Australian seaweed collections: use and misuse

The recent publication in Current Biology (21:1828-1832) by Wernberg et al. (2011) utilized historical herbarium specimen records sourced from Australia's Virtual Herbarium to suggest that the distribution ranges of numerous seaweed species are shifting southward, purportedly as a response to climate change. We contend that the paper is seriously flawed for several reasons: an inappropriate interpretation of herbarium records, the absence of ground-truthing to confirm actual extirpation, and an, incorrect interpretation of collection effort. The major assumptions of the paper are that herbarium records from a particular period are equivalent to a species' distribution in that period, and that the absence of a herbarium specimen from subsequent periods is evidence for the species' local extinction. No effort, however, has been made to validate these assumptions. Herbarium specimens are not collected in a systematic way, geographically or otherwise. Collectors are often selective in their taxonomic coverage, and there has never been a program of extensive seaweed collection over a defined period. Collections are often built up ad hoc as opportunities and funding arise. Thus the absence of a herbarium record is potentially due to any number of factors, the least likely being a local extinction. Moreover, none of the results were tested by revisiting sites and checking for the presence of supposedly locally extinct species. We also suggest that the claimed northwards collection effort bias, used in support of the conclusions, is grossly oversimplified. When examined in detail the collection effort is actually skewed southward and is potentially the underlying cause for the perceived range contractions.