Precipitation alters interactions among multiple trophic levels in a semi-arid food web
Precipitation is a key abiotic factor that can strongly affect the diversity and productivity of plant communities and higher trophic levels. Climate change is altering precipitation patterns, but the consequences for the functioning ecosystems are not yet fully understood. In semi-arid grassland ecosystems in particular, changes in precipitation may have strong impacts on food webs through both direct and indirect species interactions. Here we seek to understand how precipitation levels affect the functioning of food webs by studying an ‘endangered’ semi-arid grassland ecosystem in central California. From 2007 to 2012, we monitored soil properties, native and exotic plant composition and biomass, invertebrate diversity and biomass, lizard abundance, and rodent density in 30 140x140m² sites in the Carrizo Plain National Monument. We use structural equation modeling to estimate interaction strengths among species groups, and we divided our dataset into dry (2007-2009, 2012) and wet (2010-2011) years to investigate the effects of major changes in precipitation regimes on these interactions.
All species groups responded strongly to annual variation in precipitation. However, the functioning of the food web differed substantially in dry versus wet years. In dry years, precipitation levels favored forbs and in turn rodent density, whereas wet years favored grasses and negatively affected lizard density. Precipitation altered the importance of bottom-up and top-down forces, with relatively strong interactions among plants and invertebrates in dry years and relatively strong interactions involving higher trophic levels in wet years. Our findings highlight that precipitation can alter food webs via direct and indirect ecological pathways. This study advances our general understanding of how climate change may affect the functioning of food webs, and results may assist native and endangered species conservation efforts in the Carrizo Plain National Monument.