Impact of land-use change on flower-visiting insect communities on an oceanic island

Land-use change has profoundly impacted pollinator communities throughout the world. However, the processes through which it acts on pollinator diversity and composition are still poorly understood, especially in highly vulnerable island ecosystems.

In this study, we investigated the distribution, abundance, richness and composition of flower-visiting insects to assess their response to land-use change in Terceira Island (Azores).

Flower-visiting insects were sampled over 2 years using a standardised protocol along 50 transects across five different habitats corresponding to a land-use gradient. Insect species were classified as indigenous or exotics. We assessed changes across habitats using multiple diversity indices, species abundance distribution models (SAD) and species composition metrics (β-diversity), along with plant species composition.

We observed that indigenous flower-visiting insects were dominant, both in abundance and species richness, across the entire land-use gradient. Species diversity varied only slightly across the gradient. SADs were lognormal in all habitats, with very few truly common and rare flower-visiting insects and a prevalence of species of intermediate abundance. Species replacement was significantly higher mainly between the two most contrasting habitats (i.e. natural forests and intensive pastures) but was significantly correlated with species replacement of host plant species across the gradient.

Our results revealed that the Azorean flower-visiting insect communities were highly simplified across the entire gradient with little difference between habitats. In the absence of strong exotic competitors, indigenous flower-visiting insects expand their range and occupy new anthropogenic habitats, also facilitating the expansion of a large number of exotic plant species.