Janzen-Connell hypothesis
Encyclopedia
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, 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. 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. 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.
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).
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.
of a disease is dependent on three variables such that:
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.
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.
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
in tropical rainforests. It was published independently in the early 1970s by Daniel Janzen
Daniel Janzen
Daniel Hunt Janzen is an evolutionary ecologist, naturalist, and conservationist and the son of a previous Director of the US Fish and Wildlife Service...
and Joseph Connell. 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
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
-specific predators or pathogens. 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
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
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
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
, or fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
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
Black Cherry
Prunus serotina, commonly called black cherry, wild black cherry, rum cherry, or mountain black cherry, is a woody plant species belonging to the genus Prunus...
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
Black Cherry
Prunus serotina, commonly called black cherry, wild black cherry, rum cherry, or mountain black cherry, is a woody plant species belonging to the genus Prunus...
tree are most commonly found and increase most rapidly in high-density cherry areas. So, those cherry seedlings farther from the parent are less likely to encounter a Cherry tree parasite like Pythium
Pythium
Pythium is a genus of parasitic oomycete. Most species are plant parasites, but Pythium insidiosum is an important pathogen of animals...
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
Temperate forest
Temperate forests correspond to forest concentrations formed in the northern hemisphere. Main characteristics include: wide leaves, big and tall trees and non seasonal vegetation...
species whose growth patterns can still be explained by the Janzen-Connell hypothesis.
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 seedsSEEDS
SEEDS is a voluntary organisation registered under the Societies Act of India....
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 hypothesis
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
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. 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
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
, 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.
Disease dynamics and tree density
Plant pathogens follow infectious disease dynamics. The basic reproductive rate of a disease is dependent on three variables such that:Where β is the transmission rate or infectiousness of the disease, L is the average infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
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.Concerns with the Janzen-Connell hypothesis
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 speciesKeystone speciesA keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and...
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 successFecundityFecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...
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.
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 PanamaPanamaPanama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...
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 diversityLatitudinal gradients in species diversityThe increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal diversity gradient , is one of the most widely recognized patterns in ecology. Put another way, in the present day localities at lower latitudes generally have more...
. - 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 allelopathyAllelopathyAllelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial or detrimental effects on the target organisms...
were also suggested in some cases.
Dissenting Research
Studies questioning the Janzen-Connell hypothesis:- Hyatt et al. examined the hypothesis across a number of tree species (rather than focusing on the effects on just one) and found no support for the distance-dependence of the hypothesis and concluded that the Janzen-Connell hypothesis was not a mechanism for diversity. They did find however that in temperate settings, distance from parent reduced survivorship. But in tropical settings there was a slight positive correlation between the improved competitive ability of seeds and distance from parent.
- A 1994 study by Burkey found that seed predation did not follow a pattern supporting that of the Janzen-Connell hypothesis along a meaningful scale. Seeds within 1 metre from the trunk were highly predated. However, seeds reached their peak density while still under the canopy of their parent. The authors concluded that seed predation did not follow 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:- Heavy predation may keep some species rare and widely spaced, and these species may also be the best competitors. If so, these species are most likely to form dense aggregations that would reduce diversity in local areas. But due to their rarity or a belief that rare species are not regulated by density-dependence, these species may be the ones least likely to be studied.
- This means that the failure to find Janzen-Connell effects for what could be hundreds of tree species does not reject the hypothesis, as ecologists are missing the few key species where it does apply. However, dissenting findings do reduce the importance of the hypothesis for explaining overall coexistence of a number species.
- For example, Hyatt et al. (2003) found that there were “individual cases of conformity to the hypothesis,” which is all that is needed for the hypothesis to work if the specific cases represent tree species that are excellent competitors, highly shade tolerant, habitat generalists, or some combination of these traits that would allow these species to otherwise dominate the ecosystem.
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.