The travelling seed

Skyrail Nature Diary: July 2010

There are a number of contributing factors to a rainforest's existence. Seed dispersal is one of them. Plants have limited mobility and consequently rely upon a variety of dispersal methods to transport their seeds from one place to another.

There are three methods of seed dispersal: animals, wind and water. Dispersal by animals is the main method. This is accomplished by an animal eating the fruit and depositing the seed at a later date, or by the seed attaching itself to the animal and transported from one location to another.

Apart from animals dispersing seeds, the wind and waterways are also used for transport. Silky Oaks (Grevillea robusta) have papery seeds and are transported easily by the wind, as do species of the Flindersia family - related to citrus trees and have gherkin shaped fruits. Transport via waterways is not as common. In saying this, Idiot Fruit (Idiospermum australiense) from the Daintree Rainforest (Australia) is known to spread moderately along Olive Creek, as its original animal disperser is believed to be extinct.

Small fruits are usually spread by birds and flying foxes, as they can easily reach the fruits while they are still attached to the tree, high up in the canopy. Figs have the highest availability of any fruit in the forest, due to their year round production and are dispersed substantially via birds and flying foxes.

Flying foxes (image 2) rely on their smell and vision to locate fruit, as they have no echolocation. Because of this, they are able to disperse seeds over large distances. They also travel in groups, often hundreds or thousands at a time, so the quantity of seeds being dispersed is tremendous. When feeding, flying foxes chew on the fruit and suck out the juice. They do not swallow the fruit fibres as they find them difficult to digest. Sadly, due to habitat loss, numbers have been depleted.

Rats and ants (image 3) are also known to help with the dispersal of seeds. Rats will bury food to eat at a later date and then forget about it. Ants will transport seeds underground, leaving the seeds in an excellent position to germinate.

Bigger fruits rely on bigger animals like the Cassowary (image 4). The Southern Cassowary (Casuarius casuarius) is one of Australia's largest land animals and plays a unique role in the ecology of the World Heritage listed rainforest in tropical Australia. Cassowaries have amazing appetites - one specimen at Hartley's Creek (between Cairns and Port Douglas, Australia) reportedly ate 20 mangoes in two minutes!

Cassowaries are a keystone species, which means they are vital for seed dispersal in the rainforest. Unfortunately they are being depleted rapidly, which is a huge concern. Over 150 plants depend solely on the Cassowary for future generations - especially the larger fruit species. Some of the fruit they can digest is so toxic that no other animal can eat them. The Cassowary's unique digestive system makes it immune to the toxicity of such plants. These particular plants wouldn't be able to disperse their seeds any other way if the Cassowary was gone. What this means, in the long-term, is that the forest areas would shrink drastically resulting in other plant and animal species going extinct from habitat and food depletion.

The intrinsic processes of rainforests rely on seed dispersal for their existence. Conservation is key to making sure these processes are not disturbed.

Photo courtesy of the Wet Tropics Management Authority: Image 2, Copyright Mike Trenerry.