Isolating the single effects and net balance of negative and positive species effects in complex interaction networks is a necessary step for understanding community dynamics. Facilitation and competition have both been found to operate in harsh environments, but their relative strength may be predicted to change along gradients of herbivory. Moreover, facilitation effects through habitat amelioration and protection from herbivory may act together determining the outcome of neighborhood plant-plant interactions. We tested the hypothesis that grazing pressure alters the balance of positive and negative interactions between palatable and unpalatable species by increasing the strength of positive indirect effects mediated by associational resistance to herbivory. We conducted a two-year factorial experiment in which distance (i.e., spatial association) from the nearest unpalatable neighbor (Stipa speciosa) and root competition were manipulated for two palatable grasses (Poa ligularis and Bromus pictus), at three levels of sheep grazing (none, moderate, and high) in a Patagonian steppe community. We found that grazing shifted the effect of Stipa on both palatable grasses, from negative (competition) in the absence of grazing to positive (facilitation) under increasing herbivore pressure. In ungrazed sites, belowground competition was the dominant interaction, as shown by a significant reduction in performance of palatable grasses transplanted near to Stipa tussocks. In grazed sites, biomass of palatable plants was greater near than far from Stipa regardless of competition treatment. Proximity to Stipa reduced the amount of herbivory suffered by palatable grasses, an indirect effect that was stronger under moderate than under intense grazing. Our results demonstrate that facilitation, resulting mainly from protection against herbivory, is the overriding effect produced by unpalatable neighbors on palatable grasses in this rangeland community. This finding challenges the common view that abiotic stress amelioration should be the predominant type of facilitation in arid environments and highlights the role of herbivory in modulating complex neighborhood plant interactions in grazing systems.

Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs (‘consumer-controlled’). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food (‘resource-controlled’). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk.

Since many arid ecosystems are overstocked with domestic herbivores, biotic stress could have a stronger influence in modulating the balance of species interactions than expected from the stress gradient hypothesis (SGH). Here we tested a priori predictions about the effect of grazing on species interactions and fine scale spatial structure of grasses in water‐limited ecosystems. We used detailed vegetation mapping and spatial analysis, and performed a field experiment where the direct and indirect components of positive interactions were disentangled to provide evidence of links between process and pattern. We found associational resistance (biotic refuge) to be the dominant process in grazing situations, while competition, instead of direct facilitation, seemed to govern grass spatial patterns when herbivore pressure was relaxed. These results suggest that facilitation between grasses in arid communities may be related to herbivory rather than nurse plant effects. Associational resistance tends to have the strongest effect on spatial aggregation of species at intermediate grazing pressure. Results suggest that contrary to SGH, this physical clustering of species decreased when grazing pressure reached their maximum levels. Positive associations remained significant only when palatability differences between neighbours is large, suggesting that managing stocking rate is a key factor determining the persistence of herbivory refuges. These refuges are potential foci to initiate population recovery of high quality forage species in arid degraded areas.

A proposed refinement to the stress-gradient hypothesis requires consideration of the strategies of the interacting species and the characteristics of the stress factors. While the strength and direction of these interactions can be predicted for different ecosystems, this idea remains largely untested in the field. We performed a manipulative field experiment complemented with a descriptive study to test the predictions in a natural setting that represents the extreme end of a precipitation gradient. There, wind driven desiccation and water availability are the main stressors (non-resource and resource-based stresses, respectively). We evaluated the interaction between the shrub and grasses that are dominant in the Patagonian steppe. The species had differences in morpho-functional traits and drought tolerance that fit into the C-S axis of Grime's strategies. We experimentally separated root zones to limit direct competition for soil moisture and reduce the resource-based stress on grasses. We also manipulated the distance to shrubs to evaluate non-resource stress amelioration by canopies (e.g., sun and wind) on grasses. Finally, we evaluated the distribution of naturally established C and S grasses in the neighborhood of C and S shrubs to infer process-pattern relationships. Our growth data coincide to a large degree to the predictions. We found positive effects on the growth of beneficiaries when stress was non-resource based and when strategies differed (i.e., C -S and S -C ). We also found strong negative effects when the abiotic stress was driven by water, particularly on C grasses. Additionally, shrubs only increased the survival of grasses when strategies differed (i.e., C -S and S -C ). Our manipulative and descriptive study supported previous results that showed that stress-tolerant species are important for the persistence of competitive species at high stress. While the applicability and generality of these predictions remains to be tested with more field experiments, some ecological factors, such as stress types and species traits, can explain much of the variation in how dominant shrubs and grasses interact in this extreme arid environment. Moreover, this framework could be extended to specifically test the importance of facilitation under different levels of stress.

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Summary1. Dioecious plants frequently face sex-related resource allocation trade-offs. Differential traits of male and female plants can have important consequences that influence their direct and indirect interactions with neighbours and herbivores. 2. Gender differences in traits have been considered to be inherently linked to sex-biased ratios along environmental stress gradients. However, the mechanisms behind the variation in sex ratios with increasing biotic stress remain elusive. 3. We linked the results of process-based field experiments with fine-scale spatial patterns of naturally established plants (both sexes) at different grazing intensities. We demonstrated that slowgrowing females were better defended against insects and small mammals but were more susceptible than males to competition from neighbours. 4. In large-herbivore exclosures, we found that sex-biased spatial patterns at the microsite scale can be predicted from gender-specific traits, with females more segregated from competitors (unpalatable grasses) than males. Traits related to growth/defence trade-off in females, coupled with their higher competition costs than males, suggest that gender dimorphism in traits can directly influence species interactions and spatial organization. 5. In the presence of large herbivores, female individuals were preferentially browsed despite their larger investment in antiherbivore compounds. The greater distance of females from unpalatable competitors was the main determinant of female-biased consumption by domestic herbivores. Females growing close to neighbours successfully eluded browsing but also faced competition. Scaling up to a population level, population sex ratios drift from female-to male-bias with increasing domestic grazing intensity. 6. Synthesis: The sexual dimorphism in traits of a dioecious bunchgrass results in differential herbivory by natural enemies, differential competitive ability and differential spatial distribution of sexes. Domestic grazers disrupt this balance by shifting species interactions and their spatial organization. Changes in dioecious plant populations towards male-biased sex ratios with grazing intensity could not be predicted solely by gender-specific traits.

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1. Species loss due to an increasing the number of added nutrients has been explained by both light competition through biomass increase and by niche dimension reduction as a result of species-specific limiting soil resources trade-offs. Disturbances, by reducing community biomass, species dominance and increasing light availabil-The study was conducted in a natural grassland near Pila County,