Unpacking multi-trophic herbivore-grass-endophyte interactions: feedbacks across different scales in vegetation responses to Soay sheep herbivory

Abstract

Grazing can induce changes in both plant productivity and nutritional quality, which may subsequently influence herbivore carrying capacity. While research on Soay sheep (Ovis aries L.) dynamics on Hirta Island in the St. Kilda archipelago has elucidated the complexity of population drivers, including parasites, the role of herbivore-generated feedbacks as an intrinsic regulating factor remains unclear. The sheep lack large predators and every 3–9 years undergo population crashes (overcompensatory mortality). We investigated the effects of grazing on (1) sward productivity and (2) quality (toxicity) of the primary forage species, red fescue (Festuca rubra L.), which is highly infected by an alkaloid-synthesizing fungal endophyte. Grazing had a negative impact on both forage quantity and quality. At higher sheep densities, impacts on sward growth were magnified, resulting in a nonlinear relationship with plant productivity. Simultaneously, endophyte hyphal load (and by inference, toxicity) peaked close to the time of a crash. A greenhouse experiment showed that alkaloid concentration in F. rubra increased in response to artificial defoliation. We conclude that at high sheep densities, grazing-mediated reductions in productivity, together with sustained alkaloid production, are likely to influence sheep dynamics. Future research should consider the interactive effects of forage toxicity, quantity, and nutritional content.

Age specific impacts of vegetation functional traits on gastro-intestinal nematode parasite burdens in a large herbivore

“…The study area contains approximately 30% of the island's total sheep population (Clutton-Brock et al, 1992). The vegetation of the study area is dominated by Holcus lanatus-Agrostis capillaris (HA) grassland on free draining, fertile soils, particularly around the village, Molinia caeruleadominated grassland on areas receiving drainage, and Calluna vulgaris-dominated heath (wet and dry) on the steeper slopes around the bay (Jewell et al 1974;Vicari et al, 2018) (Supplementary Figure S1). HA grassland has the highest live standing-crop biomass of the area (Crawley et al 2004;Crawley et al, 2021) and is preferred by Soay sheep (Jones et al, 2006).…”

Density-Dependent Effects of Simultaneous Root and Floral Herbivory on Plant Fitness and Defense

“…Herbivore effects can be density-dependent, that is, the effects vary with their density. Examples abound in the case of large herbivorous mammals, in which the non-linear relationship between herbivore density/intensity and plant damage is well established [ 18 , 19 , 20 , 21 , 22 ]. Density-dependent effects may also occur in response to insect herbivory, although in this case the existing studies are scarcer.…”