Pathogens of plants are extremely important but little-known elements of tropical ecosystems. These pathogens include fungi, bacteria, protozoa, viruses, and worms (nematodes, in particular). They play a variety of roles – regulating populations, restricting plant distribution, reducing/increasing diversity, creating gaps in the canopy, regulating the reproduction and growth of the host, and affecting the availability of food/shelter for animals. In so doing pathogens are instrumental in normal community functioning (as endemic pathogens), but when they undergo population surges – and become epidemic – they can have dramatic and sometimes catastrophic effects on the forest. This occurs when the pathogen takes advantage of susceptibility in the host plant and a conducive environment; such conditions are usually brought about by human activities or extreme climatic conditions. Humans remove natural controls on plant pathogens by disturbing the natural ecosystems during the establishment of agricultural systems, by monocropping, with pollution, road construction, and the introduction of foreign species, which can act as hosts for pathogens, and by the introduction of foreign pathogens. The last-named have been the cause of the most extreme disease episodes (Gilbert and Hubbell, 1996).

When individuals of a single species live close together, pathogens can easily move from one to another, and mortality is high. Light-dependent tea seedlings in the Amazon grouped in open areas have a high mortality rate due to the “witches’ broom” fungus, so that only a few trees survive to become adults. The survivors are widely spaced, and so, by means of a pathogen, the establishment of a monoculture is prevented (Lodge, et al., 1996). Pathogens can also create canopy gaps (by killing certain individuals in an area) and promote successional events and diversity. Interestingly, Percy et al. (2002) found that trees responded to elevated levels of CO2 and ozone (two greenhouse gases) by increasing production of waxes, which provide leaves with protection from pathogens, as well as other chemical protective agents.