Demographic synchrony increases volatility and extinction risk in human populations

Area of Science:

• Demography
• Sociology
• Ecology

Background:

• Cooperation is thought to stabilize populations by pooling risk and buffering uncertainty.
• Demographic theory suggests larger populations are more stable due to reduced growth volatility.
• However, human societies of all sizes are prone to collapse, a pattern not fully explained by current theory.

Purpose of the Study:

• To propose and test a general mechanism explaining population collapse despite cooperation.
• To investigate the role of demographic synchrony in population volatility.
• To challenge the assumption of demographic independence in large populations.

Main Methods:

• Analysis of over 200 longitudinal population records from indigenous societies in the Brazilian Amazon.
• Development of a synchrony-extended stochastic model incorporating shared environmental exposure, social coupling, and spatial overlap.
• Comparison of observed population volatility with predictions from demographic independence models.

Main Results:

• Cooperation generates demographic synchrony, leading to correlated survival and reproduction among individuals.
• This synchrony inflates aggregate population volatility, causing it to decline slower with population size than predicted.
• Observed patterns imply that populations behave demographically as if composed of fewer independent units.

Conclusions:

• The cooperation-synchrony paradox: institutions that reduce individual risk can increase systemic fragility.
• Shared environmental and social factors create demographic correlations that undermine the stabilizing effects of scale.
• This mechanism likely applies across species, where cooperation can lead to systemic vulnerability by reducing effective demographic dimensionality.