Download Distributional Ecology of New Guinea Birds

Distributional Ecology of New Guinea Birds
Author(s): Jared M. Diamond
Reviewed work(s):
Source: Science, New Series, Vol. 179, No. 4075 (Feb. 23, 1973), pp. 759-769
Published by: American Association for the Advancement of Science
Stable URL: http://www.jstor.org/stable/1735788 .
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Species lliversity on Islands
If one were to count all the animal
or plant species occurring within an
area of I hectare, the result would vary
greatly dependng on the location of
the census area. The species total would
generally be much higher in the tropics
than in the temperate zones, higher at
the base of a mountainthan at the summit, higher on a large island than on
a small island, and higher on an island
near a continent than on a remote
Recent ecological and biogeographicaltheoriescan
island. It is important to understand
be tested on the bird communitiesof New Guinea.
this variation if, for example, one is
establishinga system of national parks
to
ensure survival of as many native
JaredM. Diamond
species as possible. Islands such as
those of the southwest Pacific lend
themselves well as test areas for a
quantitativetheory of species diversity
As indicated by frequent references understood.New Guinea has served as because each island represents a sepain the syntheses of zoogeography by the birdcolonizationsourcefor the thou- rate experiment, and because most inWallace;(l), of evolution by Mayr (2), sands of islandsof the southwestPacific; sular variation in species diversity can
and of ecology by MacArthur (3), the and New Guinea itself behaves as an be predicted from values of only two
tropicalisland-continentof New Guinea "islandarchipeSagos'
for montane birds, or three readily measuredvariables.We
and its birds have played a special role since its mountain ranges are isolated shall see that the diversity of bird
in advancing our understandingof ani- from each other by a i'sea" of unin- species on most Pacific islands is in a
mal populations. This role developed habitablelowlands.The numberof bird state of dynamic equilibrium that is,
partlybecause birdsare the best known, species on these oceanic islands and the diversity is determined by the
most easily observed and identifiedani- mountain islands varies with area and island's present physical characteristics
mals, and partly because of unique isolation, providing innumerable "ex- and is independentof the island's hisadvantagesof New Guinea itself New periments of nature" whereby the tory. However, on some islands, the
Guinea provides a range of habitats niche of a given species can be studied species diversity may also reflect the
from tropicalrainforest to glacierswith- as a function of the competing species island's recent history.
in distances of less than 16 kilometers, pool.
The number of land and freshwater
a range of elevations of over 50()0
Within the past decade, new para- bird species coexisting on each tropical
meters, and an equatorialposition that digms introsluced by MacArthur and island of the southwest Pacific varies
minimizes seasonal migration with its Wilson and their co-workers(3-7) have from 1 for some isolatedkatolls up to
associated complications. The rugged revolutionized our understanding of 513 for New Guinea itself. This variatopography, which isolates populations some central questionsof ecology, such tion is due partly to the greater variety
in adjacent valleys or on adjacent as: Why do diSerent localities support of habitatspresenton the largerislands.
mountains, has promoted speciation very different numbers of animal or However, even within a given habitat
within small areas of a single land mass plant species? What determines the type (for example, in tropical lowland
by essentiallythe same mechanismsthat distribution of a given species? How rain forest) there are great differences
underlie speciation on large continents. do related species manage to coexist? among islands in the number of bird
In New Guinea'sexpanses of forest un- These questions are not only of basic species to be found. These differences
disturbed by man, niche interrelations scientific interest but are also of prac- are largely predictablefrom an island's
retain a simplicity and beauty lost in tical importance in formulating con- area, its distancefrom New Guinea, and
altered environments,and distributional servation policies. Furthermore, the its elevation (13).
patterns illustrating many intermediate concepts of MacArthurand Wilson are
Figure 1 shows the number of bird
stages in evolution and in niche dis- proving increasingly helpful in under- species S occurring at sea level on
placement are readily identified. The standing human populations. In this islands between 8 and 500 km from
number of breeding bird species, 513, article I discuss these questions in the New Guinea, as a function of island
is large enough to give rise to the com- light of my studies of New Guinea area A (expressedas squarekilometers).
plex interactions characteristicof con- birds, conducted during six expeditions Over a 3-millionfold range of areas the
tinental faunas, but not s-o large as to to New Guinea and other islands of the results fit the power functionbe overwhelming.In spite of the physi- southwestPac;fic (8-15). A recent book
cal difficuIties of exploration in New discussesin detail many of the examples
5-12.3A022
(l)
Guinea, the distributionand taxonomy summarizedhere (15). Many patterns with an average error of 19 percent.
of its bird species are by now fairly well observed in New Guineaibirdsare rele- Thus, a tenfold increase in area invant to other groups of animals in other creases species diversity by somewhat
The author is professor of physiology at the
parts of the worlds especially in the less than a factor of 2. The numbered
School of Medicine, University of California,
Los Angeles 90024.
tropics.
deviant points represent islands in var23 FEBRUARY 1973
7ss
DistributionalEcology
of New GuineaBirds
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As a test of the equilibriumtheory, t represents time. Let us assume conin 1968 and 1969 I resurveyedthe land stant coefficients Ki and Ke, (expressed
and freshwater birds of a temperate as year- l ), respectivelyarchi,oelago and of a tropical island
(3)
E= Ke S(t)
whose birds had been surveyed50 years
(4)
I = Ki [S -S(t)]
previously:the nine ChannelIslands off dS/dt = l-E
KiS*:southern California (9), and Karkar
(Ki + Kc)S(t) (5)
Island off northern New Guinea (l 2).
is the mainland species pool.
