Together we stand, divided we fall: Effects of livestock grazing on vegetation patches in a desert community

Prepared by Yamila Ivón Pelliza, Anahi Rocío Fernandez, Hugo Saiz & Mariana Tadey

Grazed area under study in Patagonian Monte Desert, Neuquén province, Patagonia, Argentina. Photo credit: Ivón Pelliza.

Dryland vegetation structure and ecosystem functioning are mainly driven by the interactions among plant species. Plant-plant interactions are key in vegetation patch formation, since species survival relay on local spatial associations among individuals that enhance site conditions, such as higher moisture, lower temperature and nutrient accumulation. However, the ecosystem´s equilibrium can be easily disrupted by disturbances, such as livestock grazing. In most environments, when a disturbance is over, ecological succession occurs, and early-successional plant species are the first to colonize the newly available space. Over time, species replacement occurs until a new equilibrium or climax is reached, usually dominated by strong competitive, late-successional species. However, what happens when a disturbance persists over the years? Which type of successional species are the most affected, and which ones survive or dominate the space? These are the questions that continuous unmanaged livestock grazing implicates for drylands. We investigated how plant-plant interactions vary across a grazing gradient considering the influence of species successional role on vegetation structure. Our study was conducted in the Patagonian Monte Desert, a xerophytic shrubland occupying almost a third of the Argentinian surface. To study vegetation community, we took high-quality photographs in ten rangelands subjected to continuous grazing, but with different livestock densities. In each photograph, we determined plant species identity and patch structure (cover, abundance and richness) and the number of vegetation patches. We classified species based on their successional role as early, intermediate and late species. We analysed the data applying regressions and spatial association network analysis.

Our outcomes corroborated that livestock grazing modified plant community structure, reducing plant cover, abundance and richness. We observed that increased livestock density disrupted vegetation patch structure, simplifying plant associations and increasing isolated individuals. As we expected, based on species traits and lifeforms, early species were the least affected and late species were the most affected in terms of cover, abundance and richness. However, livestock grazing similarly decreased species richness independently of their successional role. Finally, in spatial association networks, early species showed more links and greater participation than the rest, highlighting their importance as colonizers and their capability of benefiting the establishment of other species.

Examples of aerial photographs for low stocking level (upper panel), intermediate stocking level (middle panel) and high stocking level (bottom panel) in Patagonian Monte Desert, Neuquén province, Patagonia, Argentina. Photo credit: Anahi Fernandez, Mariana Tadey and Ivón Pelliza.

Our results suggest that persistent grazing in the Monte Desert would lead to a “non-return effect” if early species are drastically affected. Declining species richness and abundance can decrease the chances of species to interact with each other, reducing the likelihood of combinations and patch formations. Network analysis revealed that late species were present in most plant associations, suggesting their importance for path structure, probably functioning as facilitators and/or protecting other species from grazing. Therefore, we suggest that restoration practices should focus on early species, while conservation strategies should focus on late species. In conclusion, considering the successional role of plant response to continuous disturbance provides valuable information on ecosystem dynamics, helping to understand how degradation occurs and, therefore, how to recover it, contributing to management, restoration and conservation strategies.

This is a plain language summary for the paper of Pelliza et al. published in the Journal of Vegetation Science (