Analysis and Interpretation of Long-Term Studies Investigating Responses to Climate Change

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Publisher Summary

This chapter discusses potential problems inherent to the interpretation of long-term data and, in particular, how these might serve as an explanation of heterogeneity in documented responses to climate change. These problems range from the choice of variables, to problems of sampling and analysis, and problems of interpretation of potential mechanisms and assumptions underlying the inference drawn. The aim is not to be critical of particular studies or methods employed, but rather to present a broad overview of the problems that may be encountered in observational long-term studies of wild bird populations. The chapter presents a number of methodological considerations. These include questions of which phenological and climatic variable to use, the effects of differences in capture probability on estimates of phenotypic variables, and questions about representativeness of the data collected. It also discusses ways to distinguish between change in phenotype due to phenotypic plasticity and genetic change. In addition, the chapter mentions that gene flow may cause a temporal change in phenotype by yet another mechanism.

Section snippets

Summary

Effects of recent climate change in mean phenotypic values of different traits in wild bird populations are already apparent, but we are still largely ignorant about the mechanism underlying these changes. Likewise, numerous methodological issues complicating the interpretation of the observed patterns have emerged, but few of these have been widely recognised. Here, we review some of these problems inherent to long-term studies of wild bird populations in the context of understanding climate

Choice of Phenological Variables

Many studies have investigated first arrival date, first breeding date or other measures of earliness in terms of phenology. A smaller number of studies have investigated means, medians or other measures of centrality of frequency distributions. The approach of using first dates has a number of potential problems. First, with only a single observation contributing to the estimate, sampling errors for first dates are considerably larger than for means or medians. Second, measures of first dates

Interpretational Considerations

Three different mechanisms may give rise to directional change in time in the mean phenotype of a population. These include (1) phenotypic plasticity, (2) micro-evolutionary change and (3) gene flow from genetically distinct populations (Hoffmann and Parsons, 1997). All these mechanisms, which are not mutually exclusive, may cause a shift in the mean phenotype of a population and serve as a means of coping with climate change (Hoffmann and Parsons, 1997). Consequently, our understanding of

Conclusions and Further Directions

In our treatment, we have focused on how the patterns seen in analyses of long-term time series can only be understood when pattern and process are considered simultaneously. Thus, in order to understand causes and consequences of climate change on bird populations, we need not only high quality long-term data sets but also rigorous analytical protocols and methods to account for possible biases in the data. Likewise, our understanding of the mechanisms underlying observed changes (or lack

Acknowledgements

P. Berthold, W. Fiedler and two reviewers kindly provided constructive comments on earlier version of this manuscript. J. M. has been supported by Academy of Finland.

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