Review: Mora, Chittaro, Sale, Kritzer, Ludsin (2003) Patterns and processes in reef fish diversity. Nature, 421:933-936.

Feature Paper: DOWNLOAD * Mora, Chittaro, Sale, Kritzer, Ludsin (2003) Patterns and processes in reef fish diversity. Nature, 421:933-936.

Author Abstract: A central aim of ecology is to explain the heterogeneous distribution of biodiversity on earth. As expectations of diversity loss grow, this understanding is also critical for effective management and conservation. Although explanations for biodiversity patterns are still a matter for intense debate, they have often been considered to be scale-dependent. At large geographical scales, biogeographers have suggested that variation in species richness results from factors such as area, temperature, environmental stability, and geological processes, among many others. From the species pools generated by these large-scale processes, community ecologists have suggested that local-scale assembly of communities is achieved through processes such as competition, predation, recruitment, disturbances and immigration. Here we analyse hypotheses on speciation and dispersal for reef fish from the Indian and Pacific oceans and show how dispersal from a major centre of origination can simultaneously account for both large-scale gradients in species richness and the structure of local communities.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: Today we look at a good paper from the eminent journal Nature about the near-global (Indian and Pacific Oceans only) biogeography of reef fishes. The authors examined latitudinal and longitudinal diversity gradients, which for reef fishes and corals are known to peak in diversity within the "coral triangle" area centered between the Philippines and eastern Indonesia through Papua New Guinea to the Solomon Islands, summarized in the figure below.

The authors go on to determine which of "three major, yet different, hypotheses invoking speciation and dispersal [that] have been suggested to explain these large-scale patterns" seems the most plausible given their analyses. The three hypotheses are:

1. "The Centre-of-Origin hypothesis suggests that the IPR [Indo-Pacific Region] is a major centre of speciation from which species disperse to marginal locations."
2. "The Centre-of-Overlap hypothesis proposes that the high diversity in the IPR is due to the overlapping of faunas from several biogeographic provinces."
3. "the Centre-of-Accumulation hypothesis states that speciation occurs in several areas peripheral to the IPR and that species extend their ranges to the IPR by prevailing currents. A variant to the Centre-of-Accumulation hypothesis holds that after extending into the IPR, many species have suffered range reductions through the loss of populations marginal to the IPR."
4. To test the last two hypotheses, the authors note that those hypotheses "state that only the tails of species’ ranges extend into the IPR. Consequently, most species should have their range midpoints marginal to the IPR, resulting in bimodal or multi-modal distributions."
5. The authors' examination of their data revealed "plots of species mid-ranges (both longitudinal and latitudinal) show nonrandom unimodal distributions with peaks coinciding with the geographical position of the IPR (Fig. 1c, d). These distributions rule out these two hypotheses and provide support for the Centre-of-Origin hypothesis. They also support, to some extent, the variant of the Centre-of-Accumulation hypothesis because range reductions through loss of peripheral populations would shift mid-ranges towards the IPR, and, if extensive, this could result in a unimodal distribution of midranges in the vicinity of the IPR."
6. The next task of the authors was to determine whether the Centre-of-Origin hypothesis was more reasonable than the variation of the Centre-of-Accumulation hypothesis, or vice-versa. To answer this question, the authors note that while the former hypothesis predicts "speciation within the IPR" the latter hypothesis predicts "speciation in locations marginal to the IPR."
7. Rather than examining the fossil records to determine zones of speciation, the authors look at patterns of endemism. The authors "assume that centers 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."
8. The authors mapped endemism of reef fish in the figure below.

The authors note that the IPR has the highest concentration of endemism in the Indian and Pacific oceans. Therefore, the authors conclude that "this result supports the IPR as a major centre of speciation and confirms the expectation of the Centre-of-Origin hypothesis."

Another cause for high levels of endemism might be that the IPR has "among the highest number of islands per unit of geographical area makes it a place where allopatric speciation might be frequent, especially when considering patterns of recent geological sea level change."

The authors tested various other predictions about the Centre-of-Origin hypothesis specifically and while I'll leave it to readers to look up the original paper, I'll summarize by saying that the authors found additional support for the Centre-of-Origin hypothesis.

The authors' take-home message is that "no location contributes as much to the overall alpha diversity of the Indian and Pacific oceans as does the IPR." However, I should note that certain organisms (such as algae) do not share the same diversity patterns as reef fish and corals, but considering how integral both corals and fishes are to coral reefs, obviously conservation priority should be placed on areas of high diversity (e.g., the Indo-West Pacific "coral triangle") and areas of particular uniqueness (e.g., the noted high-endemism regions in Figure 2 above). 

Next we'll examine the diversity patterns of algae since it is important to review organisms that don't share common diversity patterns since they too are integral to determining any underlying processes that may affect the distribution of algae as well as corals and fishes.