Rainforests plants produce a wide range of seeds that can be dispersed by wind, water, gravity, animals and people. Seed dispersal is vital for plants long term survival and ability to find a suitable place to grow. Bright fleshy fruits are a special feature of Wet Tropics rainforests.
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 dispersers of seeds by:
One of the most important groups for seed dispersal are the pigeons. The rainforest pigeons tend to be nomadic, moving around to take advantage of locally available fruit. Fruit bats are also important long-distance dispersal agents. Cassowaries are the main animal dispersal agent for many large-fruited trees and can carry them long distance, and up hills! cassowaries are thought to help transport over 150 different fruits, and some of the larger fruits rely solely on the cassowary as an animal disperser. If a seed disperser such as the cassowary should ever become rare, the plant species dependent upon it would also be affected.
Rainforest trees with seeds adapted to dispersal by fruit eaters (frugivores) have traits which encourage certain dispersal agents and discourage others. Fruits with bird-dispersal traits are generally vibrantly coloured 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 (for example, unripe fruits are often green), spines and chemicals which make the unripe fruits unpalatable or poisonous to potential consumers.
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 saltwater such as mangroves.
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.
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 seeds uphill, which highlights the importance of the cassowary as a dispersal agent (and as a keystone species) in Wet Tropics rainforests.
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.
You can read lots more about rainforest fruit dispersal in Tropical Topics.