Pollination and Fruit Dispersal

Skyrail Nature Diary: May 2014

This month we’ll be looking at pollination and fruit dispersal. Plants are unable to move and can’t reproduce without assistance. Pollination is the act of getting a plant’s pollen to the ovary. In the rainforest most plants are what we call ‘dioecious’, which means that each plant has both sexes on the same specimen. Except in certain rare cases, most of these trees need to be cross pollinated to avoid inbreeding. Most plants have insects, birds and bats as their main pollinators. Flowers are designed to attract these pollinators and not, strictly speaking, for our enjoyment. Red flowers tend to lack scent and usually attract birds, which tend to have a very bad sense of smell. Fragrant flowers attract most other pollinators. Foul smelling ones generally attract flies who think they’ve found some rotting flesh!

There are some interesting variations due to a number of insects and some birds which are able to see ultraviolet radiation. Ultraviolet is a part of the light spectrum that we humans are unable to see. This is important to white flowers, because although several of them may look almost identical to us humans, insects and birds can tell them apart thanks to the ultraviolet patterns on the petals.

Some flowers have unusual problems with regards to getting pollinated. Figs have their flowers enclosed in what we call a head, the fig itself. Not only that but the figs are monoecious, that is, they have separate sexes. They solve this problem by attracting special fig wasps that lay their eggs inside the fig. As a matter of fact the male fig wasps never leave the fig. The females are generally born in the male figs and are attracted to the female figs. Here they enter the tight opening and have their wings ripped off before depositing their collected pollen onto the female flowers. They mate with the emerging blind, flightless males and lay their eggs in special gall flowers inside the fig. Then both wasps die and are absorbed by the fig. Some trees have flowers on their trunks and are accessed by small mammals in addition to the above animals. Many flowers have special shapes that only certain animals can access, like orchids and various tubular flowers. About 10% of the plants in the rainforest are pollinated by the wind, the most obvious ones being the palms.

The ripe fruits resulting from the pollination are dispersed (spread out) by animals or the wind as described below.

Fleshy fruits are dispersed by quite a few animals. They are ingested and the seeds pass through the gut unharmed. Some are very particular about which animals disperse their fruits and use a concoction of toxins to achieve this end. Big fruits generally don’t go very far once they drop onto the ground and would easily end up in very crowded conditions if they all germinated straight away. To avoid this they contain chemicals in their seed coats that prevent germination. Then they are eaten, often by cassowaries, and are carried somewhere else before being voided into a hopefully better site.

Rats also eat some of the big fruits but tend to destroy the seeds. To avoid this they are often presented with an absolute glut of fruit in one go. Since they can’t eat all of it, the rats tend to bury some of the surplus for later harvesting. A significant number of these fruits germinate in the meantime. Small fruits are generally dispersed by birds on the wing, especially the fruit doves, like the Wompoo Pigeon. The most important small fruits by far are the figs, which can sometimes make up to 20% of the biomass in the rainforest. More importantly, figs fruit throughout the year, quite unlike most plants in the rainforest. This creates a reliable source of food for several frugivorous birds that would otherwise starve.

Burrs are less common than fleshy fruits but quite important nonetheless, especially along waterways where animals often congregate for obvious reasons. For burrs to be effective they need to target birds and mammals that have feathers and furs the burrs can attach to. Reptiles and amphibians are generally poor candidates for this kind of transport with their slippery scales and smooth skin. Most of these fruits are produced by herbaceous plants that are low enough to the ground to be snagged by a passing animal. Further inland some of the grasses employ this strategy as well. There are some trees that produce burrs despite this and passing birds are the hosts in this case. The Macarangas are a prime example of this particular strategy.

Wind dispersal is a far more chancy way of dispersing your seeds than employing animals to do the dirty work for you. There are ways around this. As a rule of thumb, plants that disperse their seeds using the wind as an agent produce huge numbers of seeds to ensure that at least some of them survive the hazards along the way. These seeds have to be small for this to work or use special structures to improve their chances of being blown away some distance. Structures enhancing transport by wind include samaras (“wings”), exemplified by Tulip Oaks, hairs, exemplified by Milky Pine and papery halos, exemplified by Rose. The best example of plants that produce tiny seeds without special structures, are the orchids. Some orchids produce in excess of one million seeds per fruit, each fruit essentially a decorated bag that shrivel up when ripe, releasing its smoky contents into the wind.


By Tore Lien Linde