The interdisciplinary field of sustainability science addresses human impacts on Earth’s life support systems and whether future generations will have adequate access to natural resources and services provided by functioning ecosystems. Human societies depend heavily on ecosystems, while at the same time impacting their function through overharvest, loss of habitat, pollution, nutrient eutrophication, and climate change, among others. Models of coupled human-natural systems can provide a framework to express our best causal understanding of interactions. Ecosystem models, long used to express our understanding of ecosystem structure and function, could be useful in coupled models, but what characteristics should they have to be useful for this purpose? I investigated this in the context of a case study of a subsistence village and its dependence on a Forest Commons in the developing world, a temperate forest in India. I created a systems dynamics model that included forest growth and succession, the harvest of forest products for cattle feeding and fuelwood, the production of crops and milk using cattle dung as fertilizer, and the conversion of forest to cropland. I used the model to analyze scenarios of variable human impacts and consequences for future flows of ecosystem services.
Results/Conclusions
Preliminary results using the case study model showed that when removal of forest canopy tissues for cattle feed slowly ramped up to 10% per year of the biomass from late-successional tree species, or 850 kg / ha y of dry mass at peak removal, the forest was converted to dominance by early-successional species within 120 years. Crop and milk production, key measures of human livelihood, peaked in year 50, followed by a slow decline for the next 150 years because in this case late-successional vegetation was needed for cattle feed. In contrast, when biomass removal was limited to 5% per year of late-successional canopy biomass, the late-successional forest maintained dominance and continued to provide cattle feed, crop and milk production indefinitely. Within 40 years the coupled human-natural system reached a sustainable steady state. This case study shows that for use in such an analysis, ecosystem models need to capture the dominant ecological processes that could give rise to state changes under different levels of human impact and must also capture ecosystem dynamics and flows of essential ecosystem services over time scales from decades to a century or longer.
