Road Ecology Center
Invertebrates – a forgotten group of animals in infrastructure planning? Butterflies as tools and model organisms in Sweden
- Author(s): Askling, John
- Bergman, Karl-Olof
- et al.
There is a growing concern about the ecological effects of roads and railways on animals. There is increased mortality due to road kills, changes in movement patterns and changes in the physical environment in areas affected by infrastructure. A majority of all studies have been on larger mammals. There are also a growing number of studies on smaller animals like birds, amphibians and small mammals. However, the studies of invertebrates are few in comparison with vertebrates, and the knowledge of the effects of infrastructure on this group is limited. The importance of also including invertebrates in the studies of infrastructure is evident. First of all, this group of animals is the richest of species that exists. They are also ecologically important. In Sweden, a majority of the red-listed species are invertebrates. Of 4,120 red-listed species, fully 2,337 are invertebrates. Their generation times are fast, which also makes the response on changes in their environment fast, compared to mammals and birds. For that reason, invertebrates can be expected to give an indication earlier than mammals if an area is negatively affected by infrastructure. Butterflies have several traits that make them suitable as model organisms to represent the invertebrates when studying problems due to infrastructure. In Sweden, they inhabit one of the most species-rich habitats: floral-rich semi-natural grasslands and open deciduous forests. This habitat has decreased 82 percent since 1880. Today it contains more than 1,000 red-listed species in Sweden. The habitat is sensitive to further fragmentation due to effects of infrastructure. There is a need to identify species that are dependent on these landscapes and that are possible to monitor. Butterflies are good candidates. It is relatively easy and cheap to catch and mark a large number of butterflies. They are active in daytime, and it is easy to put marks on their wings with simple equipment. Since some butterfly species are sensitive to habitat fragmentation and occur in species-rich habitats they may act as indicators of biologically rich landscapes and, therefore, as model organisms in infrastructure planning. Many butterflies are dependent on systems of patches, and that contact between them and the area of the patches are key elements in the preservation. In one study we examined the changes in the butterfly fauna across a gradient from an intensively managed agricultural landscape with a large amount of open fields to a landscape rich in semi-natural grasslands and deciduous forests. The study took place in the province of Östergötland in southeast of Sweden. About 70 percent of the species showed a positive response to the amount of semi-natural grasslands and open deciduous forests in the landscape. More species showed a significant response at the landscape level compared to at the site-level (e.g., site area). There seems to be clear thresholds in area demands where a small increase in the amount of habitats has large effect on occupancy probability. If you look at single species, the value for 50 percent probability of occurrence varied between 3-10 percent grasslands and deciduous forests for the seven species where the landscape factor was positively significant. For these species, there was a sharp drop in probability of occurrence at the thresholds. The individual species and groups of species that show clear thresholds in area demands can be used as indicators of biologically rich landscapes. In this study the whole group of the family Zygaenidae and the fritillaries may be used as indicators. In another study we investigated the barrier effect by marking and recapturing butterflies along the motorway E4 in southeast Sweden. The motorway was surrounded by semi-natural pastures with portions of deciduous trees on both sides. Every capture of an individual was positioned by GPS, and by plotting the data in a GIS application we could analyze the dispersal ability and the flight direction of most of the species. The results showed that there were large differences between species regarding the dispersal ability. We used the data set to simulate a new data set of random movements. For the Ringlet (Aphantopus hyperantus) we expected 12 percent crossings of the motorway but (we) found only 3 percent in our field data. The barrier effect was therefore calculated to 75 percent for the Ringlet. The migratory species Green-veined white (Pieris napi) showed another pattern, and we both expected and found 14 percent crossings. The main conclusions from this study of using butterflies in the planning process are: • Butterflies can be used to identify rich landscapes. • There exist critical thresholds, and infrastructure has probably the largest impact around the thresholds (3-10% of natural habitat left). • Indicator species may be used to identify rich landscapes. • Roads may act as barriers to some species. • Invertebrates are a significant group to consider in infrastructure planning. In the future the results can be used to identify potentially species rich areas without expensive field surveys, before the start of road and railway projects. Using data from aerial photos or satellites and analyzing them with thresholds for groups of species in a GIS application could give us a tool to prevent further fragmentation by infrastructure. The development of this tool is the next challenge, but there is also a need to confirm our findings in other landscapes as well. The thresholds in this study should be interpreted with some caution as the landscapes around some sites sometimes overlap each other.