A non-native woody plant compromises conservation benefits of mid-field woodlots for birds in farmland.
Agricultural intensification seriously threatens European biodiversity. To mitigate this threat, farmland habitat suitability should be improved. Mid-field woodlots surrounded by intensively cultivated fields or grasslands, represent one of the promising tools for such improvement. However, woodlots can be dominated by non-native plants and their impacts on woodlot function as refugees for biodiversity remains unclear. Therefore, we studied the impact of the invasive black locust (Robinia pseudoacacia L.) on bird species richness in woodlots in intensively cultivated Central European farmland. We surveyed 27 mid-field woodlots (13 covered by native vegetation and 14 dominated by black locust) in the Czech Republic, relating the species richness of all birds, habitat specialists and habitat generalists to woodlot characteristics: black locust occurrence, distance to the main forest, age of the woodlot, vegetation structure, arthropod biomass (representing food supply for birds), surrounding habitats and type of crops in which they are embedded. Our results suggest that the woodlots covered by the black locust had lower species richness of all birds, habitat specialists and habitat generalists than the woodlots covered by native tree species. The negative impact of the invasive woody plant on birds was likely driven by changes in vegetation structure since such woodlots had lower coverage of the shrub layer and less continuous canopy. This mechanism was particularly supported in habitat generalists, whereas habitat specialists additionally showed positive relationships to the other vegetation characteristics (namely the coverage of herbs in the ground layer and the proportion of medium-sized trees). Our study shows that even though mid-field woodlots host numerous bird species, their role in the delivery of conservation benefits for breeding birds is compromised if non-native woody plants dominate the vegetation. Therefore, from the perspective of breeding birds' conservation, the native woody plant species should be used for woodlot establishment.