Ab Hingston

Genetic pollution of native eucalypt gene pools—identifying the risks

The contamination of native-eucalypt gene pools via exotic pollen is of concern as (i) pollen dispersal is believed to be much more widespread than seed dispersal, (ii) reproductive barriers are often weak between closely related species, (iii) European settlement has already had a major impact on Australias eucalypt woodlands and mallee, (iv) there has been a rapid expansion of eucalypt plantations and restoration plantings in Australia and (v) Australia is the custodian of an internationally important genetic resource. Pollen flow between plantation and native eucalypt species has already been reported and implementation of strategies to minimise the risk and consequences of genetic pollution is important if Australian forestry is to be considered sustainable. The risks associated with the introduction of non-native species, provenances and hybrids include direct effects on the gene pool through genetic pollution as well as indirect effects on dependent biodiversity. In many cases, the risk of genetic pollution will be small due to strong barriers to hybridisation between distantly related species, differences in flowering time or poor fitness of hybrids. There is no risk of hybridisation between species from the different major eucalypt genera and/or subgenera (e.g. symphyomyrts, monocalypts, eudesmids, bloodwoods and angophora). The main plantation species are symphyomyrts and within this subgenus, the probability of successful hybridisation generally decreases with increasing taxonomic distance between species. The planting of non-local provenances or improved material within the range of native populations has the potential to have an impact on local gene pools to varying degrees, indicating the requirement for the adoption of management strategies to reduce this risk. Naturally small or remnant populations are at particular risk. A framework for assessment of the risk of genetic pollution is developed herein.

Displacement of Tasmanian native megachilid bees by the recently introduced bumblebee Bombus terrestris (Linnaeus, 1758) (Hymenoptera : Apidae)

The assertion that the recently introduced large earth bumblebee, Bombus terrestris, does not compete with other bees was investigated by examining the impact of B. terrestris on the foraging of two species of Chalicodoma(Megachilidae) on flowers of Gompholobium huegelii (Fabaceae). Chalicodoma spp. spent less time at each flower during the afternoon in quadrats where B. terrestris foraged than at quadrats from which B. terrestriswas excluded, indicating that standing crops of nectar were depleted by B. terrestris during the course of the day. This decline in resources was associated with reductions in the numbers of flowers visited, and the amount of time spent foraging, by Chalicodoma spp. It is proposed that the native bees avoided foraging in the quadrat where B. terrestris occurred as a response to reduced foraging efficiency in that situation. Thus, B. terrestris displaced these two species of Chalicodoma through competition for a limited resource. The high densities at which B. terrestris occurred, together with its ability to forage at lower ambient temperatures than the native bees, exacerbated the impact of this exotic species. The possibility that this will adversely affect pollination in G. huegelii is also discussed.

The swift parrot, Lathamus discolor (Psittacidae), social bees (Apidae) and native insects as pollinators of Eucalyptus globulus ssp. globulus (Myrtaceae)

It has been argued that the production of sufficient nectar to attract bird pollinators would evolve if the fitness benefits accruing from pollination services by birds, compared with insects, outweighed the cost of increased allocation of photosynthate to nectar. This hypothesis implies that the pollination services provided by birds must be considerably better than those provided by insects with which the plant has evolved. Consistent with this, we found that the endangered native swift parrot Lathamus discolor (Shaw) was a very effective pollinator of the native tree Eucalyptus globulus Labill. in Tasmania, facilitating an average of 76% of the maximum possible seed set for open-pollinated flowers in just one visit to a flower, whereas single flower visits by native insects did not facilitate any seed production. Flowers visited once by either species of introduced social bees, the honeybee Apis mellifera L. or the bumblebee Bombus terrestris (L.), produced less than 7% of the maximum possible seed set for open-pollinated flowers. Hence, easily managed social bees appear to be poor substitutes for bird pollinators in commercial seed orchards of this tree. We propose three possible reasons why this largely bird-pollinated tree has not evolved characters that deter insects from removing nectar.

