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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 (biogeography/Pacific /mountains) ERNST MAYR* AND JARED M. DIAMONDtt * Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138; and t Physiology Department, University of California at Medical Center, Los Angeles, Calif. 90024 Contributed by Ernst Mayr, March 3, 1976

ABSTRACT Biogeographers have long been fascinated by Northern Melanesian , 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 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 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 -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 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 (3300 ft, 1 species), (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 (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 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 montane forest, Australian tropical rainforest, and New 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 . 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. (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 (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 , which have the highest values of montane on all islands of occurrence (e.g., the 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 , Gatukai (3300 versus 2912 ft) and has much more extensive above 4000 ft on Bougainville and , 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). First, of these 14 species, 12 occur on three or The remaining unexplained variation is partly because actual fewer islands and 8 only on a single island, so that there is no or Smt values must be integers and are mostly 7 or less, leading to almost no opportunity to determine whether they would de- relatively large statistical fluctuations. scend to sea-level on some islands. Second, of the two species Eq. 1 suggests three conclusions about species distributions (Micropsitta bruijnii and the pigeon Gymnophaps [albertisii]) over habitat gradients, as exemplified by altitudinal gradients: that do occur on more than three islands and are everywhere (i) Species number increases with the length of the habitat montane, the former shares all islands of occurrence with a gradient (e.g., Smt increases with L). (ii) The greater the area lowland congener of mutually exclusive altitudinal range and in a particular habitat type, the more species will be tied to that may therefore be everywhere montane because it is everywhere habitat (e.g., Smt increases with A>2000). (iii) The greater the excluded from the lowlands by this competitor. Thus, of the 46 area in a second and different habitat type, the fewer species species with montane populations, the sole one for which there may be restricted to the first habitat (e.g., Smt decreases with is compelling evidence that it is obligately montane is Gym- A <2000), for two reasons: higher invasion rates by species derived nophaps [albertisfi], though a few additional species may also from the second habitat, and greater likelihood that species be obligately montane (e.g., Cichlornis [whitneyi]?) or nearly derived from the first habitat will expand to utilize the second so (Phylloscopus trivdrgatus?) without proof being possible. habitat. The section Ratio of Montane to Lowland Area below What interisland differences can explain why a species like will give examples of such "niche shifts" dependent on relative Turdus poliocephalus is confined to the cold, wind-swept areas of adjacent habitats. conditions of alpine grassland above 9000 ft on one island, but lives in tropical forest on sun-baked coral at sea-level on another Why? island? These differences in altitudinal range are much grosser Why are montane populations absent from the lowlands? It is than could be explained by interisland differences in the often assumed that montane species can be recognized unam- physical environment. However, the niche shifts can often be biguously as species always absent at sea-level, at least at a correlated with major differences in the biological environment particular latitude. Two alternative types of explanations are of competing species in the lowlands and mountains. Most advanced for this assumed strict confinement to mountains: Melanesian bird populations are restricted by competition to physiological adaptations to physical environmental factors such altitudinal ranges narrower than the range over which the as barometric pressure, temperature, humidity, wind, and in- populations' physiological adaptations and habitat tolerance solation (see the chapter entitled "Alpine Animals" in ref. 12); would permit it to survive in the absence of competition, as also or habitat selection based on preferred associations of plant shown by Terborgh (20,21) for Andean birds. One can identify species and on vegetational physiognomy (13). some populations that have mainly been "pushed" into the Actually, a strict distinction between montane and lowland mountains by lowland competitors, and others that have mainly species does not exist, at least for birds. In 1931, one of us (14) been "pulled" into the mountains by a paucity of