Abstract
Rangelands are governed by threshold dynamics, and factors such as drought, wildfire, and herbivory can drive change across thresholds and between ecological states. Most work on this topic has focused on shifts in a single response variable, vegetation, and little research has considered how to reconcile responses of more than one variable to determine whether a system has undergone a genuine state change. In sub-Saharan Africa, mobile overnight livestock corrals (bomas) can be used by managers to precipitate ecological transitions from areas dominated by bare ground to productive ecosystem hotspots (glades) that are attractive to wild herbivores. We asked how long bomas must be occupied by cattle before undergoing a state change, considering both plant and animal response variables, to glade ecosystem hotspots. We tested five durations of boma occupation: 0, 4, 7, 14, and 28 days. Each treatment was replicated five times, and we assessed vegetation as well as herbivore dung (as a proxy of use) at multiple time points over 3 yr following boma abandonment. Vegetation in 7-, 14-, and 28-d boma duration treatments appeared to undergo a complete transition to glade-like plant communities, whereas the shortest 4-d treatment had not converted to a glade plant community by year 3. Wildlife responses appeared to lag behind vegetation responses, with transitions to glade-like herbivore use occurring only in the longest duration (14- and 28-d) treatments. Our results show that different response variables, when considered individually, may provide incomplete or misleading information about state changes. Although shorter-occupied bomas might be effective for reducing bare ground, they may not attract enough wild herbivores to constitute crossing into an alternative state. Understanding threshold dynamics associated not only with vegetation responses but with a broader suite of response variables is challenging, but will provide a more complete representation of ecosystem function and greater opportunity for more successful ecosystem management.
Highlights
We studied thresholds and tipping points for plant and wildlife response variables in a sub-Saharan African rangeland, where overnight corralling of livestock can be used to precipitate ecological transitions from areas dominated by bare ground to productive “ecosystem hotspots” (Porensky and Veblen 2015)
During the baseline time period, boma duration treatment had no significant effects on plant community structure (Fig. 2a, b, nonmetric multidimensional scaling (NMDS) stress = 0.09, nonmetric fit R2 = 0.991; PerMANOVA F4,14 = 0.90, P = 0.6), a b c d e f though different blocks did support slightly different communities (F4,14 = 1.58, P = 0.05)
We contend that a major current challenge is to understand threshold dynamics associated with vegetation responses, but with a broader suite of response variables that together provide a more complete representation of ecosystem function
Summary
An understanding of ecological thresholds is increasingly being recognized as vital to successful ecosystem management (Stringham et al 2003, Briske et al 2005, Scheffer 2009, Bagchi et al 2013, Kachergis et al 2014), and scientists and managers have long recognized that rangelands in particular can be governed by threshold dynamics (Westoby et al 1989, van de Koppel et al 1997, Briske et al 2003, 2005) Factors such as drought, wildfire, fire suppression, and livestock use drive change across thresholds and between ecological states
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