Is Shrub Encroachment Driving the Decline of Small Mammal Diversity in Pyrenean Grasslands? A Preliminary Study

Abstract

The Pyrenean highlands hold the southernmost populations of some endemic and rare arvicoline species associated with grasslands. This area, as well as many other areas in the Mediterranean basin, has suffered from land abandonment due to socio-economic changes during the last decades. Those changes represented a reduction of the traditional livestock grazing by goats and sheep which naturally controlled the process of shrub encroachment. Today, maintenance of open habitats such as pasture grasslands needs to be performed by mechanical actions aimed at removing shrubs and woody vegetation. We document a case study on a plot sampled for five consecutive years (2017–2021) in which clear-cutting increased the cover of herbaceous vegetation by removing shrubs, potentially improving pasture quality. Also, changes in the small mammal community were detected, such as an increase in the species density and diversity (Shannon index) after clear-cutting, with the occurrence of typical grassland species (Microtus arvalis, M. lavernedii). Nonetheless, the effects were short-term (lasting two years), and then the community returned to the pre-treatment situation. Prior to intervention, the community was dominated by two generalist and widespread species (A. sylvaticus and C. russula), and the situation was similar two years after the intervention. Small increases in shrub cover produced relevant community changes by decreasing diversity and increasing dominance of the two most common species. These results suggested that shrub encroachment produced the banalization of small mammal communities dominated by generalist species.