Inclusion of biotic stress (consumer pressure) alters predictions from the stress gradient hypothesis

Abstract

Summary1. The stress gradient hypothesis (SGH) predicts a shift from net negative interactions in benign environments towards net positive in harsh environments in ecological communities. While several studies found support for the SGH, others found evidence against it, leading to a debate on how nature and strength of species interactions change along stress gradients, and to calls for new empirical and theoretical work. 2. In the latest attempt in this journal, it is successfully argued how the SGH should be expanded by considering different life strategies of species (stress tolerance versus competitive ability) and characteristics of abiotic stress (resource versus non-resource based) over wider stress gradients (opposed to low-high contrasts), but the crucial role of biotic stress by consumers is largely ignored in this refinement. 3. We point out that consumers strongly alter the outcome of species interactions in benign and harsh environments, and show how inclusion of consumer-incurred biotic stress alters the predicted outcome of interactions along resource-and non-resource-based stress gradients for stress-tolerant and competitive benefactors and beneficiaries. 4. Synthesis. New studies should include stress gradients consisting of both abiotic and biotic components to disentangle their impacts, and to improve our understanding of how species interactions change along environmental gradients.

Effects of cattle and rabbit grazing on clonal expansion of spiny shrubs in wood-pastures

“…These 'nurse' plants also protect palatable tree seedlings against large herbivores and thereby initiate the establishment of isolated forest patches in the landscape (Rousset & Lepart 1999;Bakker et al 2004;Smit et al 2005Smit et al , 2006Van Uytvanck et al 2008) in accordance with the shifting mosaic theory (Olff et al 1999). However, while the importance of nurse plants in grazed ecosystems is now well recognized (Smit, Rietkerk, & Wassen 2009), little is known about the factors driving the dynamics of nurse plants. Such knowledge is of crucial importance for a better understanding of the functioning of both ancient and newly created grazed ecosystems and their sustainable conservation management.…”

Mechanisms of plant–plant interactions: concealment from herbivores is more important than abiotic-stress mediation in an African savannah

“…Recent work on plant-plant interactions has emphasized that neighbours often act as facilitators in stressful conditions, but the role of consumers has received less attention in this literature [1]. Amelioration of abiotic stress in harsh environments is a well-supported mechanism of facilitation in many communities [2][3][4][5].…”

Shifting Species Interaction in Soil Microbial Community and Its Influence on Ecosystem Functions Modulating

“…The stress gradient hypothesis (SGH) predicts that facilitation and competition act simultaneously upon interacting species and the relative frequency of them are inversely correlated with gradients of environmental stress [1,2]. As a general conceptual model, SGH stimulated most of the research on facilitation over the last 20 years [3] and also led to a still-ongoing debate on its generality [2].…”

“…As a general conceptual model, SGH stimulated most of the research on facilitation over the last 20 years [3] and also led to a still-ongoing debate on its generality [2]. Although not without controversy, the net stressful impacts were useful for predicting the species interactions along abiotic gradients and for estimating the dominant factors in organizing communities [4].…”