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Numbers of mammals on the Malaysian islands

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Numbers of mammals on the Malaysian islands

J.L. Harrison (1965) applies the logarithmic series model of Fisher, Corbet and Williams to mammal collections from the islands surrounding the Malay Peninsula, asking whether the number of species on an island can be predicted from its area alone. Working from Gibson-Hill’s unfinished check-list and museum expedition records, Harrison demonstrates that while mainland forest samples conform to a consistent index of diversity (α ≈ 12), the offshore islands outside the 10-fathom contour carry a fundamentally impoverished fauna (α ≈ 7) that cannot be explained by the standard Sunda Shelf submergence model.

Summary

Harrison’s central problem is one of sampling completeness. The logarithmic series relates the number of species (S) to the number of specimens (N) through a parameter α, the index of diversity, which is a property of the underlying population. For mainland areas, where collecting has been sustained over decades, the “true” fauna can be approximated, and the data fit a curve with α = 12 when N is expressed as area in units of 1,000 m² (p. 27–29). The islands, however, have each been visited only one to three times for a week or so of intensive collecting, so the recorded species lists represent only a fraction of the true fauna. Harrison therefore cannot simply overlay island data on the mainland curve; instead he groups islands by their bathymetric position and exploration history, and shows that each group lies along a parallel curve with a shifted log N axis (p. 35–36).

The critical finding is that the inshore islands (within the 5-fathom contour: Singapore, Penang, Pangkor, Pisang) and the North Borneo islands (Malawali, Banggi, Balambangan) have indices of diversity close to 10, and Bangka and Billiton close to 12, meaning their faunas are essentially samples of the mainland fauna reduced only by incomplete sampling. But the offshore islands—those of the Johore-Pahang archipelago, the Trengganu archipelago, Langkawi, and the Natunas, all near or outside the 10-fathom contour—consistently show α ≈ 7, a value confirmed by individual collection data (e.g. Chasen’s Bungaran collection yielding α = 7.1; Sirhassan α = 8.6; Banggi α = 7.2) (p. 34, 36). This lower diversity cannot be attributed to sampling error alone; it reflects a genuinely different faunal composition.

Harrison acknowledges the method’s limitations explicitly: it has failed to produce a definitive answer to the original question of how many species should occur on an island of given size, but it has identified a statistically robust faunal discontinuity at the 10-fathom contour that challenges the simple geological narrative of uniform sea-level rise isolating all islands from a single peninsular source (p. 39, 41).

Key Findings

  • Mainland Malaysian forest samples (Borneo, Malaya, Sabah, Selangor, Bukit Lagong, S.B. Experiment) all conform to a logarithmic series with α = 12, allowing species richness to be predicted from area in 1,000 m² units (p. 28–29).
  • The S.B. Experiment at Sungei Buloh (14 hectares, 11 months) yielded 205 marked animals of 13 species, supporting a population density of approximately one mammal per 1,000 m² (p. 27).
  • Offshore islands (Johore-Pahang, Trengganu, Langkawi, Natunas; outside the 10-fathom contour) show a consistently lower index of diversity of α ≈ 7, confirmed by individual collections: Bungaran 130 specimens/21 species/24 days (α = 7.1); Sirhassan 133 specimens/24 species/2 months (α = 8.6); Banggi 112 specimens/20 species/9 days (α = 7.2) (p. 34, 36).
  • Inshore islands (Singapore, Penang, Pisang) and North Borneo islands (Banggi, Balambangan, Malawali) show α ≈ 10, statistically indistinguishable from the mainland fauna once sampling incompleteness is corrected (p. 35–36, 41).
  • Pangkor Island shows a severe faunal deficiency (χ² = 12.6 against the inshore-island expectation of 41 species; only 18 recorded), consistent with the volcanic-ash depopulation hypothesis of Harrison & Hendrickson (1963) (p. 38–39, 42).
  • Sibu Island departs significantly from the offshore-island expectation (1 species recorded vs. 7.7 expected; χ² = 5.8), with no evidence of a proper zoological expedition having worked the island (p. 37).

Conclusion

Harrison’s definitive takeaway is that the offshore islands of the Straits of Malacca and the South China Sea, lying outside the 10-fathom contour, possess a mammal fauna of fundamentally lower diversity than would be expected for their area if they were simple fragments of the Sunda Shelf peninsula. This faunal discontinuity implies that their biogeographic history is more complex than the standard model of uniform sea-level rise allows—either the islands experienced additional isolation events, or the source mainland fauna from which they were derived was itself different from the present one (p. 36, 41–42).

Context

  • The paper draws on the Raffles Museum (Singapore) and Malayan Museums collection records, Gibson-Hill’s unpublished check-list of Malayan mammals, and published expedition accounts by Chasen (1935, 1940) and Chasen & Kloss (1931).
  • Historiographically, the paper sits at the intersection of mid-century statistical ecology and Sundaic biogeography, providing one of the first quantitative tests of the Scrivenor (1949) sea-level model using the Fisher-Corbet-Williams logarithmic series, and directly informing the volcanic-ash depopulation hypothesis developed with Hendrickson (1963).

References