The impact of rabbit grazing and soil moisture variation on sapling growth and survival in a semi arid Mediterranean ecosystem
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Plant-plant interactions have been proposed as fundamental mechanisms in semiarid ecosystems. Yet little is known on how increased consumer pressure effects plant-plant interactions. Therefore this study aimed to investigate how facilitative effects changed under grazing regimes. We examined the effect of rabbit grazing and microsite variation on: 1) soil moisture 2) sapling growth and survival. Moreover we measured how sapling growth and survival changed along a rabbit grazing gradient. This was tested through a sapling transplant experiment where measurements of soil moisture, survival and height of saplings (Anthyllis cytisoides) were taken under the canopy of shrubs (Artimisia barrelieri) and in interpatches in a semiarid Mediterranean ecosystem. As we found that replicates were submitted to differences in rabbit activity we were able to mimic a grazing gradient. Results showed that rabbit grazing had a negative effect on saplings, decreasing height and survival. Microclimate amelioration and associational resistance, reduced negative effects of grazing, causing microsite variation of growth and survival. Along our rabbit herbivory gradient there was an increase in facilitation. However we found that under our highest grazing pressure, in contrast to the stress gradient hypothesis (SGH), facilitation declined. Although this decline, there remained a positive net effect for both growth and survival. We conclude that grazing by rabbits is a large driver affecting the spatial and temporal variation of sapling growth and survival and with increasing biotic stress facilitation is more likely to follow a humped-shape pattern than monotonically increase as predicted by the SGH.