On each island I found that between where S:
(dS/dt = O), the species
equilibrium
At
17 and 62 percentof the species present
given by
is
S(.(,
diversity
50 years ago had disappeared,and an
approximatelyequal number of species
KiS/(Ki + Kc)
absent 50 years ago had immigrated.
(6)
Relaxation to equilibriumfrom an iniThus, the species diversityhad remained
tial species diversity S(O) that differs
in dynamic equilibrium;Terborgh and
St, is described by:
from
Faaborg (17) obtained similar results
for the birds of Mona Island in the
[S(t) - S(sq]/[S(O) - Se] = e tXtr (7)
West Sndies.While a few of the extinctions and immigrationsin these studies where "relaxationtime" tl. is given by
were related to effects of man, most of
(8)
the changes were of the random kind
expected in the abselace of a human The relaxation time is the length of
role. As predicted f rom colonization time requiredfor the departureof spetheory (4), most of the extinctions in- cies diversity from equilibrium,IS(t)volved populations that were rare 50 S(ll, to relax to 1/e (or 36.8 percent)of
years ago because of such factors as the initial departure,15(°)-Se,ll, where
recency of colonization, small island e is the base of naturallogarithms.Actu>N= (12.3) ( I + 0.089 L/ 1000) X
size, presence of well-establishedcom- ally, this treatmentis only a crude apreal situation, be(e-D/'"")(A0 22) (2) peting species or small area or margi- proximation of the
nal suitabilityof island habitat. Among cause K( and Ki prove to be functions.
Similarly, species diversity in other tlaeChannelIslandsthe calculatedmini- Of S(t). However, by applying Eqs.
plant and animal groups on other mum turnover rates, expressed as the 3 to 8 to island faunas we can deduce
islands increases with island area and percentageof islandspecies immigrating the form of these functions, stipulate
decreases with distance from the colo- or becoming extinct per year, range some conditionsfor an improvedmodel,
nization source. These two trends con- from 1.2 percent per year for the and come to some conclusions about
stitute the fundamental law of island smallest or most barren islands down the distribution of bird species.
As an example of an "experimentof
biogeography.The exponent of area is to 0.3 percent per year for the largest
that permitsestimationof S(0),
nature"
generally in the range 0.20 to 0.34 island.
pair of values of S(t) and t, and
one
S(.<l,
Karfor
A
values
and
S
the
Since
(4. l 6).
To explain this law, MacArthurand kar fit Eq. l and lie on the general pat- hence calculation of tl., I will describe
Wilson (4), and Preston (16), suggested tern of Fig. 1, it is probable that bird the situation on a land-bridge island,
that insular species diversity represents peies diversity is near equilibriumon Misol. During the most recent Pleistoa dynamic equilibrium between im- most southwest Pacific islands. How- cene glaciation, when much water was
migration and extinction. Islands con- ever, there are some points in Fig. 1 sequesteredin glaciersand sea level was
stantly receive immigrantsdue to ran- that deviate conspicuously from the about 100 m below its present stand
dom dispersal of individuals from general pattern. These prove to be as- (18), Misol was partof the New Guinea
mainlands, and the immigration rate sociated with islands on which natural mainland and must have supported
should increase with island area and processes (such as volcanic explosions virtually the full New Guinea lowland
proximity to the colonization source. or changing sea level) have displaced avifauna of 325 species. Since the sevOn the other hand, island populations S from its equilibrium value at some ering of the New Guinea-to-Misol
risk extinction due to competition and approximatelyk nown recent time. On l-and bridge by rising .sea level about
random fluctuationsin population size, such islands S must be gradually re- 10,000 years ago, S(t) on Misol must
and extinction rates should be highest turning to equilibriumas a result of a have been relaxing toward the value
for the smallest islands with the small- temporaryimbalancebetween immigra- expected for an "oceanic island"of the
est populations. Thus, at equilibrium, tion and extinction; this process -may same area, as a result of extinction exwhen the species immigrationrate I on be termed "relaxation."Analysis of the ceeding immigration.Figure 1 (points
an island equals the species extinction islands on which species diversity is marked +) shows that Misol and other
rate E, there should be more species undergoing relaxation provides further large land-bridgeislands are still superon a large island than on a small island,- insight into immigration, extinction, saturated -that is, their present S(t)
values are still considerablyin excess of
and more on an island near to other and effects of island history (13).
