California annual grass invaders: the drivers or passengers of change?

Abstract

The dominance of invasive species is often assumed to reflect their competitive superiority over displaced native species. However, invasive species may be abundant because of their greater tolerance to anthropogenic impacts accompanying their introduction. Thus, invasive species can either be the drivers or passengers of change.We distinguish between these two possibilities in California grasslands currently dominated by Mediterranean annuals (exotics) and subjected to livestock grazing since European settlement. We focused on native annual grasses and forbs, an understudied species-rich component of the California flora, and Mediterranean annual grasses, currently dominant and among the first non-native plants introduced to the area.We established a field experiment with fenced and unfenced blocks in a cattle pasture. We measured concentrations of limiting resources (nitrogen, phosphorus, light and soil moisture) in monoculture plots as an index of competitive ability (i.e. R*). We then quantified grazing impacts on biomass and seed production in grazed vs. ungrazed monoculture plots. Finally, we measured biomass and seed production of each species competing in mixture plots, in the presence and absence of grazers.We found that native and exotic species did not differ in R* indices of competitive ability, i.e. concentrations of limiting resources in ungrazed native monoculture plots did not differ from concentrations in ungrazed exotic monoculture plots. By contrast, exotic annuals suffered less from grazing than native annuals, perhaps reflecting their longer evolutionary history with cattle grazing. Consistent with these results, native and exotic annuals were equally abundant in ungrazed mixtures, but exotic species overwhelmingly dominated grazed mixtures.Species able to draw down nitrogen and light to lower levels in monocultures (i.e. those with lower R* values) dominated biomass and seeds in mixed plots without grazers. However, R* did not predict the relative abundance of species in grazed plots. Moreover, the relative abundance of species in mixtures did not correlate with grazing impacts on their monocultures, implying that grazing alters inter-specific competitive dynamics.Synthesis. We demonstrate that the displacement of native annuals by Mediterranean annual grasses in California may largely have been driven by cattle grazing.

Trade‐off between early emergence and herbivore susceptibility mediates exotic success in an experimental California plant community

“…Our results suggest that, in areas characterized by rapid exotic emergence, grazing during the earliest period of the growing season is likely to be another effective management strategy for improving native performance relative to exotic competitors. This is consistent with findings of studies in California showing that grazing in the winter/spring (early season) leads to greater decreases in the abundance of early‐active exotics (Skaer et al., 2013), whereas grazing applied in the spring/summer (late season), at which point many exotic species are no longer active, leads to greater negative impacts on native abundance (Hille Ris Lambers et al., 2010). However, early‐season grazing is unlikely to be effective where exotic species do not benefit from priority effects (Funk, Hoffacker, & Matzek, 2015).…”

Incidence of viruses and virus-like diseases of watermelons and pumpkins in Uganda, a hitherto none-investigated pathosystem

“…Shortlived plant hosts of vectored plant viruses attract and amplify vector populations, and subsequent high virus incidence in long-lived hosts. A classic example is demonstrated under a natural ecosystem by introducing annual grass species that have overturned the dominance of perennial native bunchgrasses in California grasslands via increased infection with aphid-borne Barley/Cereal yellow dwarf viruses (Malmstrom et al, 2005;Borer et al, 2007;HilleRisLambers et al, 2010). Also, feeding preferences or feeding behaviour of insect vectors of plant viruses can be altered after exposure to infected plants and acquisition of virus (Stafford et al, 2011;Ingwell et al, 2012;MorenoDelafuente et al, 2013;Rajabaskar et al, 2014).…”

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

“…However, because exotics usually establish under disturbed conditions where resource availability tends to be higher, it is difficult to determine if the exotics themselves are causing diversity decline (i.e., are drivers), or if exotics are associated with conditions such as disturbance and high resource availability that reduce diversity (i.e., are passengers, Seabloom et al 2003;MacDougall and Turkington 2005;Hille Ris Lambers et al 2010;Price et al 2011). A meta-analysis of trait differences between native and non-native species showed that non-natives tend to have traits that are associated with rapid aboveground growth (Leishman et al 2007).…”