Acacia ligulata
Sandhill Wattle (Acacia ligulata) is very common right across semi-arid Australia. We love it because it is easy to collect and clean the seed, and it establishes well from direct seeding. But it turns out Acacia ligulata is a bit of a rock star.
Most wattle seeds have an aril; a fleshy covering over part, or all, of the seed. Arils encourage dispersal of the seed by animals. Normally, bird-dispersed wattles have fatty, colourful arils, whereas ant-dispersed wattles seeds have white arils. But Acacia ligulata is unusual because seeds are attractive to both birds and ants (1). I know the ants were happy with my dropped seed and I certainly had to push the Mallee Ringnecks out of the way when picking the seed. It is popular with Regent Parrots too.
The popular arils is thought to be a bet-hedging strategy. The Acacia ligulata seeds which are eaten by birds have their dormancy broken and are ready to germinate immediately within the favourable microhabitat of bird poo. Whereas seeds which are distributed by ants, remain dormant until there’s a specific event which signals a suitable seed bed, such as a fire where the heat breaks the seed dormancy (2).
Mallee Ringneck
Acacia ligulata is also weird because the seed is polymorphic; the aril can be either orange, red or yellow. This is driven by genetics and a complex of abiotic and biotic factors. Although the aril color does not influence seed mass, seedling emergence or survival, the red morph is more common as temperature decreases and rainfall increases (3).
There is also a difference in the composition of the different colored arils: carotenoids are more than four times higher in red versus yellow arils. Animals can’t make their own carotenoids so must obtain them from their diet. Different carotenoids contribute to things like functioning of visual systems, as well as body coloration in birds. So, the various seed dispersers, seed predators and aril thieves of Acacia ligulata might be choosing between different colored arils. These “consumer preferences” probably contribute to maintaining the seed polymorphism (3, 4, 5).
For a long time, Acacia ligulata held the record for the longest ant-mediated seed dispersal event (180 m). Meat ants Iridomyrmex viridiaeneus carried the seed to the nest, stole the aril and dumped the seed a huge distance from the parent plant (6). The distance a seed is dispersed increases the probability that the seed will germinate but usually seed dispersal events by ants are tiny, less than 5 m (7). The distance which ants take seeds in the Australian arid zone is stunning. The record is now held by Acacia karina, a whopping 425 m, detected by microsatellite markers and parentage assignment (8). I reckon that’s cheating and Acacia ligulata should still be the record holder. Seed dispersal event should be recorded by laboriously following the ant carrying the seed through the bush, not by genetics!
Acacia ligulata is the pin-up species for studies which look at recruitment failure in rare arid zone wattles. Why is Acacia ligulata thriving, when other wattles like Purple-wood Wattle Acacia carneorum continually fail to recruit? (9) In addition to the polymorphic arils, which contribute to the seed finding a suitable germination niche, Acacia ligulata has profuse flowering, is generously pollinated by the feral honeybee and has abundant and regular seed set. In contrast, the rare wattles are more closely dependent on native pollinators, require high rainfall to produce seeds and have slow-growing seedlings which are susceptible to high grazing pressure for longer. Most importantly it seems the small and isolated populations of rare species are negatively impacting the population genetics (10).
Finally, Acacia ligulata is a medicinal plant used by Australian Aborigines. Studies have found antibacterial properties: An infusion from bark and leaf extracts relieve coughs and are active against Streptococcus pyogenes (the bacterium which causes infections such as tonsillitis and rheumatic fever). Pod extracts showed cytotoxic effects against cancer cells, particularly melanoma (11) and have antibacterial, antioxidant, anticancer and antidiabetic activities (12).
Phew! A rock star I say. Another day I’ll share how we extract delicious witchetty grubs from Acacia ligulata branches.
References
Dispersal Adaptations of Some Acacia Species in the Australian Arid Zone https://esajournals.onlinelibrary.wiley.com/doi/abs/10.2307/1938312
Bet‐hedging and germination in the Australian arid zone shrub Acacia ligulata https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1442-9993.2000.01047.x
Fruit colour polymorphism in Acacia ligulata: seed and seedling performance, clinal patterns, and chemical variation https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.711.6178&rep=rep1&type=pdf
Evidence for simple genetic control of a fruit-colour polymorphism in Acacia ligulate https://www.researchgate.net/publication/228717075_Evidence_for_simple_genetic_control_of_a_fruit-colour_polymorphism_in_Acacia_ligulata
Linking frugivores to the dynamics of a fruit color polymorphism https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.3732/ajb.92.5.859
Dispersal for distance? Acacia ligulata seeds and meat ants Iridomyrmex viridiaeneus https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1442-9993.2002.01216.x
Ants as diaspore removers of non-myrmecochorous plants: a meta-analysis
The critical role of ants in the extensive dispersal of Acacia seeds revealed by genetic parentage assignment https://researchrepository.murdoch.edu.au/id/eprint/28166/1/role-of-ants-in-dispersal-of-Acacia-seeds.pdf
Can the pollination biology and floral ontogeny of the threatened Acacia carneorum explain its lack of reproductive success? https://link.springer.com/article/10.1007/s11284-013-1117-y
Reproduction of threatened, long lived semi arid Acacia within highly fragmented stands in far western NSW https://ro.uow.edu.au/cgi/viewcontent.cgi?article=5983&context=theses
Biological activity and LC-MS/MS profiling of extracts from the Australian medicinal plant Acacia ligulata (Fabaceae) https://www.tandfonline.com/doi/abs/10.1080/14786419.2017.1318383
Faast and Weinstein (2019) Plant-derived medicinal entomochemicals: an integrated approach to biodiscovery in Australia https://onlinelibrary.wiley.com/doi/full/10.1111/aen.12433