In terrestrial ecosystems, land-use change is one of the greatest threats to biodiversity. It has been increasingly acknowledged that land-use changes modify the structures of interspecific interaction networks that are vital to community dynamics and stability. However, these effects have typically been explored by evaluating a single type of interaction network. Therefore, it remains unclear whether land-use changes affect different types of interaction networks in a similar manner. In addition, as earlier studies have mainly focused on land-use changes towards overuse (e.g. intensification), the effects of land-use changes towards underuse (e.g. abandonment) on interaction networks are rarely explored.
We investigated herbivory and pollination networks simultaneously in the semi-natural grasslands formed in a Japanese agricultural landscape. The semi-natural grasslands were associated with either of the three types of agricultural land-uses (abandonment, extensive management, and intensive management). We quantified the structures of the interaction networks using several indices (connectance, diversity, evenness, generality, H2’, and robustness) and compared them between different land-use types.
Results/Conclusions
Both land-use abandonment and intensification similarly led to the reduction of plant and insect species richness. However, the structures of herbivory and pollination networks showed different responses to land-use changes. There was a marked contrast in the effects on network generality; while herbivory networks were less generalized (i.e. fewer number of host plant species interacting with an insect species) in intensified grasslands, pollinator networks were less generalized in abandoned grasslands. Pollination networks were less diverse in abandoned grasslands, while we found no significant effect of land-use changes on herbivory network diversity. Furthermore, the mechanisms behind the changes in interaction networks were also different between herbivory and pollination networks. The change in pollination network generality was induced by changes in plant species richness and species composition due to abandonment, while the change in herbivory network generality was induced directly by land-use intensification.
The present study demonstrates that agricultural land-use changes affect the herbivory and pollination networks in contrasting ways and suggests the importance of assessing multiple types of interaction networks for biodiversity conservation in plant-insect systems. Our results also highlight the under-appreciated importance of maintaining habitats with extensive intensity of management.