habitats with conservation relevance adjoining the site; Temp., average of air temperature during survey; a, first butterfly survey; b, second butterfly survey; c, grasshopper survey
Habitat variables
Date of visit (1996)
SiteSwiss grid positionVeg. str.Habitat area (ha)AWH (ha)HCRTemp. (°C)abc
Ml 1699 200/218 8503161·2328·5022·86 June22 July
Ml 2697 500/224 5003707·0355·4018·814 June31 July3 Sept
Ml 3725 850/234 9004101·295·30·3117·417 June4 Aug4 Aug
Ml 4702 800/222 0007209·4302·00·1017·711 July16 Aug16 Aug
Mm 1702 800/222 00036312·822·30·1817·36 June21 July3 Sept
Mm 2691 400/214 0003682·7286·2023·012 June29 July29 July
Mm 3749 000/230 7505742·7197·30·2016·93 July5 Aug5 Aug
Mm 4735 850/241 0005202·476·10·0920·217 July23 Aug23 Aug
Mh 1699 800/209 0002771·01058·90·7418·88 June24 July6 Sept
Mh 2692 100/213 2503529·1281·80·2318·115 June1 Aug1 Aug
Mh 3696 950/210 8504705·3935·30·7819·74 July8 Aug8 Aug
Mh 4746 850/225 5006464·6161·60·4114·718 July26 Aug26 Aug
Gl 1704 200/221 4002175·1276·30·5223·87 June23 July5 Sept
Gl 2691 600/214 7501790·9299·80·1321·012 June28 July28 July
Gl 3703 600/222 50042710·8320·0021·65 July10 Aug10 Aug
Gl 4712 200/220 7006091·4215·40·1619·713 July18 Aug18 Aug
Gm 1728 250/230 1002571·2134·7017·34 June19 July17 Sept
Gm 2705 600/213 2502886·2310·50·2117·59 June25 July5 Sept
Gm 3707 200/220 40050011·9255·90·4317·412 July17 Aug17 Aug
Gm 4712 200/220 7005203·7195·40·3424·114 July19 Aug19 Aug
Gh 1728 000/229 00021015·4112·60·4414·25 June20 July
Gh 2701 450/215 9502972·5453·40·3217·610 June26 July
Gh 3739 500/212 8505151·298·00·4519·919 June9 Aug9 Aug
Gh 4716 000/222 20044513·3202·10·5419·715 July20 Aug20 Aug

Environmental parameters

In addition to the factors detailed above, vegetation structure was measured as another indicator of habitat quality. The nature of the habitats adjoining the site, as well as the additional area of wetland habitat within a radius of 4 km, were assessed as a further aspect of habitat fragmentation.

The index of vegetation structure was the product of vegetation height and vegetation density measured at 10 sample plots of 1-m radius. Two plots were selected from graminoid-rich patches (Carex davalliana Sm., C. panicea L., Molinia caerulea (L.) Moench, Parnassia palustris L., Primula farinosa L., Succisa pratensis Moench, Tofieldia caliculatea (L.) Wahlenb.) and two from tall-herb dominated patches [Cirsium oleraceum (L.) Scop., C. palustre (L.) Scop., Filipendula ulmaria (L.) Maxim., Ranunculus aconitifolius L., Trollius europeaus L.]. To define the location of these plots, the observer walked through the site for between 1 and 5 minutes. If this took the observer to the wrong vegetation type, he continued, 10 steps at a time, until he was in one of the correct vegetation types. The fifth plot was placed at the centre of the most common additional vegetation type. Vegetation height was taken as the average of the maximal plant height and the estimated height containing 75% of the above-ground vegetation. To estimate vegetation density, a variation of the point intercept method was used. The number of contacts of the vegetation with a metal rod positioned vertically at four places in each plot was counted. The product of the averages of vegetation height and vegetation density was then used as the index of vegetation structure for one site. This index was positively correlated with 1995 estimates of standing crop (dry biomass per m2; R2 = 0·31, n = 23, P < 0·01; D. Pauli & M. Peintinger, unpublished data).

Arthropods can move between a habitat island and its surroundings, and can migrate between several habitat islands. We therefore investigated the quality of the habitats adjoining the study sites. Despite the great heterogeneity of surrounding habitats, we used the following two simple categories: (1) habitats with conservation relevance adjoining the site (nutrient-poor meadows mown once or twice a year, steep pastures with low grazing intensity, set-aside grasslands, unmanaged forest clearings, mixed forests with bushy understorey, open forests with at least 60% herbaceous cover, and diverse forest edges); (2) other habitats adjoining the site (mostly intensive agricultural or silvicultural habitats). On maps of the sites, the borders were divided into sections corresponding to the two groups and the proportion of adjoining habitats with conservation relevance was calculated. This covariable was positively correlated with altitude (R2 = 0·40, n = 24, P < 0·001).

