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

Why bother?

• Moral/ethical reasons• Medicine: Antibiotics– Tropical frogs– Silk moths, salamanders,

snakes, sharks, honeybees• Ecosystem function– Higher diversity = higher

productivity

Speciation & coexistence

• Allopatric speciation• Sympatric speciation– Requires coexistence

Coexistence in Animals

Main niche: prey (resource)

Gause’s Exclusion principle• “The struggle for existence” 1934• Discovered antibiotic used in WWII

1910-1986

Two (or more) similar species cannot coexist on a single resource

Plants

• Limited range of resources– Light, water, nutrients

• Rain forest– Panama 320spp 50ha

• Grassland– UK, 35spp 1m2

WHAT PROCESSES MAINTAIN THE DIVERSITY?

8

Interactions

• Mutualism + +• Competition - -• Predation/parasitism - +• Amensalism 0 -• Commensalism 0 +

Arbuscular

• Crop plants• Herbs, trees• Roots appear

normal• Staining shows fungi

inside the roots structure

• Endo-symbiont Clover

1. Vesicles (storage)2. Hyphae3. 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

• Root (darker)• Fungus (white) on root hairs

Nitrogen cycle

Legumes

• N essential for life• DNA; RNA; amino acids• Fixed by many plants• Symbiosis– Rhizobia (nodules)

• Legumes release N into soil on death.

• Farmers crop rotation

Nitrogen fixing bacteria

Nitrogen fixing bacteria

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

Early advantage

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

New theory

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.