The explanatory power of biogeographical patterns: a reply to de Bruyn et al

Authors


Abstract

Confusion between evidence and hypothesis in biogeographical studies was the focus of our recent Guest Editorial (Parenti & Ebach, 2013, Journal of Biogeography, 40, 813–820). That editorial was critiqued by de Bruyn et al. (2013, Journal of Biogeography, doi: 10.1111/jbi.12166) to whom we reply briefly here. Despite our shared goals – to understand what lives where and why – we argue from different philosophical premises. Although we may have little common ground, such debate encourages the good health of the field of biogeography.

Modern historical biogeographical studies fall into one of two kinds: those that generate explanations and those that discover patterns (Ebach & Humphries, 2002). We argue that discovery of patterns, such as that demonstrated through the shared components of areagrams, is primary in historical biogeography and that the generation of explanations logically follows the discovery of the patterns. Our approach to biogeography is presented in our recent book (Parenti & Ebach, 2009), to which we refer the interested reader. We expanded one topic in that book on the relationship between evidence and hypothesis in biogeography in a Guest Editorial (Parenti & Ebach, 2013), to which de Bruyn et al. (2013) have responded. We welcome this response as it encourages discussion and debate of some of the principles of historical biogeography and further exposes the assumptions made when proposing biogeographical hypotheses. We choose part of one sentence from the response of de Bruyn et al. (2013) to begin our comments. De Bruyn et al. (2013) ‘…believe that palaeogeographical reconstructions by geological experts… for the IAA [Indo-Australian Archipelago] and Sulawesi are the most suitable framework within which to test biogeographical hypotheses, as opposed to the use of hypothetical… relationships of taxa (cladograms), by biologists, to reconstruct palaeogeography'. We disagree with this statement for several reasons.

First, biological and geological patterns are independent. They provide reciprocal illumination (Rosen, 1978); they shed light on each other, but do not test, and therefore cannot reject, each other. The information contained in biogeographical distributions should not be discounted: it tipped the scale in favour of continental drift away from the long-held notion of a stable Earth (e.g. McCarthy, 2009), not to mention that it played a pivotal role in the formulation of a theory of biological evolution (e.g. Wallace, 1863).

Second, biogeographical regions are defined by the taxa that live in them (as opposed to biomes or ecoregions which are defined by climatic and ecological factors). We recognize two regions, Indo-Malayan (Wallace, 1863) and Pandora (Parenti & Ebach, 2010), which together range from east Africa/Madagascar to the central Pacific. They were allopatric in the Oligocene, but today overlap, in part, notably in the centre of the Indo-Australian Archipelago. We drew them on a map of the Oligocene to demonstrate that they were separate regions at that time and also to illustrate that they include both marine and terrestrial areas. Pandora covers a portion of what was formerly Gondwana, as well as the seas between those areas. On a modern map, Pandoran distributions look disjunct.

Pandora is a biogeographical region, like the Austral Realm (Morrone, 2002) or Bassian Region (see Ebach et al., 2013; http://www.bioatlas.info). It is not a palaeogeographical reconstruction of a former landmass, like Gondwana or Laurasia. Pandora spans the land and the sea (see diagnosis and description in Parenti & Ebach, 2010, p. 313). It is named after the South Pandora Ridge, a chain of seamounts and banks in the North Fiji Basin (see Ebach et al., 2008).

Biogeographical regions are entities to be explained. To test the utility of Pandora means to find taxa that are endemic to Pandora. For example, the close relationship of east African taxa to those in La Réunion, east of Madagascar, and Fiji reflects Pandora, as in the distribution of mynoglenine spiders (Frick & Scharff, 2013, figure 7). An analogue would be to note distributions of endemic clades throughout the southern oceans, and the lands in them, to confirm the Austral Realm as a distinct biogeographical region. Likewise, the distribution of blind, obligate cave gobies in south-western Madagascar and north-western Australia (Chakrabarty et al., 2012) is further support for the biogeographical region Pandora. The age estimates for differentiation among taxa cannot refute that observation.

Sulawesi is a geological and biological composite; part of it is in Pandora, part of it is in the Indo-Malayan region (Parenti & Ebach, 2010, figure 15.6). Debates about whether Sulawesi taxa have been distributed by vicariance or dispersal are secondary to the recognition of these distribution patterns, although de Bruyn et al. (2013) disagree. Further, we would not reject a biogeographical hypothesis because it does not agree with a geological or palaeogeographical hypothesis – even one proposed by an acknowledged expert – or with an age of divergence estimated using molecular methods. Estimates of lineage divergence times using molecular methods require stringent fossil calibration strategies, as de Bruyn et al. (2013) note, which have been absent from many recent biogeographical analyses that claim to have refuted vicariance (see especially, Heads, 2012; Sauquet et al., 2012).

Patterns of endemism on oceanic islands are routinely interpreted based on whether the islands were emergent (= assumed vicariance) or submerged (= assumed transoceanic dispersal), but this, too, is secondary to recognizing a pattern. For example, McCarthy (2009) relates the well-known conundrum surrounding evolution of the marine iguanas of the Galápagos, a lineage considered to be older, at 10–20 million years (Myr) old, than the modern islands, which are 1–5 Myr old (Rassmann et al., 2004): how could marine iguanas have developed features such as algae-eating and salt-excretion mechanisms if they did not evolve along with the archipelago? An answer came from geologists who confirmed the hypothesis that islands along the continuous string of seamounts from South America to the Galápagos have been emergent for the past 17 Myr (Werner & Hoernle, 2003) and possibly longer (Hoernle et al., 2002; see also Parent et al., 2008).

Finally, we cannot second de Bruyn et al.'s (2013) endorsement of Waters et al. (2013) ‘…to move beyond the “ancient vicariance” argument to explain each and every biogeographical scenario'. This is one way to dismiss panbiogeographical studies sensu Croizat (1958) and more recently those of Heads (2005, 2011) and others that we find instructive. More importantly, it fixes the debate on processes of individual taxa (e.g. vicariance versus dispersal) rather than on shared patterns of taxa within a biota. We, too, aim to move the debate forward: from the persistent dispersal versus vicariance argument to a period of discovery of biotic history.

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