Do silica-based defences drive plant-herbivore dynamics?

了解推动动物种群数量变化的因素是生态学的基础。许多食草动物种群在丰度上表现出规律的振荡，即所谓的周期，通常认为这是由于捕食者丰度的匹配振荡，而不是草食动物的食物植物的任何变化。食物质量被认为不会以一种可能导致循环的方式对食草动物做出反应，但我们已经发现了一种新的方式，即植物质量的变化可能会导致食草动物种群的循环。这一机制以前没有被考虑过，但它可以适用于广泛的植物-草食系统。我们的新想法被称为二氧化硅诱导假说。持续的重度放牧会导致草中二氧化硅水平的增加，因此食草动物随后会经历营养可获得性的降低。这降低了它们的生长和繁殖率，从而减缓了第二年的人口增长速度。最终，种群数量下降到只有少量草食的水平，因此树叶中的二氧化硅水平也会下降，因为产生的树叶防御较差。食草动物再次能够获得草中的营养，它们的生长和繁殖再次增加。我们相信这种机制可以在许多植物-草食系统中运行，特别是以草和其他含有高水平二氧化硅的植物为基础的系统。硅是地球上第二丰富的矿物，在所有植物中都大量存在，所以我们提出的机制具有广泛的相关性和重要意义。我们已经从实验室实验和实地观察中获得了一些支持我们想法的证据。在这个项目中，我们的目标是第一次在大规模的野外实验中测试这一潜在的机制。首先，我们将测定田鼠数量多的地区的草中二氧化硅水平，并将其与田鼠数量低的地区的二氧化硅水平进行比较。如果我们的想法是正确的，那么在田鼠数量增加的地区，二氧化硅水平应该会下降，反之亦然。然后我们将建立一个实验来测量不同放牧水平下二氧化硅增加的速度和幅度，我们还将测量二氧化硅防御水平下降的速度。然后，我们将通过将田鼠转移到我们之前诱导高二氧化硅水平的地区来测试我们的想法，并看看在这些地区喂养对它们的生长和繁殖有何影响。这些实验将评估植物防御系统的变化是否会导致草食动物丰度的变化，并帮助我们更好地了解草与其草食动物之间的相互作用。有许多重要的草原系统支持广泛的草食动物，包括稀有物种和牲畜，因此该项目将对保护和可持续农业有用。

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.