Theory 04/Ecology & Biodiversity /Community ecology/3 min
Stability Is Response Diversity
The insurance hypothesis says diversity stabilises. The kind of diversity matters more than the amount.
Ecosystem stability is set by response diversity, the variation in how species react to the same environmental driver, rather than by species richness; richness is a proxy that fails exactly when response diversity is low.
01The evidence
- Response diversity was identified as the property that determines ecosystem resilience to environmental change, with the argument that species differing in their response to disturbance provide insurance in a way that sheer numbers do not.Ecology Letters review
- The concept has since been operationalised into measurable indices, which is the precondition for testing it as a predictor rather than using it as a metaphor.Measurement of response diversity
02The tension
The biodiversity-stability relationship is one of the most studied in ecology and one of the least consistent. Some systems show strong stabilisation with richness, others show none, and the discrepancy is usually attributed to context. Context is not an explanation.
03The theory
Aggregate stability depends on the variance of the summed response, which depends on the covariances among species, not on the number of species. Adding a species that responds exactly like an existing one changes richness and leaves covariance untouched, so it adds no stability.
Response diversity is the variable that sets those covariances. Two communities with the same richness but different response diversity should differ in stability by a predictable amount, and a community that loses response diversity at constant richness should lose stability.
This explains the inconsistency in the literature. Studies that sample richness gradients also sample response diversity gradients, usually unintentionally, so the correlation between richness and stability is a correlation between two proxies with a shared cause. When the proxies diverge, the relationship disappears.
The applied consequence is that restoration should be scored on response diversity rather than on species counts. A planting mix chosen for functional spread will be more stable than a longer list chosen for taxonomic coverage, and that is a testable claim about real restoration sites.
04What this is not
The insurance hypothesis says diversity stabilises, with richness as the operative variable. This says response diversity is operative and richness is a correlate, which predicts specific failures of the richness account.
05Predictions
- P1Response diversity should predict stability better than richness across existing datasets, including those where richness failed.
- P2Removing response diversity at constant richness should reduce stability as much as removing species.
- P3Adding species with identical responses should add no measurable stability.
06What would refute it
If richness retains predictive power after response diversity is included, the proxy is doing real work and the theory is redundant.
07What it would change
It would change how restoration success is measured, from counting species to measuring the spread of responses.
08Evidence base
- Response diversity determines the resilience of ecosystems to environmental change — Ecology Lettershttps://europepmc.org/article/MED/23217173
- How to measure response diversity — methods paperhttps://researchmap.jp/samrpjross/published_papers/41569813?lang=ja