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Why are tropical rainforests so diverse?

Alas Purwo National Park, Timur Java, IndonesiaThis title is shared with one paragraph of one of my favourite readings, perfect for friday late afternoon: The Encyclopedia of Biodiversity. I report here some of the concepts recalled in that paragraph by I.M. Turner since I found them definitely interesting.

Tropical rainforests are the most species rich terrestrial ecosystems especially in terms of vascular plants: vascular plants species richness in tropical rainforests amounts to more than half of the estimated total. Let’s wonder why?

Biological diversity is the balance between speciation and extinction processes. Therefore these ecosystems should be characterized by more rapid speciation and/or a slower rate of extinction. It is possible that the benign and equable climate of the tropics makes it less likely that a species will become extinct.

From an ecological perspective species occupying separate niches have reached a competitive equilibrium that allows numerous species to co-occur. However if we think about plants simple requirements (light, water, nutrients) it is difficult to imagine a number of niches comparable to the number of species occurring in tropical rainforests, even if we take into account reproduction and regeneration requirements. It is more plausible that ecologically similar species can co-occur.

If competitive equilibrium has not been obtained then something must be preventing competitive exclusion. One possible process by which this could happen is through frequency-dependent mortality, setting an upper limit on the abundance of a species and thus making it unlikely that it can totally exclude other species from the landscape (see Janzen-Connel effect for an in-depth description of this process).

This compensatory mortality, often due to seed and seedling predators and pests and pathogens may increase in intensity with rainfall bacause pest and pathogens populations are more spread in an ever-wet forest.

Another view recalls the fact that most species are so infrequent that very rarely come into direct competition with other ecologically similar sparse species. Therefore many potential episodes of interspecific competition may be won by default because no adversaries were present at the right time and place.

At present we have no simple answer to the title’s question, nor does it seems likely that there is a simple answer. Probably all the factors mentioned  and possibly others not yet considered are operating within any rainforest.

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