Enhancing gardens as habitats for flower-visiting aerial insects (pollinators): should we plant native or exotic species?
Domestic gardens typically consist of a mixture of native and non-native plants which support biodiversity and provide valuable ecosystem services, particularly in urban environments. Many gardeners wish to encourage biodiversity by choosing appropriate plant taxa. The value of native and non-native plants in supporting animal biodiversity is, however, largely unknown. The relative value of native and non-native garden plants to invertebrates was investigated in a replicated field experiment. Plots (deliberately akin to garden borders) were planted with one of three treatments, representing assemblages of plants based on origin (native, near-native and exotic). Invertebrates and resource measurements were recorded over four years. This paper reports the abundance of flower-visiting aerial insects ('pollinators') associated with the three plant assemblages. For all pollinator groups on all treatments, greater floral resource resulted in an increase in visits. There was, however, a greater abundance of total pollinators recorded on native and near-native treatments compared with the exotic plots. Short-tongued bumblebees followed the same pattern whilst more hoverflies were recorded on the native treatment than the other treatments, and more honeybees on the near-native treatment. There was no difference between treatments in abundance of long-tongued bumblebees or solitary bees. The lack of difference in solitary bee abundance between treatments was probably due to a third of individuals from this group being recorded on one exotic plant species. The number of flower visitors corresponded to the peak flowering period of the treatments, that is there were fewer flower visitors to the exotic treatment compared with the other treatments in early summer but relatively more later in the season. Synthesis and applications. This experiment has demonstrated that utilizing plants from only a single region of origin (i.e. nativeness) may not be an optimal strategy for resource provision for pollinating insects in gardens. Gardens can be enhanced as a habitat by planting a variety of flowering plants, biased towards native and near-native species but with a selection of exotics to extend the flowering season and potentially provide resources for specialist groups.