Rapid in situ Adaptation to Changed Precipitation in Annual Plants Species

Abstract

The ability of species populations to adapt to high rates of environmental changes is important for their persistence, especially under severe decrease in precipitation as predicted for the Mediterranean Basin due to global warming. The question, if adaptation occurs fast enough is important for the assessment of the impact that climate change has on biodiversity. In a Mediterranean and a semi-arid site in Israel, the annual rainfall was manipulated in situ (+/- 30% precipitation), corresponding to climate projections. Thus, our experiments provide a realistic scenario to study adaptive processes in species populations within their communities. In the 8th year of manipulation, a set of common annual plant species was used to measure life-history traits such as phenology, vegetative growth and reproduction related traits. In the field, we studied the direct impact of increased and decreased rainfall on phenotypic variation and subsequent greenhouse experiments allowed us to distinguish between plastic response and genetic adaptation. The trends between the sites served as a proxy to predict adaptive responses to water availability. Accordingly, we expected that a reduction in precipitation will lead to reduced vegetative growth associated with earlier flowering, and higher reproductive allocation. Results from the greenhouse experiments revealed inherited trait shifts in five out of eight studied species. The measurement of trait variation under a greenhouse irrigation gradient demonstrated that higher reproductive allocation and smaller diaspores contributed to higher fitness under drought. The immediate response to changes in water availability in the field, however, resulted in trait variation that did not resemble the trends between sites, and instead, could be attributed to stress tolerance strategies. Our results show that in situ adaptation occurred after only few years of precipitation changes, and indicate that the predicted changes due to global warming can cause a severe selection pressure. The finding of inherited shifts in half of the studied species demonstrates the importance of rapid adaptation for the species diversity of these communities.