The effect of grazing on the spatial heterogeneity of vegetation

Abstract

Grazing can alter the spatial heterogeneity of vegetation, influencing ecosystem processes and biodiversity. Our objective was to identify why grazing causes increases in the spatial heterogeneity of vegetation in some cases, but decreases in others. The immediate effect of grazing on heterogeneity depends on the interaction between the spatial pattern of grazing and the pre-existing spatial pattern of vegetation. Depending on the scale of observation and on the factors that determine animal distribution, grazing patterns may be stronger or weaker than vegetation patterns, or may mirror the spatial structure of vegetation. For each possible interaction between these patterns, we make a prediction about resulting changes in the spatial heterogeneity of vegetation. Case studies from the literature support our predictions, although ecosystems characterized by strong plant-soil interactions present important exceptions. While the processes by which grazing causes increases in heterogeneity are clear, how grazing leads to decreases in heterogeneity is less so. To explore how grazing can consistently dampen the fine-scale spatial patterns of competing plant species, we built a cell-based simulation model that features two competing plant species, different grazing patterns, and different sources of vegetation pattern. Only the simulations that included neighborhood interactions as a source of vegetation pattern produced results consistent with the predictions we derived from the literature review.

Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

“…Large trees, thus, contribute to species diversity in savannah systems, in combination with smaller trees and open grassland. A high plant species diversity and heterogeneity on different spatial scales is an important feature in savannah systems and has probably caused the high abundance and diversity of herbivore species in these landscapes (Adler et al 2001).…”

“…African savannah systems are known for their spatial heterogeneity (du Toit et al 2003), which is reflected in their specific vegetation structure and composition (Augustine 2003) as well as in the spatial distribution and composition of ungulate species (Adler et al 2001). Savannahs are described as a continuous grass layer interspersed with trees (Sankaran et al 2004), and their spatial heterogeneity strongly depends on the scale of observation (Adler et al 2001).…”

“…Savannahs are described as a continuous grass layer interspersed with trees (Sankaran et al 2004), and their spatial heterogeneity strongly depends on the scale of observation (Adler et al 2001). At a landscape scale, the patchiness in spatial nutrient distribution, vegetation structure, and tree species composition has been the focus of many studies (Vetaas 1992;Scholes and Archer 1997;Sankaran et al 2004).…”

Overgrazing in the Montado? The need for monitoring grazing pressure at paddock scale

“…Interactions of livestock with the ecosystem imply the choice of grazing location through discrimination and selection among vegetation patterns (Adler et al 2001). Decisions on feeding areas may be affected by feed abundance and also other valued assets such as watering points, location of supplementary feed distribution including mineral blocks, refuge and shade (Bailey et al 1998).…”

“…These variations of attractiveness of resources in space and time create a spatial mosaic pattern of animal distribution. The consequent uneven grazing will affect vegetation dynamics (Alados et al 2004) and can increase or decrease pasture spatial heterogeneity (Adler et al 2001).…”