Ngonghala, Calistus; De Leo, Giulio; Pascual, Mercedes; Keenan, Donald C; Dobson, Andrew P.; Bonds, Matthew H. 2017. General ecological models for human subsistence, health and poverty. Nature Ecology & Evolution. (Abstract) (Online version)
The worldâ€™s rural poor rely heavily on their immediate natural environment for subsistence and suffer high rates of morbidity and mortality from infectious diseases. We present a general framework for modelling subsistence and health of the rural poor by coupling simple dynamic models of population ecology with those for economic growth. The models show that feedbacks between the biological and economic systems can lead to a state of persistent poverty. Analyses of a wide range of specific systems under alternative assumptions show the existence of three possible regimes corresponding to a globally stable development equilibrium, a globally stable poverty equilibrium and bistability. Bistability consistently emerges as a property of generalized diseaseâ€“economic systems for about a fifth of the feasible parameter space. The overall proportion of parameters leading to poverty is larger than that resulting in healthy/wealthy development. All the systems are found to be most sensitive to human disease parameters. The framework highlights feedbacks, processes and parameters that are important to measure in studies of rural poverty to identify effective pathways towards sustainable development. The conclusion of the Millennium Development era in 2015 provided benchmarks for human development, including a target to reduce extreme poverty by 50%1. As nearly a billion people in the world still subsist below the international poverty line2, there remains considerable debate over general causes of persistent extreme poverty. Approximately 70% of the poor in sub-Saharan Africa and Southeast Asia subsist from primary resource extraction: agriculture, timber and fishing3. Concomitant to this, around 35% suffer from chronic malnutrition and more than 75% die from infectious diseases4. As the global health community has broadened its priorities in light of the sustainable development goals and a movement for planetary health5, these statistics underline the importance of understanding ecological foundations of economic development based on two core principles: (1) the capital of the poor is often biological in the form of crops, livestock, forests, wildlife, soils and fisheries3,6,7,8; and (2) the dynamics of capital is embedded within systems of ecological interactions or food webs that include pathogens of humans and their biological resources9,10,11. â€˜Poverty trapsâ€™ commonly refer to the idea that accumulating wealth requires a minimum amount of wealth (for example, beyond subsistence), such that there is enough to be saved and invested for the future. Modern (neoclassical) economic growth theory has shown how poverty traps can arise from nonlinear processes in the growth rate of capital (or wealth). Most models of poverty traps are phenomenologicalâ€”that is, based on qualitative assumptions about these nonlinear processesâ€”and are rarely derived from explicit understanding of the underlying feedbacks that reinforce poverty, such as disease and resource scarcity9,12,13. Owing to effects on child development and labour productivity, the role of health conditions (particularly disease and malnutrition) as a driver of poverty traps has gained increasing attention9,14,15. It has also been recognized that poverty is an important risk factor for acquiring and succumbing to disease16. The intuitive argument for these coupled diseaseâ€“poverty systems is that escaping from such traps is difficult for the rural poor, who are highly susceptible to infectious diseases and rely heavily on subsistence agriculture. Notably, human health and resource dynamics are determined by biological processes that are well studied in the scientific literature. Models of the ecological basis of human livelihoods can accordingly be coupled explicitly with economics to identify dynamics that are based on fewer, simpler and more evidence-based assumptions that are rooted in scientific knowledge. Using simple toy models based on such principles, a recent study11 demonstrated that under specific assumptions, ecologically driven poverty traps can be formed. These models were not analysed broadly to provide a general understanding of how such models behave and did not reveal general conditions that lead to poverty traps. Here, we present the first general theoretical framework of its kind where ecological, economic and epidemiological factors leading to persistent poverty are generalized and broadly analysed. We show that systems of capital (renewable resources, human capital, physical capital) and natural enemies (for example, infectious diseases and pests) can be described with two fundamental equations, comparable to predatorâ€“prey models in population biology. We use this general modelling structure as a blueprint to derive a library of models of increasing complexity to represent specific ecological, economic and epidemiological systems that investigators can apply to systems of interest. These models are then parameterized from country-level data and analysed over the feasible parameter space to explore qualitative and quantitative properties of different regimes of economic development and human health. The analyses show that poverty traps, defined as self-reinforcing (that is, stable equilibrium) systems of poverty, are general features of the models.