Did British breeding birds move north in the late 20th century?

Abstract

Contemporary climate change is the biggest experiment ever conducted by humans on a planetary scale, and its impact on the redistribution of life is potentially huge (e.g., Barnosky et al. Nature 471:51–57, 2011, Pereira et al. Science 330:1496–1501, 2010). An accurate diagnosis of the effects of climate change on the distributions of species requires, firstly, that methods used for detection of distributional changes are able to distinguish between directional and non-directional changes and, secondly, that they are able to tease apart distributional changes driven by natural population dynamics from changes driven by external forcing (climatic or non-climatic). We ask how appropriate are methods commonly used to detect directional shifts on species range changes. We compare a widely used range-shift detection method previously used to demonstrate that climate change caused British breeding bird distributions to move northwards with alternative approaches that more comprehensively examine directionality in range changes. We find that once range dynamics are examined across all geographical quadrants in Britain, and in contrast with previous reports, no clear directional patterns of range shift emerge for this period. Some of the methods typically used for examining species range shifts are prone to false positive errors, whereby directional range shifts are detected when in fact they did not occur. Without entering the discussion of what is more important to avoid (false negative errors, whereby directional range shifts pass unnoticed by analysis, or false positive errors), we argue that methods exist to determine whether range changes are directional or non-directional (a prerequisite to discern the causes of range changes).