Why are there so many species?
Theory says that where species compete for the same few resources, one should win. In the ocean, thousands coexist. The "paradox of the plankton".
The question
A simple rule of ecology, the competitive exclusion principle, says that two species competing for exactly the same limited resource cannot coexist for long: the slightly better competitor will eventually drive the other out. Laboratory experiments since the 1930s, starting with Georgy Gause's work on microorganisms, have often confirmed it.
In 1961 the ecologist G. Evelyn Hutchinson pointed out a striking problem. Phytoplankton — the microscopic algae floating in lakes and oceans — all need the same few things: light, carbon dioxide and a handful of nutrients such as nitrogen, phosphorus, silicon and iron. The water they live in is well mixed and looks much the same everywhere. By the rule, a few species at most should win. Yet a single sample of water can hold dozens of species, and the oceans hold many thousands. He called this "the paradox of the plankton".
The puzzle reaches far beyond plankton. A single hectare of tropical rainforest can contain hundreds of tree species, all needing light, water and the same soil nutrients. Why does nature sustain so much diversity?
What we know
- The diversity is even greater than Hutchinson knew. The Tara Oceans expedition, sequencing DNA from plankton across the world's oceans, reported in 2015 about 150,000 distinct types of eukaryotic plankton, mostly single-celled, in the sunlit upper ocean alone; about a third of them could not be assigned to any known group.
- Competition for resources is real and predictable. In 1977 David Tilman grew two species of diatoms together and showed that the outcome of their competition for phosphate and silicate could be predicted from how each species used the two resources.
- Mathematical models show that competition does not have to settle down. In 1999 Jef Huisman and Franz Weissing showed that when species compete for three or more resources, their populations can oscillate or behave chaotically indefinitely, allowing far more species to persist than there are resources.
- Such chaos has been observed. In 2008 a laboratory plankton community sampled for more than six years showed chaotic fluctuations in species numbers, never settling to equilibrium.
- Many other factors help species coexist: predators and viruses that preferentially attack the commonest species, changing seasons and weather, and differences in how species respond to the same conditions.
Main hypotheses
The world never reaches equilibrium
Hutchinson's own suggestion: conditions change faster than competition can finish its work. Before one species can exclude another, the season, the temperature or the nutrient supply changes and favours someone else.
Winners are attacked
Viruses, grazers and diseases tend to strike the most abundant species hardest. This "kill the winner" effect keeps any one species from dominating and leaves room for others. A similar mechanism, the Janzen–Connell hypothesis, is thought to maintain the diversity of rainforest trees.
Internal chaos
Even in a constant environment, competition among many species for several resources can generate endless fluctuations, so no species ever wins for good.
Neutrality
According to neutral theory, developed by Stephen Hubbell, species may be roughly equal competitors, and their numbers drift by chance, much as gene frequencies drift in a population. Diversity is then maintained by a balance between extinction and the arrival of new species.
Where understanding ends
We have many mechanisms that can explain coexistence, and all of them work in models or experiments. What we don't know is how much each one actually contributes in a real lake, ocean or forest. Natural communities are too large, too variable and too long-lived to test each mechanism separately.
There is also a deeper difficulty. Neutral theory and theories based on differences between species often predict very similar patterns of abundance, so field data alone frequently cannot tell them apart.
And the question matters beyond ecology. Without knowing what maintains diversity, we cannot predict confidently how ecosystems will respond to warming, pollution or the loss of particular species.
What would settle it
- Long-term observations of natural communities combined with experiments that switch off one mechanism at a time.
- Theory that predicts not only that species coexist, but how many coexist and in what proportions, and is confirmed by data it was not fitted to.
- Genetic and ecological surveys, such as those of ocean plankton, linked to the conditions each species lives in, showing which differences between species actually allow them to coexist.
Sources
- Gause, G. F. (1934). The Struggle for Existence. Williams & Wilkins, Baltimore.
- Hardin, G. (1960). The competitive exclusion principle. Science, 131(3409), 1292–1297.
- Hutchinson, G. E. (1961). The paradox of the plankton. The American Naturalist, 95(882), 137–145.
- Janzen, D. H. (1970). Herbivores and the number of tree species in tropical forests. The American Naturalist, 104(940), 501–528.
- Tilman, D. (1977). Resource competition between plankton algae: an experimental and theoretical approach. Ecology, 58(2), 338–348.
- Valencia, R., Balslev, H., & Paz y Miño C., G. (1994). High tree alpha-diversity in Amazonian Ecuador. Biodiversity and Conservation, 3(1), 21–28.
- Huisman, J., & Weissing, F. J. (1999). Biodiversity of plankton by species oscillations and chaos. Nature, 402(6760), 407–410.
- Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics, 31, 343–366.
- Thingstad, T. F. (2000). Elements of a theory for the mechanisms controlling abundance, diversity, and biogeochemical role of lytic bacterial viruses in aquatic systems. Limnology and Oceanography, 45(6), 1320–1328.
- Hubbell, S. P. (2001). The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press.
- Benincà, E., et al. (2008). Chaos in a long-term experiment with a plankton community. Nature, 451(7180), 822–825.
- de Vargas, C., et al. (2015). Eukaryotic plankton diversity in the sunlit ocean. Science, 348(6237), 1261605.