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Janzen–Connell hypothesis
The Janzen–Connell hypothesis is a widely accepted explanation for the maintenance of tree species biodiversity in tropical rainforests. It was published independently in the early 1970s by Daniel Janzen〔Janzen, Daniel H. "Herbivores and the Number of Tree Species in Tropical Forests." The American Naturalist. 104.940 (1970).〕 and Joseph Connell.〔Connell, J.H. "On the role of natural enemies in preventing competitive exclusion in some marine animals and in rain forest trees." In: Dynamics of Population. Ed. P.J. Den Boer and G.R. Gradwell. Wageningen: Pudoc, 1970.〕 According to their hypothesis, there is a density- or distance-dependent factor in recruitment of seedlings from adults of tropical tree species due to host-specific predators or pathogens.〔Carson, Walter A. and Stefan A. Schnitzer. Tropical Forest Community Ecology. Blackwell Publishing: Hoboken, NJ, 2008.〕 These predators/pathogens that specifically target a species make the areas directly surrounding that parent tree (the seed producing tree) inhospitable for the survival of seedlings. In other words, the hypothesis states that specialist predators, pests, and pathogens keep key plant species rare enough to reduce their competitive ability enough so as to make space available for many other species. A tree predator is an herbivore of some kind, often tropical insects, that consume parts of the tree for food. An example of a tree predator are the beetles in the ''Chrysomelidae'' family commonly referred to as Leaf Beetles. A tree pathogen refers to a disease caused by a bacteria, virus, or fungus that infects the tree. A tree parasite could also be classified as a pathogen or a predator of a species.
According to the Janzen–Connell hypothesis, those seedlings that are farthest from their parent have a competitive advantage as they do not suffer from the species-specific predators that are found more commonly around the parent. For example, the Black Cherry tree (''Prunus serotina'') seedlings farther from their parent tree are surrounded by fewer other cherry tree seedlings – in other words, they are less dense. Diseases and herbivores that prey on the Black Cherry tree are most commonly found and increase most rapidly in high-density cherry areas.〔Packer, Alissa and Keith Clay. "Soil Pathogens and Spatial Patterns of Seedling Mortality in a Temperate Tree." Nature. 404. (2000): 278–281.〕 So, those cherry seedlings farther from the parent are less likely to encounter a Cherry tree parasite like ''Pythium'' and will more likely survive to adulthood in comparison to the cherry seedlings close to the parent.
This mechanism has been proposed as promoting diversity of forests as it promotes survival of a number of different plant species within one localized region. While previously thought to explain the high diversity of tropical forests in particular, subsequent research has demonstrated the applicability of the Janzen–Connell hypothesis in temperate settings as well. The Black Cherry is one such example of a temperate forest species whose growth patterns can still be explained by the Janzen–Connell hypothesis.
==History==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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