Plants - Seeds For The Future

The production of fleshy fruits and their consumption by a variety of animals is a distinctive and profoundly important phenomenon of the Wet Tropics rainforest.

In this relationship, plants provide nutritious tissue around the seed, and animals eat these fruits then regurgitate, defecate or drop the seeds some distance from the parent tree. This spreads offspring in lower densities over a greater area, giving them a greater chance of survival. This reliance of rainforest plants on animals is very different to the plants of Australia's eucalypt woodlands, which depend mostly on wind and gravity. The result is a colourful array of rainforest fruits which delight rainforest visitors.

 
Animals can act as disperses of seeds by:

• eating fruit and seed and passing the seeds undamaged in faeces
• eating the fruit but discarding the seed
• transporting seeds which get caught or stick to the body of the animal

One of the most important groups for seed dispersal are the pigeons, for which Australia is a major centre of diversity. The rainforest species tend to be nomadic, moving around to take advantage of locally available fruit. Fruit bats are also important long-distance dispersal agents. Cassowaries are considered the main animal dispersal agent for many large-fruited trees. If an animal dispersal agent should ever become rare, the plant species dependent upon it would also be affected.

Rainforest trees with seeds adapted to dispersal by fruit eaters have traits which encourage certain dispersal agents and discourage others. Fruits with bird-dispersal traits are generally vibrantly coloured black, blue, red, orange or white. Bat-dispersed fruits are usually duller colours such as browns, greens or yellows.

Plants have also evolved character traits which protect immature fruits from being eaten, including camouflage (e.g. unripe fruits are often green), spines and chemicals which make the unripe fruits unpalatable or poisonous to potential consumers.

 
Wind dispersal

Seeds which glide in a still environment are well represented amongst trees and lianes of tropical rainforests. Although wind dispersed seeds are common among canopy and emergent trees where both wind and height enhance the potential dispersal distance, it is also found in some tree species of the sub-canopy. Wind dispersed seeds are usually grey or brown, mimicking the colour of dead plant tissue.

 
Water dispersal

Dispersal of seed by water is basically confined to rainforest trees fringing watercourses. The woody material enclosing the seed of some tree species can float while the actual seed remains viable for considerable periods. This is a necessary requirement for species often found in riparian rainforests and species near salt water such as mangroves.

 
Gravity dispersal

While rolling down slopes may seem trivial, it is possibly the only means of dispersal for some species with large seeds. Generally, only a select few animals with a large gape can disperse large seeded species which highlights the importance of the cassowary as a dispersal agent (and as a keystone species) in Wet Tropics rainforests.

 
Seed dispersal must take place quickly

Seeds of many species of the primary forest have no dormancy period and lose viability quickly, remaining fertile for only a few weeks. Even seeds of the wider ranging secondary forests remain viable for only a few months and seeds which are enclosed by a fleshy fruit generally cannot tolerate prolonged desiccation.

 
The material on this page is from 'Repairing the Rainforest' by Dr Steve Goosem and Nigel Tucker. This book is available from the Wet Tropics Management Authority > see Wet Tropics Products.