7 Plant defences
Watch the 'Plant Predators' programme from 05.03-12.07 and make notes in answer to the following questions.
(a) In what ways do plants shown in this sequence protect themselves against their predators?
(b) How do tapirs avoid being poisoned?
(c) What is the purpose of the pika's haymaking activity?
(a) Plants may have spines and spikes on their leaves and stems (acacias have particularly fierce spines amongst their leaves, see Figure 4) and they may secrete poisons or have toxic chemicals inside their cells.
(b) Tapirs avoid being poisoned by being selective about the type of leaves that they eat and also by eating a mix of leaves, so that if poisons are present, they take in only a small amount of any one. They also seek out and eat quantities of a clay that is rich in kaolin [p. 91]. In their stomach, the kaolin binds to other chemicals, including any toxins which might be present in the food. (In humans, 'Kaolin and Morphine' preparations are still in occasional use as treatment for digestive upsets.)
(c) Pikas collect a variety of plant material, different sorts of leaves and flowers, and store it all in a 'hay pile'. By the end of the season, a pika may have amassed 30 kg of hay, some of it stolen from the haystacks of its neighbours (and from visiting TV presenters). Amongst the plants that pikas collect, there are some whose leaves contain poisons such as phenols that act as preservatives, so the 'hay' stays fresh for the pikas to eat over the winter period. Presumably, as with the tapirs, this mixed diet also means that pikas are unlikely to take in an 'overdose' of any one particular poisonous compound. Many other browsing species use a similar strategy, eating small quantities of a wide variety of plants, rather than stripping all the leaves from one species. And before we leave the subject of pikas, note that they are lagomorphs, and they too indulge in coprophagy. You might like to go back and add them to your completed Table 2.
Grasses have evolved some protection against their predators. What form does this protection take?
In many species of grass, there are tiny blades of silica (similar in composition to sand grains and glass) projecting from the edges of their leaves, which make the grass gritty and wear down the teeth of animals that eat it [p. 122]. As I mentioned earlier, the molar teeth of elephants are constantly replaced as they are worn down by chewing the food. For the same reason, horses have very long teeth, which emerge slowly, remaining about the same size as they gradually wear down with age.