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A tiny fraction of all species forms most of nature: Rarity as a sticky state

Summary

In nature, whether you look at forests, oceans, or human gut bacteria, a surprising pattern emerges: just a few percent of species make up most of what we see. Scientists discovered this happens because being rare is like being stuck in a sticky spot—rare species stay rare due to the mathematics of population growth, not because they’re inferior. However, these rare species aren’t useless; they act as backup species that can take over if a dominant species crashes, keeping ecosystems stable during tough times.

Background

Ecological communities across diverse taxa exhibit hyperdominance, where a small fraction of species accounts for most biomass. This phenomenon has been observed universally across bacteria, plants, fish, fungi, and other organisms, yet the mechanistic explanation remains unclear. The paper investigates why rarity appears to be a persistent state in natural communities.

Objective

The study aims to explain why a tiny fraction of species dominates most natural communities while the vast majority remains rare. The researchers propose that rarity acts as a ‘sticky state’ in neutral communities experiencing stochastic environmental fluctuations and differential responses to stressors.

Results

Simulations demonstrate that in neutral communities with multiplicative environmental noise, rarity becomes a stable attractor due to proportionally lower absolute rates of change for rare species. Analysis across communities shows consistently that only 3-4% of species account for 50% of biomass. Microbial time series reveal that dominant species occasionally shift to rare species, supporting the insurance hypothesis of biodiversity.

Conclusion

Hyperdominance emerges through stochastic processes in neutrally competing species rather than through competitive superiority. Rare species, despite functional equivalence, provide insurance value through response diversity and differential sensitivity to environmental stressors. The stickiness of rarity has major implications for understanding ecosystem stability and the preservation of biodiversity.
  • Published in:Proceedings of the National Academy of Sciences U.S.A.,
  • Study Type:Observational Study with Mathematical Modeling,
  • Source: PMID: 38165929, DOI: 10.1073/pnas.2221791120
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