Plants are faced with a dilemma; while they need to attract beneficial pollinators and seed dispensers, they must also minimise the damage caused by the marauding army of herbivores. Without some form of protection the trees would be stripped bare and smaller plants would be completely devastated, and because plants stand still, they cannot run away. This is as true in Amazonian rainforest as it is in Northern coniferous forest.
As the architects of every medieval
castle understood if you are fixed in place, your past experiences
(natural selection) lead you to develop (evolve) defences. Furthermore,
which defences are present at any given time reflect at least
four quite different processes: how many resources you have to
build and maintain them, the traits of past and present attackers,
and the structure of the entire edifice when a new defence is
being considered. Finally, by the possession of ever more defences,
the inhabitants of the castle are rendered ever more immobile
(both physically and culturally), and the defences become ever
more a long-term investment, and the castle becomes more vulnerable
if the defences are breached. Natural selection has generated
the above pattern in plants; over millions of years they have
evolved an array of defensive compounds that make their parts
(leaves, flowers, stems, roots and fruits) distasteful or poisonous
to predators. In response, however, the animals that feed on
them have evolved over successive generations a range of measures
to overcome these compounds and can eat the plant safely. The
plant in turn develops its defences still further which will protect
it until the predator again 'learns' to overcome it - and so on.
The result can be a sort of ongoing chemical "arms race"
between plant and predator. Often this increasing specialisation
means that a herbivore is able to cope with the compounds present
in only one species of plant, and so the "war" can be
narrowed to one plant species and one insect species. The phenomenon
of plant defences has great relevance for human beings because
the substances concerned are often the "useful" chemicals
of the rainforest, such as those which form the base for important
The tree trunk offers an especially
clear example of the variety of defences available to plants.
Starting at the outside, the bark is dead tissue thoroughly laced
with compounds in their active forms. Furthermore, the bark has
virtually no nutrients in it to compensate for the damage it might
cause to an animal that fed on it. Virtually no herbivores (except
for some termites) consume dead bark on the standing tree.
Just below the bark, however, lies
an area of expanding and nutrient-rich tissues (bark cambium)
which may be rich in defences but in a finely structured manner.
In a living and healthy tree, the defences in this area are particularly
active. However, when a tree is stressed and the resin or latex
pressure falls, this portion of the trunk is quickly invaded by
a variety of boring beetles that were previously kept out by the
active defences, where they find a thin shell of very nutrient-rich
tissues that they compete for with many other organisms.
The tree trunk offers an especially clear example of the variety of defences available to plants. Starting at the outside, the bark is dead tissue thoroughly laced with compounds in their active forms. Furthermore, the bark has virtually no nutrients in it to compensate for the damage it might cause to an animal that fed on it. Virtually no herbivores (except for some termites) consume dead bark on the standing tree.
Just below the bark, however, lies an area of expanding and nutrient-rich tissues (bark cambium) which may be rich in defences but in a finely structured manner. In a living and healthy tree, the defences in this area are particularly active. However, when a tree is stressed and the resin or latex pressure falls, this portion of the trunk is quickly invaded by a variety of boring beetles that were previously kept out by the active defences, where they find a thin shell of very nutrient-rich tissues that they compete for with many other organisms.
Moving into the xylem (water conducting cells - sap wood) we enter an area whose primary defences is its high water content, its largely indigestible cellulose structure, and its position sandwiched between the active defences of the bark area and the heartwood . Once the outer defences are broken through during tree death, the sapwood becomes the home and food of a great variety of animals that can digest cellulose through micro-organisms in their digestive systems. It is a striking characteristic of all of these animals that there can be no co-evolution between them and their host plant, since the host plant is either already dead or consigned to death when they enter.
The heartwood is at the centre of the trunk, the beautiful and highly valued part of the log that finds its way into beautiful furniture and veneers. This portion of the tree has died and as part of the ageing process, the cells were filled with tannins, terpenes, lactones, alkaloids, and a wide variety of other compounds in their active unbound and unglycosilated state. When a tree falls it is usual for the sapwood and surrounding materials to be degraded within a few months or a year, while the heartwood persists as a high quality log, degrading only very slowly as rainwater gradually removes the defence compounds and allows fungi and termites to invade.
