Bi-stability, compromised resilience, and state-shift indicators: Abrupt shift of tallgrass prairie to degraded shrubland

Background/Question/Methods Abrupt state shifts are one of the most dramatic and difficult to reverse ecosystem transformations, but are also among the toughest to predict, quantify, and understand mechanistically. Woody encroachment—the worldwide phenomenon of native woody plants expanding into long-standing grasslands and savannas—may represent a state-shift, reflecting bi-stability between grass and shrub-dominated states. Understanding the trajectory, thresholds, and drivers of woody encroachment is critical, because woody encroachment reduces biodiversity, alters ecosystem services, and disrupts important economic enterprises. To determine if woody encroachment in tallgrass prairie is bi-stable and susceptible to abrupt state-shifts, we applied Bayesian and frequentist statistical tools to long-term (26 year) temporal and spatial data-sets of shrub cover in tallgrass prairie. 

Results/Conclusions We found that: 1) tallgrass prairie under-went an abrupt to shrubland in one year, across a diverse array of fire frequencies, levels of grazing, and topography; 2) changes in shrub cover over time were explained by bi-stable population growth models and had little relationship with climate; 3) abrupt increases in shrub cover were proceeded by higher rates of seedling establishment, but were the direct result of positive feedback-driven expansion by individual shrubs; 4) resilience of tallgrass prairie declines with the frequency of small shrubs. Taken together with shrub-mediated reversals of fire dynamics and belowground competition, our results strongly suggest that tallgrass prairie is a bi-stable ecosystem, susceptible to abrupt shifts now and into the future.  In this system, increasing establishment rates of juveniles served as an early indicator of state-shifts, which may be common to other bi-stable ecosystems.