Feature Paper: DOWNLOAD * Stoddart (1992) Biogeography of the tropical Pacific. Pacific Science, 46(2):276-293.
Author Abstract: Many previous biogeographic regionalizations of the islands and reefs of the tropical Pacific are unsatisfactory: the regions as defined are heterogeneous, localities with unlike biotas are grouped together, and those with similar characteristics are placed in separate categories. Often distinctions appear to be based on cultural or political rather than biogeographic considerations. Criteria are defined for the establishment of biogeographic boundaries. lnstead of the hierarchical schemes often utilized, it is proposed that the basis of biogeographic regionalization be typological. A distinction is made between the biogeographic characteristics of atolIs and other reef islands, elevated limestone (makatea) islands, and high (often volcanic) islands. It is concluded that if the first two categories are filtered out, the treatment of the biogeography of the third group and hence the regionalization of the Pacific becomes relatively unproblematical.
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'll complete our look at some applications of biogeography aimed at applying global biodiversity analyses towards conservation and management of those resources.
As mentioned last week, the official download link for the Stoddart article is a TXT file and misses the figures that I believe are an integral part of the article, which aims to provide a visualization of different "schemes" meant to classify the Pacific Ocean islands that tend to be overlooked with many biogeography schemes. In fact, most maps of the world divide the globe through the Pacific, while it would make more sense geographically to divide it through the Atlantic Ocean.
Because of these reasons, I've decided to post every figure from the article in my review, which I hope is acceptable to the publishers for two reasons:
The publisher (University of Hawaii) has provided a freely-accessible TXT file (lacking figures) of this article on the official download link at the beginning of this review; and
The article is from 1992 and doesn't provide any "breaking-edge" research, at least now (being nearly 20 years old).
Okay, on to the review. To illustrate how important the figures are, I won't even discuss the majority of points detailed in the text but rather, I'll provided commentary and summaries about each figure in turn, which should suffice to provide an excellent review of the entire article in any case.
Figure 1. "Biogeographic regionalization of the Pacific (after Gressitt 1956, 1961)." As Stoddart points out, "Gressitt's [biogeographical] scheme involves a hierarchy of regions, subregions, divisions, and subdivisions, though how these are related is not immediately apparent from his map." The major biogeographical divisions that Gressitt uses (islands that he felt were best related to each other in terms of distributions of plants and animals on land and in the water) totaled 12, with major divisions for the main Japanese Islands, the Ryukyus, the South China Sea, Papua New Guinea, eastern Australia, New Zealand, and 6 subregions encompassing the Pacific Islands. He grouped Micronesia together from Palau to Johnston Atoll, and grouped Eastern Polynesia together (the Line Islands, some of the Cook Islands, all of French Polynesia, and Pitcairn and Easter Island), with smaller unique regions set aside for 1) The Hawaiian Islands; 2) Southern Polynesia from the Phoenix Islands south through Samoa to the western Cook Islands; 3) Melanesia east of the Solomon Islands; and 4) New Caledonia. However, as Stoddart mentions, Gressitt placed too much emphasis on human culture and communities in demarcating his divisions (e.g., Polynesian VS Micronesian) rather than just plants, but in Stoddart's view, this stance isn't unique to Gressitt. Rather, "there is a persistent and indeed illogical tradition in Pacific insular biogeography that places weight on the comparatively recent human settlement of the area and the cultural differences between island groups that have resulted." However, sometimes one must be careful when looking at plant and animal checklists for the Pacific, as those cultural aspects (i.e., the habitation of humans) have often resulted in certain animals (and sometimes plants) going extinct through human activities. Thus, one must know (which is impossible with 100% certainty), what the "proto-Pacific" floras and faunas were (i.e., the plant and animal compositions before humans arrived).
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Figure 2. "Biogeographic regionalization of the Pacific (after Thorne 1963)." Comparing this scheme to Figure 1 above, one can see that a lot of the southwest Pacific biogeography remains the same (with a few shifts of boundaries) but that the central Pacific is more generalized. Otherwise, it follow the same methodology and downfalls of Figure 1.
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Figure 3. "Biogeographic regionalization of the Pacific based on the distribution of the Cypraeidae (Mollusca) (after Shilder 1961)." In this Figure, Shilder only looked at one group of mollusks, the cowries. Based purely on the distributions of various taxa of cowries, Shilder created the following biogeographic divisions of the Pacific, with all islands within a given division relatively related to each other in terms of cowry species than islands in another division.
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Figure 4. "Biogeographic regionalization of the Pacific based on Udvardy (1975)." The main criticism Stoddart brings up is how heterogenous Udvardy's scheme was, especially in division VI where the equatorial Phoenix islands are grouped with the subtropical Kermadec Islands. Likewise, New Caledonia is grouped with Norfolk Island and Lord Howe, quite dissimilar areas.
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Figure 5. "Biogeographic provinces of the area covered by the South Pacific Commission (after Dahl 1979, 1980)." Dahl's scheme seems to be based significantly on political units (because the regionalization was limited to a political division in the first place, the South Pacific Commission. However, Stoddart notes that a few islands that transcend political boundaries are grouped together reasonably (e.g., Lord Howe, Norfolk, and the Kermadec islands).
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Figure 6. "Distribution of atolls and reef islands in the tropical Pacific. Numbers refer to mean annual rainfall distribution in meters (dashed lines). Black circles are treeless dry islands." This scheme avoids all biological components and purely looks at physical characteristics of island elevation. The boundaries of atolls and low reef islands are circled and rainfall data are plotted within the region noted.
