Effect of land use intensification on specialization in plant–floral visitor interaction networks in the Pampas of Argentina

Abstract

Land use intensification reduces natural areas and alters landscape heterogeneity diminishing species richness and changing ecosystem functionality. The analysis of interaction networks is a useful tool for estimating the degree of change experienced by ecosystems. Here, plant–floral visitor networks from fragments of grassland restored by agricultural activities discontinued over a long time were compared with networks from agriculturalized fragments nearby. The hypothesis evaluated was that land use intensification changes the structure of the original networks by modifying species richness and the network degree of specialization. The study was carried out at three sites located over a 700 km transect across the provinces of Buenos Aires and La Pampa, Argentina. At each site, four fragments were selected (two restored and two agricultural). Five monthly samplings were taken between November and March along three consecutive years. Entomophilous plants and their floral visitors were identified and all plant–floral visitor interactions were recorded. With these data, richness of plants and visitors were calculated, interaction networks were constructed and the degree of specialization was calculated for each network. Land use intensification caused changes in the community composition and degree of specialization. When cropping was the predominant farming enterprise, richness of floral visitors and plants decreased as compared to restored fragments. When cattle production was the predominant land use, richness of floral visitors remained stable. However, at one of the sites plants richness was 48% higher in the grazed fragments than in those restored. In the agricultural fragments, the abundance of units of floral attraction due to exotic plants increased, but the visitation rates were the same as in the restored fragments. The networks in the agricultural fragments were more generalist than in restored fragments, in agreement with previous studies on the effect of land use intensification on natural systems.