A. H. Fitter

The varying success of invaders.

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Ploughing up the wood-wide web?

Key species groups that affect major ecological processes are vital components of community diversity. Many such key groups are found in the soil, including the mycorrhizal fungi that may connect plants into a functional “wood-wide web”. Arbuscular mycorrhizal associations are formed by fungi of the order Glomales with 90% of land plant families, and many arbuscular mycorrhizal fungi are thought to have a broad host range. Here we show that, despite this broad host range, the diversity of arbuscular mycorrhizal fungi is strikingly low in arable sites compared with a woodland.

An arbuscular mycorrhizal fungus accelerates decomposition and acquires nitrogen directly from organic material

Arbuscular mycorrhizal fungi (order Glomales), which form mycorrhizal symbioses with two out of three of all plant species, are believed to be obligate biotrophs that are wholly dependent on the plant partner for their carbon supply. It is thought that they possess no degradative capability and that they are unable to decompose complex organic molecules, the form in which most soil nutrients occur. Earlier suggestions that they could exist saprotrophically were based on observation of hyphal proliferation on organic materials. In contrast, other mycorrhizal types have been shown to acquire nitrogen directly from organic sources. Here we show that the arbuscular mycorrhizal symbiosis can both enhance decomposition of and increase nitrogen capture from complex organic material (grass leaves) in soil. Hyphal growth of the fungal partner was increased in the presence of the organic material, independently of the host plant.

Biodiversity and ecosystem services: a multilayered relationship

The relationship between biodiversity and the rapidly expanding research and policy field of ecosystem services is confused and is damaging efforts to create coherent policy. Using the widely accepted Convention on Biological Diversity definition of biodiversity and work for the U.K. National Ecosystem Assessment we show that biodiversity has key roles at all levels of the ecosystem service hierarchy: as a regulator of underpinning ecosystem processes, as a final ecosystem service and as a good that is subject to valuation, whether economic or otherwise. Ecosystem science and practice has not yet absorbed the lessons of this complex relationship, which suggests an urgent need to develop the interdisciplinary science of ecosystem management bringing together ecologists, conservation biologists, resource economists and others.

The characters of successful invaders

The character&tics of invading species were explored using the Ecological Flora Database, and by using comparisons available in other studies. Invasion success has been related to both abundance and distribution in the native range, which are themselves related. Abundance is probably the key variable. In the British flora invading species are characterized more by their distribution and morphology than by their life history and reproductive behaviour. The size of the plant and characters related to propagule pressure created by humans come out as important correlates of success. For biological characters, genetic studies and the phenomenon of boom-and-bust both suggest that the critical interaction between species and habitat is often subtle. Nevertheless, studies on pines and squirrels show that such important biological characters can be determined. Copyright

Multi-functionality and biodiversity in arbuscular mycorrhizas.

Plant roots in natural ecosystems are typically colonized by a wide range of fungi. Some of these are pathogenic, others appear to be opportunistic and have no apparent impact, while mycorrhizal fungi are generally regarded as mutualistic. Of the various types of mycorrhizal fungi, the arbuscular mycorrhizal (AM) association is by far the most abundant and widespread. While the most widely accepted model of AM function depends upon plants benefiting from the facilitation of phosphorus uptake, recent data from field-based studies in temperate ecosystems indicate that only plant species with poorly branched root systems benefit from AM fungi in this way: species with highly branched root systems may benefit in other ways, such as by being protected against root pathogenic fungi. These two responses apparently represent extremes along a continuum of AM benefit determined by root system architecture.

Arbuscular mycorrhizal community composition associated with two plant species in a grassland ecosystem

Arbuscular mycorrhizal (AM) fungi are biotrophic symbionts colonizing about two‐thirds of land plant species and found in all ecosystems. They are of major importance in plant nutrient supply and their diversity is suggested to be an important determinant of plant community composition. The diversity of the AM fungal community composition in the roots of two plant species (Agrostis capillaris and Trifolium repens) that co‐occurred in the same grassland ecosystem was characterized using molecular techniques. We analysed the small subunit (SSU) ribosomal RNA gene amplified from a total root DNA extract using AM fungal‐specific primers. A total of 2001 cloned fragments from 47 root samples obtained on four dates were analysed by restriction fragment length polymorphism, and 121 of them were sequenced. The diversity found was high: a total of 24 different phylotypes (groups of phylogenetically related sequences) colonized the roots of the two host species. Phylogenetic analyses demonstrate that 19 of these phylotypes belonged to the Glomaceae, three to the Acaulosporaceae and two to the Gigasporaceae. Our study reveals clearly that the AM fungal community colonizing T. repens differed from that colonizing A. capillaris, providing evidence for AM fungal host preference. In addition, our results reveal dynamic changes in the AM fungal community through time.

