Small ‘Dunbar’ groups are the basis for all larger-scale societies, and how they work dictates social responses to change and stress. Entropy is an ever-present threat to all complex systems and is opposed by the regeneration of relationships between the entities that comprise them, represented by their information content. Genocide and wildfire are extreme examples of system dissolution, but the piecemeal erosion of integrity is more usual and can be coped with at low-enough rates. But social and ecological stresses now often exceed these rates, and ways to buffer change and accelerate system regeneration are needed. Determinants of fragility and strength include trophic structure, keystone species, co-evolved dependencies, physical structure and water (for ecological systems), and peace, war, resources, livelihoods, meaning, demoralisation, roles, alienation and language (for social ones). In human-ecological systems these combine into factors such as ‘profitability of sale’ and social and ecological ‘sustainability of production’. Knowing how systems become weaker and stronger yields guidance on the design of investments that will reliably promote strength at all levels of society and the biosphere.