The respective rates of immigration the equilibrium value .for oceanic
land than on a remote island. FurthermoreSspecies turn-overrates at equilib- and extinction, I and E (expressed as islands, although considerablyless than
rium should decrease with increasing species per year), depend on the in- the initial value of 325. Misol has an
stantaneousspecies diversityS(t), where area of 2040 km2 and should have 65
island area and remoten-ess.
ious stages of "relaxation,"as will be
explained.
l n Fig. 2 l ha\e plotted, as a function of island distance D from New
Gllillea, the ratio of an island's actual
S at sea level to the S value predicted
from the island's area and Eq. l. This
ratio decreases exponentially with distance, by a factor of 2 for each 2600
km from New Guinea. Thus, the most
remote islands of the southwest Pacific
(Mangareva and the islands of the
Pitcairngroup, 8()()0to 9200 km from
New Guinea) have a bird species diversity only 12 percent that of islands of
similar size near New Guinea.
The mollntains of the higher southwest Pacific islands harbor additional
bird species not occurring at sea level.
On the average, each 1000 m of elevation L enriches an island's avifauna by
a number of montane species equal to
8.9 percent of its avifauna at sea level.
Thus, bird species diversityon the New
Guinea satellite islands may be summarized by the empirical formula
Seq
=
tl
=
(Ki
+
Ke)
l
SC lENCE, VOL. 179
760
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lowland species at equilibrium (from
Eq. l) but actually has 135. If we take
Eq. 7 and substituteS(0) = 325, S(t) =
135, Seq= 65, and t = 10,000 years,
we obtain a relaxation time of 7,600
years for the avifauna of Misol.
Relaxation times can also be estimated from four other types of "experimentsof nature."These are: the fission of a large island into two smaller
islandsbecause of risingsea levels flooding a- low isthmus; contraction in an
island'sarea becauseof risingsea levels;
gradual extinction, from land-bridge
islands, of relict populations of those
New Guinea lowland species that never
cross water gaps of more than 8 km
or even 50 m (such species must have
reached the islands at the time of the
land bridge, and were isolated without
possibility of recolonization after submergence of the land bridge) (l3); and
recolonization of volcanic islands such
as Krakatau, Long? and Ritter, after
cataclysmic eruptions totally destroyed
the fauna.
From ca]culatedrelaxationtimes obtained by these methods for 19 New
Guinea satellite islands, the following
conclusions can be drawn(13). I(i)With
decreasing island area, extinction rates
increase (yielding shorter calculated tr
values) because of smaller population
sizes. Thus, among land-bridge islands
the calculated tr for the'relict populations decreases from 9000 years for an
island of area 7800 km2 (Aru), to 6;l00
yearsfor 450 km2(Batanta),and to 2630
yearsifor 145 km2 (Pulu Adi). Species
diversities on small land-bridgeislands
do not show an excess over equilibrium
values (Fig. 1) because relaxationtimes
for these islands are much shorter than
the 10,000 years that have elapsed since
severing of the land bridges. (ii) On
supersaturated islands certain species
are especially prone to extinction and
tend consistentlyto disappearfirst. The
resultingreleaseof the remainingspecies
from competition tends to increase
their population densities and to decrease their risk of extinction.Expressed
mathematically,K} on a given island
is an increasing function of Sl(t). (iii)
The -species that arrive first -at an
initia]l.yempty island tend to be'certain
species with consistently superior disperlsal abilityn such as those that
1.00-\!
characteristically colonize mainland
"second-growth"habitats(transientvegetational stages during regrowth of a
forest clea'ring).Subsequentimmigrants
are drawn from a mainland species
pool comprised of progressivelypoorer
colonists. Exp-ressedmathematically,K
on'a given island is a decreasingfunction of S(t). ;(iY) The probability of
extinction is much higher for recently
arrived immigrants 'that still have a
low population'density, than for established species'that-have saturatedavailable island habitats.
Patchiness of Species Distributions
.
.
.
.
Among the characteristicsof tropical
species that'distinguishthem from their
high-latitudecounterparts'arethe lesser
tendency of tropical'species to disperse,
their subjection to greater niche compression' by interspecific competition,
and their lower extinction and higher
speciation rates. All these characteristics contribute to the striking tropical
phenomenon called "patchiness"(3, 7,
15). Whereas the local presence or
.
.
.
.
*
*4
320-
*.
*/
0.50-
/
160-
78
i+t 5
'
+
3+
0.25._O
t)
Q
2 1O2s13_/
80
cn
14@
/
*H
O'
=
cn
X
_
<
3
',_
<;
O
0.125
,
Z
/?
}10
w
l
100
1000
10,000
100,000
l
1,000,000
Area (km2)
Distance (km)
Fig. 1 (left). Number of resident land and freshwater bird species on New Guinea ssatellite islands, plotted as an funotion of
island area' on a' double logarithmic scale. Symbols: *, islands on whch species diversity is presumed to be at equiliblrium
(the' remaining, numbered islands are in various s-tages of "relaoxation"after displacement of species diversity from an equilibrium value); , exploded volcanic island!s (1, Long); O, contracted islands (2, Goodenough; 3, Fergusson);' + land-bridge
islands (4, Aru;- 5, Waigeu; -6, lapen; 7, S;alawati; 8, Misol; 9, Batanta; 10, Pulu' Adi; 11, 'Ron; 12, {Sohildpadj- d3, island
fragments split ofT from a larger -island by flooding of an isthmus ( 13, Batjan- 14, Amboina- 15, New Hanover- 16, Tidore).
