Australasian Plant Conservation
Originally published in Australasian Plant Conservation 22(2) September - November 2013, p 5-7
Plants and pollinators - an essential partnership in subtropical rainforest
Geoff Williams1 and Paul Adam2
1Australian Museum; 2* School of Biological, Earth and Environmental Sciences, University of NSW
*Corresponding author email: firstname.lastname@example.org
Even apparently smooth shiny beetles can carry pollen – Scanning Electron Microscope image of mixed pollen load on the hypomeron of the beetle Castiarina acuminata (Buprestidae).
Photo: G. Williams
Polistes humilus (Vespideae) on inflorescence of Alphitonia excelsa. Photo: G. Williams
Despite the increased representation of rainforest in formal reserves since the 1980s (Adam 1987, Kitching et al. 2010) many rainforest stands, both within reserves or in private ownership, exist as disjunct fragments. This is particularly the case in the coastal lowland rainforests.
Small patches of rainforest experience all the problems of physically isolated fragments of any community type. Can small rainforest stands remain viable in the long term? Designating sites as reserves, or entering into conservation agreements with private landholders, will not of itself ensure survival, without consideration being given to maintaining ecological processes.
The viability of stands depends on sustaining plant reproduction at the species level, not just maintaining a semblance of rainforest structure and tree cover. To date most management strategies fail to consider the dynamics of recruitment and survival of individual species.
In flowering plants, pollination is essential for seed production. The popular literature and visual media often give the impression that there are many highly specific, often bizarre, pollination systems in rainforest. Whilst there are instances of ecologically specialised plant and pollinator mutualisms, for example in the rainforest understorey and for epiphytic orchids, worldwide these are relatively rare, even in floristically complex tropical rainforests.
Most flowering subtropical rainforest trees and shrubs have small white or cream coloured flowers with open floral structures generally accessible to a range of visitors. Individually, each flower produces only tiny amounts of floral resources that might benefit any insect or other animal that visits.
Most trees, shrubs and vines in subtropical rainforest, are pollinated by generalists, principally small insects less than 5 mm in length that rarely exhibit an obligatory attraction to any one species of flowering plant (Williams and Adam 2010). Such insect dominated generalist pollination systems predominate throughout the world’s rainforests.
The major groups of insects participating in generalist pollination strategies are beetles, flies, wasps and bees, though native bees are not as significant as is popularly thought. Many of the participants are relatively inefficient pollinators, and the frequency of movements between and within flowering plants is low. Some flower visitors are thieves, devouring pollen and flower structures with no obvious reproductive benefit to the plant, yet others, such as various large beetles and wasps, and widely foraging solitary bees are important in transferring pollen over large distances. And even small, individually inefficient pollinators compensate by their vast numbers in effecting pollination.
Flexibility in the recruitment of pollinators provides ecological resilience. Most plants are likely to achieve reliable levels of pollination, and consequent fertilisation, even if certain pollinators become locally extinct or their populations dramatically fluctuate within individual rainforest fragments. The survival of rainforest over geological time reflects this flexibility.
In a species rich community, where individuals of a particular plant species may be at low densities, how do species ‘ensure’ that sufficient pollen reaches a stigma of the same species and is not lost through dispersal to flowers of different species?
There are two major means by which the odds of successful reproduction are improved:
- At the community level flowers are available for much of the year, although there is a peak spring – summer flowering period; when most invertebrate pollinators are active as adults. This helps to maintain the pool of potential pollinators. However, within particular time windows only a proportion of the plant species will be in flower. In seeking to attract pollinators, a species is not necessarily competing with all other species in the community.
- Success in attracting pollinators is achieved through ‘advertising’. Although the flowers of many subtropical rainforest trees are small they are frequently aggregated into inflorescences, and mass flowering throughout the canopy creates a strong visual signal attracting pollinators.
Issues of conservation
There are significant issues for plant reproduction in rainforest fragments. Not all species are self-compatible so pollen transport between individuals is required for fertilization. Low gene flow between and within populations in isolated fragments in highly modified landscapes is likely to lead to lower reproductive success and heightened vulnerability to environmental perturbations.
