Studies have documented climate change–induced shifts in species distributions but uncertainties associated with data and methods are typically unexplored. We reviewed 240 reports of climate-related species-range shifts and classified them based on three criteria. We ask whether observed distributional shifts are compared against random expectations, whether multicausal factors are examined on equal footing, and whether studies provide sufficient documentation to enable replication. We found that only ~12.1% of studies compare distributional shifts across multiple directions, ~1.6% distinguish observed patterns from random expectations, and ~19.66% examine multicausal factors. Last, ~75.5% of studies report sufficient data and results to allow replication. We show that despite gradual improvements over time, there is scope for raising standards in data and methods within reports of climate-change induced shifts in species distribution. Accurate reporting is important because policy responses depend on them. Flawed assessments can fuel criticism and divert scarce resources for biodiversity to competing priorities.
Species are reportedly shifting their distributions poleward and upward in several parts of the world in response to climate change. The extent to which other factors might play a role driving these changes is still unclear. Land‐cover change is a major cause of distributional changes, but it cannot be discarded that distributional dynamics might be at times caused by other mechanisms (e.g. dispersal, ecological drift). Using observed changes in the distribution of 82 breeding birds in Great Britain between three time periods 1968–72 (t1), 1988–91 (t2) and 2007–2011 (t3), we examine whether observed bird range shifts between t1‐t2 and t1‐t3 are best explained by climate change or land‐cover change, or whether they are not distinguishable from what would be expected by chance. We found that range shifts across the rear edge of northerly distributed species in Great Britain are best explained by climate change, while shifts across the leading edge of southerly distributed species are best explained by changes in land‐cover. In contrast, at the northern and southern edges of Great Britain, range dynamics could not be distinguished from that expected by chance. The latter observation could be a consequence of boundary effects limiting the direction and magnitude of range changes, stochastic demographic mechanisms neither associated with climate nor land‐cover change or with complex interactions among factors. Our results reinforce the view that comprehensive assessments of climate change effects on species range shifts need to examine alternative drivers of change on equal footing and that null models can help assess whether observed patterns could have arisen by chance alone.
Background: 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. Main: 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. Conclusions: 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 nondirectional (a prerequisite to discern the causes of range changes). Background
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