Arbuscular Mycorrhiza Protect an Annual Grass from Root Pathogenic Fungi in the Field

1 Seedlings of the annual grass Vulpia ciliata ssp. ambigua were inoculated in the laboratory with a factorial combination of the cosmopolitan root pathogen Fusarium oxysporum and an arbuscular mycorrhizal (AM) fungus (a Glomus sp.) before being planted out into a natural population of V. ciliata at Mildenhall, UK, from which both fungi had been isolated. 2 At both 62 and 90 days after transplantation, inoculation with Glomus sp. had not increased plant P concentrations, but had protected the plants from the deleterious effects of F. oxysporum infection on shoot and root growth, apparently by suppressing pathogen development in roots. The effects of Glomus sp. on plant performance were negligible in the absence of F. oxysporum. 3 After transplantation, comparisons made of the root-infecting mycofloras of uninoculated plants and plants inoculated only with Glomus sp. showed that the latter developed fewer naturally occurring infections of F. oxysporum and Embellisia chlamydospora, two species of fungi which are correlated with reductions in fecundity in natural populations of V. ciliata. 4 These results confirm conclusions from previous experiments that the main benefit supplied by AM fungi to V. ciliata is in protection from pathogenic fungi, rather than improved P uptake, and indicate that AM colonization significantly alters the rootinfecting mycoflora of V. ciliata. We propose that AM fungi may confer similar benefits in other plant species, which may account for the difficulty in demonstrating a benefit of AM fungi to the P nutrition of host plant species under natural conditions.

Co-existing grass species have distinctive arbuscular mycorrhizal communities

Arbuscular mycorrhizal (AM) fungi are biotrophic symbionts colonizing the majority of land plants, and are of major importance in plant nutrient supply. Their diversity is suggested to be an important determinant of plant community structure, but the influence of host‐plant and environmental factors on AM fungal community in plant roots is poorly documented. Using the terminal restriction fragment length polymorphism (T‐RFLP) strategy, the diversity of AM fungi was assessed in 89 roots of three grass species (Agrostis capillaris, Festuca rubra, Poa pratensis) that co‐occurred in the same plots of a field experiment. The impact of different soil amendments (nitrogen, lime, nitrogen and lime) and insecticide application on AM fungal community was also studied. The level of diversity found in AM fungal communities using the T‐RFLP strategy was consistent with previous studies based on clone libraries. Our results clearly confirm that an AM fungal host‐plant preference exists, even between different grass species. AM communities colonizing A. capillaris were statistically different from the others (P < 0.05). Although grass species evenness changed in amended soils, AM fungal community composition in roots of a given grass species remained stable. Conversely, in plots where insecticide was applied, we found higher AM fungal diversity and, in F. rubra roots, a statistically different AM fungal community.

Relative roles of niche and neutral processes in structuring a soil microbial community

Most attempts to identify the processes that structure natural communities have focused on conspicuous macroorganisms whereas the processes responsible for structuring microbial communities remain relatively unknown. Two main theories explaining these processes have emerged; niche theory, which highlights the importance of deterministic processes, and neutral theory, which focuses on stochastic processes. We examined whether neutral or niche-based mechanisms best explain the composition and structure of communities of a functionally important soil microbe, the arbuscular mycorrhizal (AM) fungi. Using molecular techniques, we surveyed AM fungi from 425 individual plants of 28 plant species along a soil pH gradient. There was evidence that both niche and neutral processes structured this community. Species abundances fitted the zero-sum multinomial distribution and there was evidence of dispersal limitation, both indicators of neutral processes. However, we found stronger support that niche differentiation based on abiotic soil factors, primarily pH, was structuring the AM fungal community. Host plant species affected AM fungal community composition negligibly compared to soil pH. We conclude that although niche partitioning was the primary mechanism regulating the composition and diversity of natural AM fungal communities, these communities are also influenced by stochastic-neutral processes. This study represents one of the most comprehensive investigations of community-level processes acting on soil microbes; revealing a community that although influenced by stochastic processes, still responded in a predictable manner to a major abiotic niche axis, soil pH. The strong response to environmental factors of this community highlights the susceptibility of soil microbes to environmental change.