General principles of herbivory
Herbivory is an ancient and important ecological relationship. Herbivorous insects alone represent almost a third of all living species (Greenwood 1990, Wehling and Thompson 1999). In natural ecosystems roughly 10% of all plant resources are lost to herbivory (Schoonhoven et al 1998). The battle can be seen from either side, the animals and their attack on plants, or plants and their defences against animals.
Many theories have been advanced that try to generalize rules of herbivory and explain patterns observed in nature. Animal herbivory strategies were defined in ideas like the ‘feeding specialist theory’ where those animals that specialized on particular plants performed much more efficiently than generalists; those generalists, however, could choose from a wider range of food (Howe and Westley 1990). Many theories have been put forward to explain the situation from the other side of the war. In the seventies, authors such as Feeny, Rhoades and Coates suggested plants being either ‘apparant’ or ‘non/un-apparent’; the former plants were obvious and common and employed ‘quantitive’ physical defences such as toughness, whereas the latter plants were sparse and defended themselves with ‘qualitive’ defences such as toxins (Harborne 1997). This was then later challenged by authors such as Coley (1983) and theories including ‘resource availability’ that saw plant defences as a consequence of what was available in that particular environment. Further theories exploring plant defence included the ‘plant stress hypothesis’ by White (Floater 1997) and ‘induced defence response’.
Physical defences are quite often external and usually obvious to human eyes. The simple toughnesss of leaves and their waxy coating can act as an effective barrier against small chewing mouthparts, especially of earlier smaller instars (Norris 1991). Even the simple and common pattern of the serrated edge of leaves may slow down an insect herbivore. Spikes and stinging hairs may also act as a defence; although this may be more effective against vertebrate predators, with smaller insects simply chewing around them.
Chemical and internal composition is the second type of defence. Chemical compounds in the plant that are used for normal functioning are known as primary compounds; secondary compounds are those that have evolved for other uses, such as defence (Harborne 1997). Resins, tannins and silica can render the plant indigestible, essential oils can acts as feeding deterrents, and alkaloids can act as direct toxins (Norris 1991).