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

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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.