Endemism and evolution in the Coral Triangle: a call for clarity

Authors

  • David R. Bellwood,

    Corresponding author
    1. School of Marine and Tropical Biology, and Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, Queensland, Australia
      *E-mail: david.bellwood@jcu.edu.au
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  • Christopher P. Meyer

    1. Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
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*E-mail: david.bellwood@jcu.edu.au

Abstract

In a recent paper (Bellwood & Meyer, Journal of Biogeography, 2009, 36, 569–576), we critically evaluated the utility of marine endemics for marking the geographical origins of species. In reply, Briggs (2009) identified two issues that needed clarification: (1) whether endemics are assumed to mark the geographical origins of species or areas of exceptionally high rates of origination, and (2) whether our evaluation of the role of endemics disproves the centre of origin hypothesis. Of these two issues, the first can be clearly resolved by recourse to the original literature that explicitly states that it has been assumed that endemics do indeed mark the probable sites of origin of species. The second is equally clear: our evidence does not and can not disprove the centre of origin theory. We suggest, however, that the current data, and endemics in particular, provide limited support for centre of origin theories and that they are more consistent with some centre of accumulation theories. The Indo-Australian Archipelago (IAA; Coral Triangle) therefore appears to be an area where species persist, a centre of survival, regardless of the site of origin of species.

In response to our recent paper on the Indo-Australian Archipelago (IAA) biodiversity hotspot and the significance of endemics (Bellwood & Meyer, 2009), Briggs (2009) appears to have misunderstood our central message. Indeed, it was to avoid such misunderstandings that we quoted rather than paraphrased Mora et al.’s (2003)Nature paper, which explicitly stated ‘We assume that centres of endemism contain a preponderance of recently derived species that are yet to expand their ranges (neo-endemics) and thus provide insights into areas where species are most likely to originate.’ Our interpretation of this was that they assumed that endemics often mark the geographical origins of species. Our stated objective, therefore, was to examine this assumption and evaluate the utility of marine endemics for identifying the presumed geographical origins of species. We did not directly address the issue of rates of origination.

Briggs has been a stalwart defender of centre of origin theories (Briggs, 1999) and his response to our paper is entirely consistent with this stance. However, his comments have raised a number of serious issues. They highlight the problems faced with marine biodiversity hotspots: a lack of clarity with regard to definitions, evidence and assumptions. There is a clear need to understand how we define hotspots, what evidence there is available to help explain the origins and maintenance of both hotspots and their component species, and the assumptions we make, whether implicit or explicit, in evaluating competing hypotheses.

The IAA marine biodiversity hotspot has been a topic of debate and discussion for decades. We cannot even agree on a name for the region, nor on its exact dimensions. In a thorough review of the topic, Hoeksema (2007) lists 17 alternative names. The one thing all workers appear to agree on is that this is indeed a hotspot. But even this means different things to different authors. For example, Roberts et al. (2002) based their definition on the number of endemics and environmental vulnerability, reflecting terrestrial definitions. Most other workers designated the area as a hotspot based on the exceptional species richness in the area, whether in absolute terms or relative to mid-domain expectations (Connolly et al., 2003; Bellwood et al., 2005; reviewed by Hoeksema, 2007).

However, these discussions go beyond mere semantics or academic interest, and the role of endemics is key to the problem of evaluating hotspots. Endemics are not just used as evidence for the three main competing hypotheses regarding the origin and maintenance of the IAA hotspot (centre of origin, overlap, or accumulation), but also have far-reaching implications for conservation. A hotspot that is defined largely on the basis of endemics will require significantly different conservation strategies from one that is a product of overlapping ranges of species with geographical ranges that span a quarter to two-thirds of the equatorial tropics (cf. Hughes et al., 2002; Connolly et al., 2003; Orme et al., 2005). Our central question was, therefore, what can we understand from the current data on endemics in the IAA and across the Indo-Pacific?

As acknowledged by Briggs (2009), previous studies have repeatedly demonstrated that most endemics are peripheral (outside the IAA). We were interested in their relative ages. Briggs (2009) asserts, without any dating evidence, that endemics in Indonesian waters are ‘neo-endemics that were recently derived’, while many peripheral species are ‘phylogenetic relicts’ and that Bellwood & Meyer (2009) do not offer counter-evidence. This was one of the specific goals of our paper: to examine the estimated ages of endemic taxa. Although preliminary, our results were consistent within and amongst taxa (fishes and cowries). The endemics (including those in the IAA) were of a comparable age to other species (i.e. not recently derived). Furthermore, most were pre-Pleistocene, arising before the presumed cladogenic impacts of sea-level changes (reviewed in Hoeksema, 2007; Renema et al., 2008).

We concluded that care is needed when making assumptions about endemics in the marine realm. One cannot necessarily assume that most endemics are young or that they mark the geographical origins of a species. Mora et al. (2003) are commendable in that they explicitly stated their basic assumption: that most endemics in the IAA are relatively young. Although this is undoubtedly correct in some cases (and we outline several groups in which this is highly likely), we suggest that this may not always be a reasonable assumption for widely dispersing species. Importantly, it is these widely dispersing species (corals, fishes) that are most frequently used to categorize the IAA as a marine hotspot.

Contrary to Briggs’s (2009) assertion that our evident intent was to ‘disprove the [centre of origin] theory’, our intent was merely to critically evaluate a commonly used form of data: endemics. The three traditional hypotheses – centre of origin, centre of overlap and centre of accumulation – are not mutually exclusive, and all have undoubtedly contributed to the patterns we see (Paulay, 1997; Bellwood & Wainwright, 2002). However, it is becoming increasingly difficult to separate the relative contributions of the three hypotheses. The original power ascribed to endemics as being able to resolve these hypotheses, extending from Potts (1985) to Mora et al. (2003), is becoming increasingly less convincing as molecular data give species origins between 2 and 20 Ma. Some species may even pre-date the formation of the IAA hotspot (Renema et al., 2008). We are left trying to identify the geographical origins of species that are millions of years old, with dispersive larvae and the potential to spread over half of the equatorial tropics. Indeed, the most parsimonious approach may be to change the question, from origins to maintenance. The IAA appears to operate primarily as a centre of survival (Potts, 1985; Barber & Bellwood, 2005; Williams, 2007), whether the species arise within or outside the hotspot, or, most likely, both (cf. Rocha et al., 2008).

Jack Briggs continues to play an invaluable role in biogeography by providing a consistent and often contrasting view of marine biogeography. Through his dedication to the centre of origin theories he has encouraged us to evaluate the available evidence carefully. At the very least, we hope that these discussions have helped to highlight potential limitations with endemics, the value of temporal information, and the challenges that lie ahead, especially given the urgent need to protect coral reef ecosystems and/or biodiversity whether inside or outside hotspots.

Editor: John Lambshead

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