Proc. Nati. Acad. Sci. USA Vol. 73, No. 5, pp. 1765-1769, May 1976 Zoology Birds on islands in the sky: Origin of the montane avifauna of Northern Melanesia (biogeography/Pacific Ocean/mountains) ERNST MAYR* AND JARED M. DIAMONDtt * Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138; and t Physiology Department, University of California at Los Angeles Medical Center, Los Angeles, Calif. 90024 Contributed by Ernst Mayr, March 3, 1976 ABSTRACT Biogeographers have long been fascinated by Northern Melanesian birds, other papers having discussed the the disjunct distributions of species stranded on mountaintops. species-area relation (7) and species-distance relation This paper analyzes, for the montane bird populations of (8). Northern Melanesian islands, how many such populations there How many? are, why they are restricted to mountains, and how they dis- persed to mountains. The number of populations increases with Table 1 of ref. 7 lists, for each Solomon island, the number of island elevation and with montane area, and decreases with land and fresh-water bird species occurring in the lowlands lowland area, exemplifying the problem of continental species (Siow), the number confined on that island to the mountains diversity. Most species with montane populations on some (Smt), and the island area A (square miles) and elevation L island(s) have sea-level populations on some other island(s). (feet). Elsewhere (7, 9) we showed that variation in A and in These altitudinal niche shifts can be variously related to inter- island differences either in altitudinal distribution of area or island isolation accounts for 98% of the variation in SIo., Smt else in competitive pressure in the lowlands or mountains. Re- consists of all populations that do not normally reach sea-level striction of Northern Melanesian bird populations to mountains on a given island, regardless of whether their lower altitudinal is more often due to lowland competitors than to inability to limit is 1000 ft or 4000 ft. Thirteen islands have one or more survive under the physical conditions of the lowlands. Of four montane populations, the largest number on a single island possible mechanisms for the origin of a montane population being 23. These 13 islands constitute all Solomon islands that (referred to as jumping, land-bridge crossing, tric ling, and push-pull shifts), only the first and last have been significant for exceed 2600 ft in elevation; Smt is 0 for all lower islands. All 13 Northern Melanesian birds. of these islands are "central" ones (as defined in ref. 8), for which the effect of variation in isolation on species number is When early naturalists from Linnaeus (1) to Darwin (2) began negligible. Higher islands prove to harbor more montane to catalogue species and plot their localities of occurrence, they populations: linear regression of Smt on L explains 87% of the were struck by discontinuous distributions of many alpine variation in Smt. But obvious examples of unexplained variation species. These species are confined to summits of high moun- emerge from comparison of Gatukai (2912 ft, Smt = 3 species) tains, but may recur on numerous summits separated by ex- with New Georgia (3300 ft, 1 species), Malaita (4200 ft, 7 panses of intervening lowlands in which these species are absent. species) with Ysabel (4100 ft. 3 species), or Ganonga (2800 ft, The problems posed by the faunas and floras of such "islands 3 species) with Vella Lavella (2600 ft, 1 species). in the sky" have fascinated modern biogeographers as they did An attempt to account for more of the variation in Smt is in- Linnaeus and Darwin. Are montane species completely teresting because it exemplifies the problem of "continental stranded on their islands of habitat, or can they disperse? Are species diversity," i.e., patterns of species number along a they confined to mountaintops because of habitat taboos, habitat gradient within a single land mass (10). Just as island physiological inability to survive in the warmer lowlands, or area affects the total number of species on an island at equi- competition from species better adapted to lowland conditions? librium, so the area of a particular habitat within a continent Did they reach their biogeographic eyries by dispersing directly or island must affect the number of species that have immi- from summit to summit, by trickling through the lowlands grated or evolved to become tied to that habitat (11). This is why under present-day climatic conditions, by migrating over the small areas of South American temperate forest, African vanished bridges of montane vegetation during cool Pleistocene montane forest, Australian tropical rainforest, and New Guinea periods, or by evolving from lowland ancestors? Do montane alpine grassland are relatively poor in total species as well as in species represent dead-ends of evolutionary "taxon cycles" (3)? species