The Flowering of Australia’s Rainforests

Invertebrates are essential in pollinating our rainforests – but how do climate change, fire, fragmentation, invasive species and destructive pathogens impact pollination networks? Dr Geoff Williams OAM, AM, explores how in this Second Edition of The Flowering of Australia’s Rainforests.

Drawing on 50 years of field and research experience by Australian Museum Research Associate and author Geoff Williams, the recently published Second Edition of The Flowering of Australia’s Rainforests provides a comprehensive introduction to the pollination ecology, evolution and conservation of Australian rainforest plants in a world context, with particular emphasis on subtropical rainforests and their associated pollinators. Subtitled Pollination Ecology and Plant Evolution this significantly expanded volume includes new information on the impacts of climate change, fire, fragmentation, invasive species and destructive pathogens. The book features approximately 150 colour photos of plants, pollinators and rainforest ecosystems. In keeping with the role played by invertebrates as principal agents of pollination in Australia’s rainforests, it provides an extensive discussion of plant-invertebrate mutualisms and co-evolution.

The reproduction strategies deployed by Australia’s rainforest plants are diverse. The evolutionary history of individual plant species and families, and the pollinator relationships that have evolved within them, are equally so. Although rainforest ecosystems retain examples of non-flowering plants, for example towering emergent conifers and ‘archaic’ cycads of the understorey, the flowering plants (the angiosperms) are the dominating species to be encountered in all forest strata. The pollination of conifers is reliant on wind to mobilise and distribute pollen grains, however, cycads have developed mutualisms with insects, frequently beetles, as agents of pollen transfer.

Among the angiosperms there are species reliant on single or few pollinators. These include specialised pollination relationships with minute thrips, as in the shrub Wilkiea huegeliana, and the small trees of the genus Eupomatia that are wholly dependent on tiny species of Elleschodes weevils for pollination.

Eupomatia are members of the evolutionarily conservative Eupomatiaceae, a numerically small family largely restricted to eastern Australia. Their flowers emit strong fragrances that attract pollinating beetles, and after pollination the fused flower structures fall away as single units, carrying the beetles’ eggs to the ground, there to develop in the soil and from which new adults will later emerge. But in all rainforests, ‘generalist’ pollination ecologies prevail – that is, plants recruit from a broad suite of potential pollinators. These can be mixed assemblages of birds, blossom bats and insects, as in the North Queensland Syzygium cormiflorum (Myrtaceae), or more typically pollinator assemblages comprised exclusively of insects, of which native bees may only constitute a small proportion. In subtropical rainforests most pollinating insects, collectively cohorts of Diptera, Coleoptera and Hymenoptera, are smaller than 7 mm in size. Though size reduces an individual’s ability to carry large pollen loads this is made up for by absolute numbers.

By comparison vertebrates play a lesser role than they do in non-rainforest communities, though nevertheless are important agents of pollination for some plant families with highly adapted flower forms such as the Proteaceae and Castanospermum australe of the Fabaceae. Vertebrate-adapted flowers are often brightly coloured, have floral structures that tend to be more complex in form, and produce nectar in greater volume. Invertebrate-adapted flowers are more commonly white or creamish-white in colour, and usually possess shallow readily accessible flowers which individually produce small amounts of nectar. Flowers with tubular corollas suggest adaptation for pollination by birds (notably meliphagid honeyeaters) and long-tongued Lepidoptera (though apart from hawkmoths the role of moths is poorly understood). In general however, there is great variety in floral shape, colour, nectar volume, nectar type and structural presentation of flowers within rainforest strata. Consequently the flowers of many rainforest plants give little clear morphological insight as to what specific pollinators their flowers are seeking to attract.

Globally rainforests continue to be a focus of conservation concern, not least owing to the ongoing clearing of tropical forests that threatens the world’s biodiversity and consequent devastation to pollinators and the crucial pollination services that these provide. Australia has not been immune to the clearing of rainforest (and associated eucalypt-dominated wet sclerophyll forest), with the impact of these historical losses further exacerbated by the cataclysmic landscape scale bushfires and extreme drought events that devastated vast swathes of eastern Australia’s forests in 2019-2020. How these forests, and their prior pollination networks, may recover is uncertain.

Despite the popular appreciation of our remaining rainforests, especially since the 1970s, plant-pollinator relationships and pollinator dependencies for the great majority of Australia’s species remain poorly studied, with information on much of what we do know being relatively inaccessible in specialist journals. This Second Edition of The Flowering of Australia’s Rainforests makes a timely contribution to our understanding of the nature and function of Australia’s and the world’s pollinator fauna, plant-reproduction dependencies, and the evolutionary pathways that have brought them to their present state and function. The hope is that the book will serve to encourage further research into, and the conservation of, the rainforest heritage that we still retain.