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Afrikaanse Inligting
How to Safeguard Seed from Predators: Proteas do it their way
Penny
Mustart, UCT.
You know that many Proteaceae are serotinous (they store seed from the crops of successive years in their canopies). Seeds are stored for many years in fire-resistant cones, or cone-like structures. These seeds are important in two respects. Firstly, nitrogen and phosphorous are highly concentrated in them making these seeds an attractive food source for rodents, birds and insects in an otherwise nutrient-poor environment. Secondly, many serotinous proteas are killed by fire: the seed released from their cones at this time are thus the only link between the pre- and post-fire generations. Seeds are clearly a precious commodity. It makes sense for the adult plants to safeguard the seeds from insects and other predators whilst storing them on their canopies.
Australian proteas have thick, woody structures enclosing the seeds, and in this way they are mechanically protected against attack from cockatoo birds with their strong, stout beaks. The nut-like follicles of Hakea species (PAN 8: 14) and the monstrous cone-like structures of Banksia species (PAN 4: 10) are similar in strategy to using the Tower of London to protect the Crown Jewels. South African serotinous proteas are less paranoic about burglar-proofing their valuable assets. In Aulax species the branches form a cup-like structure enclosing the seeds, and in Leucadendron those familiar cones are woody bracts each protecting a seed (PAN 6: 9). Neither of these strategies approaches the "heavy arm" tactics of their Australian relatives, probably because we do not have any bird-equivalent of the cockatoos. However, our proteas do show extensive invasion by insects: this is particularly evident in Protea. In fact, serotinous proteas appear to have provided no protection for their seeds at all - a dashing cavalier approach in the face of all-out insect attack. Protea seeds are tightly packed like eggs into the bottom of an egg box, and then left fully exposed to the dangers of piracy. Some species, for example, Pr obtu and Pr repe have pointed cone-like structures formed from the involucral bracts closing over the inflorescence after it has flowered. Yet the contents of these seed heads show signs of extensive damage from insects that had arrived before the bracts had closed over. Other insects bore in from below the receptacle. So how do sufficient Protea seeds escape this plunder over the years?
Protea seeds play a form of hide and seek. Of all the florets in the flower heads, few (1-30%) actually form fertile, embryo-containing seeds. Most florets develop into infertile, empty, wooden structures almost indistinguishable from the fertile seeds until cracked open (PAN 4: 16). Tests have shown that insect larvae munch through Protea seed heads indiscriminately eating fertile and infertile seeds - suggesting that they cannot tell the difference (PAN 10: 14). In this way the effect of damage is diluted and some fertile seeds escape been eaten.
But this is not the whole story. These few fertile seeds are not randomly arranged amongst the infertile ones, but occur in small clusters of two to six seeds. I discovered this pattern while spending many tedious hours counting the numbers of fertile seeds in seed heads for some other research purpose. It seemed then to me that the seeds were positioned in small groups. A statistical test subsequently showed this clustering pattern to be highly significant - it was not a random happening. In other biological systems it has been shown that if the prey are aggregated, then they have a greater chance of escaping attack than if more randomly, or evenly, arranged. Thus aphids living on cabbage leaves escape being parasitized more effectively when they occur in patches, than when evenly distributed on the leaf. The strategy appears to be very successful in Protea. I have seen old seed heads (stored on the plant for more than four years) with extensive insect damage leaving only about three or four uneaten seeds on the receptacle - often all of these are fertile seeds. Their hide and seek game was successful.
In this unique way, proteas ensure that there are enough seeds to ensure adequate germination levels after fire. Proteas do it their way!
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