Indicator patches: exploiting spatial heterogeneity to improve monitoring systems

Abstract

When choosing which environmental attributes to monitor in assessing disturbance, it is important to consider not only which metric will provide the most sensitive indicator of disturbance, but also the spatial considerations of where in the landscape that metric will be most responsive to change. Degradation in landscapes is often unevenly expressed because: (i) disturbance is spatially localised, (ii) landscape elements differ in their sensitivity to disturbance, and (iii) degradation following localised disturbance is spatially contagious. The spatial heterogeneity of degradation has proven to be a key obstacle to rangeland monitoring (e.g. where the initiating processes of broad-scale degradation are concentrated in landscape locations that are not detected by surveys) but can also provide opportunities to focus monitoring efforts. We propose that the effectiveness of monitoring could be enhanced by identifying and selectively monitoring ‘indicator patches’, i.e. specific landscape locations that provide the most management-relevant and timely information about the consequences of a monitored disturbance. We tested and demonstrated the utility of the ‘indicator patch’ concept in the rangelands of the Succulent Karoo in southern Africa. We contrasted the grazing response of dominant ‘representative’ vegetation, with responses of interspersed patches of distinct vegetation associated with zoogenic mounds. Since mound vegetation is more palatable and preferentially grazed by sheep, we tested whether mounds could serve as ‘indicator patches’ in providing a sensitive measure of grazing disturbance. Percentage canopy cover measurements in dominant off-mound vegetation provided a poor indicator of grazing disturbance (although more intensive plant size measurements did reveal grazing impacts on plant population dynamics). In contrast, vegetation on mounds displayed patterns of changes in species abundances that were easier to detect and useful for interpreting and quantifying the effects of grazing. Mound vegetation could, therefore, be used as ‘indicator patches’ and targeted for exclusive sampling as a sensitive method for monitoring rangeland condition and detecting early warnings of vegetation change. This approach could be widely employed to better harness the extensive knowledge base regarding the patchy, spatially localised nature of degradation-initiating processes in numerous other landscapes. Routinely incorporating this understanding into the design of monitoring programs could improve the effectiveness of sampling effort, allow detection of more subtle trends (changes), and provide earlier warning of impending degradation so remedial action can be taken before degradation becomes severe and widespread.

Grazing ecology and the conservation of the Caldenal rangelands, Argentina

“…Prevention of transition to undesirable regimes requires the monitoring of ecological indicators that can be used to assess the ecological status and trends in the health of the ecosystem and its component parts. Examples of such indicators are vigour and size of perennial grasses, abundance of annual species, seedling establishment of undesirable species, size and connectivity of denuded soil patches, and soil aggregate stability (Pellant et al, 2005;Chartier and Rostagno, 2006;Stokes et al, 2009;Kaghergis et al, 2011;L opez et al, 2011;Sasaki et al, 2011). Ecological indicators serve to identify "at risk communities", which should emphasize the necessity to initiate preventive management actions that block undesirable changes in the structure and functioning of the ecosystem.…”

Termite activity, not grazing, is the main determinant of spatial variation in savanna herbaceous vegetation

“…2008; Sileshi et al. 2010), diversity (Anderson, Ritchie & McNaughton 2007; Moe, Mobæk & Narmo 2009) and species composition (Stokes et al. 2009; Sileshi et al.…”

Grazing Intensity and Spatial Heterogeneity in Bare Soil in a Grazing-Resistant Grassland

“…Spatial patterns in the distribution of vegetation cover have been proposed as indicators of rangeland condition because loss of vegetation cover can initially occur in particular landscape positions and then spread laterally (Bestelmeyer et al 2006;Kefi et al 2007;Stokes et al 2009). However, the ways that grazers influence spatial heterogeneity in vegetation cover are complex and difficult to predict, as it often reflects an interaction between factors influencing animal foraging behavior and the inherent vegetation patterns that would occur in the absence of herbivory (reviewed by Adler et al 2001).…”