woensdag 21 maart 2012

Disturbance and diversity in grass

Romanian field with 43 species in 0.1 square meter.

A reader with no name left a link to a news article on a study on comparative plant biodiversity between ecosystems. The paper claims to be looking at biodiversity world records but that is really not that interesting; rainforests are the biodiversity hotspots of the world, what is fascinating is that when the size of the area researched shrinks grasslands rise to the fore with a spectacular number of species crammed in a tiny space. Examples: a 49-square-meter patch of Czech grassland contained 131 plant species, a patch of Argentine grassland of 1 square meter contained 89 species,  0.1 square meter of Romanian grassland contained 43 species.
 Even more fascinating is the reason for this diversity: human disturbance, read: 10.000 years of mowing. Citing 'Plant species richness: the world records'
The co-existence of large numbers of species is also of theoretical importance as a challenge to the ‘Paradox of the Plankton’. The principle of Gause states that two species occupying the same niche cannot co-exist long term, so how do 942 plant species co-exist in 1 ha of tropical rain forest? Can there be 942 niches?

The difference in vegetation type at which record richnesses are known parallels the size of the plants – grass tillers vs rain forest trees – but may also reflect intrinsic differences in the community. The high-richness short grasslands are all subject to repeated disturbance – mowing, grazing or fire – and this leads to more symmetric competition, and hence slower competitive exclusion. The most common management of these grasslands has been by mowing, practiced regularly for many years. For one of these extraordinarily rich, semi-dry grasslands, that in the Czech part of the White Carpathians, which holds the record at five spatial scales, continuity as a managed grassland since Neolithic times has been suggested, giving thousands of years for the immigration and sorting of species and for evolution to occur. Tropical rain forests have a more stable environment, the disturbances being mainly occasional windthrow. Their richness has been explained in many ways, including continuous speciation in a ‘stable’ ecosystem and high energy input. Wright (2002) highlighted niche differentiation, pest pressure and lifehistory differences.

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