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Discussion

The results show that density and/or species richness of butterflies and burnet moths was influenced by both landscape composition and local habitat quality. From the local habitat perspective, grassy elements like road verges, field margins and non-grazed grasslands were most important habitats. Also outcrops and clearings were important, as they had high density of butterflies and burnet moths.

We see a strong correlation between habitat quality and butterfly and burnet moth density. But even more interesting in this study is the significant effect of the landscape composition. Depending on the surrounding habitats in the matrix, density of butterflies and burnet moths varies in the local habitats. Largest effect on the local habitat was found at a scale of 3000 m in radii.

The importance of the landscape and surrounding matrix have been shown in several studies. Jules and Shahani (2003) review evidence for plants and show that different matrix types (e.g. clear-cuts and agricultural land) can act in different ways to alter resources availability and movement of pollinators, seed dispersal and herbivores.  In pre-alpine calcareous grasslands surrounded by landscape dominated of forest Krämer et al. (2012) found higher number of specialist species, which also are found in a similar study by Öckinger et al. (2012) that compare species richness in grassland surrounded of forest or arable land.  The effect of landscape composition at a broader scale is also confirmed by similar studies of mine. Bergman et al. (2004) found a significant effect of surrounding landscape at a 5000 m scale on occurrence of individual butterflies and total butterfly species richness along a gradient from a landscape dominated by arable land to a landscape dominated by deciduous forest and semi-natural grasslands. Along this gradient, the occurrence increased with increasing deciduous forest and semi-natural grasslands in the landscape. Botham et al. (2015) investigated the importance of habitat area and habitat diversity for Lepidoptera communities across an agricultural gradient. They found largest effect at 2000 m with regard to habitat size, and at 500 m scale with regard to habitat diversity. These results indicate the importance of a broader landscape perspective during conservation.

Botham et al. (2015) found abundance most strongly affected by habitat size and habitat diversity, and that heterogeneity in the landscape have a positive impact on species richness and diversity is shown in many studies . The greater range of resources and microclimates may generate more stable population dynamics. The small variation in a landscape where agriculture, urbanization and forestry develops and create a more homogenous landscape may be a problem. The importance of a heterogenic landscape have especially been found on farms, where species richness have been significantly higher when there were a small-scale heterogeneity in the surroundings. A heterogeneous farm harbour an even interspersed distribution of different habitats, e.g arable fields, pastures and forests.

Today the typical agricultural landscape contains far less than 20 % of habitat suitable for butterflies. The grassy elements are therefore especially important in arable landscapes where they play an important role in butterfly dispersal, even though they may not be sufficient to preserve specialist and sedentary species.Butterflies are found to decrease with isolation from grassland patches in line with the mainland-island or source-sink hypothesis. Semi-natural grasslands are historically dependent on agriculture but as the efficiency of agricultural land and changing livestock the cultivated land area increase, while these grasslands are becoming more isolated.

In contrast to arable land, a matrix dominated by forests show a positive effect on butterfly and burnet moth density, which also is supported by others. At the landscape scale, woodland patches provide additional herbaceous habitats such as edges, clearings, glades and openings, which increase the heterogeneity in the matrix.  It is shown that an increase from 0 to 20% in woodland cover in the immediate surroundings of semi-natural grasslands enhanced butterfly richness by 13% and abundance by 16%.


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Last updated: 05/05/16