Dispersal of rainforest seeds by frugivorous birds

Brisbane Rainforest Action & Information Network

DISPERSAL OF RAINFOREST SEEDS BY FRUGIVOROUS BIRDS
A talk given by Dr. Ronda Green to the Queensland Ornithological Society Inc. in May.
(The following is based on Stuart Pell's write-up in QOSI newsletter).

Frugivory is a mutualism - an activity that benefits the two organisms concerned. In this case plants get their seeds dispersed and the birds get a meal.

Pro's and con's of frugivory - from the bird's point of view.

Advantages. Fruits are conspicuous and can be found easily; they are often succulent; and they can be a good source of pigment.
Disadvantages. Fruits are generally low in protein; they can be toxic; they are often found in restricted areas; they are seasonal and there can sometimes be crop failures. Frugivorous birds therefore have to cope with 'lean' seasons.

Pro's and con's of frugivory - from the plant's standpoint.
Wide dispersal of its seeds has some definite advantages viz. seeds can reach other suitable areas; competition from the parent tree is reduced; clumping of seeds is avoided, making it more difficult for seed-eating animals (e.g. rodents) to find them. It has been found that seeds germinate better when stripped of the flesh of the fruit. Similarly, seeds in droppings germinate better than from the whole fruit.

How do plants adapt for attracting the right dispersers and getting them to disperse plenty of seeds and which birds are the best dispersers?
Well, it seems that not all birds that eat fruit are useful in dispersing seed. There are 'mashers', 'peckers', 'grinders and digesters' and 'swallowers or gulpers'. Generally, those birds that swallow fruit and pass the seed undigested are useful dispersers. Birds that grind and digest seed, birds that mash the fruit and lose seed and birds that just peck at fruit, are not good dispersers of seed. Topknot Pigeons and Wompoo, Rose-crowned and Superb Fruit-doves do not grind and digest seeds, so they are passed and dispersed effectively. Parrots, on the other hand, chew seeds, so the seeds do not get through the intestine intact and are not viable when dispersed. Many species of bird disperse small fruit seed well (e.g. Figbird, Lewin's Honeyeater, Bowerbirds, and Catbirds) but they are not as effective at dispersing larger seeds.

Much of this information on feeding technique and efficiency came from fieldwork undertaken at various study sites around Queensland, including sites at Lamington National Park and in the Border Ranges. Fruiting trees were observed over four hour periods during which bird species using the tree, arrival and departure times, feeding rate and feeding technique were recorded. From these observations it was possible to determine the most popular trees for frugivorous birds and the bird species which were effective seed dispersers.

How do trees attract the right birds for seed dispersal?
Birds have very good colour vision and it has been postulated that fruit colour is important in attracting birds to fruit. In nature, red, black and blue are the most common fruit colours. Ronda ran a series of experiments using beads of different colours (firmly attached to a plastic frame) in order to discover which colours were most attractive to highly frugivorous birds, and to various Honeyeaters. Surprisingly, orange was the most favoured colour generally, although frugivorous birds also favoured black. None preferred red as first choice! The trials did not conclusively sort out the importance of fruit colour in frugivory - it may be that the shade of red (e.g. orangey-red) is important. Taste may also be of importance. It may be that an unattractive taste discourages 'mashers' or 'peckers' from eating the fruit (these birds are not effective dispersers) while 'gulpers' or 'swallowers' might eat the fruit in quantity. The latter birds are effective dispersers. However, this possible adaption for good dispersion is still speculative.

How do plants adapt to ensure that birds disperse plenty of their seeds?
Possibilities here are: -

Particular seeds may have a laxative effect and may have a rapid passage through the gut, resulting in increased consumption and dispersion.
Large numbers of seeds are present in soft flesh; flesh may adhere tightly to the seed coat.

Further, it has been noticed that frugivorous birds tend to stay for only short periods feeding in any particular tree. This is good for dispersion but why does it occur? Is it due to a build-up of mild toxins or might the fruit be thirst inducing? Clear answers are not yet available to these questions.

Conservation
Finally, Rhonda discussed the conservation aspects of her work and the possibility of using Frugivores to aid forest restoration in areas with difficult access. Frugivores could possibly be used as cheap spreaders of threatened rainforest tree species. Details were given of research underway at Canungra in which QOSI volunteers have been involved.

Further information
'Frugivory Updates' and a Home Page on the Net are available at:
http://www.ens.gu.edu.au/ecology/fruity/frughome.htm

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