A biogeographic hypothesis that host‑specific natural enemies (e.g., pathogens, herbivores) cause elevated mortality of conspecific offspring at short distances or high local densities near parent plants, producing negative distance‑ or density‑dependent recruitment that reduces local dominance and helps maintain plant species coexistence.

Principle

Host‑specific negative density/distance dependence in early life stages lowers local recruitment of common species and thereby promotes coexistence by preventing competitive exclusion at small spatial scales.

Demonstration

Illustrative scenario → Situation: In a tropical tree assemblage, seedlings of Species A near adult conspecifics are frequently attacked by a fungus that rarely affects heterospecific seedlings. Recognition: Observers record higher seedling mortality within a few metres of adult A. Action: Recruitment surveys account for distance from parent trees. Consequence: Fewer A recruits near parents relative to farther distances, allowing rarer species to establish and increasing local species richness.

Misapplication

Asserting the hypothesis explains all patterns of plant diversity or attributing observed negative density effects to Janzen–Connell mechanisms when they are actually caused by habitat heterogeneity, interspecific competition, or generalist enemies; the error is conflating pattern with a specific host‑specific enemy mechanism.

Consequence

When operative, the mechanism reduces local recruitment of abundant conspecifics, alters spatial distribution of recruits, and implies that management altering enemy specificity or host densities (e.g., monoculture planting) can change community composition and regeneration dynamics.

Reversal

Where host‑specific natural enemies are absent, weak, or overwhelmed by abiotic filtering, dispersal limitation, or strong competitive dominance, the predicted negative density/distance effect will be weak or absent and coexistence must be explained by other mechanisms.

Boundary

Clearly within: plant communities where empirically detected, species‑specific antagonists raise mortality of nearby conspecific juveniles. Boundary case: systems with both host‑specific and generalist enemies—interpretation depends on relative strength. Clearly outside: diversity patterns driven primarily by heterogeneous habitat preferences or neutral drift without evidence of host‑specific mortality.

Semantic Tension

Negative density/distance dependence (enemy‑mediated coexistence) ↔ habitat heterogeneity and niche differentiation as alternative or complementary explanations of species coexistence.

Synthesis

The hypothesis makes explicit how a localized, host‑specific mortality process at early life stages can scale up to maintain community diversity; its explanatory power depends on enemy specificity, spatial scale, and the relative strength of other structuring processes.