Butterfly species richness in the north-west Mediterranean Basin:: the role of natural and human-induced factors

Aim We developed a model enabling us to evaluate the contribution of both natural and human-related factors to butterfly species richness in Catalonia, a Mediterranean area that harbours one of the most diverse butterfly faunas in Europe. Location The study was carried out in Catalonia (north-east Iberian Peninsula), a region of 31,930 km(2) lying between the Pyrenees, the Ebro depression and the Mediterranean sea. Methods Data from the Catalan Butterfly Monitoring Scheme were used to assess butterfly species richness from 55 transects spread all over the region. Three groups of environmental variables likely to affect the presence of butterfly species were calculated, above all from geographic information system data: (1) climatology and topography, (2) vegetation structure and (3) human disturbance. Because climatic and topographic variables are expected to be strongly correlated, we first performed a principal component analysis (PCA) to create a summarizing factor that would account for most of the variance within this set of variables. Subsequently, a backward stepwise multiple regression was performed in order to assess the effects of environmental factors on butterfly species richness. Results A total of 131 species were detected in the monitoring transects, representing 75.7% of the butterfly fauna known from Catalonia. Mean species richness per transect and per year was 41.4, although values varied greatly among sites (range: 14-76.8). The final regression model explained more than 80% of the total variance, which indicated a strong association between butterfly species richness and the studied environmental factors. The model revealed the very important contribution of climatic and topographic variables, which were combined into a single factor in the PCA. In contrast to what has been found in other, more northerly countries, species richness was negatively correlated with temperature and positively correlated with rainfall, except for extreme cold and wet conditions. This may be a consequence of the predictably adverse effects of the Mediterranean summer drought on herbivorous insects, and the fact that the limits of distribution of many butterflies correlate well with climatic variables. Human disturbance (defined as the proportion of urban and agricultural landscape cover in buffer areas of 5 km around the transect sites) was the second most important predictor for species richness. We found that a significant decrease in species numbers was associated with an increase in human pressure, a finding that indicates that not only building development, but also modern-day agricultural practices are detrimental to the conservation of Mediterranean butterflies. Surprisingly, vegetation variables had an almost negligible effect on butterfly species richness. Main conclusions Our findings strongly indicate that the current motors of global change will have a negative effect on Mediterranean butterfly assemblages. First, changes in land-use are transforming and fragmenting the landscape into an inhospitable and less permeable matrix for butterflies. Secondly, the negative correlation between species richness and temperature will lead to a predictable loss of diversity over the coming years, as predicted in the most plausible scenarios of climate change. Considering the high butterfly richness characterizing the Mediterranean Basin, this future trend will pose a serious threat to biodiversity.