Range Condition Assessment and the Concept of Thresholds: A Viewpoint

Abstract

Dissatisfaction persists with current approaches to range condition and trend assessment. Sometimes assessed condition does not truly represent the past or the potential of range. One of the likely causes is a failure to reexamine and change if necessary the theoretical basis of assessment, in line with developing understanding of ecological processes. The concept of thresholds of environmental change appears to provide a reasonable alternative in some circumstances to the concepts of gradual retrogression and secondary succession which are currently accepted. I suggest that environmental change can be discontinuous, with thresholds between alternative states. Once a threshold is crossed to a more degraded state, the former state cannot be attained without significant management effort, such as prescribed burning, ploughing, or herbicide application, rather than simple grazing control. Examination of data from extensive monitoring programs and from a study of grazing impact, as well as more general sources of information, indicates that thresholds of change may be identifiable in arid rangelands. A practical means of monitoring proximity to thresholds is available and, with the aid of multivariate analysis, the effects of spatial variability and season can be separated from those of management. The potential of this approach deserves investigation in a wider variety of environments.

Vegetation responses following wildfire on grazed and ungrazed sagebrush semi-desert

“…The state-and-transition model (Westoby et al 1989a(Westoby et al , 1989b implies that plant community composition makes dramatic changes only during times of unusual environmental influences. Furthermore, the species composition of differing plant communities in particular states, on a particular ecological site, fluctuate within defined limits (Treshow and Allan 1985), which can also be expressed as several domains of attraction, or boundedness (Lewontin 1969, Friedel 1991, Crowley 1992 …”

Impact of pastoral land use on central Namibian savanna vegetation - Methods for indictor development and perspectives for their application

“…Eine Annäherung an das Problem der Degradation besteht darin, Indikatoren zu identifizieren, die die Intaktheit oder eine Verschlechterung des Weidezustands anzeigen. Trotz der schon seit den 1930er Jahren im südlichen Afrika etablierten weideökologischen Forschung (Hoffman et al 1999) gibt es für die ariden Teile des Subkontinents, insbesondere den namibischen Teil der Nama Karoo, wenig gesicherte wissenschaftliche Erkenntnisse über den Zusammenhang von veränderter Weidenutzung und Dynamik der Vegetation (Palmer & Hoffman 1997 (Friedel 1991;Bestelmeyer 2006;Sasaki et al 2008;Briske et al 2010). (Haarmeyer et al 2010).…”

Sahelian Rangeland Development; A Catastrophe?

“…There are 2 ways to formulate this property in relation to the rangeland ecosystem (Lockwood and Lockwood 1993). The first is the formulation that transitions between states are virtually irreversible without substantial human intervention (Friedel 1991). As mentioned earlier, the transition from a vegetation community of annual herbs to another state is not necessarily possible within a reasonable period of time (several years) if the soil has to recover from soil degradation such as crust formation or erosion.…”

“…Another way to formulate hysteresis is to envisage "decline" and "recovery" as following different paths or trajectories (Friedel 1991, Laycock 1991, as was earlier recognized in semiarid pastoral ecosystems [e.g. by Breman et al (1980), Sinclair and Fryxell (1985), Ellis and Swift (1988) and Behnke and Scoones (1992)].…”