Grasses bite back
How plant defences can affect animal populations.
The player will show in this paragraph
Sue Hartley
Professor of Ecology
Global food stocks are at their lowest for decades and agricultural production is falling in the face of climate change and other pressures.
We depend on grasses and grazing animals for food, so there has never been a more important time to understand the interactions between grasses and the animals that feed on them. Our research at Sussex is reassessing how grasses defend themselves against grazing animals, including insect pests.
‘Sheep may safely graze’, thought Johann Sebastian Bach, and what could be more harmless than grass? But eating grass is trickier than it appears. Grasses contain silica taken up from the soil and deposited in their leaves. Our studies provide the first evidence that silica has wide-ranging impacts on many herbivores, including voles and sheep. It deters feeding and reduces their ability to extract nutrients from food, slowing their growth and reproduction. Some herbivores even lose weight when fed high-silica diets.
Vole population growth is predominantly determined by the growth rate and body mass of females. If silica reduces these, it will have a major effect on vole abundance. Could silica defences be the key to understanding what causes the cyclic fluctuations in grass-feeding mammal populations? If so, this would be a major scientific advance: for eight decades, ecologists have tried to understand what causes these cycles.
But to influence population cycles, the effect of silica must increase when vole populations increase – in order to drive them down – and decrease when populations are low – to allow them to recover. Could silica defences work in this way? Our studies discovered that silica is an inducible defence, one whose levels are dramatically increased after damage by herbivores. Silica levels in grasses grazed by voles quadrupled compared with those in undamaged plants. Grasses clipped with scissors did not respond in the same way, suggesting that a specific cue from vole feeding elicits the response. Significantly, we found the silica induction response only when plants suffered intense grazing over many months – the sort of time period required to produce cycles.
We propose that changing silica levels, in response to damage by grazing, may play a role in driving vole population cycles. During years of rapid population growth, grazing intensity increases, causing the induction of silica defences. The increased silica levels reduce the growth and reproduction of female voles and population growth slows. As the population declines, so does the grazing pressure, leading to a relaxation of silica defences and a population recovery. Our initial observations support these ideas: silica levels are twice as high in areas where vole populations are beginning to decline than they are in areas where populations are increasing.
Our research isn’t just relevant to ecologists. If we can understand how grazing animals can be affected by the food that they eat, and how grasses can become more resistant to pests, we may help to combat falling food stocks and the decline in agricultural production.
