For years, scientists have wondered why rainforests are so diverse. Competition for resources should lead one species to dominate and exclude the others, but this doesn’t happen to the extent it should. The following sheds some light on the mystery and the theory seems to be supported by available evidence.
The Reason Why Rainforests Are So Diverse
If disease causing agents, named pathogens, affected only one species or killed some species, while others tolerated or were immune to them, then only specific plants would be killed when growing in the pathogen’s vicinity. If the pathogen was not well dispersed and lived in a localised area, it could provide a mechanism where less competitive species would survive due to the disease debilitating dominant species. If the plants themselves were dispersal limited and there were numerous pathogens with different plant preferences, it would help explain tropical diversity.
Summary – Why Rainforests Are Diverse
So why are rainforests so diverse? The answer is because disease stops one or a few species from dominating in certain areas, allowing other species to live and grow that would otherwise be out-competed and erased. Therefore, it seems disease is the caretaker of tropical plant diversity.
Support For The Janzen Connell Hypothesis
The hypothesis is strongest if plants killed are either seeds or juveniles, while adults are left unharmed, and since its inception, the Janzen Connell hypothesis has been supported by numerous tropical studies (Harms et al 2000; Bell et al., 2006; Bagchi et al, 2010; Mangan et al. 2010) as well as for temperate regions (Packer & Clay, 2000; Petermann et al., 2008), although to a lesser extent. As well as pathogens, insect herbivores have also been implicated for maintaining tropical plant diversity (Sullivan, 2003).
