An empirical assessment of non-equilibrium theory and state-and-transition models with a long- term (1953-2009) vegetation record from the Sonoran Desert

Background/Question/Methods

Resilience-based ecological frameworks, including state-and-transition models (STM), are being increasingly called upon to guide ecosystem management and inform natural resource policy. Yet multiple challenges continue to impede their application, including limited empirical assessments of non-equilibrium theory, infrequent evaluation of expert-based STMs against long-term empirical data, and uncertainty in identifying early-warning signals of pending thresholds. We evaluated long-term term (1953-2009) vegetation data from the Sonoran Desert to address how well the inherent empirical trends are captured by the theoretical constructs of non-equilibrium theory and STMs. 

Results/Conclusions

Seven statistically distinct plant communities were identified using model-based clustering and all instances of abrupt compositional transitions among communities were recorded. The temporal dynamics of species composition illustrate four major patterns – high stability, random drift, reversibility, and irreversible change that corroborate the occurrence of both equilibrium and non-equilibrium dynamics in this system. Degree of compositional change during transitions was positively skewed indicating that even modest amounts of turnover could result in transitions. Transitions were more frequent between communities with similar species pools than between more distant communities. Frequency of transitions and the degree of spatial correlation in vegetation composition were both positively correlated with fluctuations in mean annual precipitation, with the greatest number of transitions occurring during wet periods. This provides new insights into the spatio-temporal dimension of ecological resilience, and aligns with the hypothesis that spatial correlation can serve as an early-warning signal of imminent thresholds, which can, at least in part, be linked to climatic drivers. The vegetation dynamics observed in this record indicate that conventional expressions of resilience and transitions in STMs may account for only a fraction of the complex dynamics that exist in ecosystems. A network of sites with long-term empirical data may provide ecological benchmarks to support emerging resilience-based management by guiding development of rules for model construction and corroborating the assumptions of existing STMs.