The straight line was fitted by leiast'mean squ'aresthrough points for all 'is]ands except the'land-bridge islands. No,te that the
number of species increases with area; and thait deviations Sforrelaxing islands are more marked for 'llargeislands than for small
' Fi'g. 2 (right). Ordiislands, 'because of high extinction rates and short relaxation times on the latter. [After Diamond (13)]
nate (logarithmic scale), number of resident land and freshwater bird 'species (S) on tropical southwest Pacific islands more than
500 km from New Guinea,' divided by number of species expected' on an is]and of equivalent area iess 'than 500 km from
New Guinea (calculated from Eq. 1). Abscissa^,island distance from New Guinea. The'graph shows that species diversity decl eases by a factor of 2 per 2600 ktn. [After Diamond ( l3')g
761
23 FEBRUARY 1973
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6°S-
l
IJSS
-
l
s
l
Fig. 3. Distribution of the Papuan tree creeper, Cliszzactezis 1eucopSzaea, in the mountains of New Guinea. Although mountains and forests with similar tree bark extend
uninterruptedfor 1600 km, and although there is no other New Guinea bird in the
same family, this bark-feeding species has a distribution gap (unshaded area) of 400
km in the middle of its range (hatched area). [After Diamond (15)1
absence of temperate-zonespecies can
generally be predicted from knowledge
of their particularhabitat requirements,
many tropical species are patchily distribu-tedwith respect to the available
habitat-that is, they may be absent
at a considerable fraction of the localitles oSering a suitable habitat for
them. Temperate-zone biologists, because they are rarely confronted with
examples of such patterns, often
assume that the evidence for patchiness
in the tropics can be dismissed as
an artifact of inadequate exploration
or insufficiently understood habitat
requirements.To prove that a species
actually is locally absent rather than
just overiooked is certainly more difficult than to prove that it is present.
Fortunately, the documentation of
patchy distributionsin New Guinea has
been facilitated by many New Guinea
natives who possess a detailed knowledge of local birds. Some natives were
able to name (in their native languages)
and accurately describe in advance all
bird species that I eventually located
in their areas; they could distinguish
at a distance obscure sibling species in
such taxonomically difficult genera as
Sericornis; and could accurately describe other species known to them only
from single individuals observed up
to 10 years previously (14, 15). When
such "walking encyclopedias" of bird
lore confirm the permanent local
absenceof a species that is regularlyencotlnteredin other areas, one can have
corfidence that the species is actually
absent and not merely overlooked. It
is even more difficult to prove that a
locality really does meet all of a bird's
762
habitat requirements, since a skeptic
can always claim that some unspecified
factor has been overlooked. However,
many of the examples of patchy distribution that I describebrieflybelow and
in detail elsewhere (15) involve wellstudied species which appear to have
distinct and readity defined requirements, and which are ubiquitous in
habitats meeting these requirementsin
many geographical areas of New
Guinea. Since there do seem to be
some generalizations emerging about
patchiness, it is becoming increasingly
unnecessary to invoke unspecified factors as an explanation.
There are four main types of patchy
distribution.
1) Distributionalgaps in a continuous
habitat.The ICentralDividing Range of
New Guinea provides an uninterrupted
expanse of montane forest for 1600
km. Nevertheless, 18 montane bird
species that would otherwise be uniformly distributedhave a distributional
gap of severalhundredkilometerssomewhere along the Central Range (Fig.
3 ) . For instance, the finch Lonchura
montana occurs commonly in large
flocks throughoutthe alpine grasslands
of western and eastern New Guinea,
where it is the only specialized seedeating bird, yet it is absent in the alpine
grasslandof centralNew Guinea,which
has similar grass and an otherwise
similar avifauna (15). Large distributional gaps in a continuoushabitat also
occur in the New Guinea lowlands.
The interpretationof these remarkable
patterns is discussed below.
2) Very local distributions.The distributionsof many species are patchy to
an extremedegree, such that they occur
at a few widely separatedlocalities but
are absent in similar habitatsover most
of New Guinea (15). For instance, the
flycatcher Poecilodryas placens is
known from six scattered areas, where
it inhabitsrain forestswith a well-shaded
understoryof small saplings in locally
flat terrain up to 1000 m above sea
level. The species is absent from hundreds of well-explored iocalities with
similarhabitatelsewherein New Guinea.
Most of these very local species in
New Guinea fall Into one of two categories: distinctive "monotypic genera"
with no close relatives (that is, genera
consisting only of a single isolated
species), or members of large genera
consisting of many eccalogicallysimilar
species. The highly fragmented distributions of these species suggest that
they are slowly becoming extinct, either
because they are the last survivors of
unsuccessful evolutionary lines (monotypic genera), or because they cannot
compete with several ecologically similar relativesin the same genus.