montane pointed out that the thrush Turdus poliocephalus and the fly- competitors, as follows. (i) Populations Pushed into the Mountains by a Single Lowland Competitor. Most altitudinal niche shifts discussed Values of A >2ooo/A for the 13 islands with montane <2000 avifaunas in the recent ecological literature are correlated with the are: Bougainville, 0.41; Guadalcanal, 0.33; Kolombangara, 0.18; Malaita, 0.096; Rendova, 0.081; San Cristobal, 0.080; , 0.072; presence or absence of a single close relative that represents by Ganonga, 0.032; Ysabel, 0.029; Gatukai, 0.021; Choiseul, 0.017; New far the major competitor (15-20). A Northern Melanesian ex- Georgia, 0.012; Vella Lavella, 0.0075. ample is that on and New Ireland, which the Downloaded by guest on September 23, 2021 Zoology: Mayr and Diamond Proc. Natl. Acad. Sci. USA 73 (1976) 1767 closely related lorikeets Vini rubrigularis and V. placentis poliocephalus in a tendency to be montane on species-rich share, these two species have altitudinal ranges that are largely Melanesian islands, but to occur at sea-level on species-poor mutually exclusive, the former species being confined to the islands. In all these cases, variation in diffuse competition in mountains above about 1500 ft. On Karkar, colonized only by the lowlands may contribute to altitudinal shifts, such that V. rubrigularis, it descends to sea-level; on Tolokiwa, colonized species are pushed out of the lowlands on species-rich islands. only by V. placentas, it.extends to the summit at 4650 ft (Fig. (iii) Species Pulled into the Mountains. The mountains of 39 of ref. 19). Evidently, V. placentis has pushed V. ubrigularis New Guinea harbor about 200 montane species, most of which out of the lowlands. Further examples in Northern Melanesia are endemic to New Guinea at the species level or higher and are provided by the variable altitudinal floors of Myzomela must have had a long history of adaptation to montane condi- cruentata (correlated with presence or absence of M. eryth- tions. The situation is very different in the mountains of romelas; Fig. 41 of ref. 19) and of Zosterops ugiensi (correlated Northern Melanesia. Because the total area of mountains in with presence or absence of Z. metcalfei). Five additional Northern Melanesia is small, populations that are confined to montane species of Northern Melanesia are confined to one or the mountains there have high extinction rates (see ref. 23), and a few islands, on each of which their altitudinal range abuts but only nine have survived long enough to become endemic to scarcely overlaps that of a lowland congener (Micropsitta Northern Melanesia at the full species level. In addition, New bruijnfi-M. fpusio], Pachycephala implicata-P. pectoralis, Guinea montane species are poor over-water colonists, and only Rhipidura dromnei-R. rufifrons, Zosterops murphyi-Z. ren- about 13 of them now have conspecifics or recognizable de- dovae, and possibly Aplonis brunneicapilla-A metallica, in scendants in the mountains of any Northern Melanesian island each case citing the montane species first and the lowland (see paragraph Jumping below). No single island has more than species second). Although the failure of these five montane seven of these preadapted New Guinea montane populations, species to reach an island without the lowland congener has nor more than two of the old montane endemics of Northern removed the opportunity to detect interisland niche shifts, the Melanesia. Because Northern Melanesian mountains thus have mutually exclusive altitudinal ranges on the shared islands make far fewer montane specialists than New Guinea mountains, it likely that confinement to the mountains is due to push from Northern Melanesian mountains also have far lower total a single competitor in these cases as well. population densities of birds, by up to a factor of 10(24); that (ii) Populations Pushed into the Mountains by Diffuse is, density compensation for the missing New Guinea specialists Competition in the Lowlands. While niche shifts due to by the few montane specialists of Northern Melanesia is very presence or absence of a single competitor provide clear illus- incomplete (see refs. 10, 22,24, and 25 for discussion of density trative examples, they probably account for only a small frac- compensation). As a result of these low densities, dozens of tion of all the niche shifts associated with differences in com- species that are confined on New Guinea to the low- petitive pressure. More often, the competition that a given lands by competing montane specialists expand to high eleva- species faces is not due overwhelmingly to a single other species tions in Northern Melanesia, even though they may be less well with high potential niche overlap, but instead to "diffuse adapted to higher elevations. competition," the summed effect of smaller niche overlaps with Because a species cannot be good at everything, these upward many species (see the community matrices and dendrograms expansions and adaptations to the mountains tend to come at in Appendix B of ref. 22). Naturally, it is far more difficult to the expense of competitive ability in