Declining diversity in response to intensifying midwinter drought in a Californian grassland

Background/Question/Methods

Local ecological communities represent the scale at which species coexist and share resources, and at which diversity has been experimentally shown to underlie stability and other community properties.  Globally, local community diversity has shown a mixture of increases and decreases over recent decades, and these changes have relatively seldom been linked to climatic trends.  In a heterogeneous California grassland, we measured plant diversity from 2000-2014 at both the local community (5 m²) and landscape (27 km²) scales, across multiple functional groups and soil environments.  We also measured climatic trends, focusing on precipitation in midwinter (Dec-Feb), the time when annual plants are present as small seedlings.  We used simple regressions to test for temporal trends in grassland diversity and climate, and an autoregressive time series model to link changes in diversity to changes in midwinter precipitation.

 Results/Conclusions

Grassland species richness declined significantly over the 15-year period at both local and landscape scales. Within the most numerous and rapidly declining group, native annual forbs, communities disproportionately lost species with high specific leaf area, an indicator of drought intolerance. Time-series models directly linked the loss of species to the significant long-term decline in midwinter precipitation, which was also accompanied by declines in cloudiness and humidity. Additional evidence ruled out other potential causes of diversity loss, such as grazing, fire, N deposition, invasion, and the recent 2013-2014 drought. This study is among the first demonstrations of climate-driven directional loss of species diversity in ecological communities in a natural (non-experimental) setting. Such losses, which may also foreshadow larger-scale extinctions, may be especially likely in semiarid regions that are undergoing further aridification