Mechanisms of Species Coexistence: Twelve Explanations for Hutchinson's 'paradox of the Plankton': Evidence from New Zealand Plant Communities

A problem perplexing many ecologists studying natural communities has been how the species of a community coexist without competitive exclusion occurring. Hutchinson named this the 'Paradox of the Plankton', though the question has been asked especially for tropical rain forest and coral reef communities. Twelve mechanisms have been proposed to explain the paradox: 1. Niche Diversification, 7. Initial Patch Composition 2. Pest Pressure, 8. Spatial Mass Effect 3. Equal Chance, 9. Circular Competitive Networks, 4. Gradual Climate Change, 10. Cyclic Succession, 5. Intermediate-timescale Disturbance, 11. Aggregation, 6. Life History Differences, 12. Stabilising Coevolution. Evidence is reviewed for the relative importance of these twelve mechanisms in New Zealand indigenous vegetation. From the available evidence, it is suggested that (4) Gradual Climate Change is probably a major explanation of the Paradox of the Plankton in New Zealand. (10) Cyclic Successional processes are probably involved in this change, though there is so far only speculation for N.Z. (8) Spatial Mass Effect and (1) Niche Diversification are probably also important, though poorly documented. Aggregation (11) may facilitate these processes. (5) Intermediate-timescale Disturbance is probably unimportant for within-community coexistence, with the notable exception of small forest gaps. Elements of (3) Equal Chance and (6) Life History Differences are probably involved in this, though there is no evidence available for N.Z. Some of the elements necessary for Pest Pressure (2) appear to occur. (7) Initial Patch Composition, (9) Circular Competitive Networks, and (12) Stabilising Coevolution are probably unimportant, or even non-existent.