The impact of coupled human-environment system dynamics on alternative stable states of a forest-grassland mosaic ecosystem

Background/Question/Methods

Mosaic ecosystems can exhibit alternative stables states, whereby for the same environmental parameters the ecosystem could equally well reside either in one state or another state depending on the initial conditions of the system.  Forest-grassland mosaics have been shown to exemplify this phenomenon, which has been captured by mathematical models.  Because such ecosystems can be readily ‘flipped’ from one state to another, the potential human impact on these ecosystems is large.  Despite this, the impact of changes in mosaic ecosystem states on human perceptions of conservation priorities, and the resulting feedback of changing human activities back onto mosaic ecosystem dynamics (coupled human-environment dynamics), has not been investigated.  Here we explore the question of how human-environment system dynamics change the stability properties of forest-grassland mosaic ecosystems.  We develop a mathematical model that combines a simplified forest-grassland mosaic model with a dynamic model of rarity-based perceptions of forest/grassland value and how those perceptions influence the state of the mosaic. We analyze the local stability properties and numerical solutions of the model. 

Results/Conclusions

We find that human influence can have a significant influence on stability properties of the mosaic ecosystem.  Surprisingly, weak to moderate human influence can actually increase the region of parameter space where bistable solutions are possible.  However, when human influence is strong, the bistability properties of the forest-grassland mosaic are lost and forest and grassland can either co-exist at a single, stable equilibrium, or their relative abundance can oscillate over time.  Moreover, in this parameter regime, a perturbation (such as unilaterally increasing forest cover in a short period of time) can shift the system from a stable state to a state of unstable dynamics (very large oscillations).  We conclude that human-environment dynamics can significantly alter the fate of bistable forest-grassland mosaic ecosystems.  Therefore, in understanding the long-term influence of humans on mosaic ecosystems, the human factor should be viewed as dynamic, responsive element rather than as a fixed, unchanging entity.