Pollination services provided by various size classes of flower visitors to Eucalyptus globulus ssp. globulus (Myrtaceae)

Flowers of the commercially important tree Eucalyptus globulus Labill. ssp. globulus were visited by a wide variety of insects and birds within its natural distribution. Flowers were visited so frequently that most available nectar was consumed, but seed production within 5 m of the ground was consistently far less than the maximum possible, indicating the presence of large numbers of inefficient pollinators and few efficient pollinators. Pollen limitation was more severe on fully self-incompatible trees than on partially self-compatible trees, demonstrating that pollinator inefficiency resulted from infrequent outcrossing rather than inability to deposit pollen on stigmas. The flower visitors that were responsible for almost all nectar consumption from flowers within 5 m of the ground were insects that were able to permeate cages with 5-mm apertures but not cages with 1-mm apertures, the most abundant of which was the introduced honeybee Apis mellifera L. These insects contributed less than 20% of the maximum possible seed set, indicating that they were inefficient pollinators. Birds and smaller insects made lesser contributions to seed production, but consumed little nectar within 5 m of the ground. However, anthophilous birds appeared to mostly forage higher in the trees and probably consumed more nectar from, and provided more pollination services to, flowers higher in the trees.

Is the exotic bumblebee Bombus terrestris really invading Tasmanian native vegetation

There has been a great deal of disagreement surrounding the capacity of Bombus terrestris to invade Tasmanian native vegetation. This paper reviews the conflicting findings of previous surveys of the invasion of Tasmania by B. terrestris, and presents new data from the 2004–2005 austral summer. From this, it is clear that B. terrestris has extensively invaded Tasmanian native vegetation. The new data provide strong evidence that B. terrestris is breeding in native vegetation in every region of Tasmania. More than 10 bumblebees were seen in one day at 153 locations in native vegetation, including 42 locations within 10 National Parks and 38 locations within the Tasmanian Wilderness World Heritage Area. Nests of B. terrestris were also found within two National Parks. These findings suggest that B. terrestris would also invade native vegetation in non-arid temperate regions of the Australian mainland, if it is introduced there.

Does the introduced bumblebee, Bombus terrestris (Apidae), prefer flowers of introduced or native plants in Australia?

Proponents of importation of the European bumblebee, Bombus terrestris (L.), into Australia for pollination of commercial greenhouse crops argue that this species will have little impact on Australian native ecosystems because it prefers to forage on flowers of introduced species of plants rather than Australian native plants. However, data presented as evidence of preference for introduced plants have been equivocal. This study compared the attractiveness of introduced and Australian native plants to free-foraging B. terrestris in a garden at the interface between an urban area and native vegetation in the Australian island of Tasmania, where a feral population of B. terrestris had been established for over 10 years. No evidence was found to support the proposal that B. terrestris forages on flowers of introduced plants in preference to those of Australian native plants. The numbers of B. terrestris seen foraging per 1000 flowers did not differ significantly between introduced plants and Australian native plants, and the preferred food sources of B. terrestris included flowers of both introduced and Australian native species. Because B. terrestris forages frequently on many species of both introduced and native plants, assessments of its ecological impacts must include the effects of altered pollination on recruitment rates in both introduced weeds and native plants, and reduced quantities of nectar and pollen of native plants on recruitment rates of dependent fauna.

Pollinators in seed orchards of Eucalyptus nitens (Myrtaceae)

Flowers of the commercially important tree Eucalyptus nitens (Deane & Maiden) Maiden were visited by a diverse array of insects, but not by birds, in Tasmanian seed orchards. Most species of insects that visited the flowers of E. nitens are likely to be effective pollinators because all common species of visitors carried many grains of Eucalyptus pollen, and the open floral structure facilitates frequent insect contact with stigmas. Seed production also suggested that a wide variety of insects were effective pollinators because flowers were consistently well pollinated, despite differences in flower-visitor communities among orchards and particular branches of flowers. The generalised entomophilous pollination system of E. nitens suggests that effective pollinators should occur in seed orchards of this tree throughout the world, provided that flowering occurs at a time of year conducive to insect activity. Although a wide variety of insects appear to be effective pollinators of E. nitens, introduced honeybees, Apis mellifera L., that are often deployed as pollinators in seed orchards were consistently not attracted to the flowers. The reliance on wild insects as pollinators suggests that seed production in E. nitens may benefit from reduced use of broad-spectrum insecticides in, and near, seed orchards.