The third variable was the additional area of wetland within a radius of 4 km around each site. This was measured with the help of the software Toposcop Standard-Version 1.0.4. (WSL, Birmensdorf, Switzerland) on printed maps containing all the fens in the inventory (Broggi 1990). We calculated the areal extent of all additional wetland around the centre of each study site, excluding the site itself.

Finally, during each visit the air temperature was recorded at 5-min intervals with a Campbell CR10 data logger (Campbell Scientific Ltd, Sutton Bonigton, UK) and Cr-Al-thermocouples (Bakrona AG, Basel, Switzerland). The average temperature decreased with altitude from 20·4 °C at the lower altitudes to 17·8 °C at the higher altitudes, but this decrease was not significant (P = 0·15). This covariable was evaluated to allow corrections for temperature-based differences among recording days for arthropod diversity. However, no measure of arthropod diversity correlated significantly with the average air temperature during the visit to each site. This confirmed that observations of arthropods were independent of the temperature under the defined monitoring conditions.

Arthropods

To obtain general information about the arthropods living in the above-ground vegetation of the fens, they were collected with insect nets in 10 different plots – five plots at each of the two visits per site. These plots were at the same positions as the points selected for measuring the vegetation structure. The net was swept 10 times at equal intensity through the vegetation. The collected arthropods were determined to the order (suborder for the Hemiptera) and counted. From these data we calculated the total abundance of arthropods and the Shannon diversity index H at the order level. The number of observed taxa was not a suitable measure because of its small variation among sites.

Grasshoppers

The number of grasshopper species was determined at two scales at each site. First, we established a species list for the whole site. All species of the Tettigoniidae, Gryllidae, Catantopidae and Acrididae observed during the 2 days of study at every site were recorded. This overall species richness at site level is called species richness in the following text. Secondly, we determined the number of species in six transects to obtain an average measure of species density per unit area of 20 m2. For each transect all species found within a band of 1 × 20 m on a 60-second walk were considered. With this method, the number of species at the transect level was assessed with the same intensity at all sites, independent of their different total areas. This measure is called species density of the grasshoppers. To define the position of the transects, the site was divided into six approximately equal parts and the transect was placed through the centre of each part. Site Ml 1 was completely mown before the field visit to census the grasshoppers and at the sites Gh 1 and Gh 2 snow fell before the visit (Table 1). Therefore, these three sites were not considered in the analysis of the species density of grasshoppers.

Butterflies

Species richness and species density of butterflies were determined using the same procedure as for grasshoppers. To ensure comparable data, counts of butterflies were only done under favourable monitoring conditions (Pollard & Yates 1990). On site Gh 1, these conditions were not found because of strong winds, cold temperature or thunderstorms during five different occasions when the site was visited. Therefore, this site was not included in the analysis of the butterflies. To determine species richness at site level, all species of the Papilionidae, Pieridae, Nymphalidae, Satyridae, Lycaenidae and Hesperidae were recorded during the two visits. Additionally, the Zygaenidae were recorded as Zygaena sp. Based on the habitat preferences of the caterpillars and the adults (Lepidopterologen-Arbeitsgruppe 1991), the species were classified as ‘wetland indicator’ and ‘other species’. Wetland indicators either depend entirely on wetlands or their caterpillars regularly live in at least two types of wetlands. Data for species density were only collected from 11 July to 17 August 1996 during one visit at three transects per site. The low density of butterflies made it necessary to use larger transects of 10 × ≈540 m (10-min walks of 600 steps with ≈0·9 m per step) than for the grasshoppers. Therefore, species density for the butterflies was the average number of species observed per unit area of c. 5400 m2 during 10 minutes.

Statistical analysis

We used ancova models with the two factors altitudinal level and farming management, the two covariables area of habitat and vegetation structure, and if necessary further covariables to determine the relationship between these habitat features and the obtained measures of animal diversity and abundance. The statistical models were calculated with the program JMP using type-III sum of squares (SAS Institute 1994). Because the factorial design was balanced and the first two covariables were not correlated with the factors or with each other, these results were generally similar to those obtained with type-I sum of squares. The influence of the proportion of adjoining habitat with conservation relevance was tested with a simple regression analysis because of its correlation with altitude.

