Annals of Botany

All bees are equal – but some are more equal than others

Honey bees are good pollinators - but they may pose a threat to endangered plant species.
Bee pollination in the endangered Iris atropurpurea
Bee pollination in the endangered Iris atropurpurea

The conservation of biodiversity has become one of the major goals of present environmental policies, given the current pace of global species loss. Recent assessments estimate that one in five of the world’s plant species is facing extinction (Kew/Natural History Museum/IUCN, 2010). Plant–pollinator interactions play a significant role in maintaining the functional integrity of most terrestrial ecosystems. There is mounting evidence of large scale global declines of wild bee populations and parallel diversity declines of bees and insect–pollinated plants across Europe. Landscape modification, habitat fragmentation and the associated decline in suitable forage plants have been pinpointed as major global threats to bee diversity and pollination services.

Assessing the effectiveness of different flower visitors for pollination and subsequent seed-set is central to addressing questions about the ecology and evolution of plant reproduction. Pollinator effectiveness is a function of multiple interacting traits of both flower and flower visitors, influenced by animal behaviour, morphology and relative sizes. Quantity (visitation rates) and quality components (pollen removal and deposition) of pollination service have been widely used in the literature to rank individual pollinators and to determine overall pollinator importance.

Honey-bees (Apis mellifera; Apidae) vary widely in their effectiveness as pollinators of native plants and are often inferior pollinators compared with other native flower visitors, and in some cases are responsible for a reduction in seed-set. In natural plant communities, honey-bees are less important as pollinators than they are in agricultural systems, because in most regions honey-bees are not a native species. Although honey-bees are native to the Mediterranean region of Israel, beekeepers maintain very high bee densities. Consequently, their establishment and spread, particularly into nature reserves, has the potential to reduce the pool of resources available to other native bee communities, and negatively impact on plant fitness, including rare and endangered species.

Primary floral resources utilized by bees include pollen, nectar, oil, resin and gums, although flowers functioning as a shelter or sleeping place could also be considered a possible reward offered by a particular plant. Flowers have been utilized as night shelters by a variety of solitary male bee species. In many species of aculeate Hymenoptera, males form sleeping aggregations on exposed plants in the late afternoon or early evening and disperse early the following morning.

Iris atropurpurea is a Red List species endemic to the coastal plain of Israel. Some populations are threatened by extinction, and about one-third have already disappeared. Previous studies have reported that the main visitors to Oncocyclus irises are male eucerine bees, but until now there has been no attempt to quantify the pollination services of these bees directly. Studying the reproductive biology of Iris atropurpurea is important given its rarity and conservation status. New research presents data on observations and experiments in which the pollination effectiveness of honey-bees was compared with that of male solitary bees on both male and female fitness components. Three questions were asked.

  1. What are the main flower visitors of I. atropurpurea?
  2. Which bee species are the most effective pollinators in terms of visitation rate, pollen deposition and pollen removal?
  3. Are honey-bees as effective as solitary bees as pollinators of I. atropurpurea?

The results show main wild pollinators of Iris atropurpurea are medium-sized male eucerine bees. Although a wide diversity of other insects regularly sheltered in the flower tunnels, these visitors were not considered as pollinators because they did not carry pollen. Honey-bees were found to be as effective as night-sheltering bees at pollinating this species. In the presence of honey-bees, male eucerine bees were low removal–low deposition pollinators, whereas honey-bees were high removal–low deposition pollinators. Even though overall, both bee taxa were equally effective pollinators, honey-bees not only have the potential to reduce the amount of pollen available for plant reproduction, they also have the potential to reduce the amount of resources available to solitary bee communities. The results of this study have potential implications for the conservation of this highly endangered plant species if hives are permitted inside reserves, where the bulk of Oncocyclus iris species are protected.

 

The endangered Iris atropurpurea (Iridaceae) in Israel: honey-bees, night-sheltering male bees and female solitary bees as pollinators. (2013) Annals of Botany 111 (3): 395-407
Abstract
The coastal plain of Israel hosts the last few remaining populations of the endemic Iris atropurpurea (Iridaceae), a Red List species of high conservation priority. The flowers offer no nectar reward. Here the role of night-sheltering male solitary bees, honey-bees and female solitary bees as pollinators of I. atropurpurea is documented. Breeding system, floral longevity, stigma receptivity, visitation rates, pollen loads, pollen deposition and removal and fruit- and seed-set were investigated. The main wild pollinators of this plant are male eucerine bees, and to a lesser extent, but with the potential to transfer pollen, female solitary bees. Honey-bees were found to be frequent diurnal visitors; they removed large quantities of pollen and were as effective as male sheltering bees at pollinating this species. The low density of pollen carried by male solitary bees was attributed to grooming activities, pollen displacement when bees aggregated together in flowers and pollen depletion by honey-bees. In the population free of honey-bee hives, male bees carried significantly more pollen grains on their bodies. Results from pollen analysis and pollen deposited on stigmas suggest that inadequate pollination may be an important factor limiting fruit-set. In the presence of honey-bees, eucerine bees were low removal–low deposition pollinators, whereas honey-bees were high removal–low deposition pollinators, because they removed large amounts into corbiculae and deposited relatively little onto receptive stigmas. Even though overall, both bee taxa were equally effective pollinators, we suggest that honey-bees have the potential to reduce the amount of pollen available for plant reproduction, and to reduce the amount of resources available to solitary bee communities. The results of this study have potential implications for the conservation of this highly endangered plant species if hives are permitted inside reserves, where the bulk of Oncocyclus iris species are protected.

 

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