Snack time in the trees

We heard the sound of a squeaky door in the canopy above as we climbed through an old stand of Oyster Bay Pine (Callitris rhomboidea) followed by the sound of something dropping in the dry leaf litter below. Upon closer inspection, we found what all the fuss was about, a messy litter of broken Callitris cones on the ground and chewed-up cones still attached to the trees. Seeds are a staple food for Gang Gang Cockatoos, and we happened to disturb snack time for these two birds.

The chewed up cones of the Oyster Bay Pine is a common feature in semi-arid woodland of western Victoria.

Seed predation is super common in many Australian plant families and no doubt many of you have inadvertently seen this if you've collected Acacia seed. I very nearly crashed my car after I left a wool bale of Acacia seed in the back of my old 1994 Corolla and became very distracted by all the interesting bugs crawling all over the inside of the car. This started as something of a side interest for me and culminated in a significant chunk of my Masters thesis back in 2020 and a paper in 2023.

Desktop X-ray imaging is beginning to open a world of possibilities. Here we have a wasp still hidden within the seed of the rare Limestone Blue Wattle.

But what about the plants? Plant seeds are more than just a tasty, convenient snack for animals, they are less passive than we might think. Many plants employ some pretty incredible methods to avoid, deter or maim those pesky seed predators. Some plant species employ chemical warfare or spines to avoid seed predation. And in Western Australia, a species of Hakea uses leaves that resemble young woody fruit to trick Cockatoos into leaving its seeds in peace. In some cases, seed predation by animals can have a big impact on the plants that feed them, especially those that are rare and endangered. This is something we think we're observing in some of the savanna and woodland Silver Banksia (Banksia marginata) trees in small fragments in the Western basalt plains of Victoria. Yellow-tailed Black Cockatoos are a common feature in many of these remaining stands of Silver Banksia where they spend much of summer munching on Banksia fruit in small isolated stands of trees. Because of the isolated nature of many Silver Banksia populations, we think that the Cockatoos are spending more time foraging and depleting the local seed crop. In fact, this is exactly what we found with a number of populations experiencing nearly 100% removal of fruit. As you can see from the images below their foraging can be quite destructive so the loss of much of the seed is inevitable. This means fewer and fewer seeds for recruitment and over time a population is going to spiral closer towards extinction. Seed predation is a really cool and interesting interaction but can become harmful for rare plants in fragmented populations. Silver Banksia is probably one of the luckier species with an army of passionate volunteers spending much of their time protecting remnants and planting new populations. Planting and revegetation activities by Landcare and Friends groups may even allow seed predators to spread their foraging across larger populations or more connected fragments and thereby allowing more seed to hit the ground and replace older trees. I would love to hear about your observations or thoughts on seed predations so feel free to comment below.

From left to right: Banksia cones looking plump, ripe and ready to be munched; Cockatoo dinner time is a messy affair and fairly destructive; Some cones survive relatively intact but they don't open once removed from the tree.

Further Reading

Coffey, K., Benkman, C.W. and Milligan, B.G. (1999), The adaptive significance of spines on pine cones. Ecology, 80: 1221-1229

Groom, P. K., Lamont, B. B. & Duff, H. C. (1994). Self-crypsis in Hakea trifurcata as an aviangranivore deterrent. Functional Ecology, 8, 110-117 Guimarães, P.R., José, J., Galetti, M. & José Roberto Trigo, J.R. (2003) Quinolizidine Alkaloids in Ormosia arborea Seeds Inhibit Predation But Not Hoarding by Agoutis (Dasyprocta leporina). Journal of Chemical Ecology, 29, 1065–1072