To check the appropriateness of the statistical model, the distribution of residuals was compared with the normal distribution using the Shapiro–Wilk test and plotted against the fitted values to visually check the homogeneity of variance. The covariables area, vegetation structure and area of wetland within 4 km were subsequently log-transformed for all final analyses.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Arthropods

A total of 5488 arthropods out of 15 taxonomic groups was collected. Diptera and Auchenorrhyncha were caught most frequently with 30·8% and 21·7%, respectively. On average, 228·7 individuals out of 9·1 taxonomic groups were caught at one site with 100 sweeps of the insect net. At this high taxonomic level, sensitivity to most of the measured environmental parameters was low. Only the vegetation structure showed a correlation with the arthropod diversity at the order level, indicating increasing diversity with increasing vegetation structure (P < 0·01, Table 2, Fig. 1a). The abundance of all arthropods, however, was not related to vegetation structure (P > 0·1), but it decreased significantly with increasing altitude from a mean of 249 animals per site at low altitude to 236 animals at the intermediate level and to 191 animals at high altitude.

Table 2.  Influence of vegetation structure, habitat area, management and altitude on (a) arthropod diversity at order level and abundance, and on species richness/species density of (b) grasshoppers and (c) butterflies. Only significant interactions are included and linear effects of altitude were tested when altitude showed a significant effect
(a) Arthropods Diversity at order level (R2 = 0·40)(b) Grasshoppers Species richness (R2 = 0·53)(c) Butterflies Species richness (R2 = 0·62)
Source of varianced.f.SSFSloped.f.SSFSloped.fSSFSlope
Log (habitat area)10·01570·5 13·5711·7 156·985·3*3·89
Log (veg. structure)10·28208·5**0·7110·2744·8*4·418·200·8 
Management10·00030·01 10·2500·1 116·591·6 
Altitude20·06110·9 228·5866·7** 2138·656·5** 
Linear effect 128·57813·4** 1106·8310·03** 
Altitude × log
(habitat area)  2112·625·3*l = 7; m = 8; h = −4
Error180·6002  1838·309  15159·69  
Total abundance of arthropods (R2 = 0·42)Species density (R2 = 0·56)Species density (R2 = 0·33)
Source of varianced.f.SSFSloped.f.SSFSloped.fSSFSlope
  1. (*P < 0·1; *P < 0·05; **P < 0·01.

Log (habitat area)135102·2 10·47761·2 10·5960·2 
Log (veg. structure)136382·3 11·92024·7*2·1111·2224·3 (*)−5·0
Management117381·1 12·02955·0* 114·4945·6* 
Altitude2123403·9* 21·35061·7 21·4610·3 
Linear effect1113127·2*   
Manag. × log
(habitat area) 12·52906·2*M = −0·3; G = 1·4 
Error1828427  145·6993  1744·075  
image

Figure 1. Relationship between the vegetation structure and (a) the diversity of arthropods at order level, (b) the species density of grasshoppers, and (c) the species density of butterflies. The sites Gh 1 and Mh 4 were the only sites with an average air temperature below 15 °C during the survey. Site Gl 2 was the smallest site in the study.

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Grasshoppers

A total of 16 species of grasshoppers was recorded at the study sites, with an average of 7·25 species per site (ranging from 4 to 10 species). The mean of the species density was 2·9 species per 20 m2. In 82% of all counts, between two and four species were found on the transects. Species richness decreased from a mean of 8·6 species per site at low altitude to 6·1 species at high altitude (Table 2, Fig. 2a). Species density, however, did not vary significantly between the three altitudinal levels (P > 0·4). With regard to habitat quality, species richness as well as species density were positively related to the vegetation structure (Fig. 1b). Significantly more grasshopper species occurred in transects at grazed than at mown sites (3·05 > 2·77, P < 0·05, Fig. 2c). On average, no attribute of habitat fragmentation appeared to influence the grasshoppers. However, species density increased with habitat area in grazed sites and decreased with habitat area in mown sites (P < 0·05 for interaction term habitat area × management; Fig. 3a). Generally, grasshoppers showed similar but more differentiated responses to the measured habitat variables, as did all the orders of arthropods taken together. The main difference between species richness and species density was that the first was sensitive to the altitude and the second was sensitive to habitat management.

image

Figure 2. Contrasting responses of species richness and species density of grasshoppers (a, c) and butterflies (b, d) to altitude and management. The boxes represent quantile plots that contain 50% of the points with the median between.