As the leaves of plants mature they become tougher and more resistant to insect attack; some plants shield their leaves with a thick, waxy cuticle. Others have evolved forms of physical defence, such as thorns or spines. The trunks of many forest trees, including palms and the ginseng family have a thick armour of sharp spines to prevent larger animals from reaching their leaves, flowers and fruits. The vast majority of plants, however, do not have these visible forms of defence: they are protected by various types of chemical compounds, which course through every part of then.
These chemical defences take a huge variety of forms, and are remarkably effective. The leaves of some plants, for instance, have stinging hairs that cause an immediate and severe burning sensation on the skin. Others, especially of the sumac family, contain chemicals that can cause an allergic reaction in the intestines of a herbivore that is strong enough to prevent it from trying to eat the plant again. Other leaves contain substances that, while harmless themselves, are phototoxic, meaning that they react with ultraviolet light to form poisonous compounds that disable or kill the unfortunate predator. However, most of the protective chemical compounds used in plants merely taste unpleasant, deterring predators at first bite - though they may have an increasingly poisonous effect if the animal continues to eat the plant.
Some plants protect themselves using cunning tricks with chemicals. Certain plants in the legume family, for instance, produce amino acids which, though different, are structurally similar to the amino acids found in proteins; more than 1,200 legumes are known to contain canavanine (a non-protein amino acid that is related to the protein amino acid arginine). Most insects, on eating the plant, will incorporate the wrong amino acid into their proteins in place of arginine. The "false" proteins do not work correctly, and as a result the insects die. The bruchid beetle, however, uses the seed of one such plant as a nursery for its offspring. The larvae feed on the seeds freely, because they manufacture a protein-building enzyme that can tell the difference between canavanine and arginine. The beetle even manages to metabolise the canavanine for use as food.
More extraordinary still are the plant compounds that interfere with the very life cycles of their predators. Before reaching maturity, insects often go through a complex series of larval and adult stages and several moults, all of which are controlled by juvenile and moulting hormones. Some plants contain large amounts of the juvenile hormone of predator insects, causing them to remain in the larval stage and ultimately, to die without reproducing. Other plants do the opposite, producing substances that overcome the effects of juvenile hormones. The result of this is that the insects skip some vital larval stages and become adults too early. Still further plants contain compounds that interfere with the moulting process. Similar techniques are used to deter mammalian herbivores by a group of plants containing substances that imitate the effects of oestrogen, thus lowering the mammals' fertility rates.
Lethal poisons are also used in this struggle for survival. Some plants contain cyanogenic glycosides that, when the cells are disrupted by a predator's bite, give off hydrogen cyanide (prussic acid), a toxin that inhibits respiration. Also, the seed coats of the castor oil plant contain one of the most toxic substances known to humanity, a protein called ricin, of which a single molecule is enough to kill an entire cell.
Many of these defensive chemicals are familiar to us - strychnine and cyanide for example are common in certain plants. Others contain bitter tannins, which we know through the astringent taste they impart to unripe fruit or red wine. Every group of plants tends to specialise in the production of a particular set of chemical compounds, trees of different species growing side by side in the forest may contain completely different defensive chemicals, so that a herbivore which can safely eat the leaves of one tree but may, be poisoned by its neighbour. In this way, plants have derived not only individual defence mechanisms but also a communal one, for it is impossible for a single predator to devastate even a small area of forest.
In the fiercely competitive environment of the forest, however, such ingenuity does not go unchallenged; it appears that a sort of chemical 'arms race' is being fought by the plants and their insect predators. In response to the chemicals that the plant produces to deter them, the insects evolve detoxification mechanisms or other means of overcoming the protective poisons. The plants, in turn, now evolve new chemical defences. In time the insects' remarkable adaptive abilities will overcome these too, and so the escalation continues. The results of this unresolved war are ever more specialised relationships between insects and plants. Fewer and fewer predators are able to feed on any one species of tree, and each predator may feed on fewer and fewer host plants.
