DIRECT IN SITU MEASUREMENT OF RESOURCE COMPETITION BY PLANTS ALONG ENVIRONMENTAL GRADIENTS

在任何生物群落中（如森林、珊瑚礁、鸟群），个体必须与其他个体竞争食物、空间和配偶。那些能够比竞争对手获得更多的人往往会留下更多的后代。它们最终会以牺牲竞争能力较弱个体的后代为代价，主宰整个群体。这是查尔斯·达尔文的自然选择进化论的一个关键特征。他称之为“生存斗争”。了解这种斗争是如何进行的，是我们理解自然界的基础。生态学家非常重视个体之间的竞争，认为这可能解释群落构成的模式，也是决定进化成功的一个因素。尽管观察和衡量实际竞争是非常困难的，通常是不可能的。相反，生态学家依靠各种间接测量（不同密度下动物种群增长率的变化；有邻居和没有邻居的植物生长的变化；等等）来推断竞争相对于其他因素的重要性。例如，这种间接方法的多样性是对竞争在植物群落中的作用没有普遍共识的原因之一。分歧几乎没有得到解决的迹象，需要一种新的方法来直接衡量工厂之间的竞争。本项目将采用这种方法。植物竞争光、水或养分。我们可以用其中任何一种来表示竞争，但我们选择看一种基本营养物质，氮(N)。使用氮的好处是植物需要大量的氮，而土壤（大多数植物从土壤中获取营养）通常含有的氮太少。这意味着在自然条件下，植物通常会竞争氮，至少在它们生长周期中生长最快、最需要氮的那一部分（英国的春末夏初）。使用N的另一个好处是，它以两种不同的稳定同位素存在，正常的“轻”氮14N和罕见的“重”氮15N。我们将人为地将植物利用的土壤氮素形态中铵态氮和硝态氮的含量增加到已知的量。通过测量这个量，以及在土壤上生长的植物的15N含量，我们将能够计算出植物吸收了多少铵和硝酸盐。如果我们在相同的土壤上种植植物，我们将使用这种方法，第一次能够测量植物之间的实际竞争。通过使用相同或不同物种的植物组合，单独的植物和混合的植物，以及通过给土壤施肥来控制可用的氮量，我们将能够测试我们对竞争的直接测量是如何受到植物邻居的影响的，同样重要的是，竞争（氮吸收）是如何影响邻居的。和通过比较测量条件下有利于植物生长与衡量经济增长可能会受到严重的环境条件,我们可以看看竞争作为生态过程的重要性取决于在哪里发生,许多生态学家们希望看到问题解决。我们计划在自然环境梯度的两端进行所需的大型实验来测试这些想法，从阿伯丁接近海平面的温和地点到苏格兰高地布雷马尔更严重、海拔更高的地点。我们将使用一种常见的草，鸭蹄草（Dactylis glomerata）和车前草（Plantago lanceolata）作为我们的试验物种。两个实验将涉及在两个地点的户外种植这些植物，但在盆栽中，这样我们就可以控制土壤条件和邻近个体的密度。我们还计划进行一个包括这些物种的田间试验，看看盆栽试验的结果是否可以预测在更自然的条件下可能发生的情况。

In any community of living organisms (e.g., a forest, coral reef, bird colony), individuals must compete with others for food, space and mates. Those able to obtain more than their rivals tend to leave more offspring. These can eventually dominate the community at the expense of the descendants of less competitive individuals. This is a key feature of Charles Darwin's theory of evolution by natural selection. He called it the 'struggle for existence'. Understanding how this struggle works is fundamental to our understanding of the natural world. Ecologists attach great importance to competition between individuals as a possible explanation for the patterns seen in the composition of communities and as a factor that determines evolutionary success. This is despite the fact that it is very difficult, often impossible, to observe and measure actual competition as it happens. Instead ecologists rely on a variety of indirect measurements (changes in animal population growth rate at different densities; changes in the growth of plants with and without neighbours; and so on) to infer the importance of competition relative to other factors. This variety of indirect approaches is one reason why there is no general agreement about the role of competition in plant communities, for example. The disagreements show little sign of being resolved, and a new approach is needed that allows plant competition to be measured directly. This project will use such an approach. Plants compete for light, water or nutrients. We could use any of these to indicate competition, but we have chosen to look at one essential nutrient, nitrogen (N). The beauty of using N is that plants need a lot of it and soils (from where most plants get their nutrients) usually contain too little of it. This means that plants are often likely to compete for N under natural conditions, at least during that part of their growth cycle when they are growing most rapidly and need most N (late Spring/early Summer in the UK). Another benefit of using N is that it exists as two distinct stable isotopes, normal 'light' N, 14N, and rare 'heavy' form, 15N. We will artificially enrich the 15N content of the soil N forms that plants use, ammonium and nitrate, by a known amount. By measuring that amount, and the resulting 15N content of the plants growing on that soil, we will be able to calculate how much of the ammonium and nitrate the plants have taken up. If we grow plants together on the same soil we will, using this method, be able for the first time to measure actual competition between plants. By using combinations of plants of the same or different species, isolated plants as well as mixtures, and by manipulating the amount of N available by fertilising the soil, we will be able to test how our direct measure of competition is influenced by a plant's neighbours and, equally important, how competition (N uptake) influences the neighbours. And by comparing measurements made under conditions favourable for plant growth with those measured where growth is likely to be hampered by severe environmental conditions, we will be able to see if the importance of competition as an ecological process depends on where it happens, a question that many ecologists would like to see resolved. We plan to run the large experiments needed to test these ideas at either end of a natural environmental gradient, from a benign site close to sea level at Aberdeen to a more severe, higher altitude site at Braemar in the Scottish Highlands. We will use a common grass, cocksfoot (Dactylis glomerata), and ribwort plantain (Plantago lanceolata) as our test species. Two experiments will involve growing these plants outdoors at the two sites, but in pots so that we can control soil conditions and the densities of neighbouring individuals. We also plan a field experiment which will include these species to see if the results from the pot experiments can predict those likely to occur under more natural conditions.