the lowlands. In some cases identify and substantiate niche shifts due to changes in diffuse the colonists actually abandon the lowlands and become con- competition than due to presence or absence of a single com- fined to the mountains. This outcome will be favored if the petitor, but two methods permit clear identification in some release from competition that a colonist encounters on going cases: recognition of "assembly rules" which associate presence from New Guinea to Northern Melanesia is greater in the or absence of one species with particular combinations of mountains than in the lowlands: i.e., the colonist is pulled up competitors, and recognition of niche shifts closely linked to into the mountains. For example, the montane flycatcher changes in the total number of competing species (19). For Rhipidura dahli, an endemic allospecies of the Bismarck example, the flycatcher Rhipidurafuliginosa occurs at sea-level mountains, is derived from R. rufidorsa of the New Guinea on numerous Pacific islands which it shares with no or one lowlands, which R. rufidorsa shares with about five congeners. congener, but is montane on San Cristobal, which it shares with It at first seems mysterious that R. rufidorsa -- R. dahli should three lowland congeners. The altitudinal floor of Turdus pol- have abandoned the Bismarck lowlands, where it would have iocephalus is directly correlated with the total number of bird encountered only two competing congeners compared to the species on the island. Because the local number of species at a five on New Guinea, until one realizes that the Bismarck given altitude on a given island decreases with altitude, a species mountains contain no congeners at all compared to four in the sensitive to diffuse competition should tend to be confined to New Guinea mountains. Thus, a complete shift into the higher altitudes on species-rich islands than on species-poor mountains was favored over a more limited expansion into the islands, in order that it encounter no more than a certain mountains while retaining the lowlands. Similar compromises number of competing species on each island. For instance, the probably explain the derivation of Northern Melanesian effect of the enormous variation in the altitudinal floor of montane populations of the endemic allospecies Accipiter Turdus poliocephalus in the eastern parts of its range is that this princeps, Ducula melanochroa, Reinwardtoena crassirostris, thrush nowhere shares its habitat with more than 30 other Henicophaps foersteri, Vini meeki, and perhaps Rhipidura species, a limit that is not exceeded even at sea-level on Rennell malaitae, and even of some nonendemic montane populations, but that is exceeded at all altitudes below 90(0 ft on New from New Guinea lowland ancestors. Guinea (Fig. 42 of ref. 19). Five species that are montane on (iv) Ratio of Montane to Lowland Area. As discussed in the species-rich New Guinea (Accipiter [melanochlamys], Galli- preceding paragraphs, most altitudinal niche shifts in Northern columba beccarii, Coracina lineata, Monarcha [frater], Ar- Melanesian birds can be related to interisland variation in tamus [maximus]) descend to sea-level on every Northern competition. However, these considerations fail to explain why Melanesian island of occurrence. Twelve other Northern Me- the cuckoo-shrike Coracina caledonica and five pigeon species lanesian species resemble Rhipidura fuliginosa and Turdus (Ptilinopus solomonensis, Ducula brenchleyi, Reinwardtoena Downloaded by guest on September 23, 2021 1768 Zoology: Mayr and Diamond Proc. Natl. Acad. Sci. USA 73 (1976) crassirostris, Columba vitiensis, C. pallidiceps) occur at sea- level on some smaller Solomon islands but are confined to ele- vations above 3000 ft on the largest Solomon islands, Bou- JUMPING gainville and Guadalcanal, even though montane habitats on these two islands contain as many or more competitors of the six species involved than do the lowlands of the smaller islands. What does distinguish Bougainville and Guadalcanal is that, of all the mountainous islands of the Solomons, these two have \,1* LAND-BRIDGE by far the greatest montane areas (A>2000 958 and 501 square CROSSING miles, respectively, versus 0.8-145 square miles for the other /-1p islands), and the highest ratios of montane to lowland areas (A>2000/A<2000 0.41 and 0.33, versus 0.01-0.18 for the other islands). Insofar as natural selection maximizes population size, altitudinal niche shifts should depend not only on the relative number of competitors that a species faces in the lowlands and TRICKLING mountains of the two islands compared, but also on the relative areas (hence carrying capacities) in the mountains and lowlands. Hence, because of the high ratio of montane to lowland area on Bougainville and Guadalcanal and despite numerous mon- tane competitors, the six above-mentioned species could max- imize population size by shifting into the mountains, and PUSH-PULL abandoned the lowlands as a result of the difficulties of re- maining adapted to a wide range of altitudes. Similar consid- erations of relative areas help explain finer differences in the FIG. 1. Four