Seed dispersal is the critical period when a plant sends its progeny out into the world. Plants have evolved various means to get their seeds to suitable places for germination and growth.
Some seeds have structures that allow them to be moved by forces such as wind and water while others are moved by animals including mammals, birds and insects. Some have no apparent mechanisms and fall on the ground near the parent plant.
Dispersal by animals
Mammals are also involved in seed dispersal. Foxes eat and disperse the fleshy fruits of the exotic African olive *Olea europea subsp. cuspidata and African Boxthorn *Lycium ferocissimum. We have some evidence that Swamp wallabies or Wallaroos may disperse small amounts of native species e.g. Centipeda minima and Persicaria decipiens but mainly exotics e.g. *Solanum nigrum, *Sida rhombifolia and *Setaria gracilis, in their scats. Some species may be dispersed by rabbits. We have observed Solanum prinophyllum germinating on rabbit latrines.
Many seeds, particularly grasses, may adhere to fur and be carried distances. Some grass species e.g. Themeda triandra, Austrostipa setacea, Aristida ramosa have spiky awns and barbed appendages that may adhere to animals, while other species e.g. Chloris ventricosa, Bothriochloa macra, Dichelachne micrantha have rough surfaces that may adhere. The native carrot Daucus glochidiatus has hooked spines. The seeds of Plantago gaudichaudii and Linum marginale, may be sticky when wet, aiding dispersal.
Human movement of many seeds on socks, vehicles, lawn mowers, etc may also be important, particularly for weed species. Woodland species you may collect inadvertently include natives, Microlaena stipoides, Aristida ramosa, Desmodium brachypodum, Lagenophora gracilis and weeds like *Bidens pilosa, *Paspalum dilatatum.
Plants with fleshy fruits such as Einadia nutans subsp linifolia, Eremophila debilis, Dianella longifolia and Exocarpus cupressiformis appear to be bird-dispersed. African Olive *Olea europea subsp cuspidata are bird dispersed and are more abundant under large trees. We have seen the fleshy fruits of Einadia nutans subsp. linifolia being moved by ants.
Dispersal by wind
Some seeds are moved long distances by wind and water. Large plumed seeds on some of the vines like Parsonsia straminea and Clematis glycinoides and the weed *Araujia sericifera. Small hairy pappus hairs assist some daisies to disperse e.g. Senecio quadridentatus, Vernonia cinerea, Vittadinia sulcata and the weeds *Senecio madagascariensis and *Sonchus oleraceus. Dodonaea viscosa has winged fruit but they probably only result in localised dispersal.
Some species have mechanisms for local dispersal. Some have food bodies (eliasomes) that are attractive to ants, who move the seed and remove the food body e.g. Acacia implexa, Cymbonotus lawsonianus, Other species have fascinating ballistic dispersal mechanisms e.g. Oxalis perennans, Daviesia ulicifolia, Indigofera australis, Lotus australis, Geranium homeanum.
The long-reaching spidery inflorescence of grass seeds may place the seed beyond the immediate location of the parent plant. Clustering of grass plants in open areas suggests this method of operation, e.g. Elymus scaber, Themeda australis.
Species without dispersal mechanisms
Some species have seeds that lack dispersal mechanisms and some of these are very tiny e.g. Pterostylis curta and may be dispersed by wind, but others are larger and appear to fall mainly close to the adult plant e.g. Solenogyne dominii. However given that adult plants have already found suitable growing conditions it may be advantageous to the seed to stay in the appropriate habitat close to home e.g. Pimelea spicata, Plantago gaudichaudii.
Can the groundlayer plants cross the road?
- Species with no special morphology, e.g. Wahlenbergia stricta, or ballistic devices, drop seed beneath or within a few metres of the parent.
- Ants generally move seeds with food bodies up to 5 metres away, with a few long-distance exceptions - 77 metres in one case! Wings and plumes may help disperse seeds further, most reported distances are less than 100 metres from tree height, but when parent plants are small herbs like the daisy Rhodanthe anthemoides the distance travelled is more limited. Dust-like spores and orchid seeds may be carried much further by wind than other seeds without adaptations.
- Animals would have played a role in dispersing seeds with adhesive devices such as the spines on seeds of the burr daisy, Calotis lappulacea. However, native animal populations have been severely depleted, and they are now missing or confined within the one remnant. The potential for long distance travel by animals is blocked by impassable obstacles between today’s fragments.
- Fleshy fruits may still be carried between nearby remnants by birds or possibly mammals, although the chances of a bird arriving in one fragment with fruit from another are of course severely reduced compared with the possibilities before fragmentation. Only 10% of species in Cumberland Plain Woodland produce fleshy fruits.
Seeds of most Cumberland Plain Woodland plants are small, and lack means for long-distance dispersal, especially between today’s isolated fragmented remnants. Most can’t cross a road.
Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.