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Testing intra-site transfer functions: an example using chironomids and water depth |
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| Authors: | Gaute Velle Richard J Telford Oliver Heiri Joshua Kurek H John B Birks |
| Institution: | 1. Bergen Museum, University of Bergen, Post Box 7800, 5020, Bergen, Norway 2. Uni Environment, Uni Research, Thorm?hlensgt. 49b, 5006, Bergen, Norway 3. Department of Biology, University of Bergen, Post Box 7803, 5020, Bergen, Norway 4. Bjerknes Centre for Climate Research, All??gaten 55, 5007, Bergen, Norway 5. Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern, Altenbergrain 21, 3013, Bern, Switzerland 6. Department of Biology, Queen??s University, 116 Barrie St., Kingston, ON, K7L 3N6, Canada 7. School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK 8. Environmental Change Research Centre, University College London, London, WC1E 6BT, UK |
| Abstract: | Most calibration data sets used to infer past environmental conditions from biological proxies are derived from many sites. An alternative strategy is to derive the calibration data set from within a single site. Transfer functions derived from such intra-site calibration data sets are usually applied to fossil assemblages from the focal lake, but a recent development has been to apply these transfer functions to other sites. Transfer functions derived from intra-site calibration data sets can have impressive cross-validation performance, but that gives little indication of their performance when applied to other sites. Here, we develop transfer functions for lake depth from intra-lake chironomid calibration data sets in Norway and Alaska and test the resulting models by cross-validation and against known depth in external lakes. Lake depth is a statistically significant predictor of chironomid assemblages at all these lakes, and most intra-lake transfer functions perform reasonably well under cross-validation, but their performance against external data is erratic. Downcore reconstructions from transfer functions developed on different lakes are dissimilar. Ignoring the poorly performing transfer functions, only 3 of 14 downcore reconstructions are statistically significant. Few assemblages downcore had good modern analogues in the calibration data set, even when the core was from the same lake as the calibration data set. We conclude that intra-site calibration data sets can find site-specific rather than general relationships between species and the environment and thus should be applied with care and to external sites only after careful and critical validation. |
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