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

1.  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. 2.  Previous work suggested that B/CYDVs could mediate apparent competition between invasive exotic grasses and native bunchgrasses in California. 3.  To investigate the potential significance of virus-mediated mortality for early survivorship of bunchgrass seedlings, we compared the separate and combined effects of virus infection, competition and simulated grazing in a field experiment. We infected two species of young bunchgrasses that show different sensitivity to B/CYDV infection, subjected them to competition with three different densities of exotic annuals crossed with two clipping treatments, and monitored their growth and first-year survivorship. 4.  Although virus infection alone did not reduce first-year survivorship, it halved the survivorship of bunchgrasses competing with exotics. Within an environment in which competition strongly reduces seedling survivorship (as in natural grasslands), virus infection therefore has the power to cause additional seedling mortality and alter patterns of establishment. 5.  Surprisingly, clipping did not reduce bunchgrass survivorship further, but rather doubled it and disproportionately increased survivorship of infected bunchgrasses. 6.  Together with previous work, these findings show that B/CYDVs can be potentially powerful elements influencing species interactions in natural grasslands. 7.  More generally, our findings demonstrate the potential significance of multitrophic interactions in virus ecology. Although sometimes treated collectively as plant ‘predators’, viruses and herbivores may exert influences that are distinctly different, even counteracting

Long-term variation in the breeding diets of macaroni and eastern rockhopper penguins at Marion Island (1994–2018)

Populations of the macaroni penguin Eudyptes chrysolophus and the eastern rockhopper penguin E. filholi breeding at Marion Island (Prince Edward Islands) in the sub-Antarctic Indian Ocean decreased from 1994 to 2018. We examined their diets when rearing chicks during this period. There was substantial overlap in the diets of the two species, with crustaceans, dominated by the euphausiids Thysanoessa vicina and Euphausia vallentini, making up >80% of the diets by number and >60% by mass over the study period. The lanternfishes Krefftichthys anderssoni and Protomyctophum tenisoni were the most-commonly consumed fish in the diet of breeding macaroni penguins and dominated in their diet in three of the 25 years (1994, 1998 and 2014); they were also the most commonly consumed fish by eastern rockhopper penguins. Macaroni penguins consumed more of the amphipod crustacean Themisto gaudichaudii and the lanternfish Electrona carlsbergi as well as a greater diversity of and larger-sized fish than did eastern rockhopper penguins. The horsefish Zanclorhynchus spinifer was found in substantial amounts in the diet of eastern rockhopper penguins in 1996 and 1997. Despite annual variations in relative prey contributions to the diets, there were no significant long-term changes in the diet of either penguin species over the study period or when compared with an earlier assessment in 1982. We conclude that changes in the relative proportions of prey in the diets of these penguin species during breeding are unlikely to account for the recent declines in these penguin populations