The Serventy Memorial Lecture is held every year by the WA Naturalists’ Club in honour of the outstanding contribution made by the Serventy family to Natural History in Australia and to our Club. The aim of the Serventy Memorial fund is to encourage young people to study natural history. Before the talk, Roz Hart reminded us about the work of the various members of the Serventy family, starting with the foundation of the Club by Dom Serventy in 1924.
Why do our bird-pollinated eucalypts occur mostly on poor soils? Our speaker for this year’s SML, Professor Stephen Hopper, has been doing a lot of detective work to try to solve this problem.
Steve was Director of Kings Park and Botanic Garden from 1992 till 2004, and after that held an academic position at UWA. For the past six years he was Director and Chief Scientist of the Royal Botanic Gardens at Kew. Kew is a World Heritage site, caring for the world’s largest botanical and fungal collections. Currently he is Winthrop Professor of Biodiversity at UWA in Albany. Steve is an active field botanist and conservation biologist. His research interests include eucalypts, orchids, kangaroo-paws and their relatives, granite outcrop plants and “OCBIL” landscapes (old, climatically-buffered infertile landscapes). In 2012, Steve was awarded Companion of the Order of Australia for his international research leadership.
Stephen began by acknowledging our Club’s founder Dom Serventy’s book Birds of Western Australia and his use of Aboriginal names for the birds. Early WA botanist James Drummond referred to “honey-sucking birds”. We now know that the South West of WA is the world’s prime hotspot for bird-pollinated plants. It is mostly our honeyeaters, chats and lorikeets that feed on nectar. Here, 15% of plants are bird-pollinated (and in rare plants 40%), followed by 6% in our central desert region, 4% in southern Africa, another 4% in South America and 1% in North America. Europe has none. Our south-west is also one of 25 global hotspots for biodiversity.
Conservation requires knowledge of habitats, and to understand habitats, Steve said, we need to understand processes such as pollination. Most pollination is selfed, but outcrosses produce a better seed set, so plants employ various means to discriminate against their own pollen reaching the ovary and fertilising the plant it came from. The evolution of bird-pollination mechanisms involves a dynamic partnership between plants and birds, balancing optimal foraging by birds with optimal resource allocation by plants to achieve pollination. As a rule, birds will go for any nectar; exclusive plant-pollinator relationships are not as common as generalist systems. This makes it likely that plants will receive pollen from other species, and that of course is not in the plant’s interest.
Steve’s approach to nature is exemplified by the story he told us about two gum trees he saw in blossom recently from his kitchen window: a Coastal Moort Eucalyptus utilis and a Red-flowering Gum Corymbia ficifolia. He noticed that the first was full of insects and the second was full of birds. This illustrates the way some eucalypts specialise in bird-pollination and others in insect pollination, and begs the question as to why this is so.
Birds are bigger and more mobile than insects, are active in cold weather, have high energy requirements and better visual acuity. Grant and Grant, in “Hummingbirds and their Flowers” say that for plants to use birds they would need to display vivid colours (including red which is not noticed by insects), produce copious amounts of dilute nectar, have mechanisms to exclude insects such as tubular, pendulous flowers and lack of scent, be tough so as to protect themselves from injury, and have floral architecture suitable for birds to use for feeding while effecting pollination.
An analysis was done on a big, prolifically flowering Jarrah tree in Kings Park. Practically all the pollinators were insects. Then he looked at work in South Australia on three eucalypt species: E obliqua, E leucoxylon and E cosmophylla. In the order listed here, they were found to be progressively more specialised in bird-pollination and less in insect-pollination. The progression showed an increasing nectar production, a lower density of flowers and lower feeding rates by honeyeaters. That meant that the birds were able to collect more nectar from fewer flowers in a given time than they would in the other species.
A 35 year case study was done of the two subspecies of Eucalyptus caesia: subspecies caesia, and subspecies magna. The latter has fewer and bigger flowers, with double the number of stamens. The evidence suggested that strong natural selection had occurred in Eucalyptus caesia favouring birds by packing large quantities of dilute nectar produced day and night in fewer larger flowers per plant. This increased the likelihood of a significant number of honeyeater movements between trees, thus effecting outcross pollination, and the production of higher quality seeds. Up to 8 species of honeyeater were observed working flowers for nectar in E. caesia, In recent years, arid-country species such as Black Honeyeaters and White-fronted Honeyeaters have moved further south on granite outcrops occupied by E. caesia.
Another study in 1980 was done on Eucalyptus stoatei near Ravensthorpe. This tree has a pendulous, tubular flower that excludes insects, and is probably bird-pollinated exclusively. Honeyeaters visited 1 to 5 flowers per tree and then flew off to another tree – often not even to neighbouring trees. Because of this, a high rate of out-crossing is inferred, and this inference is confirmed by genetic tests that showed 82% outcrossing – the highest then recorded for eucalypts.
The question arises as to the evolutionary advantage of such extreme specialisation. Steve quoted the “James Effect”, after UWA biologist Sid James – that small, long-lived populations need mechanisms to preserve genetic diversity in the face of prolonged inbreeding.” E. caesia and E. stoatei both grow in small, dissected populations, in very poor soils on “islands” of granite outcrops and lateritic plateaux. Could it be that birds, being more mobile than insects, serve to take their pollen further, and thus serve as an outbreeding mechanism for these isolated populations of trees?
Of the eucalypts, only 5% are predominantly bird-pollinated, most endemic to the old landscapes of south-west Australia. Steve’s work with eucalypts seems to confirm the suggestion by Oswald Sargent, an early member of our club, that they seem to be evolving towards bird pollination. A grant is under review to apply DNA-based paternity analysis, in order to help answer these questions. This will be useful in helping conservation and bush regeneration.
Steve is not only a scientist with a deep interest in his work, but also a true naturalist. And as he pointed out, if you stay still for long enough, Nature will come to you.
Mike Gregson.
