Ecological Loop Demography: A Systematic Feedback Theory of Population Decline and Structural Ageing

<p dir="ltr">Human populations are undergoing a structural shift toward older, slower-growing, and ultimately contracting configurations as fertility falls below replacement, childbearing is postponed, and longevity rises. These changes are commonly framed as policy failures or temporary anomalies. This paper argues instead that they reflect endogenous, system-level demographic dynamics analogous to late-successional ecological processes. It introduces Ecological Loop Demography (ELD), a framework that models population change as a stock–flow system governed by interacting reinforcing and balancing feedback loops embedded in age structure. ELD identifies four mutually reinforcing demographic loops: (1) fertility-driven cohort compression, (2) structural ageing–fertility suppression, (3) longevity-driven elder stock expansion, and (4) momentum exhaustion through natural increase collapse. Once activated, these loops shift population dynamics from flow-dominated growth to stock-dominated contraction, rendering fertility recovery increasingly ineffective. In ELD, fertility and mortality rates function as flows that modulate demographic change, while age-structured cohort sizes constitute dominant stocks that change slowly and generate inertia. As populations age, outcomes become increasingly governed by stock dynamics rather than by short-term rate changes, producing declining policy elasticity and asymptotic adjustment toward smaller, older population structures. Using global demographic data (UN WPP 2024; IHME) and comparative analyses of Nigeria, Brazil, and Japan, the paper demonstrates that sustained low fertility and longevity gains generate predictable trajectories of structural ageing and population contraction largely independent of short-term policy intervention. A composite ELD index shows that fertility responsiveness collapses once age-structural feedbacks dominate. These findings suggest that population decline is not a demographic malfunction but a self-organising adjustment within biophysical limits. However, because modern democratic capitalism remains institutionally dependent on labour-force growth and continuous accumulation, demographic ageing and contraction are widely interpreted as economic crises. ELD situates contemporary demographic change within a broader ecological–political economy framework, highlighting why ageing, inequality, and democratic fragility increasingly co-evolve.</p>