How should we view temperate semi-natural grasslands? Insights from butterflies in Japan

Abstract

In humid temperate regions, it has been commonly thought that closed forests represent original states and grasslands are unstable human artifacts that persist only as “semi-natural” states. This perspective underrates grasslands, because it makes the recent decline in grassland excusable. A contrasting perspective was recently proposed for temperate Europe and North America, stating that grasslands are intrinsically natural and stable, and hence could have survived without human presence during the postglacial age. However, few quantitative analyses have been performed to test the validity of these two perspectives. Considering that the evolutionary history of species is reflected in species traits, analyzing traits of grassland species in relation to habitat stability is expected to identify the intrinsic nature of grasslands (stable or unstable) and test the validity of these two perspectives. Here, using temperate Japanese butterflies, I compared ecological traits, geographic range, endemism and habitat types in the Eurasian continent between grassland species and forest (stable habitat) and ruderal (unstable habitat) species. I found that grassland butterflies shared similar ecological traits with forest butterflies, but not with ruderal butterflies. Moreover, grassland butterflies were primarily associated with natural and stable steppes and meadows in the Eurasian continent. Furthermore, Japanese grassland species are evolutionarily unique, nurturing many endemic subspecies with narrow geographic ranges. These results indicate that grasslands should not be regarded as unstable, transient, and artificial, but as intrinsically natural and stable in humid temperate regions. These grasslands may have persisted in a stable state without human intervention during the Holocene due to natural disturbances and severe environmental conditions, which are currently immensely suppressed or altered by human. Such perspective also holds true for humid temperate regions in Europe and North America. Thus, temperate semi-natural grasslands should be conserved with high priority in these regions considering that few natural grasslands are left resulting from the present intensive land use pressures and disturbance suppression.