alternative mechanisms for the colonization of altitudinal floors of the five above-mentioned pigeon species mountaintops by montane birds. Hatched area is the zone of montane elsewhere in the Solomons, and also underlie the dependence vegetation and climate. Jumping: a single direct flight from one summit to another. Land-bridge crossing: gradual spread through the of montane species number on montane and lowland areas, as lowlands in an era of cool climate, when montane vegetation descends summarized by Eq. 1. to the lowlands. Trickling: dispersal of occasional individuals through the lowlands without the individuals' remaining to breed. Push-pull: How? colonization of a summit by an originally lowland population, impelled Virtually the whole avifauna of Northern Melanesia, including by competition from other species in the lowlands and/or by avail- the 46 montane populations, has been derived from New ability of resources used by few competitors in the mountains. Guinea, except for a few species derived from and the . However, these montane populations reached (il) Land-Bridge Crossing. Jumping obviously cannot con- the mountains by various mechanisms. In this section we tribute to the dispersal of flightless montane animals too heavy evaluate the relative importance in Northern Melanesia of four to be wind-carried, and may be unimportant for some very different mechanisms for the origin of the montane populations sedentary flying forms. How such species reached their islands (Fig. 1). The first three of.these derive one montane population in the sky was a great puzzle to early biogeographers, until the from another, the last from a lowland population. realization (29) that montane climates had descended to the (I) Jumping. Colonists from one mountain may fly or be lowlands during cool phases of the Pleistocene. Over such "land blown directly to another, without stopping in the intervening bridges" of montane vegetation the montane species could have lowlands. Early biogeographers doubted that any montane dispersed without the need for any jumps, and would then have species was capable of such direct jumps between isolated been stranded in biogeographic eyries on mountaintops as the mountains. Indeed, most are not. Of the rich montane avifauna climate warmed up again (e.g., ref. 30). However, this mech- of New Guinea (about 200 species), only about 27 are now es- anism cannot have contributed at all to the arrival of montane tablished on even a single island not connected to New Guinea species in Northern Melanesia, which (at least in geologically by a former land-bridge, and only about 13 of these species are recent times) has always been separated by water gaps from now represented by montane populations in Northern Melan- New Guinea and other colonization sources. Nor can this esia. No segment of the New Guinea avifauna has dispersed less mechanism have contributed significantly to subsequent dis- over water than the montane avifauna. However, within persal between the modern islands of Northern Melanesia, Northern Melanesia these 13 montane colonists have together because no islands with montane avifaunas had Pleistocene produced 58 montane populations, and four of the colonists connections to each other except for Bougainville-Choiseul- (Ptilinopus [rivoli], Gymnophaps [albertisii], Phylloscopus Ysabel-Guadalcanal, Kolombangara-New Georgia-Vangunu- trivsirgatus, Erythrura trichroa) have each founded eight or Gatukai, Vella Lavella-Ganonga, and possibly New Britain- more different montane populations, evidently by successive Umboi. over-water colonizations. In addition, there are 8-14 other (iil) Trickling Through the Lowlands. Individuals of species species, represented by 16-37 montane populations, that ini- that can survive long or breed only on mountaintops may still tially reached Northern Melanesian mountains by the "push- be able to survive in the lowlands for sufficient time to disperse pull" mechanism discussed above but that may then have in low numbers from one mountain to another without any long spread from mountain to mountain by jumping. Thus, jumping jumps. This mechanism may have contributed significantly to has generated much of the montane avifauna: 21-27 of the 46 dispersal of montane species among the montane ranges of New species and 74-95 of the 128 populations. Jumping has similarly Guinea because the rare individuals of montane species en- generated virtually the entire montane avifauna of Timor (26) countered in the lowlands usually prove to be immature, the and part of that of Celebes (27) and of the Venezuelan High- age class most likely to be colonists (ref. 16, pp. 30-31). For the lands (28). same reasons as land-bridge crossing, trickling may have pro- Downloaded by guest on September 23, 2021 Zoology: Mayr and Diamond Proc. Nati. Acad. Sci. USA 73 (1976) 1769

