PHD Math Defence: Regime Shifts in Ecological and Socio-ecological Systems by Ram Sigdel

CANDIDATE:   RAM SIGDEL

ABSTRACT:

Alternative stable states are common in nature, as are regime shifts that take a system from one stable state to an alternative contrasting one. Regime shifts have been explored in social models, ecological models, and coupled socio-ecological models. These models show that regime shifts may be driven either by human factors (such as social norms) or natural factors (such as a fire in forest-grassland systems). Our objective is to identify the conditions that give rise to regime shifts in a selection of ecological and socio-ecological models, and to determine which conditions foster regime shifts to sustainable outcomes. First, a model of forest growth is coupled with a social dynamics model based on the replicator equations to explore how the interplay between social norms and conservation priorities structure socio-ecological dynamics.  This is followed by a chapter on common-pool resource systems to study how different types of recruitment assumptions influence socio-ecological model dynamics. Finally, a chapter on a tree-grassland mosaic ecosystem model describes when, where, and why transitions between two alternate stable states occur, and environmental gradients determine land state composition. In both human-driven and environment-driven systems, resource growth is either measured by a threshold growth function, a constant growth rate, or logistic growth. Regime shifts are common in all three models but can emerge either from assumptions about recruitment or from socio-ecological effects such as social norms. In the socio-ecological model of forest growth, regime shifts take place through transcritical or Hopf bifurcations when injunctive social norms are present, whereas in the common-pool resource model, regime shifts take place through transcritical, fold, or Hopf bifurcations, in which the path of the regime shift is mainly determined by changes in the social learning rate, ostracism rate, and resource productivity rate. In the former case, the forest always survives and in the latter case, the resource system cannot survive a larger destruction rate value despite co-operative harvesting practices. In the fire-mediated tree-grassland mosaic model, a regime shift takes place through both transcritical and fold bifurcations in which the critical point of transition between alternate stable states is mainly determined by the sharpness transition parameters. As in the common-pool resource model, the forest-grassland mosaic cannot survive when there is a large harvesting rate and a small growth rate. Limit cycles occur in both the socio-ecological model of forest growth and the socio-ecological model of common-pool resources. In the  former case, oscillations can be removed by changing social norms, conservation values, resource growth rates and in the latter case, they might be removed by changing the social learning rate, the resource productivity rate, and the social ostracism rate. Oscillations do not occur in the mosaic model because the model does not include feedback effects from a human subsystem. We conclude that regime shifts are common across ecological and socio-ecological systems and are caused by different drivers, but across all three model systems we find that increasing resource growth rates and/or decreasing harvesting rates best generates sustainable outcomes that are far away from tipping points and oscillations. We also find that social norms and threshold-based recruitment functions tend to generate alternative stable states in social and ecological subsystems alike.