Poison in Paradise or Fruits of Death!

Poison in Paradise or Fruits of Death!Turkey Tangential By Spencer Shaw
What a dramatic headline and attention grabbing headline! There’s nothing quite like a bit of fear, to bring out the voyeur within. Given the recent removal of a White Cedar from a park in Maleny, I think however that may be timely to again look at some of our poisonous plants.Over the millennia, plants have evolved various means to stop us greedy herbivores and omnivores getting too carried away and eating them all into oblivion. Some are prickly – such as the tendrils of Lawyer cane Calamus muelleri, some are hairy – such as the fruit of Foam bark Jagera pseudorhus, some are tough – such as the leaves of Wilkea macrophylla and some are … poisonous! Well actually most if not all plants have developed some form of chemical defence systems. Take many of the leafy vegetables we eat such as spinach or silverbeet. Their leaves are laced with oxalic acid an irritant that is removed when we dispose of the water we cook them in. Quite often survival comes down to the tolerance and adaptation that animals develop to the range of chemicals that plants defend themselves with. For example 1080 or Sodium fluoroacetate  is a chemical commonly used for poisoning feral animals in Australia. It also occurs naturally as a defence chemical in some Australian plants and consequently many of our native herbivores have developed varying degrees of tolerance to it. Another interesting example of a specialist plant animal relationship is that of the Richmond Birdwing Butterfly and its host plant. The Richmond Birdwing Ornithoptera richmondia has evolved with its local food plant Pararistolochia praevenosa the Richmond Birdwing Vine. The vine P. praevenosa has a powerful chemical arsenal that keeps most other herbivores away, however the Richmond Birdwing has developed a high level of tolerance to these chemicals and its caterpillars can feed on the new growth. This degree of specialisation by plant and animal has many advantages. For the plant its toxicity allows only specialist herbivores to feed upon it and for the animal if you can adapt and specialise to the consumption of highly toxic plants you often have very little competition. The specialist adaptations of the Richmond Birdwing however have left it very vulnerable. The loss of its specific food plant to habitat clearance has been a major blow, but on a more sinister level the introduction of a South American vine species Aristolochia elegans the Dutchman’s pipe has been its greatest threat. A. elegans is related to our local Birdwing food plants but contains much higher levels of toxins – ironically, the same toxins that attract the Richmond Birdwing  in the first place! The butterflies lay their eggs on this vine but the caterpillars soon succumb to the higher levels of toxins.So far we have mainly discussed (very briefly) the chemical defences of plants against herbivores. The protection of their leaves and stems from the greedy herbivores is understandable, but what about fruit – surely they wouldn’t be poisonous when they need to be eaten, to be dispersed? Or would they?Birds and mammals are the main seed dispersers and both groups of animals have quite different digestive systems. Birds are often better seed dispersers because their digestive systems allow seeds to pass through quickly and unharmed. Many birds also appear to have developed greater tolerances or immunity to fruit toxins as opposed to many mammals that are a better guide to us as to what is and isn’t edible.Generally we can taste what is all right to eat in regard to fruit, but this is not always the case. Fruits such as those of White Cedar Melia azederach, Chain Fruit Alyxia ruscifolia or Tie Bush Wikstroemia indica don’t taste too bad at all from personal experience, but apparently are listed as poisonous. Even more insidious is the Finger cherry Rhodomyrtus macrocarpa of Nth QLD. Its tasty fruit are documented as causing blindness when consumed, however it is possible a fungus present on the skin of the fruit may cause this injury. To add more confusion to the issue many fruit that are apparently edible, taste – well let’s be honest, awful! The Native Grapes Cissus sp. are extremely astringent and to my mind are far from edible but it is quite likely that birds do not even detect this discomfort and the astringency merely acts to speed the travel of the seed through the birds gut preventing digestion of the seed.In finishing I hope I haven’t left you all feeling that our native forests are full of ‘poisonous potential’. With toxic fruit ready to jump down your throat at every bend in the trail, or stinging tree leaves waiting in ambush! After all your average domestic exotic garden can be a far more dangerous place with a whole suite of garden plants that you take for granted, being extremely  toxic! Plant poisons are in fact a part of life. Without them the herbivores would know no limits and deserts would be our domain. Hmmm, limits!  Now there’s an interesting concept for us humans?