1. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
2. Department of Biological Sciences, Imperial College at Silwood Park, Ascot, Berkshire SL5 7PY, UK
3. Institute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK
4. Present address: Department of Infectious Disease Epidemiology, Imperial College of Science Technology and Medicine, Norfolk Place, London W2 1PG, UK

Correspondence to: T. Coulson² Email: t.coulson@imperial.ac.uk

Abstract

Large-scale climatic indices such as the North Atlantic Oscillation¹ are associated with population dynamics², variation in demographic rates³ and values of phenotypic traits^{4, 5} in many species. Paradoxically, these large-scale indices can seem to be better predictors of ecological processes than local climate^{5, 6, 7, 8}. Using detailed data from a population of Soay sheep^{9, 10}, we show that high rainfall, high winds or low temperatures at any time during a 3-month period can cause mortality either immediately or lagged by a few days. Most measures of local climate used by ecologists fail to capture such complex associations between weather and ecological process, and this may help to explain why large-scale, seasonal indices of climate spanning several months can outperform local climatic factors. Furthermore, we show why an understanding of the mechanism by which climate influences population ecology is important. Through simulation we demonstrate that the timing of bad weather within a period of mortality can have an important modifying influence on intraspecific competition for food, revealing an interaction between climate and density dependence¹¹ that the use of large-scale climatic indices or inappropriate local weather variables might obscure.

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