The defensive compounds produced by plants are by definition, potent substances, otherwise the plant would be dead. Only a few compounds from rainforest plants have been studied for their useful potential, but of those that have, the results have been impressive. Plants produce fungicides, insect repellents, and pesticides to deter predators. Some have been found which, unlike many man-made chemicals, are effective against insects but harmless to mammals.
One of the most obvious examples, rubber, evolved by the plant to protect itself, is of great economic importance to many tropical countries and has never been matched by man-made substances. The purpose of latex in plants is to simply gum up the mouth parts of their predators. When these plants are damaged they produce latex, which oxidises on exposure to air to form a thick glue. Needless to say, some of their insect predators have learned to bypass this threat: biting through the plant's major veins, they drain the latex, interrupting its flow to the leaf so they can safety feed. This clever ploy is also used by some insects on leaves that contain poisonous chemicals.
This defence costs the plant considerable energy - it is noteworthy that once serious tapping of medium-sized rubber trees in a plantation begins, they increase in size at a much slower rate and produce very few seeds. This suggests that they are putting much of their resources into replacing the latex. When the latex is not being removed at frequent regular intervals, the cost to the plant of maintaining this defence system is clearly much less.
Such an unchallenged standing defence is often thought of as a form of 'insurance' against herbivores, but this is not quite the case. Buying insurance means that you will be paid back for future damages, the insurance does not prevent the accident from happening. It is more accurate to compare the rubber tree's defences with the national defence budget. Just as in this budget, there are enormous costs . Furthermore, just as in national defence, the real threat is extremely difficult to define and demonstrate unless one carries out an experiment removing the defence system.
Just as humans undoubtedly evolved the cultural trait of throwing a deadly spear many times, latex is a defence trait that appears in many different quite unrelated families of tropical plants, like the development of resin and gum produced as a defence in other sets of tropical plant families.
What do the tannins in leaves do to the herbivores that eat them? In the tanner's vat, tannins are used to cure leather. They do this by binding with collagen to render the protein in a hide unavailable to bacterial action - they 'tan' leather and stop it decaying. The simple answer used to be that in the herbivore's gut they tanned the proteins of the gut wall, the bacteria in the gut lumen and the proteins released from the leaf by the herbivore's chewing, meaning that they are unavailable to the herbivore or its microbes. However, it is now becoming clear that the many different kinds of tannins respond differently. To complicate the story even further, it seems that tannins are treated just like other defences by certain kinds of herbivores; the herbivore may actually use the tannin in its metabolism, thereby turning the tannin into a nutrient for the herbivore.
We have the herbivores to thank for more than latex, resin and tropical heartwoods - because of them, we have fruits which we enjoy. Fruits are perhaps the most complex of all the plant parts in the tropical rainforest, at least with respect to their chemistry. The biological function of a fruit is to get the seeds into the right place (i.e. a place where it can germinate and grow, preferably away from the parent plant, and to achieve this inside a fruit eating dispersing animal) and keep them out of the wrong place (i.e. near the parent plant, or more particularly inside a seed-eating herbivore). Since nearly all the herbivores are a potential threat to ripe fruits, it is the chemistry of that defence that is responsible for many of the different flavours that we enjoy or dislike in fruits.
Of course, the flavour (and colour) of fruits is also geared to the likes and dislikes of the proper dispensers as well. The striking absence of a bountiful harvest of commercial wild fruits from Amazonian rainforest (in contrast to the mangoes, mangosteens, rambutans, lichees, &c. of Southeast Asian rainforests) seems to be related to the fact that the Amazonian rainforest lacks a fauna of large primates with taste in fruit similar to that of humans.
Defensive chemicals are the active ingredients in practically all medicines that come from plants, drugs that make up 40% of those prescribed in the West. The compounds evolved by rainforest plants could, by careful breeding and genetic manipulation, be used to strengthen food plants against disease and predators in areas of the world where people are struggling to grow food. The list of possible benefits that we can reap from plants in general, and large rainforests in general nearly endless. If we allow the rainforests to survive, we could tap their wealth, established over millions of years of evolutionary warfare, for many years to come.
Copyright © Marcus Wischik 1998