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Scaling mount SAR: Commentary on Matthews et al. (2021) The Species–Area Relationship: Theory and Application

The issue of spatial scaling has grown in recent decades from a minor geeky obsession to a mainstream methodological preoccupation in virtually all fields of science. Increasingly, scientists of all stripes have been challenged to scale up their understanding of physical, chemical and even social processes from test tube or patch to global scales, driven both by the opportunity of increasingly sophisticated global data sources and by the need to address urgent and interconnected global challenges. In this respect, as in so many others, ecologists have been well ahead of the crowd: we as a field have been in the business of scaling up biodiversity for more than a century. The main tool for doing that upscaling has been the species– area relationship (SAR). The SAR has been percolating in the collective consciousness of biogeographers since the late 18th century, but the idea was formalised and brought to wider attention by the competing works of Arrhenius (1921) and Gleason (1922). In its essence, the idea is simple: more area generally contains more species. However, the subadditive scaling of biodiversity (twice as much area generally contains less than twice as many species) makes the challenge of upscaling biodiversity decidedly nontrivial, an issue that has inspired theoretical and empirical research in equal measure. It has been a full century now since the SAR first became widely appreciated among ecologists, and a quartercentury since the last major book on the topic (Mike Rosenzweig's Species Diversity in Space and Time, 1995); it is high time someone revisited the topic in light of recent work. Fortunately, Tom Matthews, Kostas Triantis and Rob Whittaker have done just that with their new book: The Species– Area Relationship: Theory and Application (2021). In many ways, Matthews et al. (2021) come in stark contrast to Rosenzweig’s (1995) volume. While Rosenzweig's book is an extensive exploration of a single author's views, the new volume includes the work of 44 contributors from 35 institutions, spanning 19 chapters and almost as many subdisciplines. The book includes the insights and perspectives of a large fraction of the key players working on this topic; its contributors list reads like a Who's Who of spatial ecology and macroecology researchers: Blackburn and BordadeÁgua, Harte and Hubbell, Šizling and Storch, Thuiller and Tjørve, Ugland and Ulrich, to name just a few tantalisingly alliterative pairs. If Rosenzweig's book feels spacious and expansive (even chatty in places), the new volume feels dense and tightly stuffed with diverse treats, and even a bit claustrophobic in places. In part that is a result of downsized pages and fonts: Rosenzweig's pages are 20% larger than those of the newer book, but the words per page are about the same. Where Rosenzweig had the opportunity to slowly build an extended discourse, each of the chapters in Matthews et al. stands alone as a separate narrowly focused account. The book is full of scattered gems: it includes the best account I have seen of the history of the SAR concept (Chapter 2), valiant attempts to generalise SARs to the scaling of ecological function (Chapter 5) and trophic networks (Chapter 12), and applications of SARs to identify biodiversity hotspots (Chapter 13) and predict extinctions (Chapters 14 and 17) and many more. However, the richness and diversity of perspectives comes at a price: despite the valiant efforts of the editors, the book as a whole does not (and indeed can't) build up to a single coherent argument. The breadth of authorship brings with it a diversity of approaches and expertise, but also inconsistencies in style and tone, and occasional overlaps and gaps. On a topic that is rife with terminological inconsistency, the editors do a good job of enforcing a common framework for all, which is largely adhered to by most authors. They simplify Scheiner’s (2003) influential typology of SARs to its most fundamental dichotomy: contrasting ISARs (‘Island’ or ‘Isolate’ SARs for analyses comparing the species richness of sets of discrete sites of different sizes) and saSACs (‘sampling area Species Accumulation Curves’, for comparisons across subsamples of different sizes nested