Do silica-based defences drive plant-herbivore dynamics?

Lead Research Organisation: University of Sussex
Department Name: Sch of Life Sciences

Abstract

Understanding the factors that drive changes in the abundance of animal populations is fundamental to ecology. Many herbivore populations show regular oscillations in abundance, known as cycles, and these are usually thought to be due to matched oscillations in the abundance of predators rather than any changes in the herbivore's food plants. Food quality is not thought to respond to herbivory in a way which could lead to cycles, but we have discovered a novel way in which changes in plant quality could cause cycles in herbivore populations. This mechanism has not been considered before but it could apply to wide range of plant-herbivore systems. Our new idea is called the silica induction hypothesis. Periods of sustained heavy grazing lead to an increase in the levels of silica in grasses, so herbivores subsequently experience reduced availability of nutrients. This reduces their growth and reproductive rate and hence slows down the rate of population growth in the following year. Eventually populations fall to a level where there is only low grazing on the grasses, so the levels of silica in the leaves also fall because less well-defended leaves are produced. Herbivores are once again able to access nutrients in the grasses and their growth and reproduction increase again. We believe this mechanism can operate in many plant-herbivore systems, particularly ones based on grasses and other plants that contain high levels of silica. Silicon is the second most abundant mineral on earth and present in significant amounts in all plants, so the mechanism we propose is of wide relevance and significance. We already have some evidence from laboratory experiments and observations in the field that support our idea. In this project we aim to test this potential mechanism for the first time in large-scale field experiments. Firstly, we will determine the silica levels in grasses in areas where vole populations are high and compare them with those in areas where vole populations are low. If our ideas are correct, silica levels should be declining in areas where vole populations are increasing and vice versa. We will then set up an experiment to measure the rate and magnitude of the increase in silica at different levels of grazing and we will also measure how quickly the levels of silica defences decrease. Then we will test our ideas by moving voles into areas where we have induced high silica levels previously and see how feeding in these areas affects their growth and reproduction. These experiments will assess whether changes in plant defences can cause changes in herbivore abundance and help us develop a better understanding of the interactions between grasses and their herbivores. There are many important grassland systems that support a wide range of herbivores, including both rare species and livestock, so this project will be useful to both conservation and sustainable agriculture.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
NE/F003137/1 06/10/2008 05/11/2010 £309,351
NE/F003137/2 Transfer NE/F003137/1 05/11/2010 05/07/2012 £114,126
 
Description Understanding the factors that drive changes in species abundance is fundamental to biology and the role that plant-based factors play in the population dynamics of mammalian herbivores has been the subject of much debate. Silica has been proposed as the primary defence in grasses and is thought to lead to increased abrasiveness of foliage so deterring feeding, as well as reducing foliage digestibility and herbivore performance. However, at present there is little direct experimental evidence to support these ideas. We tested the effects of manipulating silica levels on the abrasiveness of grasses and on the feeding preference and growth performance of field voles, specialist grass-feeding herbivores. Elevated silica levels did increase the abrasiveness of grasses and deterred feeding by voles. We also demonstrated, for the first time, that silica reduced the growth rates of both juvenile and mature female voles by reducing the nitrogen they could absorb from the foliage. Furthermore, we found that vole feeding leads to increased levels of silica in leaves, suggesting a dynamic feedback between grasses and their herbivores. We propose that silica induction due to vole grazing reduces vole performance and hence could contribute to cyclic dynamics in vole populations.
We also found evidence that in grasses induction of physical defences is both specific to herbivore feeding, as opposed to mechanical damage, and is be dependant on the amount of damage imposed. Furthermore, we showed that the magnitude of the induction response is sufficient to deter further damage and affect herbivore performance. We compared silica induction in two grass species in response to vertebrate and invertebrate damage, and to mechanical defoliation. Induction was assessed at two levels of damage over 16 months. Foliar silica content did not increase in response to mechanical defoliation, but damage by either voles or locusts resulted increases in silica content of over 400%. This increase deterred feeding by both voles and locusts. Silica induction in grasses due to repeated damage events over a prolonged period supports our previous suggestion for a possible role for silica defence in the cyclical population fluctuations observed in many grass-feeding herbivores.
We then discovered more evidence for a novel mechanism by which herbivore-induced reductions in plant quality alter herbivore life history parameters and subsequent population growth. We tested the effect of high silica levels on the population growth and individual performance of voles reared on their winter food plant, a grass called Deschampsia caespitosa. In sites where vole population density was high, silica levels in D. caespitosa leaves collected several months later were also high and vole populations subsequently declined; in sites where vole densities were low, levels of silica were low and population density increased. High silica levels in their food reduced vole body mass by 0.5% a day. Again our findings led us to argue that silica-based defences in grasses may play a key role in driving vole population cycles.
Exploitation Route The novel findings on the impact of changes in plant quality (in terms of silicon defences) on the population of small mammals will be an important addition to the understanding of the factors controlling the abundance of small mammals, some of which are rare or declining species.
Sectors Environment

 
Description Publications have been cited by other academic researchers