vided an alternative to jumping for dispersal among mountains should be replaced by the more precise concept of a-diversity on the same island of Northern Melanesia, but cannot have (species diversity at a point) and ,3-diversity (species turnover contributed at all to the initial founding of montane populations between habitats; see ref. 10). in Northern Melanesia, nor to much of their subsequent dis- between islands. We thank the governments and numerous residents of the British persal Solomon Islands and New Guinea for making the field work (iv) Push-Pull Shifts. Montane populations may be derived possible; the National Geographic Society for support; and Martin Cody from lowland ancestors that were pushed up into the mountains for comments on the manuscript. by other lowland competitors (see i-ii under heading Why?). This mechanism probably applies to 6-8 montane species and 1. Linnaeus, C. (1753) Species Plantarum (Salvius, Stockholm). 9-12 populations whose nearest relatives are in the lowlands 2. Darwin, C. (1859) in On the Origin of Species by Means of of the same island and which segregate altitudinally from this Natural Selection (Murray, London), chap. 11. relative or another lowland congener. Other montane popu- 3. Wilson, E. 0. (1961) Am. Nat. 95, 169-193. 4. E. numerous lations are derived from lowland populations that were pulled Mayr, (1931-1957) papers in Am. Mus. Novit. 5. Mayr, E. (1945) Birds of the Southwest Pacific (Macmillan, New up into the mountains by the opportunities created by the very York). low bird population densities there (see iii under heading 6. Mayr, E. & Diamond, J. M. (1976) Bull. Mus. Comp. Zool., in Why?). In this category belong some of the 16 species with 50 press. montane populations that are derived from lowland populations 7. Diamond, J. M. & Mayr, E. (1976) Proc. Natl. Acad. Sci. USA but that are not excluded from the lowlands by a single, very 73,262-266. similar relative. Because it is often difficult to trace out effects 8. Diamond, J. M., Gilpin, M. E. & Mayr, E. (1976) Proc. Natl. Acad. of varying diffuse competition, it is uncertain whether these Sci. USA 73, in press. 50 populations were predominantly pushed or pulled. 9. Gilpin, M. E. & Diamond, J. M. (1976) Proc. Natl. Acad. Sci. USA Finally, about 8-10 species with 8-11 montane populations 73, in press. 10. M. L. in and are old endemics with no close relatives (e.g., Halcyon bou- Cody, (1975) Ecology Evolution ofCommunities, eds. Cody, M. L. & Diamond, J. M. (Harvard University Press, gainvillei, "Stresemannia" bougainvdllei, "Guadalcanarsa" Cambridge), pp. 214-257. inexpectata), so that one cannot determine how they arrived 11. Vuilleumier, F. (1970) Am. Nat. 104,373-388. in the mountains. In summary, jumping has contributed slightly 12. Hesse, R., Allee, W. C. & Schmidt, K. P. (1951) EcologicalAnimal more montane species (21-27) and populations (74-95) than Geography (Wiley, New York), 2nd ed. push-pull shifts (22-24 species and 59-62 populations). 13. Clements, F. F. & Shelford, V. E. (1939) Bio-ecology (Wiley, New York). Outlook: Unsolved problems 14. Mayr, E. (1931) Am. Mus. Novit. No. 502. 15. Diamond, J. M. (1970) Proc. Natl. Acad. Sci. USA 67,529-536. Despite recognizable patterns in the distribution and evolution 16. Diamond, J. M. (1972) Adfauna of the Eastern Highlands of of the montane avifauna, a bewildering range of differences New Guinea (Nuttall Ornithological Club, Cambridge). among species remains unexplained. Why does one montane 17. Diamond, J. M. (1973) Science 179, 759-769. species colonize readily (e.g., Phylloscopus trivrgatus), while 18. Diamond, J. M. (1974) Condor 77,14-23. 19. Diamond, J. M. (1975) in Ecology and Evolution ofCommuni- a close relative does not (P. amoenus)? Why can one species ties, eds. Cody, M. L. & Diamond, J. M. (Harvard University expand its niche in the absence of a competitor (e.g., Vini ru- Press, Cambridge), pp. 342-444. brigularis), while a close relative cannot (V. meeki)? Why does 20. Terborgh, J. W. & Weske, J. S. (1975) Ecology 56,562-576. one species have great ecological flexibility (e.g., Turdus pol- 21. Terborgh, J. W. (1971) Ecology 52,23-40. iocephalus), while another does not (e.g., Gymnophaps [al- 22. Cody, M. L. (1974) Competition and the Structure of Bird bertisii])? Communities (Princeton University Press, Princeton, N.J.). To account quantitatively for continental species diversity 23. Mayr, E. (1965) Science 150, 1587-1588. patterns, the distribution of species along a habitat gradient 24. Diamond, J. M. (1970) Proc. Natl. Acad. Sci. USA 67,1715-1721. within a single land mass, remains one of the major unsolved 25. MacArthur, R. H., Diamond, J. M. & Karr, J. R. (1972) Ecology problems of ecology. Altitudinal gradients pose this problem 53,330-342. 26. Mayr, E. (1944) Bull. Am. Mus. Nat. Hist. 83, in an especially clear form, but Eq. 1 is no more than a 123-194. crude 27. Stresemann, E. (1939) J. Orn. 87, 299-425. first approach to a Eq. 1 formulation. quantifies species number 28. Mayr, E. & Phelps, W. J., Jr. (1967) Bull. Am. Mus. Nat. Hist. and habitat area only dichotomously (Smt versus Slow, A>2000 136,269-328. versus A<2000), whereas both should be analyzed as continuous 29. Forbes, E. (1846) Geol. Surv. G.B. Eng. Wales 1,336-432. functions of altitude. Our discussion of "species tied to a habitat" 30. Brown, J. H. (1971) Am. Nat. 105,467-478. Downloaded by guest on September 23, 2021