3 ) Complementary checkerboard
ranges. Some local absences of species
are correlated with the presence of
ecologically similar congeners, indicating that one-to-one competitive exclusion occurs in New Guinea mainland
habitats as on oceanic islands. For example, the extensive midmontanegrasslands, which are a by-productof human
agricultureduringthe last few centuries,
have been colonized in irregularcheckerboardfashionby eight Lonchurafinch
species native to other habitats. Each
midmontane area supports only' one
finch species over a considerablelocal
range of grass types and heights, altitudes, and rainfall conditions,'but the
areas inhabitedby a given species are
often scattered hundreds of kilometers
apart. Evidence indicates that in each
instance the first arrival became established over a local area and was able to
exclude potential subsequent colonists
of the seven other species, but the
identity of each locally successful
colonist depended partly on chance
(15). In the slightlymore complex situation illustrated in Fig. 4 ("compound
checkerboard exclusion"), each local
area can supportcoexisting populations
of any two species out of three potential
colonists, and the identity of the locally
missing third species varies irregularly.
4) Distributionalislands on a mainland. Comparisonof mountainsseveral
kilometersapart within the same range
invariably reveals faunal differences
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SCIENCE, VOL. 179
that cannot be explained by differences
in habitat (15). For example, species
characteristicof lowerlmontane forest
at 800 to 1100 m were compared on
four peaks of the North Coastal Range.
In order of decreasing.area and summit elevation, the mountains and their
numberof lower montanespecies were:
Menawa (1890 m high), 45 lower montane species at 800 to 1100 m; Nibo
(1560 m), 36 species; Somoro (1420 m),
34 species; and Turu (1140 m), 26 species. The four mountainshad structurally similar forest at 800 to 1100 m, but
the smaller mountains had less area of
such forest and, correlated with this,
fewer bi'rdspecies characteristic-of this
forest. Similar findings have been described for Andean birds and North
American mammals ( 19 ) . These patterns are reminiscent of the fundamental species: area relation of island
biogeography(see page 760). Evidently,
dispersal rates of birds betwee.n'New
Guinea mountainsseparatedfrom each
other by valleys a few kilometers wide
are so low that the peaks behave as
islands.
Significanceof Patchiness
While much remains bafRing about
patchy distributions in the tropics,
such distributionsappear to be caused
by the synergistic'effects of two characteristics.of tropical species comparedto
temperate-zone species. These are 'the
lower dispersalrates of tropical species,
which prolong the existence of distributional gaps as temporary or nonequilibriumphenomena;and the greater
pressure from interspecificcompetition,
which stabilizes gaps' as indefinitely
miaintainedor equiiibrium phenomena.
That the mean dispersal distances of
many New Guinea bird species are less
than'several kilometers is indicated (i)
by the absences of montane species on
mountains separatedby several kilometers from populations of the same
species on other mountains;(ii) by the
numerous species absent on oceanic
islands a few kilometers or even only a
few meters from the New Guinea mainland (13); and (iii) by the presence of
distinct subspecies or even semispecies
on mountains separatedby valleys several kilometers wide or on islands
separated by straits several kilometers
wide. Once a species has become locally extinct for whateverreason, immigration from populations in immediately
contiguousareas is so slow that distribu23 FEBRUARY 1973
Fig. 4. Compound checkerboard exclusion: distributions of three Melidecteshoneyochromelas;B, M. belfordi;R, M. sufocrissalissuperspecies. Most
eaters. O, MeZidectes
t two species with mutually exclusive altimountainous areas of New Guinea SUppO1
tudinal ranges. At each locality depicted on the map of New Guinea, the letters above
and below indicate the species present at higher and lower altitudes, respectively. The
identity of the locally missing thind species is subject to irregular geographical variation.
tional gaps in a continuous habitat may
persist for long periods of time, albeit
as nonequilibrium phenomena. For
example, the finch Lonchura montana
(discussedabove3may have disappeared
from the alpine grassland of central
New Guinea severalmillenniaago, when
the area of this habitat was reduced by
encroachmentof forest; and this finch
may dispersetoo slowly to have refilled
the whole gap since then. In the more
seasonal temperate latitudes, by contrast,the annualnorth-southmigrations,
and the postbreeding wanderings of
many nonmigratoryspecies, flood suitable habitats with potential colonists of
most species each iyear. Mortality due
to climatic fluctuations in temperate
Iatitudesplaces a premium on dispersai
ability to recolonize vacated territories,
whereas the greater stability of the
tropics selects against dispersal.
Effects of interspecific competition
on local distributionare easiest to recognize in the one-to-one situation of
simple checkerboard exclusion, where
species A, for example, occurs only in
the absence of species B and vice versa,
or in compound checkerboard exclusion, where species A occurs only in the
absence of either species B or species C.
