Misconceptions about analyses of Australian seaweed collections

Abstract:

One of the greatest impediments to detecting changes in species distributions in response to ocean warming is the lack of baseline data. In a recent article, we compared old (1940–1959) and new (1990–2009) herbarium records of Australian seaweeds and found a net southward shift in the latitude of northernmost collections of temperate species, implying a flora-wide poleward retreat over the past five decades. Huisman & Millar (2013) criticised our methods, contending that a comparison of herbarium records from different time periods cannot be used to infer changes in species distributions without field-based validation. However, our analysis compared the median position of extreme records of random species from random locations rather than focusing on particular species and their possible loss from specific sites. Hence, ground-truthing ‘extinctions’ are of limited value to the interpretation of our analysis. Moreover, subtidal ground-truthing over biogeographic scales is not logistically possible and even runs counter to entire disciplines (e.g. palaeontology, extinction biology and biogeography) that assess hypotheses of extinction and shifting distributions. Huisman & Millar also questioned the direction of biases in the data set. We show here that patterns of collection effort should have produced an apparent shift northward in the absence of a true shift southward. Even if herbaria were not designed for the purpose of detecting species’ range changes, we contend that such collections can contain useful information on the distribution of species across space and time.

Key Words::

• Climate change
• Collection bias
• Global warming
• Herbarium data
• Macroalgae
• Range shift

ACKNOWLEDGEMENTS

We thank the seven anonymous reviewers for their common support and advice on how to improve this rebuttal. Our initial work on seaweed herbarium specimens was supported by the Australian Research Council and New Zealand Vegetation Function Network. T.W., B.D.R., C.F.D.G., C.J.A.B., and S.D.C. further acknowledge support from the Australian Research Council.