restricted to that habitat, compared to the structurally We shall explore these and related questions for the montane similar but larger expanses of Australian temperate forest, New avifauna of Northern Melanesia, which consists of the Bismarck Guinea montane forest, African tropical rainforest, and South and Solomon archipelagoes east of New Guinea. This avifauna American alpine grassland. In seeking correlations between offers the advantages that it is well known taxonomically and number of land and fresh-water bird species and island area for distributionally (4-6), and that one of us (J.M.D.) has been able whole islands, one can neglect the adjacent "habitats" of the to determine altitudinal ranges of most species on many of the sea completely, because feeding by marine birds in terrestrial islands during a series of expeditions. We discuss in turn how habitats and vice versa is negligible. The corresponding conti- many populations are montane on each island, why they are nental problem is more complex: the number of species tied to restricted to mountains, and how they dispersed to mountains. habitat type X depends not only on the area of that habitat, but This paper is the second in a series on ecology and evolution of also on the area of habitat types Y, Z, etc., the abilities of species derived from habitats Y and Z to invade habitat X and compete Abbreviations: ft. foot (feet) (1 foot = 0.305 m). 1 square mile = 2.59 with its specialists, and the ability of habitat X's specialists to km2. utilize the adjacent habitat types. t Address reprint requests to J. M. D. These considerations account for much of the variation in 1765 Downloaded by guest on September 23, 2021 1766 Zoology: Mayr and Diamond Proc. Natl. Acad. Sci. USA 73 (1976) Smt not explained by L alone: Smt increases with montane area catcher Rhipidura [spiloderaI are montane on some islands but and decreases with lowland area, as measured by the areas are lowland inhabitants of other islands in the same archipelago above and below 2000 ft on modern 1:50,000 contour maps of of the tropical Southwest Pacific. The frequency of such local the Solomons (A>20oo and A<2000, in square miles). § For ex- geographical variation in altitudinal range, or altitudinal "niche ample, New Georgia and Vella Lavella, which have the lowest shifts," has been appreciated only recently (e.g., 15-20). In this values of Smt/L of the 13 mountainous islands inTablel of ref. regard, the 46 bird superspecies that are represented by at least 7, also have relatively the least area at higher elevations (lowest one montane population in Northern Melanesia display a value of A>2000/A<200o). Conversely, Bougainville, Guadal- complete spectrum of behavior, from 14 superspecies that are canal, and Kolombangara, which have the highest values of montane on all islands of occurrence (e.g., the parrot Mi- Smt/L, have relatively the most area at higher elevations. cropsitta bruijnii), to superspecies that are montane on some Malaita and Ysabel are similar in elevation (4200 versus 4100 islands but in the lowlands of others (e.g., the pigeon Rein- ft) and in total area (1663 versus 1581 square miles), but Malaita wardtoena [reinwardtifi]), to superspecies that are in the low- has more than twice as many montane species (7 versus 3), lands of most islands but montane on just one or two islands (e.g., correlated with its more extensive mountains (A>2000 145 versus the flycatcher Rhipidura [spilodera]). The interisland variation 45 square miles). Comparison of New Georgia and Gatukai il- in altitudinal range in some of these cases is enormous; for in- lustrates the negative correlation between lowland area and stance, Turdus poliocephalus is confined to elevations above richness of the montane avifauna (see second paragraph below 9000 ft on New Guinea, above 8000 ft on Celebes, above 7500 for suggested interpretation). New Georgia is higher than ft on Ceram, above 7000 ft on Borneo, above 5500 ft on Timor, Gatukai (3300 versus 2912 ft) and has much more extensive above 4000 ft on Bougainville and Guadalcanal, above 3400 ft mountains (A>2000 9.2 versus 0.85 square miles). Yet New on Kolombangara, and above 2500 ft on Tolokiwa, and de- Georgia has only one montane species compared with Gatukai's scends to sea-level on Rennell and numerous other islands in three, correlated with New Georgia's far more extensive low- the eastern (but not the western) part of its range. Even the lands (A>2000 19 times greater than for Gatukai). observation that 14 of the 46 montane superspecies are montane Multiple regression yields the following equation, which on all islands of occurrence overestimates, for two reasons, the accounts for 93% of the variation in Smt: proportion of species that are obligately montane (i.e., that are confined to mountains because of physiological adaptations or Smt = 49L 36(A>2000)6 / (A<2000)039 [l] habitat taboos).