The separate effects of shrubs, herbivores and winter rainfall on ephemeral communities have been studied for decades in a worldwide range of arid/semi-arid ecosystems. These three abiotic/biotic factors appear to be determining elements for the assembly of ephemeral herbaceous species in xeric environments. However, little attention has been directed to the study of ecological constraints that each exerts when they act simultaneously in natural environments. We explored the interaction of these three factors in the assembly pattern of winter ephemeral communities in semi-arid coastal Chile (north-central Chile, IV Region). Multivariate analysis of ordination, cluster, and GLM statistics were used to test the environmental effects of the main gradients of ephemeral community heterogeneity.

Results/Conclusions Shrub canopy (mainly that of Porlieria chilensis) was the most important factor for ephemeral assemblages through creation of a local patch mosaic of varying shade conditions as determined by a Plant Area Index (PAI). However, shrub influence was partially constrained by winter precipitation; in the driest years, lower water availability enhanced the effects of shrub canopy due to a general inhibition of ephemeral germination below sclerophyllous shrubs such as P. chilensis. Thus, this explains why higher winter precipitation impacts occurred under shrub canopies in wet years. Herbivore impacts are similarly dependent on shrub canopy. The effect of herbivores is greater outside than underneath canopies. Increasing winter precipitation decreased the influence of shrubs, but increased herbivore impacts. These results suggest that interactive impacts of the three factors examined simultaneously drive ephemeral species assembly via a complex but complementary series of effects in semi-arid coastal Chile. Furthermore, the final outcomes of these interactions could emerge in part from the combination of its impacts upon the presence/absence of seeds to the abundance of plants pointing to complementary ecological processes acting at different biological stages and spatial-temporal scales.