Can the intermediate disturbance hypothesis explain grazing–diversity relations at a global scale?

Abstract

In the context of grazing, the intermediate disturbance hypothesis (IDH) predicts that plant diversity peaks under moderate grazing, resulting in a hump‐shape pattern for the grazing–diversity relationship. Although this has been debated due to contradictory empirical results, the IDH is still widely accepted among rangeland ecologists. The Milchunas–Sala–Lauenroth (MSL) model predicts that in arid areas grazing affects diversity negatively regardless of grazing intensity, whereas in mesic areas grazing effects on plant diversity are the same as predicted by the IDH. Very few studies have attempted to specifically evaluate the applicability of the IDH and MSL to grazing systems at a global scale, accounting for the possible effects of climate. We conducted a meta‐analysis and vote‐counting analysis to evaluate these two hypotheses. The results of both analyses show that the IDH cannot be applied globally and that its application largely depends on aridity. The IDH prediction of a hump‐shape curve is supported in wet areas, while in dry areas there is a slight decrease in species richness with increasing grazing intensity. Overall, the MSL model correctly predicted grazing–diversity relations in both wet and dry areas. Looking at specific ecosystem types, we found that these results hold in grasslands, but not in woodlands. Differences between livestock types, not considered by the MSL model, were found to be important. Mixed sheep and goat grazing in dry areas resulted in a significant decline of species richness with grazing intensity, while grazing by sheep only had little effect on species richness. Cattle grazing and yak grazing in wet areas yielded a clear hump‐shape pattern. Therefore, we conclude that the climate‐specific MSL model better predicts the impact of grazing on diversity than the IDH in rangelands, and that the response patterns of plant richness to grazing are dependent on aridity, grazing intensity and grazer type.

Friend and foe? The effects of grassland management on global patterns of spider diversity

“…Araneidae) and Sense webs (e.g. Mygalomorph), suggest that the tridimensional vegetal structure maintained by intermediate levels of grazing can benefit web building spiders, as previously advocated (Sasaki et al, 2009;Nogueira & Pinto-da-Rocha, 2016;Gao & Carmel, 2019;Hu et al, 2019;Wang & Tang, 2019;Ferreira et al, 2020).…”

Responses and Indicators of Composition, Diversity, and Productivity of Plant Communities at Different Levels of Disturbance in a Wetland Ecosystem

“…Although a certain level of disturbance can significantly reduce productivity, it may also increase plant diversity, such as the Shannon-Wiener index in our study (see Figure 3B). This seems to support the intermediate disturbance hypothesis (IDH), which assumes that species diversity is greatest under a moderate intensity of disturbance [25][26][27][28]62,63]. However, whether this increase in diversity is our true goal needs to be considered in conjunction with the diversity indices used, as well as the productivity and sustainability of the grassland.…”

“…This shows that the diversity indices and their variability can indicate the disturbance level of the plant community from different perspectives, particularly for severely disturbed areas. Several studies have also confirmed that grazing disturbance promotes diversity [61], and that diversity was greatest in the presence of moderate disturbance [25][26][27][28]62,63]. However, some of these studies used the Shannon-Wiener index [25,28], while others used the richness index (number of species per unit area) [26,27].…”

Cascading effects of elephant-human interactions in a savanna ecosystem and the implications for ecology and conservation

“…The species richness along the Amboseli elephant gradient fits the Intermediate Disturbance Hypothesis (99), as modified by the Milchunas-Sala-Lauenroth (MSL) models (100) along a productivity gradient. Although the literature shows elephants depleting woodlands and creating browse traps of dwarfed woody vegetation in high density park populations (101), the impact of short-term intensive browsing has yet to be studied at an ecosystem scale.…”

“…Our observations in Amboseli suggest that short intense browsing due to seasonal elephant movements and in response to human activity may stimulate woody production and diversity in much the same way that short-term intensive grazing can create browsing arenas which stimulate grass production (102–104) and species richness (100). Our observations of giraffe browsing lend support to this view.…”