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image

Figure 3. Habitat area and grazing (circles) and mowing (diamonds) interact in their influence on the species density of grasshoppers (a); and habitat area and low (diamonds), medium (squares) and high (circles) altitude interact in their influence on the species richness of butterflies (b). For definition of variables, see Materials and methods.

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Butterflies

A total of 63 butterfly species was found during the study period in summer 1996. Sixteen of these were wetland indicators. On average, 20 species were observed at each site, ranging from 8 to 28 species. The mean species density of the butterflies was 8·3 species per transect of size 10 × 540 m. Species richness increased with increasing altitude from a mean of 19·6 species per site at low altitude to 20·1 species at intermediate altitude and to 23·8 species at the high altitudes (Fig. 2b, see also the interaction between altitude and habitat area shown in Fig. 3b, explained below). Species density, however, was not related to altitude but to variables of habitat quality. The vegetation structure was negatively related to the species density (Fig. 1c) and mown sites held a slightly higher species density than grazed ones, with a mean of 8·9 species per transect at the mown sites and 7·7 species at the grazed sites (P < 0·05, Fig. 2d).

In contrast to the grasshoppers, the species richness of butterflies was significantly related to the three variables concerning habitat fragmentation. First, species richness was positively related with habitat area. However, it increased only at the two lower altitudes and decreased at the high altitude from very high numbers at small sites to numbers similar to those of the lower altitudes for the larger sites (P < 0·01 for interaction term habitat area × altitude; Fig. 3b). This can be explained in relation to the quality of the surrounding habitat. In the sites with few adjoining habitats of conservation relevance, only few guest species from the surrounding areas were observed. At high altitude, however, where many semi-natural habitats (structured forests, extensively used or abandoned lands) surrounded the sites, several species typical of these habitats were also regularly observed in the fens (e.g. Erebia ligea L., Hamearis lucina L., Lycaena hippothoe L.). The small fens had a higher edge to habitat area ratio than the large fens which may explain the high species richness in small sites situated at high altitude. After fitting the variation explained by the proportion of adjoining habitats with conservation relevance (Fig. 4a), the effect of altitude was no longer significant (P < 0·05 for the proportion of adjoining habitats with conservation relevance and P > 0·4 for the effect of altitude, type-I sum of squares).

image

Figure 4. Influence of three attributes of habitat fragmentation on butterfly diversity: (a) species richness per site and the proportion of adjoining habitats with conservation relevance (HCR); (b) total number of indicator species at the site and the habitat area of the site; (c) total number of indicator species at the site and the additional area of wetland habitats within a radius of 4 km (AWH). Ml 2 had no adjoining habitats with conservation relevance, but one of the most diverse areas of moorland in Switzerland (Schwantenau) is only 1 km away. Mh 1 is situated within the largest wetland landscape of the study area (Moorlandschaft Ibergeregg, 1058·9 ha of fens within a radius of 4 km around the site).

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The analysis of the effects of habitat area and additional area of wetland within a radius of 4 km on species richness of butterflies revealed a discrepancy between species that serve as wetland indicators and other species (Table 3). The indicator species responded more strongly to the influence of habitat area than was observed for species richness of all butterflies (Table 3, Fig. 4b). In addition, more wetland indicator species were observed when large additional wetland areas occurred nearby (Fig. 4c).

Table 3.  Two ancova models (type-III sum of squares) assessing the relationships of five habitat variables with (a) the number of those butterfly species which serve as wetland indicators and (b) the number of other butterfly species. AWH: additional area of wetland habitats within a radius of 4 km
(a) Indicator species (R2 = 0·80) (b) Other species (R2 = 0·10)
Variablesd.f.SSQFd.f.SSQF
  • *

    P < 0·05;

  • **

    P < 0·01.

Log (AWH)118·25115·56**10·1480·93
Log (habitat area)19·1847·83*110·3840·44
Log (veg. structure)10·2870·2510·0140·98
Management11·1060·9412·2790·71
Altitude213·9155·93*211·7940·70
Error1618·773 16262·077 

In summary, grasshoppers and butterflies showed contrasting responses to the analysed environmental parameters. This may explain the low sensitivity of all orders of arthropods together. The 13 other taxonomic groups probably also had widely differing ecological demands and therefore contrasting responses may have cancelled each other out. Nevertheless, both the grasshoppers and the butterflies showed the same differences between species richness and species density: while species richness was a sensitive indicator of geographical attributes (altitude, three aspects of habitat fragmentation), species density was more sensitive to the habitat quality (management, vegetation structure). One exception was that both species richness and species density of grasshoppers were positively related to vegetation structure.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The usefulness of arthropods as indicators of human impact on wetlands is supported by this study because their diversity was strongly influenced by all the habitat variables that were investigated. However, the responses differed considerably among the different taxa and not all of the diversity measures were equally sensitive.

