Grasses have a secret weapon against grazing that packs a surprisingly powerful punch.
Grass can defend itself using tiny nodules of silica 鈥� and a field study of voles suggests that the silica could be causing rodent populations to boom and bust. While predators are known to drive population cycles, the new study suggests that plants can have a similarly drastic effect.
Earlier studies had shown that grasses could form deposits of silica in their leaves, apparently as a response to grazing. Voles seem to dislike eating grasses with these deposits, and the defence gets to them in other ways too 鈥� it makes them lose weight, apparently by inhibiting their digestion of proteins in the grass.
Advertisement
Important defence
鈥淧eople suspected that silica might be an important defense, but didn鈥檛 think that it was able to drive the population cycles,鈥� says study leader of the University of Sussex, Brighton, UK.
鈥淭his is the first evidence from field experiments that silica defence and vole populations might be linked,鈥漵he says.
Hartley and colleagues from the University of Aberdeen, UK, have been studying a population of voles () that live in Kielder Forest, in northern England. During the winter, the voles鈥� main food source is tufted hairgrass ().
When the number of voles was high, or had been high the previous year, the silica levels in the plants were also high 鈥� apparently a response to more grazing, Hartley鈥檚 team found. But when the voles鈥� numbers had been low before, and grazing was at low levels, the plants had less silica.
Boom and bust
Eating high-silica grass reduced vole body mass by 0.5% per day, an effect that the researchers argue could drive population cycles.
It isn鈥檛 an instant defence, Hartley says, since it seems to take several months for the grasses to grow the deposits. Nor is it a constant defence.
鈥淚t must be quite costly for the plant to extract the silicon from the soil and deposit it [in the leaves],鈥� which would make them less competitive against other grasses not eaten by the voles, Hartley says. 鈥淭he grass wouldn鈥檛 want to be on maximum alert all the time.鈥�
Other populations of voles may go through booms and busts due to predators, but in England the plants鈥� defence could be driving the cycles.
Journal reference: (DOI: 10.1098/rsbl.2008.0106)
