Does observational methods affect the observed impacts of exotic plants on flower visitors in around Macadamia orchards

Abstract

Pollination of flowering plants is mostly facilitated by insects. However, there is a global decline in the pollination services caused by invasive alien plants (IAPs), excessive use of pesticides, habitat loss, and fragmentation. IAPs affect native plants pollinators and integrate into their network interactions. Habitat loss leads to small remnant patches surrounded by a matrix of mainly agricultural activities, these remnant patches support native vegetation and provide forage to pollinators within and outside the flowering period of agricultural crops. However, these patches are also vulnerable to invasion by IAP’s. Macadamia nuts are one of the fastest growing markets in South Africa. Macadamia nut trees (Macadamia integrifolia) are mostly pollinated by wild and managed honey bees (Apis mellifera), but studies have shown that large densities not only impact negatively on pollination services but affect pollinator communities as a whole. Here we study plant flower visitation by potential pollinators in a remnant patch, invaded by exotic plants, adjacent to a macadamia orchard in a heavily transformed, commercial agricultural landscape. The impact of IAPs on flower visitor community composition (abundance and species richness) was assessed using two field methods of observation methods (camera trap and visual observation). Relative to visual observation, camera traps recorded smaller species and those that tend to avoid humans, while rarer species were more prevalent in visual observation. Therefore, camera traps observed more species at a sampling point, but less species overall. Observational methods are explained by the largest amounts of conditional variation. IAPs and native plants were visited by a larger and diverse set of flower visitors than macadamia although macadamia flower densities were twice as high and covered a much larger extent. IAPs were more generalist in the flower visitors they attracted. Native plants were more specialized in the flower visitors they attract with GLMM suggesting although not significantly were associated with more species richness. These plants were also associated with overall visitation of species. More visitation of Apis mellifera were on IAP’s and native plants which contrast with macadamia with less than 10%. However, this small amount account for 65% of all macadamia flower visitations, the remainder was largely (33%) made up by Eristalis tenax. Dominant flower visitors seemed to avoid A. mellifera by partitioning their activity in time or iii

specializing on certain plant species. The importance of IAP’s in providing forage to A. mellifera before macadamia flowering periods are noted. However, this probably has negatively impacted on other potential native pollinators of macadamias (e.g. Eristalis tenax). The removal of IAPs around mass flowering native plants such as Gymnanthemum myrianthum, should support and facilitate the second most common flower visitor of macadamia, E. tenax and flower visitors in general.