Altitude

The decline of general arthropod diversity and in species richness of grasshoppers with increasing altitude is consistent with most previous findings, indicating that the diversity of many taxa decreases with colder temperatures (Currie 1991; Thomas 1991; Kennedy 1994). Thus, Burla & Bächli (1991) found similar declines in species richness in drosophilid flies along altitudinal gradients in the Alps. However, the positive relationship of the species richness of butterflies with increasing altitude was unexpected. The general distribution of the Swiss butterflies shows that 119 species occur between 800 and 1000 m a.s.l., while only 109 species occur between 1200 and 1400 m a.s.l. (Lepidopterologen-Arbeitsgruppe 1991). Therefore, we suggest that our result is largely due to the higher proportion of habitats with conservation relevance adjoining the high-altitude sites. This would also be consistent with the observation that many butterfly species have been displaced from the Swiss lowlands because of intensification of land-use in recent years (Lepidopterologen-Arbeitsgruppe 1991).

Habitat quality

Only small differences in diversity were found between mown and grazed sites, with grasshoppers having slightly higher species densities at grazed sites and butterflies having higher species densities at mown sites. A possible explanation for the former may be a more heterogeneous patchwork of vegetation due to cattle trampling and feeding. Mown sites supported a higher abundance of important nectar plants (mean at mown sites 28·9 species, mean at grazed sites 19·5 species, R2 = 0·44, n = 23, P < 0·001, M. Peintinger, unpublished data) which presumably made these sites more attractive than grazed sites for the butterflies (Loertscher, Erhardt & Zettel 1995).

For some endangered grasshopper and butterfly species, the differences in abundance between the two management types were quite large. For example, Mecostethus grossus L. (an endangered grasshopper species restricted to wetlands) was present at six grazed sites but only two mown sites. This species apparently likes the structure of long sedges disturbed only by low-intensity trampling of cattle at the wettest parts of the wetlands. The endangered butterfly species of the genus Maculinea (M. alcon D & S., M. teleius Bergstr. and M. nausithous Bergstr.), on the other hand, were found at six mown sites and only a single (non-resident?) individual was found at one heavily grazed site. Grazing probably has a negative influence at some stage of the complicated life cycle of these species.

One grasshopper species, Tettigonia cantans Fuessly, was more common in mown than in grazed sites. The homogeneously high vegetation, richer in flowering plants at mown sites, may explain this preference (the nymphs feed, to a large extent, on flower parts rather than vegetative plant tissue, and the very mobile adults can easily leave the unsuitable short vegetation of grazed sites). The two butterfly species Clossiana selene D & S. and Boloria aquilonaris Stichel were the exceptions in their taxonomic group, because they were more common in grazed rather than mown sites (raw data not presented in the results section). For B. aquilonaris, the problem may be disturbance of the larval food plant Vaccinium oxycoccos through mowing too close to the ground on mossy parts of the wetlands, whereas it can survive at sites with only weak grazing pressure. These contrasting patterns shown by individual species indicate that no general management recommendations can be given that benefit all species of conservation value at the same time. Conservation planning itself should therefore contain an element of diversity instead of promoting only one type of management (Schmid 1996). As a precaution, avoiding fundamental change and intensification of traditional land-use may need to be considered as a framework within which site-specific recommendations can be developed.

With regard to vegetation structure, one can draw similar conclusions to those for altitude. The increases in general arthropod diversity and grasshopper diversity with increasing vegetation structure are consistent with ecological theory (Pianka 1966). Because the diversity rather than the abundance of arthropods was positively related to vegetation structure, the relatively simple measure used probably indicates resource diversity as well as resource quantity (MacArthur 1972). But again the butterfly species density showed the opposite relationship (though not every butterfly species followed the same trend in abundance). A possible explanation for this difference may be that many butterfly species depend directly on plant species typical of nutrient-poor soils with relatively low vegetation structure (Hodgson 1993). The observed grasshopper species, however, are generalists using many host plants.