Compared with its occurrence in flowering plants as a whole, dioecy (where individual plants are of one sex) is more common in rainforests. In New South Wales dioecious species commonly represent greater than 20% of the woody species in individual coastal remnants (Adam and Williams 2001). Some dioecious species, e.g. female Alchornea ilicifolia, may reproduce asexually, others can reproduce vegetatively by coppicing (e.g. Diospyros australis). For those species where pollen transfer between separate male and female plants is essential for reproduction, pollen transfer within or between stands is unlikely if:
- the population in a particular stand is reduced to individuals of only one sex, or
- the population is very small with male and female individuals widely separated.
At the population level there may be functional extinction, even if adult longevity masks the inevitability of demise unless there is active management intervention. Unfortunately it may not be possible morphologically to determine the sex of seedlings and saplings, and sexual maturity may not occur for many years, so artificial recruitment by planting is not guaranteed to improve sex ratios.
Even for hermaphroditic species, populations in fragments may be very small, with individual species being represented by only one or two plants. While existing trees may be long-lived, potential recruitment may be highly inbred unless there is gene flow by either pollen or seed dispersal between fragments.
Successful dispersal will depend upon the distance between fragments, and pollinator availability. Even when fragments are close, gene transfer between fragments may be very limited. Artificial transfer of pollen between stands is not likely to be practically feasible, but transfer of seeds, or seedlings, may be feasible – but would require very careful evaluation, and if it were to occur should be fully documented.
There is a great diversity of native bees in subtropical rainforest, though relative to the abundance of other native pollinating insects on flowers, their numbers are often not great.
Introduced honey bees (Apis mellifera) are common within rainforest fragments, either from commercial hives or from feral hives located in tree hollows. Their impact on plant pollination in rainforests is uncertain. They frequently mass to flowers during peak flowering events but observations suggest that for canopy dominating trees at least, the sheer number of available flowers means that other potential pollinators can still access some flowers.
The introduced honey bee does not necessarily move between pollen producing and nectar producing plants during flowering events. They commonly exhibit foraging constancy for either resource, and so pollen transfer may not occur to the level that their numbers might suggest. Where they forage at subcanopy levels, and at plants producing fewer flowers, then their impact may be more disadvantageous, and though their great numbers appear not to disturb foraging by larger and more robust native insects, they readily disturb and discourage many native bees, wasps, flies, butterflies, and smaller beetles. How this affects the life histories and conservation status of native insects is largely unknown.
Impacts of weeds on pollination
The extent to which the co-flowering of invading weeds such as lantana, camphor laurel and large-leaved privet, impacts on the pollination of native plants in rainforest fragments, by diverting pollinators, is not well known. The popularity of rainforest species in horticulture means that non locally indigenous species may be planted in gardens close to remnant stands, potentially competing for a limited pool of pollinators. Such plantings can also adversely impact on threatened fauna; for example exotic Aristolochia can be toxic to the larvae of the Richmond River Birdwing Butterfly.
The increase in the area of rainforest in formal reserves, or subject to conservation agreements on private lands, might give a false sense of security about the conservation status of individual species and communities. To reach long term sustainability will require greater attention to be given to maintaining ecological processes within rainforest - and reproductive processes of both plants and pollinators are of great significance.
Adam, P. (1987). New South Wales Rainforests: The Nomination for the World Heritage List. National Parks and Wildlife Service, NSW.
Adam, P. and Williams. G. (2001). Dioecy, self-compatibility and vegetative reproduction in Australian subtropical rainforest trees and shrubs. Cunninghamia 7: 89-100.
Kitching. R., Braithwaite, R. and Cavanaugh, J. (2010). Remnants of Gondwana: a Natural and Social History of the Gondwana Rainforests of Australia. Surrey Beatty and Sons Pty Ltd, Baulkham Hills.
Williams, G. and Adam, P. (2010). The Flowering of Australia’s Rainforests. CSIRO Publishing, Collingwood.