The eompetitors in these situations are
usually, t-houghnot always, close relatives within the same genus. It is more
diicult to determinethe connections in
so-calied diffuse competition,where the
absence of a species is due to the combined efdfectsof many species, each
somewhatdistantlyrelatedto the absent
species and potentially overlapping it
ecologically only in part (3, 10). A
decline in species B following an invasion of its close competitorA may permit B's other close competitor C to
increase, depressing the population of
C's competitor D and ultimately affecting species ecologically far removed
from A. Correlated with the greater
diversity of species in the tropics than
at higher latitudes is the fact that tropical species have more closely packed
niches than their temperate counterparts, such that the local survival of a
given species may be criticallydependent
on the mix of competitorsas well as on
the suitabilityof the habitat (3, pp. 231
ff3. Thus, competition may stabilize
some distributionalgaps indefinitely.
These considerations help us to understand the differing numbers of
species on differentcontinents.Low dispersal may allow localized populations
of disappearingspecies to linger, relatively undisturbedby influx of competitors, for long periods of time before
final extinction. Conversely,low dispersal favors high rates of speciation, since
the first stage of speciation depends on
the effectiveness of geographical barriers or distributionalgaps (Fig. 8). Because the number of species at equilibrium on a continent depends on a
balance between speciation rates and
extinction rates, tropical habitatsshould
be expected to have a greater number
of species at equilibriumthan habitats
at higher latitudes (Fig. 5). Furthermore, with increasing continental area,
extinction rates should decrease (because of larger population sizes and
more local refuges), and speciationrates
should increase (because of more popu763
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_
.
_
_
_
_
b
Fig. 5. Speciation rates (solid curves) and
E1
extinction rates (dashed curves3 on a
JZ sa
single land mass, as a inction of the
l/
number of species (abscissa). The ordiU
n?te is the tota1'number of species pro*ov
ir
duced through speciat-ionor lost through
extinction per 10S000 years. The intersection of the curves (N1,^1V2) determines
the number of species at equilibrium. The
S2
speciation cu'rve'is approximately linear,
o o
/
/
/
because the probability'of speciation of a
given species'is appro:ximatelyindependent
a.O
of the presence' of other species but extinction rates increase sharply as more
O
/
f
species are packed,-together, ()n a large
AS-_
*/
*
tropical land massn high' speciation 3nates
N2
N:I:
(curve Sl) and 1QW e,xtinctionrates (EI)
Number of species
yield' a high nvimberof species at equilibrium (N1). On a similar-'sizedtemperate
lancl mass or a smaller tropical Iand mass,
lower speciation rates (S2?=and higher exThe best understoodsegregating
r mechatinction rates' (E2) yield fewer species at
nism among New Guinea birdsdepends
equilibrium ( N2 ) .
their altitudinal range (Fig. 6). With
increasingelevation C. mglrinabecomes
increasinglyabundantuntil it-suddenly
disappears at 1643 m an altitude not
far above its altitude of masimum
abundance.At this elevation C. rohsttz
suddenly appears near its masimum
abundance and becomes progressively
less common toward Mt. KarinluiSs
summit. Prolonged observationsat the
transition altitude showed that the tWQ
species ,are interspecifically territorial
at 1643 mnand no individual of either
species was ever found transgressingthe
range of the other. There is no change
in vegetationat this altitude the nearest
i'ecotone' (a border 'between forest
types) lies 338 m higher. In all, the
New Guinea avifauna contains about
on altitude and involves an inlteresting 45 pairs, 13 trios and 3 quartets of
behaviolnal response.
relatedspecies which ]replaceeach other
.
,.
Altitudinxxl segregation. O]n New altitudinally with similar abruptness.
lations isolated over greater distances).
Thus, one should also expect more Guinea mountains not distur
rbed by For a given species pair the transition
species at equilibriumin a large center man, forest extends uninterrupt
:ed from altitude shows minor local variation
of speciation, such as the Amazon sea level to timberline at around 3800 correlatedwith local conditionsof rainBasin, than in a small center, such as m. Many bird species especialElythose fall, exposure, and slope but no-sysNew Guinea (Fig. 5).
witk no closely related species
s in the tematicgeographicalvariationoverNew
areanexhibit gradual changes iin abun- Guinea'. Altitudinal sequences with
dance with altitude. However in many sharp transitions also occur among
Ecological Segregationof Species
other instances one finds seque
ences of birds in the Peruvian Andes (21) and
two, three or even four closel f rela-ted on all large mountainous islands of
A central problem of ecology con- species replacing each other, abruptly Indonesia eind the southwest Pacific.
cerns community- organization: How with altitude (15)* For examptleSCrStric,ttransitionsmay be vi,olatedby
are resourcesdivided among the species teroseelis murina and CratterosceZis young birds: when one finds an indiof an ecological community and what robusta,two abundantand ver y similar vidual outside the normal altitudinal
mutual arrangements permit the c o- warblersthat glean for insects ]near the range of its species it generally proves
existence of closely relatedspecies (20)? ground dif3er ecologically m ainly in to be a juvenile or an immaturebird.
Or
X,
a
.
§e
/<
.
.
.
.
.
