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.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, 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.Carson, Walter A. and Stefan A. Schnitzer. Tropical Forest Community Ecology. Blackwell Publishing: Hoboken, NJ, 2008. 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. 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. 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. The first is that the Janzen–Connell hypothesis explains diversity at a community-wide scale, but most studies have only looked at a single species or a localized region. Most studies do not test the diversity prediction and did not determine the causes for patterns consistent with Janzen–Connell effects. As Wright (2002) pointed out, "field measurements only demonstrate that niche differences, Janzen–Connell effects, and negative density dependence occur. Implications for species coexistence and plant diversity remain conjectural." While these host-specific predators may play keystone species roles as they possibly aid in the prevention of superior tree competitors from monopolizing an area, no study has yet to examine species richness and abundance after removing natural enemies, creating a gap in the supporting research for the hypothesis. Diversity may be maintained, at least in part, by episodic outbreaks of specialized pests, which may reduce the survivorship, growth, and reproductive success of adults of a species whenever they are particularly aggregated. Thus, specialist predation also impacts adult density, rather than just juveniles which has been the focus of research efforts. Disease and predation may be just affecting overall density rather than in localized regions around adults alone. Theoretical work has suggested that distance-responsive predators are less able to promote coexistence than equivalent predators that are not distance-responsive. Many empirical studies have shown that few natural enemies are host-specific specialists, as the original hypothesis claimed. However, theoretical work has suggested that predators can have broader diets as long as they are more damaging to their primary host than other species. The responses of density-dependent predators' may not be restricted to a single trophic level, and when multiple trophic levels interact, Janzen-Connell effects may be negated. This is an example of "the enemy of my enemy is my friend" and it has been shown that the potential of host-specific insects to induce negative-density dependence in plant populations can be severely limited when the natural enemies of the seed predators are also density responsive. Supporting research There have been over 50 studies designed to test predictions of this hypothesis in tropical and other settings  and most show that many tree species exhibit patterns consistent with Janzen–Connell effects. Studies that have supported the Janzen–Connell hypothesis: A 2010 study of Panama forest reinforced that Janzen–Connell effects are also prevalent in tropical rainforests as seedlings growing close to their parents are likely to die due to microorganisms in the soil. A 2008 study by Petermann et al. in grasslands showed that soil-borne pests created a feedback very similar to the Janzen–Connell effect, supporting the hypothesis as a driver of diversity in temperate ecosystems. This study suggests that the predator/density mechanism that promotes species diversity is not contained to tropical forests alone – even if tropical ecosystems do have the highest diversity. This supports the hypothesis as a mechanism for diversity, but not as an explanation latitudinal gradients in species diversity. An examination of spatial data for 24 woody tropical rain forest plants showed either density-dependence or distance-dependence in plant offspring, supporting the hypothesis. However, other causal factors such as allelopathy were also suggested in some cases. See also ecological fitting
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.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, 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.Carson, Walter A. and Stefan A. Schnitzer. Tropical Forest Community Ecology. Blackwell Publishing: Hoboken, NJ, 2008. 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 be explained by the Janzen–Connell hypothesis. 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. 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. The Janzen–Connell hypothesis explains diversity at a community-wide scale, but most studies have only looked at a single species or a localized region. Most studies do not test the diversity prediction and did not determine the causes for patterns consistent with Janzen–Connell effects. As Wright (2002) pointed out, "field measurements only demonstrate that niche differences, Janzen–Connell effects, and negative density dependence occur. Implications for species coexistence and plant diversity remain conjectural." While these host-specific predators may play keystone species roles as they possibly aid in the prevention of superior tree competitors from monopolizing an area, no study has yet to examine species richness and abundance after removing natural enemies, creating a gap in the supporting research for the hypothesis. Diversity may be maintained, at least in part, by episodic outbreaks of specialized pests, which may reduce the survivorship, growth, and reproductive success of adults of a species whenever they are particularly aggregated. Thus, specialist predation also impacts adult density, rather than just that of juveniles, the latter of which has been the focus of research efforts. Disease and predation may be just affecting overall density rather than in localized regions around adults alone. Theoretical work has suggested that distance-responsive predators are less able to promote coexistence than equivalent predators that are not distance-responsive. Many empirical studies have shown that few natural enemies are host-specific specialists, as the original hypothesis claimed. However, theoretical work has suggested that predators can have broader diets as long as they are more damaging to their primary host than other species. The responses of density-dependent predators' may not be restricted to a single trophic level, and when multiple trophic levels interact, Janzen–Connell effects may be negated. This is an example of "the enemy of my enemy is my friend" and it has been shown that the potential of host-specific insects to induce negative-density dependence in plant populations can be severely limited when the natural enemies of the seed predators are also density responsive. Supporting research There have been over 50 studies designed to test predictions of this hypothesis in tropical and other settings , and most show that many tree species exhibit patterns consistent with Janzen–Connell effects. Studies that have supported the Janzen–Connell hypothesis: A 2010 study of forest in Panama reinforced that Janzen–Connell effects are also prevalent in tropical rainforests as seedlings growing close to their parents are likely to die due to microorganisms in the soil. A 2008 study in grasslands showed that soil-borne pests created a feedback very similar to the Janzen–Connell effect, supporting the hypothesis as a driver of diversity in temperate ecosystems. This study suggests that the predator/density mechanism that promotes species diversity is not contained to tropical forests alone , even if tropical ecosystems do have the highest diversity. This supports the hypothesis as a mechanism for diversity, but not as an explanation latitudinal gradients in species diversity. An examination of spatial data for 24 woody tropical rain forest plants showed either density-dependence or distance-dependence in plant offspring, supporting the hypothesis. However, other causal factors such as allelopathy were also suggested in some cases. See also Ecological fitting
R
seed producing
seed-producing
R
Janzen-Connell
Janzen–Connell
R
Janzen-Connell
Janzen–Connell
R
Black Cherry
black cherry
D
still
R
Where
where
D
first is that the
R
juveniles
that of juveniles, the latter of
R
Janzen-Connell
Janzen–Connell
A
,
R
Panama forest
forest in Panama
D
by Petermann et al.
R
–
,
R
ecological
Ecological