FIGURE 4.6 Changes in feeding behavior of the collembola Proistoma minuta over time when offered a choice of food items of the ectomycorrhizal fungi Alpova olicaeotinctus, Melanogaster tuberiformis, an unknown isolate M-40, or PDA agar in pairwise combinations. Source: Data from Schultz (1991).

Indirect effects of herbivory can influence fungal communities and biomass. In a study of the effects of damage to the photosynthetic apparatus of pinyon pine forest trees by the larvae of the moth Dioryctria albovitella, Gehring and Whitham (1991; 1994) found that there were trees that were both susceptible and resistant to moth attack. Reduction in photosynthate supply to roots of susceptible trees by moth larval grazing significantly reduced the number of ectomycorrhizal root tips formed on the trees compared to the herbivore-resistant trees. When herbivore grazing pressure was artificially removed, the mycorrhizal status of susceptible trees returned to that of resistant trees (Fig. 4.7). The effect of herbivory on mycorrhizal colonization of pine roots and growth of the host plant was greater in the stressed environment of an oligotrophic cinder soil than in a more nutrient rich, neighboring, sandy loam soil (Gehring and Whitham, 1994).