| dc.description.abstract | 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. | |
