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 and Joseph Connell. According to their hypothesis, host-specific herbivores, pathogens, or other natural enemies (often referred to as predators) make the areas near a parent tree (the seed producing tree) inhospitable for the survival of seedlings. These natural enemies are referred to as 'distance-responsive predators' if they kill seeds or seedlings near the parent tree, or 'density-dependent predators' if they kill seeds or seedlings where they are most abundant (which is typically near the parent tree). Such predators can prevent any one species from dominating the landscape, because if that species is too common, there will be few safe places for its seedlings to survive. However, because the predators are host-specific (also called specialists), they will not harm other tree species. As a result, if a species becomes very rare, then more predator-free areas will become available, giving that species' seedlings a competitive advantage. This negative feedback allows the tree species to coexist, and can be classified as a stabilizing mechanism. 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 and Joseph Connell. According to their hypothesis, host-specific herbivores, pathogens, or other natural enemies (often referred to as predators) make the areas near a parent tree (the seed producing tree) inhospitable for the survival of seedlings. These natural enemies are referred to as 'distance-responsive predators' if they kill seeds or seedlings near the parent tree, or 'density-dependent predators' if they kill seeds or seedlings where they are most abundant (which is typically near the parent tree). Such predators can prevent any one species from dominating the landscape, because if that species is too common, there will be few safe places for its seedlings to survive. However, because the predators are host-specific (also called specialists), they will not harm other tree species. As a result, if a species becomes very rare, then more predator-free areas will become available, giving that species' seedlings a competitive advantage. This negative feedback allows the tree species to coexist, and can be classified as a stabilizing mechanism. The Janzen-Connell hypothesis has been called a special case of keystone predation, predator partitioning or the pest pressure hypothesis. The pest pressure hypothesis states that plant diversity is maintained by specialist natural enemies. The Janzen-Connell hypothesis expands on this, by claiming that the natural enemies are not only specialists, but also are distance-responsive or density-responsive. 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. Daniel Janzen published his hypothesis in 1970 in The American Naturalist under the article 'Herbivores and the Number of Tree Species in Tropical Forests.' His hypothesis was based on the observation that in tropical forests (when compared to temperate forests), there were few new adult trees in the immediate vicinity of their parent tree. He explained the low density of tropical trees and lack of 'bunching' of tree types around parent trees for two reasons: (1) the number of seeds decline with distance from the parent tree and (2) that the adult tree, its seeds, and seedlings are a source of food for host-specific parasites and diseases.Using his observations, Janzen created a model demonstrating the probability of a seed maturation or a seedling survival as a function of distance from the parent tree (as well as total seed count, dispersal mechanism, and predatorial activity). Joseph Connell published his hypothesis in 1971 in Dynamics of Populations. Unlike Janzen, Connell proposed experiments that focused on the key prediction that exclusion of host-specific predators would cause a decrease in diversity as tree species with greater establishment or competitive ability formed low-diversity seedling and sapling communities where dominance was concentrated in a few species. He formed his hypothesis through observations in Queensland, Australia. Along with Jack Greening Tracey and Larry Johnson Webb, he mapped trees in two rainforests and observed that smaller seedlings tended to occur in single-species clumps. Smaller seedlings also exhibited greater mortality, especially when their nearest neighbor was an individual of the same species. This pattern lessened with growth and age until seedlings exhibited similar pattern diversity to adults. To reinforce these observations, Connell ran an experiment showing that adult trees have a deleterious effect on smaller trees of the same species. In another experiment, Connell found that pre-germination predation was greater on seeds near adults of the same species than those near adults of others. Through these observations, Connell suggests that each tree species has host-specific enemies that attack it and any of its offspring which are close to the parent. This emphasizes the importance of the role of predation in preventing trees from forming single-species groves, which is probably the only way in which one species of tree could exclude others by interspecies competition. Plant pathogens follow infectious disease dynamics. The basic reproductive rate ( R 0 ) {displaystyle (R_{0})} of a disease is dependent on three variables such that: R0 = βL'SWhere β is the transmission rate or infectiousness of the disease, L is the average infection time of the host, and S is the density of the host population. By decreasing any one of the variables, the reproduction rate of the disease decreases. Since seed dispersal is such that the highest density of seeds is around the parent with density decreasing with distance from the parent, the reproduction rate of a disease infecting seeds and seedlings will be highest around the parent and decrease with distance. Thus, seedlings close to the parent are likely to die due to the disease prevalence. However, seedlings farther away are less likely to encounter the disease and therefore will more likely grow into adults.