2500 -
2000
Fig. 6. Altitudinal ranges of the warblers CraterosceZisrobmsta
(*) and C. murina (0) on the'west ridge of Mt. KarimuiS
New Guinea. On the left, eacRhmark represents one individual
heard, seen, or collected at the given altitude''(the paucity of
records at 650 to 1050 m results from my. having spent little
time at this altitude). The right-hand side gives 'the relat;ve
abundance in ' the whole avlfa.una-that is the percentage of
bird individuals of all species estimated as being C. robust or
C. m7lrin. The two species replace 'each other abruptly at 1643
m, ard each species reaches'its masimum abundance near tEs
altitude. Many other .species show equally sharp'.transitions
although the altitude of masimum abundance frequently differs
frc>n the altitude 'at which the species transibion occurs
.!
-
*;
::
::#S
C. robusta
C. murina
-
*.
g g
1 SOO a
3
/
._
C5
o
O
[000
r
oW
500-
-
L__,
o
764
]__
1
t
l
I
{
_
4 6 8 10 12
S0totai
of bgrdindividuals
2
SCIENCE, VOI. 179
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Among New Guinea birds generally,
there is a characteristicDaltitudinal
dependence of population structure.
Typically, immature birds are found
in a fringe at the bottom of the
altitudinalrange; somewhat higher, one
finds immature birds plus nonbreeding
adults, with females usually appearing
at lower altitudes than males; next
comes the optimal part of the species'
range, with breeding adults of both
sexes; and, finally, another fringe of
immature birds but few adults appears
at the upper altitudinal limit of some
but not all species. This population
structure is manifested to an extreme
degree by some birds of paradise,
whose displaying adult males may becompressed into the top 180 m of the
altitudinal range, with adult females
and especially immature birds being
found up to 1000 m below the lowest
adult male.
UItimately the altitudinal sequences
are related to differential adaptations
that are continuous functions of altitude, one species being preferentially
adapted to higher altitudes, cooler temperatures,and more montanevegetation
than the other species. But the very
sharpnessof the transitionsimplies that
competition superimposes a special
behavior pattern on these differential
adaptations,since the transitionsdo not
coincide with sharp changes in temperature or vegetation. That is, each
species must be capable of surviving
over a wider altitudinal range than it
actually inhabitsand must be excluding
related species from the range in which
it is competitivelysuperiorto them. To
confirm this interpretation, the local
poptllation of one member of a sequence would have to be removed in
order to ascertainwhether the adjacent
species would expand their altitudinal
ranges. This test of competition arises
in New Guinea under two types of
naturally occurring conditions.
The first test of niche expansion
associated with relief of competition
occurs on small or isolated mountains
or islands (8, 10, 15). Because of the
dependence of species diversity on
area and isolation and because of the
random element in colonization, one
or another member of an altitudinal
sequence may be missing on such a
"mountainisland."As shown in Fig. 7,
under these circumstancesa low-altitude
species may expand into the range of
a missing high-altituderelative, a highaltitude species may expand into the
range of a missing low-altituderelative,
and a high-altitudespecies and a low23 FEBRUARY 1973
altitude species may expand simultaneously into the range of a missing
middle-altituderelative.
Reconstructionof the process of speciation provides the other test for competition and clearly demonstrates the
origin of altitudinal sequences ( 15) .
Speciation occurs when an initially
continuous population breaks up into
geographical isolates, the isolates
diverge, reestablish geographical contact and perfect their reproductive
isolating mechanisms, and finally reinvade each other's geographical ranges
if there are sufficient ecological differences to permit coexistence (2).
While the slowness of evolution rarely
permits following a given species
through this process with time, the
distribution patterns of diSerent New
Guinea montane species represent
"snapshots" of seven different stages
in a continuous speciation process
(Fig. 8):
1) A single montane species extends
from the western to the eastern end
of New Guinea, oiccupying the same
altitudinal range at all longitudes.
2) The local population in one area
dies out, so that the east-west distribution becomes discontinuous.
3) The eastern and western populations, now isolated, diverge sufficiently
that they would probably not interbreed if contact were established;hence,
they are assumed to be distinct species.
They may also develop slightly diSerent
altitudinal ranges.
4) Both populations reexpand geographically until their geographiclal
ranges abut but--donot overlap. There
is no or little interbreeding, proving
that the populations are in fact distinct
species.
S) Each species begins to expand
geographically into the range of the
other, so that there is geographical
overlap ("sympatric" distribution) for
a short distance. Within the zone of
sympatry the two species segregate
altitudinally,each being confined to the
altitudinal range in which it is competitively superior to the other. The
narrower altitudinal range inside than
outside the zone of sympatry is a clear
demonstration of niche compression
due to competition.
6 ) Expansion continues, and the
western species reaches the eastern end
of New Guinea and overrunsthe entire
geographicalrange of its eastern sibling
species. The eastern species has given
up the upper (or lower) part of its
altitudinal range throughout its whole
geographical range7 and there is no
altitudinal overlap.
7) The eastern species continues to
expand until it has reached the western
end of New Guinea. The two species
are now sympatric over the entire
length of New Guinea, with mutually
exclusive altitudinal ranges. Evolution
Mt.
