How did paradise begin? New Scientist 96 (extract)
How is it that there are 700 species of trees in a 40 m by 500 m plot in Yasuni? Terry Irwin also as Yasuni national Park collects insects by using biodegradable insecticide in 3 meter saquare collecting regions and estimates a single hectare contains 60,000 species of insects and spiders, counting ground and bark-dwelling types which miss his nets. 80% have never been seen before. They come from 14 major groups. Only 18 of his 900 beetle samples have been catalogued. Already he has 1348 species from his sample of a million and a half specimens. There are no less than 740 species of leaf hoppers on one years collection. This diversity is repeated on larger scales because the species are completely different 100 kilometres away.
Most people favour the idea that the Amazon has gone through repeated fragmentations during aridity accompanying ice ages, but the evidence for this is thin on the ground. The refugia idea appeared to coincide with diversity hotspots until Bruce Nelson of INPA figured out that the hotspots realloy coincided with more intensive collecting suggesting the diversity was wide-spread rather than lingering around ancient refugia.
Dan Nepstead began to reason that intermittent drought possibly accompanied by fires was a factor. He discovered that despite Amazonian trees standing on buttressed lateral surface roots, there are delicate myriad roots stretching down as far as 21 metres, rather than the mere 3 metres previously supposed. Four times in the last 500 years, there have been huge droughts and perhaps fires in the Amazon basin, caused by drastic El Niño episodes. In 1992 a dry year Betty Meggers studied the water budget in a patch of forest in Pará and found severe shortage with 15% leaf fall of the top canopy trees with tinder dry leaf-fall and loss of water down to 8 m. Coddington sees the factors leading to diversity as complex.
Safety in Diversity New Scientist 23 Mar 96 Chris Wills (extract)
How is it that in a region of 100 metres square of the Amazon in a region of constant temperature and abundant moisture, there are 600 trees from no less than 300 different species?
There are many theories for this, including intermittent fragmentation of the rainforest during periods of aridity, and fractal disruptions caused by falling trees and fires, but one hypothesis is the "Red Queen" one of an evolutionary race between species and their parasites developed by Douglas Green and Chris Wills and previously by Jan Gillett.
Stephen Hubble, Robin Foster and Richard Condit have studied a 0.5 sq kilometre artificial island in the Panaman Canal for 15 years, which contains 240,000 trees of 300 species. They found no greater diversity in clearings from fallen trees. Another theory proposed by Daniel Janzen and Joseph Connell suggests that a complex distribution of individuals aids survival in an environment rich in parasites, however the Panamanian species are clumped rather than spread.
Humans and other plants and animals have a rich array of diverse genes to aid resistance to parasites, which are rapidly evolving. The theory proposes that genetic diversity in such populations aids parasite resistance and causes a complex spread-out distribution of resistant individuals left over from parasitic attack which inhibits the epidemic spread of pathogens. Applying the same argument to species diversity in an environment rich in parsite diversity of bacteria, fungi, insects and protozoa, we arrive at the model of species herd-immunity. The model is supported by data form field quadrats where equally many species are resident. If a quadrat has a few common species rather than an even distribution of rareness, deaths outstrip births and vice verse for high evenness quadrats. Conversely crwoding favours disease. The researchers found little concrete evidence for other effects such as selective enhancement between complementary plant species.
Life on the Edge New Scientist 96 (extract)
Most people believe that diversity declines as virgin forest is carved into smaller blocks and islands, probaly roughly according to a fractal power law related to the size of the islands. Some people advocate corridors connecting such regions. It is important to determine how much biodiversity declines as whole tracts are cut up often into many small isolated island of vegetation which can no longer readily exchange animal species and seeds, and where random fortune can easily drive small population extinct over time.
Twenty years ago Thomas Lovejoy began an experiment near manaus to study the effects of such fragmentation. Robert Macarthur and Edward Wilson have predicted the obvious decline in diversity associated with fragmentation. Lovejoy persuaded ranchers to leave islands forming squares of 1, 10 or 100 hectares. As everyone expected some species were squeezed out of the smaller fragments. Large mixed-species flocks of insect-eating birds dispersed within 2 years of isolation in the smaller fragments leaving only a few stragglers. Many other species of insectivorous birds also disappeared over the first 3 to 6 years. Even the 100 hectare plots proved too small for species such as army ants, ant-following birds, and wide-ranging capuchin monkeys. However hummingbirds were little affected and small mammals increased by taking advantage of the insects in the trash habitat in the smaller fragments.
However rather than plot size, the edge effects of greater light wind disruption and drier conditions had more effect. Some speices better adapted to light and dryness such as Brazil nut thrived nead the edge, while Jacarandas did not survive. The nature of the surrounding territory and nearness of forest were also important. During the 80s declining fortunes and the collapse of farm land with disintegrating Amazonian soils caused more gegrowth between the patches. The matrix of secondary growth proved critical to the diversity of the patches and even to their recolonization by species such as insect-eating birds. Cecropia trees in the regenerating matrix retaining more species.