Frequency of association: a key indicator for assessing livestock grazing effects on dryland plant interactions, applicable in restoration

Abstract

In arid ecosystems, vegetation patches promote biological activity and ecosystem functioning through species interactions that mitigate adverse environmental impacts. However, overgrazing disrupts those interactions, contributing to desertification. We investigated livestock influence on spatial vegetation patterns through the modification of the frequency and type of associations (i.e. number of associated species within a vegetation patch) among types of successional species (i.e. early, intermediate, and late). We studied vegetation in nine rangelands with varying stocking rates (treatments) within the Monte Desert region. We postulated three hypotheses: (1) Grazing simplifies community structure, increasing the frequency of isolated individuals and reducing the association diversity among plant species; (2) competition between plants favors the associations among species with different ecological niche requirements, reducing competition, enhancing facilitation, and regulating the association frequency among species; (3) grazing modifies plant competition and patterns of association among species. We found that with increasing stocking rates, the frequency of isolated individuals increased while association diversity declined. Associations between species with dissimilar niche requirements (e.g. early–late) were more frequent than those with similar needs (e.g. early–early). Livestock grazing modified plant competition, leading to an increased frequency of associations among species with similar requirements. Therefore, the association frequency among types of species in these drylands clearly reflected the degradation process along a grazing gradient, aiding in understanding plant ecological interactions. The association frequency could be implemented as an indicator in diagnostic and monitoring stages of restoration processes.