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(a) Competition for resources

Resource sharing Spatial heterogeneity

Species differ in their ability to use resources

In one of many microhabitats one species is able to exclude all other species

Intraspecific and spatial aggregation increases intraspecific competition and aids coexistence Important species compete at different times

Trade-offs in ability to compete for two or more restricted resources There are as many species as there are habitats

Tilman (1982)

(a) Competition for resources

Resource sharing Spatial heterogeneity

Neighbour effect

Species differ in their ability to use resources

In one of many microhabitats one species is able to exclude all other species

Intraspecific and spatial aggregation increases intraspecific competition and aids coexistence Important species compete at different times

Trade-offs in ability to compete for two or more restricted resources There are as many species as there are habitats

Clumped distribution of plant species involved

Communities consist of pioneers and long-lived species

Heterogeneity with time

(b) Competition in niche differentiation

Regeneration niches No competition, as species differ in Many different micro-habitats requirements for regeneration Disturbance Species diversity is maximal at in termediate disturbance Lottery model Seeds colonise available microhabi tats randomly and are strong enough to ensure further colonisation

Habitat productivity and disturbance are inversely proportional Long-living, fertile species; colonisation is a function of seed production

Tilman (1982)

Pacala and Rough-garden (1982)

Pacala (1986)

Warner and Chesson (1986)

Grubb (1977) Grime (1973) Sale (1977)

reached by avoiding competition and thus by tolerating stress.

Experiments and Observations Showing Coexistence

Gigon (1994) and Gigon and Ryser (1996) listed experimental evidence of positive interactions between plants, and also provided important examples of proven and assumed positive influences. Facultative or obligate relationships between two partners are termed positive if one benefits and the other is not inhibited. It should be noted that these are relations between individuals, populations or species. Only the present, and sometimes subsequent, generations are considered if favourable conditions are indirectly created. The space is defined as a community and its site (ecosystem), as well as by directly neighbouring systems.

For a dry grassland in Switzerland, the question was asked how up to 40 different plants species manage to live together on 1 m2 without negative effects, i.e. competition, reducing the number of species (Fig. 4.3.27). This is explained by the numerous positive interactions which prove that these plant species are mutually dependent. In a simplified scheme of the system, the interactions are outlined for only seven plant species, for the field mouse (Microtus arvalis), and the clover canker fungus (Sclerotina trifo-

lium) as well as Rhizobium, mycorrhizae, pollinators and dispersers. This system is a close web of predominantly positive (direct or indirect) interactions between the participating partners.

A direct positive effect is provided, for example, by nurse plants, adult individuals which protect seedlings of their own species, or of a different species, by providing shade or because of protective thorns (Fig. 4.3.28). A positive influence is provided indirectly when a competing species is eliminated by pathogens or herbivores. Such indirect advantage occurs with Trifolium pratense in mixed cultures with Dactylis glome-rata, where the grass provides a barrier for spread of the clover canker fungus, for example. Furthermore, Dactylis has the advantage of a better nitrogen supply. In principle a short-term negative 'nutritional relationship' may be positive in the long term for pasture plants.

Direct positive influences between two plant species (from grassland communities in central Europe) without participation of other organisms are:

• physical support from neighbouring plants (e.g. for epiphytes, vines or as protection from wind damage);

• stabilisation of mobile substrates (e.g. plants which stabilise rock and scree slopes);

■ Fig. 4.3.27. Scheme of interactions between seven species of plants, a field mouse (Microtis arvalis), the fungus Scierotina trifolium, Rhizobium, mycorrhizae, pollinators and dispersers in a semi-dry grassland. (After Gigon 1994)

Taraxacum

^Trifolium prat.

I'

■ it\

Mycor-rhiza

Rhizobium

Animal distributors

Onobrychis

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Primula

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