The arguments so far in this chapter may lead one to think that there are effectively two main types of explanation (top-down and bottom-up) to what R.M.M. Crawford23 has called 'one of the marvels of nature' namely, that plants 'while providing the original source of food for all animal and microbial life are not themselves consumed to the point where they are no longer able to support their predators'. In making our case, we have largely considered insects and mammals as herbivores but we have been forced to ignore microbes as there is little in the way of relevant experiments to discuss.
Despite this lack of study, microbes may play a role in keeping the world green and some fascinating relationships are starting to be discovered. For example, one recent study showed that antibiotic-producing bacteria in southern beech trees apparently provide protection from fungal attack.41 Likewise, it has been known for some time that some species of fungi that live within grasses can produce alkaloids that can help protect both fungi and grass from attack.42 So, microbes may feature in bottom-up processes. Micro-parasites are also probably very important in many top-down processes, by potentially controlling the population size of many herbivores. Of course, it is a two-way street because microbes can facilitate as well as prevent defoliation—for example, gut-living microbes are also important in allowing many animals to detoxify plant secondary compounds21 and can also directly destroy plant material. The role of microbial processes in each of these contexts is under studied and we may find out a lot more over the next few decades.
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