In this chapter we discussed the basic processes of herbivory, parasitism and mutualism. Herbivory of shoots, leaves, roots and seeds is often non-fatal but since it is damaging, plants have adapted avoidance, tolerance and inducible defence mechanisms. Though non-fatal herbivory can alter the population dynamics of weeds (and other plants), the absence of herbivores helps to give weeds an advantage and the introduction of herbivorous biological control agents attempts to remove that advantage. The same is true for introduction of parasites for biological control of weeds, though many weeds are already parasitized. Weeds themselves can be parasites; this reinforces the idea that not all parasites are microscopic organisms like bacteria or fungi. Mutualisms are important in populations and communities, especially in terms of mycorrhizae and nitrogen-fixing bacteria. Most native plants cannot survive well without mutualists like mycorrhizae. Some weeds need mycorrhizae, but many weeds gain extra competitive advantages if mycor-rhizae are diminished in numbers since native species will be at a disadvantage.

We conclude that while studying all of these processes separately was useful, the real world is a place where the processes act antagonistically or synergistically in complicated ways. A realistic ecological study would be one that examines the community - the hundreds of plant species (many of them weeds) and millions of other species that interact in a defined area - however arbitrary the definition of the community boundaries may be. As a result of all this complexity, it is necessary to discuss how this is addressed by studying what is called 'community structure and dynamics' in Chapters 10-14.

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