Abstract
Butterfly monitoring schemes are recording programs initiated to monitor nationwide butterfly abundance and distribution patterns, often with help from volunteers. The method generates high-resolution data, but may be associated with a degree of habitat sampling bias if volunteers prefer to survey areas perceived to be high-quality butterfly habitats. This can result in habitats becoming underrepresented in the data set, leading to less information about the butterfly populations there. In the present study, we investigate the possibility of applying a spatial design used by the Swedish Bird Survey for nationwide, gridbased sampling, with a goal to get butterfly monitoring data covering a representative sample of different habitats. We surveyed four 2×2 km sampling squares, split into 100 m segments, in the southernmost region of Sweden (Scania) and four in the northernmost region (Norrbotten). The grid-based transects were compared with volunteer-selected transects in a G IS analysis using a refined Swedish version of CORINE land cover data to see how well these two transect designs represent true habitat coverage. A total of 53 km transect was monitored, resulting in 490 individuals and 29 different species recorded. We found that transect cover correlated significantly with overall land cover using both monitoring methods, though standardised transects outperformed volunteer-selected transects in habitat representation in Scania, but not in Norrbotten. Butterflies were found to aggregate significantly in specific habitats, but with contrasting results for the two geographically different regions. Grasslands in both regions generated a high number of recorded butterflies, although so did clear-cut and residential areas in Norrbotten as well. The highest number of individuals recorded per transect was found in bogs in Scania. This study emphasises the value of complementing free site selection monitoring schemes with spatially representative schemes such as the Swedish Bird Survey, and sheds some light on general habitat preferences for Swedish butterflies in two contrasting climatic regions. Copyright Elin Videvall et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. (Less)
Highlights
Butterflies are the most widely studied of all insect groups (Dennis et al 2006, van Swaay et al 2008), and their sensitivity to environmental change together with the availability of butterfly data makes this group very useful as indicators for biodiversity (New 1997, Thomas 2005, Dennis et al 2006, van Swaay et al 2015)
We evaluated similarity in habitat coverage between the region as a whole and the two transect approaches using Spearman Rank Correlation tests within each region followed by two-sample tests of correlation coefficients using Fisher ztransformed values (Zar 1999)
There was no difference in the relation between overall land cover and the two transect categories in Norrbotten (Z = 1.472, p = 0.14)
Summary
Butterflies are the most widely studied of all insect groups (Dennis et al 2006, van Swaay et al 2008), and their sensitivity to environmental change together with the availability of butterfly data makes this group very useful as indicators for biodiversity (New 1997, Thomas 2005, Dennis et al 2006, van Swaay et al 2015). Due to changes in land-use and intensification of agriculture throughout the last century there has been a loss of many open and half-open natural and semi-natural habitats (Nilsson et al 2013, Cousins et al 2015) This has had dramatic effects on many insect groups, and there has been a decline in butterfly numbers all over Europe (van Swaay et al 2006, Konvicka et al 2008, Van Dyck et al 2009, Dover et al 2011). The recently developed “European Grassland Butterfly Indicator” (van Swaay et al 2015) based on butterfly data from 22 European countries suggests that grassland butterfly populations may have declined as much as 30% since 1990
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