Habitat fragmentation

All three variables of habitat fragmentation that were measured can be assumed to affect metapopulation dynamics (Hanski 1994) and biogeographic equilibria of species richness (MacArthur & Wilson 1967). First, the habitat area of a site has been shown to influence population sizes (Harrisson, Murphy & Ehrlich 1988) and therefore extinction rate. In our case, the species richness, especially for wetland butterflies, was higher at the large sites. But the fact that habitat area was not correlated with species density indicated that habitat heterogeneity probably also had an important impact (Lack 1969).

Secondly, the proportion of adjoining habitats with conservation relevance can influence species richness by indicating the degree of connectivity and therefore the colonization probability (van Drop & Opdam 1987). Indeed, more butterfly species were found at sites with a high proportion of adjoining habitats with conservation relevance, probably due to immigration of non-breeding species from the surroundings. Therefore, the proportion of adjoining habitats with conservation relevance may be a good indicator of β-diversity. To reduce the measure of connectivity to an indicator with only one dimension, however, is undoubtedly an over-simplification of the complex problem of dispersal between habitat patches.

Thirdly, the area of wetland habitats nearby represents a source of species and therefore influences the rate of colonization. This is strongly supported here for the butterfly indicator species. They often occurred only in small populations even at larger study sites. Therefore, they may only persist if they are embedded in an intact metapopulation, which prevents local extinction by the so-called ‘rescue effect’ or allows recolonization of sites after extinction (Brown & Koderic-Brown 1977; Thomas 1994; Warren 1994).

Our results for the three variables of habitat fragmentation strongly confirm the importance of butterflies as indicators of landscape change (Erhardt & Thomas 1991). Earlier work has shown a dependence on habitat area and isolation for grasshopper species too (e.g. Decticus verrucivorus, Hjermann & Ims 1996). In our study, however, such a dependence was not observed, perhaps because only two of the recorded grasshopper species (Miramella alpina Kollar and Mecostethus grossus) were restricted to wetlands. Furthermore, compared with many butterfly species, the relatively higher population density and lower mobility of grasshoppers may also make them less sensitive to habitat fragmentation and the types of surrounding habitats than was the case for butterflies.

Conclusions for conservation

Our study indicates that no single management practice should be given general priority in conservation programmes (see also Gibson et al. 1992; Schwarzwälder et al. 1997). Grazing and mowing are both useful management tools to maintain habitats for a wide variety of insects in montane wetlands. Because of the contrasting responses of different taxa to many aspects of environmental change, recommendations for conservation should carefully consider the historical context. Where the relevant biological information is available, management recommendations can be specified for particular taxonomic groups or species at a particular site. A diversity of land-use practices (different regimes of late mowing or low-density grazing, depending on the distribution of target species among target sites) should be employed at the regional level to maintain the regional diversity of species and communities.

Variables with significant effects at the highest taxonomic level (i.e. affecting the diversity of arthropods at order level) can be used for establishing initial recommendations for management plans. In this regard, highly structured vegetation patches (e.g. tall-herb dominated communities) should be encouraged. Care should be taken, however, not to displace nutrient-poor patches for plant and butterfly specialists. The highly structured patches can, for example, be arranged as buffer zones to surrounding habitat or as connecting elements between neighbouring wetland sites. High conservation priority should then be given to any threatened species present. Within the 24 study sites, special measures need to be developed for populations of five butterfly species of international conservation concern (Boloria aquilonaris, Eurodryas aurinia Rott., Maculinea teleius, M. nausithous, M. alcon).

Further loss of wetland area and intensification of agriculture in the adjoining habitats could have very serious consequences for the butterfly species with highest conservation priority. This means that further habitat fragmentation must be avoided and that, instead, a system of connected sites should be established. The connectedness could best be enhanced by ‘permeable’ habitat, i.e. a low-intensity agricultural landscape. This would maximize the chances of conserving whole metapopulations. It is important to protect these pre-alpine regions in the face of pressure for an uphill expansion of intensive agriculture as a consequence of increasing winter temperature and longer vegetation period.

In Switzerland, protecting wetland landscapes through an amendment to the Swiss constitution is an extraordinary initiative. However, it is important that the legal instruments and management plans are now rapidly developed to set the goodwill for protection into practice. One major challenge, already highlighted by Warren (1993), will be to successfully combine large-scale planning and local site management.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank Daniela Pauli and Markus Peintinger for much help in the preparation of this work, and Matthias Diemer, Markus Fischer and three anonymous referees for valuable comments on earlier versions of the manuscript. We are especially grateful to all the farmers and landowners who allowed us to work on their land.

References

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  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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Received 11 March 1998; revision received 11 February 1999