Banksia Reproduction:

Pollination Visitors

Description

Figure 1 White-cheeked Honeyeater feeding on a banksia flower

This entry reflects on the aspects of pollination in Banksia species. Hairpin banksia (Banksia spinulosa) reproduces via dense, nectar-rich flower spikes that invite a remarkably mixed vertebrate audience—honeyeater birds by day and small, non-flying marsupials by night (Figure 1). Susan Carthew’s landmark field study in coastal heath of New South Wales set out to decide which of those visitors truly pollinate the shrub and how their behaviour affects seed output (Carthew, 2009). By following individual inflorescences over three flowering seasons, counting every feathered or furred arrival and measuring the pollen they carried, the study framed reproduction not as a single act of pollination but as a cascade: visitor identity → pollen transfer → fertilisation → fruit maturation. In doing so, it offered a model example of how basic natural-history observation can answer an ecological puzzle—namely why B. spinulosa persists in resource-poor soils despite notoriously low natural fruit set. (Wawrzyczek et al., 2024).

Analysis

Carthew’s daytime focal watches and red-light night surveys recorded roughly equal visit rates from honeyeaters (chiefly Eastern spinebills and crescent honeyeaters) and nocturnal mammals—eastern pygmy-possums, sugar gliders and brown antechinus—while insects formed < 5 % of visits (Figure 2). Single-visit trials showed mammals left the stigmas far dustier than birds because their muzzles and foreheads contacted more flowers per bout; pollen-tube assays confirmed that both guilds achieved similar fertilisation success after a visit. Nectar depletion experiments revealed mammals arrive as soon as buds open at dusk and strip up to two-thirds of self-pollen overnight, a behaviour that forces subsequent cross-pollen delivery by the first bird at dawn and so helps maintain the shrub’s high outcrossing rate despite its self-compatibility (Cunningham, 1991).

Across 50 tagged spikes, only 0.9–1.8 % of the ~1 000 flowers on each spike matured into woody follicles—numbers perfectly in line with wider meta-analyses showing average fruit-set in the genus hovers around 2 % even under unrestricted visitation (Bell, Hunter and Steed, 2022). Carthew argued that pollen shortage is therefore not the primary limitation; instead, post-pollination abortion and the costs of provisioning large, serotinous seed cones in a nutrient-poor habitat cap final yield. Later work on selective fruit abortion supports her reasoning, showing follicles with fewer ovules and poorer endosperm are preferentially shed (Vaughton and Carthew, 1993).

Figure 2 Bar plot summary of the frequency of floral visitation for the four study species by different pollinator functional groups (mammals, birds, moths) based on 11 574 hours of camera trapping surveys, highlighting between-species trend

Outcomes and Actions

This paper revolutionised thinking about Australian pollination syndromes by proving that small, non-flying mammals are not just nectar thieves but major pollen vectors, responsible for more than half of successful pollinations at many sites. Conservation managers now recognise that declines of possums or gliders—through habitat loss, feral predators or inappropriate fire regimes—could silently throttle banksia recruitment long before changes are evident in bird surveys. Conversely, the dual-guild system offers redundancy: after intense wildfires that temporarily reduce hollow-bearing trees (and thus mammal refuges), honeyeaters can meet short-term pollination demand until shrub-layer complexity returns. Practical actions therefore include (i) retaining coarse woody debris and tree hollows when planning hazard-reduction burns, (ii) controlling foxes and feral cats that suppress small-mammal numbers, and (iii) staggering prescribed-burn mosaics to ensure continuous nectar supply for both vertebrate guilds.

More broadly, Carthew’s findings caution researchers against equating “lots of visitors” with “effective pollination.” Careful measurement of pollen loads, pollen-tube growth and fruit set remains essential for diagnosing reproductive bottlenecks in other Proteaceae. For high-school or community restoration projects, the study provides an accessible story: a plant that needs

Reference list

Bell, S.A.J., Hunter, N. and Steed, A. (2022). Lack of Fire Rather than Pollinator Absence May Drive Population Decline in the Critically Endangered. Australian Journal of Botany, 70(5), pp.372–383. doi:https://doi.org/10.1071/bt21143.

Carthew, S.M. (2009). The Pollination Biology and Breeding System of Banksia Spinulosa. [Doctoral Thesis] Available at: https://ro.uow.edu.au/articles/thesis/The_pollination_biology_and_breeding_system_of_Banksia_spinulosa/27652275/1?file=50356890.

Cunningham, S.A. (1991). Experimental Evidence for Pollination of Banksia spp. by non-flying Mammals. Oecologia, 87(1), pp.86–90. doi:https://doi.org/10.1007/bf00323784.

Vaughton, G. and Carthew, S.M. (1993). Evidence for Selective Fruit Abortion in Banksia Spinulosa (Proteaceae). Biological Journal of the Linnean Society, 50(1), pp.35–46. doi:https://doi.org/10.1111/j.1095-8312.1993.tb00917.x.

Wawrzyczek, S.K., Davis, R.A., Krauss, S.L., Hoebee, S.E., Ashton, L.M. and Phillips, R.D. (2024). Pollination by birds, non-flying mammals, and European Honeybees in a Heathland shrub, Banksia catoglypta (Proteaceae). Botanical Journal of the Linnean Society, 206(288). doi:https://doi.org/10.1093/botlinnean/boae024.

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