Pollinators of Eucalyptus regnans (Myrtaceae), the world’s tallest flowering plant species

Insect visitors to the flowers of Eucalyptus regnans F.Muell. in a remnant natural stand were classified into 33 functional pollinator groups according to taxonomic affinity and body size. In total, 92% of insects caught were dipterans; however, most of these were small and did not contribute significantly to pollination. For the majority of taxa, which have short mouthparts and therefore need to intrude themselves into the flower while feeding on nectar, there was a highly significant relationship between body length and the number of E. regnans pollen grains carried on the body. Mean pollen loads ranged from 20 grains per insect for sepsid flies to 84 000 for large tachinid flies. An index of pollen-deposition potential, which is based on population size and pollen load, suggested that the larger tachinid, calliphorid and syrphid flies were the most important pollen vectors and that larger sphecid wasps also played a significant role. Many taxa appeared to contribute little to pollination because they were uncommon and/or did not carry large quantities of pollen. A convention is proposed whereby groups are weighted according to their contribution to total pollen-deposition potential. For E. regnans, a ratio of 5 Diptera/1 Hymenoptera + (Coleoptera/Lepidoptera) is described, with the taxa in parentheses contributing less than 10% of the total.

Corrigendum to: TURNER REVIEW No. 6 Genetic pollution of native eucalypt gene pools—identifying the risks

The contamination of native-eucalypt gene pools via exotic pollen is of concern as (i) pollen dispersal is believed to be much more widespread than seed dispersal, (ii) reproductive barriers are often weak between closely related species, (iii) European settlement has already had a major impact on Australias eucalypt woodlands and mallee, (iv) there has been a rapid expansion of eucalypt plantations and restoration plantings in Australia and (v) Australia is the custodian of an internationally important genetic resource. Pollen flow between plantation and native eucalypt species has already been reported and implementation of strategies to minimise the risk and consequences of genetic pollution is important if Australian forestry is to be considered sustainable. The risks associated with the introduction of non-native species, provenances and hybrids include direct effects on the gene pool through genetic pollution as well as indirect effects on dependent biodiversity. In many cases, the risk of genetic pollution will be small due to strong barriers to hybridisation between distantly related species, differences in flowering time or poor fitness of hybrids. There is no risk of hybridisation between species from the different major eucalypt genera and/or subgenera (e.g. symphyomyrts, monocalypts, eudesmids, bloodwoods and angophora). The main plantation species are symphyomyrts and within this subgenus, the probability of successful hybridisation generally decreases with increasing taxonomic distance between species. The planting of non-local provenances or improved material within the range of native populations has the potential to have an impact on local gene pools to varying degrees, indicating the requirement for the adoption of management strategies to reduce this risk. Naturally small or remnant populations are at particular risk. A framework for assessment of the risk of genetic pollution is developed herein.

Co-occurrence of RNA viruses in Tasmanian-introduced bumble bees (Bombus terrestris) and honey bees (Apis mellifera)

A number of bee RNA viruses, including Deformed wing virus (DWV), are so far unreported from Australia. These viruses can be introduced together with imported live honey bees (Apis mellifera) and their products, with other bee species, and bee parasites. Given that bee viruses have a profound impact on bee health, it is surprising that since the introduction of bumble bees (Bombus terrestris) onto Tasmania in 1992 from New Zealand, no work has been done to investigate which RNA viruses are associated with these bees. Consequently, we investigate the current prevalence of RNA viruses in B. terrestris and A. mellifera collected in south-eastern Tasmania. We did not find DWV in either A. mellifera and B. terrestris. However, both bee species shared Kashmir bee virus (KBV) and Sacbrood virus (SBV), but Black queen cell virus (BQCV) was detected only in A. mellifera. This reinforces the importance of ongoing strong regulation of the anthropogenic movement of live bees and their products.