In tropical forests all organisms are dependent to some extent on bacteria and fungi. Some animals such as wood and leaf-eating insects depend on symbiotic gut microbes to digest cellulose in their food supply, while other insects utilize fungi directly as a food source.
a. nutrient cycling:
i) Decomposition: Without microbes, organic matter on the forest floor and in the soil would never decompose. The rate at which these microorganisms decompose dead material is directly responsible for the availability of nutrients for plants. As the humidity and temperatures in rainforests are high, conditions are ideal for rapid microbial decomposition. However, the rates of decomposition will differ according to which microorganisms are present, the character of the organic matter, the physical and chemical environment of the soil and so forth. Apparently microbial and fungal populations are quite sensitive to fluctuations in soil moisture and other disturbances.
ii) Nitrogen fixation: Nitrogen fixation is an essential function of microbes in forests. Without bacteria which are capable of converting gaseous nitrogen into nitrates and nitrites which plants can utilize, rainforest soils would rapidly become depleted of this essential mineral in usable form. Many million tons of nitrogen are converted annually and added to the soil by these organisms. In the many tropical soils which are nutrient-poor, only nitrogen-fixing bacteria allow plants to survive.
b. Tree dispersion and other ecological effects: Pathogenic microbes play a role in preventing “clumping” of trees or plants of a particular species and ensuring their wide dispersal throughout the forest. When plants of one species live close together, they are subject to attacks by pathogenic agents, while if they are more widely dispersed, transmission of disease agents is more difficult. In this way, the presence of microbial and fungal pathogens play a role in structuring the composition of tropical forests, by ensuring that most individuals of a given (tree) species will be fairly widely dispersed (van der Putten, 2000). This has implications for monocrops, such as oil palm, soy beans, and rubber, which are being raised on converted rainforest land in many tropical areas. For example, the large gaps in the forests made to create oil palm forests in Malaysia and Indonesia have contributed to the spread of the root-rot fungus Ganoderma, and another root-rot fungus, Phytophthora cinnamon, spread widely after logging in Australia. Similarly, cutting in Eucalyptus forests in Australia has led to a great increase in severity of outbreaks of this pathogen (Lodge, et al., 1996; Gilbert and Hubbell, 1996). These outbreaks have many ancillary effects, including alterations in forest structure, changes in animal populations (including endangerment of rare species), and decreases in tree density.
c. Food sources: Microbes provide food for many small organisms in forests and also as agents which allow the digestion of otherwise indigestible food sources in the guts of many animals. Fungi are important food sources for some invertebrates such, as ants and their fungus gardens (see above) and beetles. On the island of Sulawesi (Celebes), 40% of the beetles feed on fungi (Lodge, et al., 1996).
d. Regulators of population size: Pathogenic microorganisms have important effects on the population size of any organisms which they infect. For instance, defoliation of green plants is restricted by the attacks of pathogens on insect predators of those plants.
e. Mycorrhizae: Many fungi are present in rainforest soils, and some form close associations with tree roots. These presumably symbiotic associations are known as mycorrhizae. Certainly the associations, in which the fungal hyphae penetrate or ensheathe the root, are very close, although they may not all be beneficial to the plant. Up to 90% of all tree roots are involved in these associations. The fungi colonize roots through the spread of the hyphae or the dispersion of spores. The exact role of mycorrhizae in forest ecology is not clear. Some believe that they are involved in nutrient capture and that much carbon and other minerals (nitrogen and phosphorus, especially) are transferred from the soil to the roots by mycorrhizal associations. In turn, the plant passes manufactured carbon compounds to the fungi, and since the mycorrhizae are themselves eaten by soil organisms, carbon is transferred rapidly from the host tree to the soil ecosystem. Mycorrhizae apparently facilitate the uptake of water by roots and increase the resistance of roots to pathogens. All in all, mycorrhizal associations appear to play key roles in growth, nutrient cycling and primary productivity in tropical rainforests. They also appear to have some control over the structure of the plant community and the course of succession. Where forest is disturbed, plants which do not form mycorrhizal associations will predominate; later, as the fungi invade the area, there will be a succession of plants which tolerate and, later, require these associations. This scenario is complicated by the fact that fungi also undergo succession, and that these changes may play a role in the successional dynamics of plant species (Bever, et al., 2001).
Mycorrhizal fungi also act as social agents, as they interconnect trees through their hyphae. This may mean that trees can transfer carbon among themselves via the fungal mat, so that trees in the shade (and thus less able to photosynthesize) are “subsidized” by well-illuminated trees. It is possible that young trees in shady environments are enabled to survive by this mechanism, at least until they can extend their branches into the canopy. It has been speculated that forests are less competitive than they appear, particularly if the mycorrhizae act to reduce competition for nutrients by equitable distribution (Read, 1977). In one experiment, tree seedlings were found to transfer carbon between species bidirectionally (Simard, et al., 1997). However, Proctor, (1995) warns that little is known about the role of mycorrhizae in tropical forests, and therefore one must be cautious about assessing the roles of these fungi in these ecosystems.
Little is known about the ecology of mycorrhizae, but they appear to have a narrow range of tolerance; some can colonize more than one species of tree; others apparently cannot. Mycorrhizae don’t seem to reform easily in disturbed or logged environments. In one experiment, seedling roots became infected with fungus only when they were in contact with living mycorrhizal-root associations during the early stages of their growth (Lodge, et al., 1996). We don’t even know if all of these associations are essential or beneficial to the tree. However, if these associations are important, as they appear to be, disturbances of forests by logging may contribute to further forest destruction by disrupting them.