Discussion & Conclusion
Habitat quality and butterfly density
Most butterfly species were negatively affected by higher amount of nectar. This can probably be explained by the nectar density being highest in the former arable land, where the dominating nectar giving despecies were e.g. Trifolium repens, Capsella bursa-pastoris, and Matricaria recutita, species that are characteristically favoured by fertilizing and tramping. The high densities of butterflies were mainly found where the density of dipsacaceae, Centaurea, and Cirsium was high. This suggests that the amount of nectar in general may not always be of great importance to butterflies, and thus not a sufficient measurement of the nectar resource availability.
The richness of nectar giving species did have a positive significant effect on only four butterfly species. In this study, the number of nectar giving species was the habitat quality factor that was most even for the different pastures. This is surprising since one can expect the diversity of nectar species being greater in unfertilized areas, grazed later in the season. At the time of plant inventory though, many plant individuals had become unidentifiable, and the recorded number of species for some areas was therefore probably underestimated.
Most butterfly species were found where the grass sward height was higher than 10 cm, and no species preferred a mean lower than 5 cm. However, even though tall vegetation has been shown to be important for butterflies, there are species that are dependent on short vegetation. For example, monophagous and oligophagous species, which have one or few host-plants, has been shown to prefer short vegetation compared to polyphagous species. One species that requires low vegetation height, and thus high grazing pressure, is the red-listed Hesperia comma, which is a warm loving species that lays its eggs on Festuca ovina surrounded by bare ground. Also in the present study, H. comma was one of the species with lower grass sward height preference.
Tree/shrub cover had a negative significant effect on most butterfly species, however, previous findings show that many species do not like openness due to higher risk of predation and wind. Also,shrubs provide a greater environmental heterogeneity for competing species to coexist in.
Timing of grazing in relation to habitat quality
As reported by several studies, timing of grazing or harvest is important for habitat quality factors and butterflies. We could see that areas with late grazing, from end of July and forward, had higher grass sward, higher densities of butterflies, Dipsacaceae, Centaurea, and Cirsium, and slightly higher densities of plant species, than areas grazed earlier in the season. The former arable land had highest densities of nectar, and if that pasture would be excluded we can see that also then the density of nectar was highest in areas with late grazing onset. With late grazing, butterflies and plants get a chance to complete their reproduction.
Effects of rotational grazing
The rotational grazing regime created structural diversity with areas with short, intermediate, and tall vegetation, and this structural heterogeneity has shown to benefit butterflies. For example, with rotational grazing, host-plants for butterfly larvae to feed on and nectar resources for adult butterflies are available for a longer time in the year, and with different climates for different years, and also with global climate changes, butterflies can shift habitats within the pasture.
This was an initial case study, and these results should be verified by further studies. Future studies should evaluate rotational grazing, but in comparison with other forms of management, and also different kinds of rotational managements. In the long term, if further studies verify rotational grazing to be beneficial for biodiversity, organisations and authorities can use the results as support in decisions and suggestions of which management is the best one to use.
There are also other factors that need to be taken into account before rotational grazing can be said to be a good form of management to benefit butterflies. For example, factors on a landscape level have shown to be of great importance for the species richness and abundance of butterflies on a local scale. Thus, by only looking on a small scale the rotational management might not give the wanted results.
Responsible for this page: Agneta Johansson
Last updated: 06/01/17