Janzen–Connell hypothesis

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.