Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Virus infection and grazing exert counteracting influences on survivorship of native bunchgrass seedlings competing with invasive exotics.

Abstract

Invasive annual grasses introduced by European settlers have largely displaced native grassland vegetation in California and now form dense stands that constrain the establishment of native perennial bunchgrass seedlings. Bunchgrass seedlings face additional pressures from both livestock grazing and barley and cereal yellow dwarf viruses (B/CYDVs), which infect both young and established grasses throughout the state. Previous work suggested that B/CYDVs could mediate apparent competition between invasive exotic grasses and native bunchgrasses in California. To investigate the potential significance of virus-mediated mortality for early survival of bunchgrass seedlings, we compared the separate and combined effects of virus infection, competition and simulated grazing in a field experiment conducted in Yolo County, California, USA. We infected 2 species of young bunchgrasses (Nassella pulchra and Elymus multisetus) that show different sensitivity to B/CYDV infection, subjected them to competition with 3 densities (0, 5000 and 28 000 seeds/m2) and of exotic annuals (Bromus hordeaceus) crossed with 2 clipping treatments, and monitored their growth and first-year survival. Although virus infection alone did not reduce first-year survival, it reduced the survival of bunchgrasses competing with exotics by 50%. Within an environment in which competition strongly reduces seedling survivorship (as in natural grasslands), virus infection, therefore, can cause additional seedling mortality and alter patterns of establishment. Clipping did not reduce bunchgrass survival further, but increased it by 2-fold it and disproportionately increased the survival of infected bunchgrasses. These findings showed that B/CYDVs can be potentially powerful elements influencing species interactions in natural grasslands. More generally, our findings demonstrated the potential significance of multitrophic interactions in virus ecology.