Climate-driven diversity loss in a grassland community

Susan P. Harrison; Elise S. Gornish; Stella Copeland

doi:10.1073/pnas.1502074112

Edited by Jonathan M. Levine, ETH Zurich, Switzerland, and accepted by the Editorial Board May 26, 2015 (received for review February 4, 2015)

Significance

Whereas a dominant conservation paradigm proposes that species are being lost from ecological communities with a consequent loss of ecosystem function, recent analyses have concluded there is no globally consistent trend toward lower community diversity. In a study of Californian grassland communities, we show that 15 years of climatic drying—consistent with the forecasts for this and other semiarid regions under climate change—have led to directional losses of plant species richness, especially of native annual forb (“wildflower”) species with traits indicative of low drought tolerance. Although many anthropogenic impacts may increase or not affect community diversity, our result underlines that declining plant community diversity may be especially likely in climates that are becoming more arid and less productive.

Abstract

Local ecological communities represent the scale at which species coexist and share resources, and at which diversity has been experimentally shown to underlie stability, productivity, invasion resistance, and other desirable community properties. Globally, community diversity shows 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 documented declining plant diversity from 2000 to 2014 at both the local community (5 m²) and landscape (27 km²) scales, across multiple functional groups and soil environments. Communities became particularly poorer in native annual forbs, which are present as small seedlings in midwinter; within native annual forbs, community composition changed toward lower representation of species with a trait indicating drought intolerance (high specific leaf area). Time series models linked diversity decline to the significant decrease in midwinter precipitation. Livestock grazing history, fire, succession, N deposition, and increases in exotic species could be ruled out as contributing causes. This finding is among the first demonstrations to our knowledge of climate-driven directional loss of species diversity in ecological communities in a natural (nonexperimental) setting. Such diversity losses, which may also foreshadow larger-scale extinctions, may be especially likely in semiarid regions that are undergoing climatic trends toward higher aridity and lower productivity.

Footnotes

↵¹To whom correspondence should be addressed. Email: spharrison{at}ucdavis.edu.

Author contributions: S.P.H. designed research; S.P.H. and S.C. performed research; S.P.H., E.S.G., and S.C. analyzed data; and S.P.H., E.S.G., and S.C. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.M.L. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1502074112/-/DCSupplemental.