Breeding-bird assemblages of calcareous grasslands and heathlands provide evidence for Common juniper (Juniperus communis) as a keystone species

Abstract

Relative to their biomass or abundance, keystone species have a disproportionate effect on many other organisms. Common juniper (Juniperus communis) is an evergreen shrub that has significant impacts on microclimate, composition of plant assemblages and herbivorous insects. However, J. communis is currently not considered a keystone species. The present study aims to provide deeper insights into the potential importance of juniper as a keystone species in its two main habitats: calcareous grasslands and heathlands. The response of breeding birds to the occurrence of Common juniper was highly similar in both habitat types. Plots with the presence of J. communis had a higher species richness and density of all and shrub-nesting species than absence plots. Additionally, presence plots exhibited a higher density (calcareous grasslands) or richness (heathlands) of threatened species than absence plots. Overall, the occurrence of scattered juniper shrubs boosted species richness and abundance of breeding birds. Responsible for this pattern were (i) the general increase in structural complexity and (ii) two (genuine) properties of the tall, evergreen shrub in particular: dense growth and prickly leaves. Due to the latter, the shrubs provide nesting sites that are strongly sheltered against predation and adverse weather. However, juniper shrubs have also an important function as song posts and perches for hunting. Additionally, the usually scattered distribution of juniper shrubs ensures a maximum territory density in calcareous grasslands and heathlands. Our study provides evidence that J. communis should be considered a keystone species in both habitat types of European conservation concern.