The diversity of natural communities of arbuscular mycorrhizal fungi: niche or neutral model?

Wherever you look there are patterns in nature. Ecologists have long sought to explain these natural patterns of distribution and abundance and two models have emerged as explanations. The traditional view is that the organisms have evolved to occupy a 'niche'. More recently, the 'neutral' model describes a populations determined by birth, death and migration rather than evolution per se, and that these processes, rather than interaction and evolution determine the distributions we observe in nature. Interactions between microbes and plants are a very important part of ecosystems.. Microbial diseases in particular have been shown to change plant communities: Dutch Elm disease is a good example of this. The most common plant microbe interaction however, is not a disease. The arbuscular mycorrhizal fungi (AMF) colonise plant roots, gaining carbon from the plant in return for a variety of benefits (e.g. nutrient uptake and improved water relations). Around 2/3 of plant species form such mycorrhizas (from Greek for 'fungus-root'), and this 'symbiosis' is found in all land-based ecosystems except Antarctica. This is not a host-specific symbiosis: most plant species capable of forming mycorrhizas can be colonised by any AMF and this is also predicted by evolutionary theory. Many surveys of the AM symbiosis in field systems, however, have consistently shown that the distribution of the AMF among the host plants is not random. This system is therefore an excellent test of the neutral vs. niche theory. The former predicts that the non-random association between plants and fungi is due to factors independent of the host/fungus interaction, such as plant and/or fungal growth, soil factors or indeed chance. The formation of a mycorrhizal symbiosis depends therefore on the meeting of the two organisms and is essentially 'neutral'. The niche theory, by contrast, proposes that the pairing of plant and fungus is due to selection in favour of that pairing because of increased fitness of either or both partners. In this case we would expect external soil properties to have much less effect. To test this we will construct models based on the two competing theories, with the predictions made by the models compared to the patterns that are actually observed in the field. We need a system that is reasonably diverse both in plant and fungal species and in soil properties. Hetchell Wood, near Leeds UK, is an ideal place for this because of its underlying geology. An acid base rock, millstone grit, is capped by Magnesian Limestone, which is much less acid. Where the limestone cap has eroded away the soil is much more acid, and a striking transition from acid to basic soil can occur over the space of a few metres. This results in a high diversity of soil and vegetation in a restricted area. We need to know the distribution of the fungal and root populations, and the properties of the soil in which they are growing. The most effective way of identifying individual roots and the fungi colonising them is to use DNA testing to generate a sequence to identify the plant and a 'fingerprint' profile of the fungi that are colonising it. We will use this technique to profile samples taken from a grid placed across a defined transition so that a map of the plants, fungi and soil properties can be constructed. The testing of the field data against the models will allow us to identify whether or not interactions between host and fungus is more important than growth in response to soil environment in determining observed patterns of AM distribution and abundance. This is a novel approach to a fundamental question in community ecology and also is directly relevant to the management of biodiversity.