Investigating the Potential Negative Impacts of Managed Honey Bees (Apis mellifera) on Native New Zealand Plant-Pollinator Communities

Abstract

The interactions formed between plants and their animal pollinators play a valuable role in maintaining ecological integrity and sustaining ecosystem function. The introduction of an exotic pollinator may disrupt native plant-pollinator. Worldwide, there is contradictory evidence as to whether honey bees cause changes to the diversity and abundance to new pollinator communities, changes to native pollinator foraging behaviour, and/or consequences to plant reproductive success. The relative impact of honey bees is likely to depend on the behavior and guilds of native bees, changes in habitat availability and seasons. The impact of introduced honey bees is likely to be greater on small and remote oceanic islands with high levels of endemism, such as Aotearoa New Zealand. Given the widespread occurrence of honey bees in New Zealand, and recent increases in bee keeping, the impacts on native plants and pollinators need to be addressed. The objective of my study was to explore the potential negative impacts of honey bees on native New Zealand plant-pollinator communities. Using a comparative observational approach as well as some field manipulations and molecular analyses, I surveyed pollinator behavior, pollination processes, and plant fitness for two widespread and taonga plants, pōhutukawa and kānuka at three different sites across New Zealand, two with honey bees (Thames and Waiheke Island), and one without honey bees (Great Mercury Island). The widespread distribution of honey bees in New Zealand makes finding study sites without bees but with comparable plants very difficult. First, I examine interspecific (between pollinator species) and intraspecific (between individuals of one pollinator species) differences in foraging behaviour, and floral resource preferences between introduced honey bees and native solitary bees to gauge the potential for competition. Negative impacts on native pollinators was also assessed by measuring the body size of native bees across the three different study sites. Second, I compared the quality and quantity of pollen on the stigmatic surface, number of pollen tubes in the style, and number of seeds within a capsule to explore questions relating to plant reproductive success in the presence of honey bees. DNA metabarcoding techniques were used to assess pollen loads carried by both honey bees and native pollinator species. Overall pollinator body size, diversity and abundance was significantly lower in sites where honey bees are present. Specifically, the diversity of pollinators visiting pōhutukawa was significantly lower at Waiheke Island than Great Mercury Island. Honey bees are poor pollinators of pōhutukawa and their dominance at Waiheke also resulted in lower conspecific pollen deposition on stigmas, lower numbers of pollen tube in the style, which correlated with reduced pōhutukawa seed set. Kānuka was pollinated almost exclusively by native bees, and there was higher pollination rates on Waiheke. This is likely due to increased intraspecific competition between Leioproctus imitatus on Waiheke Island, resulting in higher delivery of conspecific pollen to the stigmas, increased number of pollen tubes in the style, and higher reproductive success. Results from pollen DNA analyses also demonstrate honey bees’ preference for pōhutukawa, but also exotic plants in the families Apiaceae, Asteraceae, and Fabaceae. There was a significant difference in the quality and quantity of pollen carried by different pollinators. Pollen loads carried by honey bees reflected their preferences for exotic plants. Out of all the native insects, native Leioproctus carried larger pollen loads consisting of higher proportion of conspecific pollen relative to the plants that they were observed visiting. The species richness of the pollen carried by Leioproctus were significantly lower in sites where honey bees are present, especially for exotic pollen from Asteraceae. This suggests there is competition between Leioproctus and honey bees for highly rewarding exotic plant species, which may explain the observed reduced native bee body size where honey bees are present. In order to limit the potential impact of honey bees on native plant-pollinator, I urge land managers to consider site specific attributes such as conservation status of resident plant and pollinator community and flowering peak period when making decisions regarding bee hive placement. The introduction of honey bees to small offshore islands that offer refugia to from mainland threats (such as Great Mercury Island) should be discouraged.