Mutualism: Within species = co-operationInterspecificegmycorrhizae70% of land plants associated with fungi. Fungi receive sugars made from photosynthesis; plant receives soil nutrients absorbed by fungi.Competition: within and between speciesPredationAmensalism: eg asymmetric competition. Trees and grassesCommensalism: remora feeds off food left over from shark. No effect on shark.
The most widespread are the arbuscularmycorrhizas (AM), sometimes called vesicular-arbuscularmycorrhizas. These are found world-wide on many crop plants, wild herbaceous plants and trees, as well as on pteridophytes and some bryophytes. The fungi that form these mycorrhizas are members of the zygomycota, such as Glomus and Acaulospora spp. None of these can be grown in pure culture, away from their host plants. Roots appear quite normal when infected by these fungi, but staining of the root tissues reveals a substantial amount of internal colonisation (Figure 1).
Transplant experiment. Grasses grown in soil dominated by different speciesAll did better on soil from other species. Sterlization = diminished this response suggesting it was soil pathogens.
How many species? Perhaps 50 million species 287,655 plants 74,000–120,000 fungi 1,250,000 animals: 1,190,200 invertebrates: 950,000 insects, molluscs, crustaceans, etc. 58,808 vertebrates: 29,300 fish, 5,743 amphibians, 8,240 reptiles, 10,234 birds, 5,146 mammals
Arbuscular Crop plants Herbs, trees Roots appear normal Staining shows fungi inside the roots structure Endo-symbiont Clover 1. Vesicles (storage) 2. Hyphae 3. Arbuscules (exchange)
Ectomycorrhizas Broad leaved trees and conifers Pine; spruce; fir; beech; birch Most trees in London Common mushrooms and toadstools in woodland –fruiting bodies No penetration of plant cell Form sheath on the root Hyphae extend into the soil
Benefits Plant Improved uptake of soil nutrients Ecto: N Arb: P Immunity to pathogens Drought tolerance Microbe Carbon
Possible consequences Positive feedback Seedlings perform well near to parent Nurse plants Monodominance? Depends on early conditions/densities Rain forests Species rich Large patches >50% of trees of one species
Ectomycorrhizal networks Larch seedling Low nutrient (tropics nutrient cycles are faster) Soils can be nutrient poor Seedlings with access to an ECM network had greater growth (73% greater), leaf number (55% more), and survivorship (47% greater) than seedlings without such access
Negative feedbacks Build up of soil pathogens Seed predators Crop rotation Janzen-Connell hypothesis Negative frequency dependence Advantage of rarity Coexistence Seedling establishment Better further away from parent
Grassland experiment Petermann et al. (2008) Ecology, 89(9): 2399–2406
Summary Microbes important Poor soils Early establishment Modulate competition? Negative feedback Temporal variation? Stochastic model required
Competition Lotka-Volterra theory 1920’s Tested by Gause in 1930’s Coexistence most likely when: Intraspecific > interspecific competition Niche differentiation Coexistence when species most limited by themselves
References David Read in Nature (November 1998) 396, 22-23; and the full paper on pp. 69-72 in the same issue Bever (2003) New Phytologist, 157: 465-473. Bever et al. (2010) Trends in Ecology and Evolution, 25: 468-478.