- Janzen & Connell
– Charles Darwin
Humboldt and Darwin
While on a five year journey to South America in 1799, Alexander von Humboldt wrote of ‘an inexhaustible treasure trove’ of Nature. He co-authored a 3,700 page report on the journey that would inspire generations of naturalists to come, including Charles Darwin.
Tropical biodiversity is certainly impressive and the rainforest ecosystem is unique in species richness.
Unlike most monotonous temperate forests, rainforests are home to a very high number of tree species. Trees of one species are spaced far apart throughout the forest but can be close together if forest conditions are right. The world record for tree diversity was set in 1988 in the forests of Peru, near Iquitos, where around 300 tree species with trunk diameters over 10 cm were recorded in each of two soccer-pitch-sized patches of land. Since this time, comparable tree species richness has also been documented in the central Amazon. However, the trees of tropical rainforests are only the tip of the iceberg, as trees account for only 25% of plant diversity.
One of the main groups contributing a significant diversity component are the vines, but it is the angiosperms, or flowering plants, that dominate tropical biodiversity.
The mystery of tropical biodiversity has drawn researchers from all over the world to find out how so much diversity can exist in one place. If two or more species occupy the same ecological niche, competition for resources should lead one species to dominate and exclude the others. But rainforests, and especially tropical rainforests, are inhabited by hundreds of similar trees with very similar resource requirements. So what can explain this mysterious level of biodiversity?
The Janzen Connell Hypothesis
During the early 1970s, two men, Dan Janzen and Joseph Connell, began the search for the reason behind the unusual level of species richness. The Janzen Connell theory for why competitive exclusion doesn’t reduce diversity involves plant enemies, such as pathogens (disease causing agents) and it now suggests that disease is the craftsman, or at least the caretaker, of tropical biodiversity.