surfaces absorb ammonium and distribute it by diffusion. Ecology Biogeography Arbuscular mycorrhizal fungi are most frequent in plants growing on mineral soils, and are of extreme importance for plants growing in nutrient-deficient substrates such as in volcanic soil and sand dune environments. The populations of AM fungi is greatest in plant communities with high diversity such as tropical rainforests and temperate grasslands where they have many potential host plants and can take advantage of their ability to colonize a broad host range. There is a lower incidence of mycorrhizal colonization in very arid or nutrient-rich soils. Mycorrhizas have been observed in aquatic habitats; however, waterlogged soils have been shown to decrease colonization in some species. Arbuscular mycorrhizal fungi are found in 80% of plant species and have been surveyed on all continents except Antarctica. The biogeography of glomeromycota is influenced by dispersal limitation, environmental factors such as climate, soil series and soil pH, soil nutrients and plant community. While evidence from 2000 suggests that AM fungi are not specialists on their host species, studies as of 2002 have indicated that at least some fungi taxa are host specialists. The ecology of Mucoromycotinian fungi, which form ‘fine root endophyte’ arbuscular mycorrhizas is largely unknown. Response to plant communities The specificity, host range, and degree of colonization of mycorrhizal fungi are difficult to analyze in the field due to the complexity of interactions between the fungi within a root and within the system. There is no clear evidence to suggest that arbuscular mycorrhizal fungi exhibit specificity for colonization of potential AM host plant species as do fungal pathogens for their host plants. This may be due to the opposite selective pressure involved. In pathogenic relations, the host plant benefits from mutations that prevent colonization, whereas, in a mutualistic symbiotic relationship, the plant benefits from mutation that allow for colonization by AMF. However, plant species differ in the extent and dependence on colonization by certain AM fungi, and some plants may be facultative mycotrophs, while others may be obligate mycotrophs. Recently, mycorrhizal status has been linked to plant distributions, with obligate mycorrhizal plants occupying warmer, drier habitats while facultative mycorrhizal plants occupy larger ranges of habitats. The ability of the same AM fungi to colonize many species of plants has ecological implications. Plants of different species can be linked underground to a common mycelial network. One plant may provide the photosynthate carbon for