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Figure 7. "Mean annual rainfall and Fosberg zones in the west-central Pacific (Marshalls, Kiribati, and Tuvalu). Rainfall data from Taylor (1973)." This figure details how various islands from 20ºN to 20ºS fall on a range of rainfall data, with the resulting curve being divided into zones named after Fosberg, meant to delineate the similarity of islands based on rainfall coupled with latitude. As Stoddart notes, "the main controls on atoll biota in the Pacific are ecological. Atoll vegetation responds asymmetrically to rainfall extremes: it is more sensitive to drought than to wetness." Starting in Fosberg zone 1, there is insufficient rainfall to support coconuts. In zone 2, Pisonia forests begin. "In zone 3 there is Cordia, Pemphis, mixed forest, and coconuts. In zone 4 ther is Neisosperma forest and breadfruit; in zone 5 coconuts and breadfruit; and in zone 6 dense forest. Zones 7, 8, and 9 mirror zones 5, 4, and 3 to the south."
I'm skipping Figure 8, which deals with numbers of plans in the Marshall Islands based on rainfall because I think Figures 6 and 7 make it clear that rainfall is important in determining animal and plant compositions on islands. Stoddart does note that "atoll floras are unresponsive to land area (and indeed distance from presumed source area) but instead reflect rainfall [with very few exceptions]."
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Figure 9. "Raised makatea islands in the tropical Pacific. These islands are abundant west of the solid line (i.e., in the Solomon, New Guinea, Palau, Marianas, Fiji, and Tonga); black circles show makatea islands or volcanic islands with substantial raised limestone in the open Pacific." Again, this scheme merely maps out the geography of the Pacific irregardless of biota, with high limestone islands (called makatea in various Polynesian tongues) and high volcanic islands and atolls noted.
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Figure 10. "High islands in the tropical Pacific. These islands are frequent west of the continuous line; possible biogeographic boundaries within this area are indicated. East of the line individual islands or clusters of high islands are indicated. Numbers refer to mean annual sea-level rainfall in meters, though this may vary widely according to topographic situation (rainfall data from Brookfield and Hart 1966 and Taylor 1973)." Stoddart, in recognizing how the Pacific is dotted with islands but having vast water barriers between, and how atolls are different from high islands, has produced the map below to note only high islands (circled) with rainfall data provided, and some attempt at biogeographic scheming from Samoa west to New Guinea and north to Japan.
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Figure 11. "Distribution of native land bird species in the tropical Pacific (excluding the easternmost Pacific). Data from Pratt et al. (1987) and other sources." The numbers below merely show land bird (non-flying and non-migratory birds) taxonomic abundance throughout the Pacific islands, showing that New Guinea has the most species (325) followed by some other Melanesian islands (Fiji, Santa Cruz Islands) and then other scattered pockets of medium diversity elsewhere (Hawaii, Galapagos, Tonga, Samoa, Palau).
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Figure 12. "Pacific limits of the seagrass genera Thalassea (TH), Enhalus (EN), Halophila (HA), and Syringodium (SY). Data from Den Hartog (1970), with additions." The figure is self-explanatory, as is the next one.
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Figure 13. "Pacific limits of the mangrove genera Rhizophora (RH), Avicennia (AV), and Excoecaria (EX). Data from various sources."
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Figure 14. "Generic diversity of scleractinian corals in the tropical Pacific (after Coudray and Montaggioni 1983)." Since Stoddardt's day, Veron's (2000, with additions) Corals of the World treatise has refined Coudray and Montaggioni's figure below, especially in the Indo-West Pacific (barely shown below). Still, what is key to note is how diversity scales or drops off from west to east, with highest diversity around Papua New Guinea and least in the Eastern Pacific.
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Figure 15. "Distribution of numbers of taxa in the genus Strombus (Mollusca, Gastropoda); the solid line shows the eastern limits of Strombus labiatus (after Abbott 1960)." Like the cowries, the figure below notes another mollusk, the conchs, though they are less abundant than cowries both in terms of species numbers (overall diversity) and distribution.
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Figure 16. "Distribution of families of warm-water marine shorefishes in the tropical Pacific (after Springer 1982)." Of particular note is how the shorefishes (or reef fish) diversity is similar in pattern to Figure 14, coral generic diversity.
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Figure 17. "Distribution of species of damselfish (Pomacentridae) in the tropical Pacific; the solid line shows the eastern limits of Amphiprion clarkii (after Allen 1975 in Briggs 1984 and Springer 1982)." This figure refines Figure 16 above by looking only at damselfishes, one of the most diverse reef fish families, with the most widespread clownfish species marked off with the solid line. Note how the Great Barrier Reef and Papua New Guinea have the highest diversities.
To conclude, one must take Stoddart's paper with an understanding of the state of knowledge in 1992 when it was written. Nowadays, biogeographers use powerful databases to quickly generate hundreds of maps tailored to individual species or taxonomic groups in their ever-refined quest of determining global biogeography patterns. Stoddart's attempt to consolidate all relatively recent views of Pacific Island biogeography while lacking a lot of the computer tools of biogeographers today should be applauded and there are still gems to be found in his paper nearly 20 years later.
For while the modern biogeographic figures have been refined here and there for individual groups since Stoddart's day, the same basic principles Stoddart pointed out remain true: many tropical organisms have similar diversity patterns in the Pacific, but that atolls should be separated from high islands in data sets for terrestrial faunas and floras. Also, diversity tends to be greater in the tropics compared to temperate waters.
While I've skimmed over the figures above I hope that everyone spends just as much time as you did reading this review as for reviewing the figures and making your own conclusions based on the numbers and patterns shown.
I hope everyone's enjoyed this review. Next we'll look at taxonomic similarity indices, which have grown out of the need to analyze ever-vaster species checklists globally and determine how the floras and faunas of such regions are related. In other words, we'll see what biogeographers are doing now to address Stoddart's question.
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