The rangeland equilibrium-non-equilibrium debate produced several important advances in our understanding of rangeland systems. But, in their recent Review, Briske et al. ask if, collectively, these advances are still insufficient to inform the stewardship strategies necessary to sustain global rangelands? Here they provide a summary of their work.
The rangeland equilibrium-non-equilibrium debate of the late 20th Century questioned the appropriate ecological model governing the function of rangeland systems. In retrospect, an overly simplistic interpretation of herbivore carrying capacity that excluded scale and functional resource heterogeneity resided at the centre of the debate. The number of herbivores that an area can support is generally determined by the availability of key resources during the dormant season, rather than abundant resources during the growing season. Therefore, grazed ecosystems are not distinguished by whether they are equilibrial or non-equilibrial, as previously assumed, but by spatial and temporal variation in accessible resources that determine herbivore population change. The spatial heterogeneity of key resources, which are present in all grazed ecosystems, contributes to herbivore persistence by buffering population dynamics against temporal variability in resource availability. Carrying capacity can only be defined as a long-term mean in climatically stochastic environments, thereby limiting the practical applicability of the concept for management of grazed systems. Projections for increasing climate variability will further exacerbate this limitation. However, the concept does have heuristic value for investigation of how herbivore populations exploit and depend on functional resource heterogeneity.
The fundamental challenge facing the global rangeland community in the 21st Century may not be identification of a unified model of rangeland ecology as assumed during the rangeland debate. Rather, the challenge may be how to best transform rangeland social-ecological systems to provide optimal combinations of ecosystem services to meet the needs of global citizens, while improving the well-being of millions of rangeland residents who are highly dependent upon provisioning services. In a global context, the aggregate value of non-provisioning services may be of equal or greater value than those of the select provisioning services that are currently recognized. Stewardship strategies of the 20th Century are insufficient to effectively inform the contemporary challenges confronting global rangelands in the 21st Century. A comprehensive accounting of rangeland ecosystem services, supported by institutional governance and delivered as state-community partnerships, may provide the foundation for an alternative stewardship strategy.
Read the full Review,Strategies for global rangeland stewardship: Assessment through the lens of the equilibrium–non‐equilibrium debate, in Journal of Applied Ecology.