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Self Thinning Rule of Ecology - The self-thinning rule of ecology states that on a given site, as some trees increase in mass, the number of trees on the site decreases. As some of the trees die, their exudates increase and benefit soil microorganisms. This process has high survival value in a forest, but may not be effective in cities where individual trees are removed as they die. The desire for trees in our cities goes on. The trees no longer have the benefits of group defense and group protection. Dutch elm disease is an example. The disease in the natural stands did little harm. Actually, the fungi and insects were more like bonogens than pathogens because they served to maintain the self-thinning rule of ecology. However, when elms were taken out of the group and were not able to reproduce on concrete, their defense and protection mechanisms were weakened. The beneficial fungi and insects, the bonogens, were forced to become pathogens. As more trees became trapped in cities, the pathogens "had to" compete for the new energy source. Then the pathogens did what they "knew" how to do best: mutate and reproduce rapidly. Virulent forms increased and soon even healthy trees were attacked. The example is still Dutch elm disease. Think about it. We have done the same for oak wilt and for many other diseases in cities.