Explanation for tree species biodiversity in rainforests
The Janzen–Connell hypothesis is a well-known hypothesis for the maintenance of high species biodiversity in the tropics. It was published independently in the early 1970s by Daniel Janzen, who focused on tropical trees, and Joseph Connell who discussed trees and marine invertebrates. According to their hypothesis, host-specific herbivores, pathogens, or other natural enemies (sometimes also referred to as predators) make the areas near a parent tree (the seed-producing tree) inhospitable for the survival of seeds or 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. Both Janzen and Connell originally proposed that for natural enemies to increase local diversity, they must be host-specific (also called specialists) and relatively immobile, such that they disproportionately reduce the density of the more locally common tree species. This prevents any one species from becoming dominant and excluding other species through competition, allowing more species to coexist in small areas. This can be classified as a stabilizing mechanism.
Notably, Janzen–Connell effects provide a recruitment advantage to locally-rare trees, since they act primarily on seeds and seedlings. These effects promote the establishment of rare tree species, but do nothing to ensure the survival of these species post-germination. 
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.
Both Connell and Janzen, but particularly Connell, proposed that natural enemies will be more likely to prevent competitive dominance in more climatically stable environments. This lead both authors to predict that natural enemies contribute to the latitudinal diversity gradient by promoting local coexistence of many species in the warm, stable, highly productive climates of the wet tropics. This does not negate that Janzen-Connell effects may also happen in temperate forests. The black cherry is one such example of a temperate forest species whose growth patterns can be explained by the Janzen–Connell hypothesis.
History.
Daniel Janzen’s 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 predatory activity).
Joseph Connell hypothesis.
Joseph Connell published his hypothesis in 1970 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.
Effects on forest dynamics.
Disease dynamics and tree density.
Plant pathogens follow infectious disease dynamics. The basic reproductive rate formula_1 of a disease is dependent on three variables such that:
 ;br ;"R"0 
 β"LS" ;br ;
 ;br ;where 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.
Herbivory and tree density.
Specialist herbivores who consume plant matter can also be thought of as having a "transmission rate" between individuals similarly to a disease. Tree predators (especially herbivorous insects) are limited by the ease of movement. When individuals are closer together at high density, movement between trees is easier and the predators quickly spread out. However, at low tree density, predators can not find the next individual with as much ease and thus often have low transmission rates leading to less specialist predation.
Problematic aspects.
Many studies examining the Janzen–Connell hypothesis have shown supporting patterns with a number of tree species, but despite this there are also problematic aspects of the hypothesis.
Research testing the hypothesis.
Many tests of the Janzen-Connell hypothesis focus on whether seed or seedling survival is lower when they are closer to a conspecific (same-species) tree (distance dependence) or when seeds or seedlings are closer to seeds or seedlings of the same species (density dependence), rather than testing the prevalence or attack rates of natural enemies explicitly. Fewer studies assess the geographic implication that Janzen-Connell effects are stronger in the tropics.
Supporting research.
There have been many studies designed to test predictions of this hypothesis in tropical and other settings. Studies that have supported the Janzen–Connell hypothesis include:
Dissenting research.
Studies questioning the Janzen–Connell hypothesis:
Current conclusions and further research.
It is tricky to form conclusions regarding the accuracy of the Janzen–Connell hypothesis as it is difficult to falsify. This is because:
It is likely that a number of mechanisms underscore the coexistence of similar species and thus cause biodiversity in ecosystems. It is possible the Janzen–Connell hypothesis is applicable only for some species depending on species characteristics. The hypothesis may also be affected by the kind of predator or pathogen as preliminary research has shown that the hypothesis is true only when host-specific predators have limited mobility with a range less than the seed dispersal range.
References.
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