4000Mt.
may then continue in the direction of
Wilhelmina
Mt.
Michael
either
stage 8a, 8b, or 8c.
Saruwaged
8a) Stages 1 through 7 may be repeated one or two more times to yield
,.X..'.'...i..'V..X.'T--T'
series of three or four closely related
..'.-'.-."-.'-'ly.'S-iT-'..i:
.T..T,i1..Xi:0f,00i;
species sympatric over the whole of
3000-i.-0,-.-X-00-..,;i.X--0ft;:
New Guinea but occupying mutuallyexti.'---iX'TS'-0.Xt-itA.,X,-t
clusive altitudinalranges.
,.,i,,,i,,,,,,000t,t,,,,f,.;
..;;.;f
X00.;
TS,,;fE4i,!00X
8b) Two species that have become
,...t,--0f."t.,f-'T,',;d,'T,,lL;
3
sympatric
with mutually exclusive
0'-'."i't-'TiX'T-i'
iTE.-40;-0T'tT-'.iT.X.';.'tl.it
altitudinal
ranges
may diverge in other
zr:
.0T,,-T,T,';ttEt;0'S-'t;00tl-07
2000niche parameters besides altitudinal
-,000t,-;i..0f-,X,0X-0tT;t
.t,,000;-Xti.-E,;i...-,;X..t.,.
preference (for example7 they may
Tit-'t;-S't"t'.'.;.-t;-.X:-;X.:00
iS.Xt;
:ti
,0T
,:fE.t
..l,
ly
TlL,0 develop differentdiets or foraging tech-00t-'T-'T
iT-'tt.ti".t;000.-i;
niques), so that partial altitudinaloverlap as well as complete geographical
1000 /
overlap becomes possible.
8c) Each species becomes genetically
of
molded to its compressed altitudinal
Fig. 7. Altitudinal ranges of the honey- range to a degree such that the range
eaters PtiZop)ora perst)iata (diagonally now reflects innate survival ability
hatched bar) and P. guisei (solid bar) on
three New Guinea mountains. On Mt. rather than competitive compression, a
Michael, where both species are present, gap between the ranges of the two
their altitudinal ranges are mutually ex- species develops, and neither species
clusive. On Mt. Wilhelmina, where P.
expands altitudinallyon removal of the
guisei is absent, and on Mt. Saruwaged,
other.
where P. pe)striata is absent, the remainThis process seems to be the principal
ing species takes over most or all of the
mechanism by w'nichthe rich diversity
altitudinal range of its missing relative.
765
-
..
....-
-
a)
._
0..
.=L
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-
this overlap band the neighbor of a
given individualwould as likely belong
to the other species as to the same
species, whereas in the distributionsof
Fig. 6 each individual is surrounded
by individuals czf the same species. In
fact, altitudinal segregation of congeneric species on mountainousPacific
Stage
Stage
islands with fewer than 66 species
30003000always involves broad overlap bands,
C)
1
and sharp transitionsoccur only on the
2000
more species-richislands. This contrast
z
suggests two selective pressuresunderlying sharp transitions.First, in species1 000 {ooopoor areas the niche of a given spe
cies is nearly as fbroad as permitted
Oo]
loy its intrinsic adaptationsand is little
compressedby interspecificcompetitione
30003000
these conditionsthe success oiTa
7 Under
2
dispersingjuvenile in colonizing a new
2000
territory will depend mainly on visible
2000features of the environment.In species1000rich areas,'however,the task-of identi1000fying a territory that offers a high
probability of breeding success for a
Ogiven species becomes much more
-3000difficult, since success now depends
30008a critically on the presence and abun3
dance of many other competingspecies.
a
2000This in effect would requirea dispersing
s
1 M
3
juvenile to perform a faunal survey in
111111
10001000addition to merely judging the appear:
ance of the habitat. Second, a selective
OOpressure may arise from the difficulty
of finding mates in a species-richcom3000- munity where many species are rare.
30008b Thus, I postulate that, on species-rich
4
2000islands but not on species-poorislands,
2000dispersingyoung birds are programmed
1000or they learn by early
genetically,
10P0experience, to seek out habitats where
their species is already established,
OOrather than habitatswhere they will be
surrounded by individuals of other
n
30Q0300018c species. The presence of individualsof
1
5
their own species becomes the only
20002000reliable indicator that the habitat is
suitable for them. By waiting in the
juvenile fringe or "kindergarten"at
1
1000the lower or upper limit of the
f
|
' 1
O
I
altitudinal range, the juveniles are in
o
132 138 144 150
a ringsideposition from which they can
132 138 444 150
eventually seize an optimal territory
vacated by an adult.
Longitude (°E)
h
Otherspatialsegregatingmechanisms
Fig. 8. Stages in the evoltltionary transformation of one species into two species with
further ecological segregating
,I
Two
mutually exclusive altitudinal ranges. See text for details. In each diagram altitudina]
range is plotted as a function of longitude for some species or pairs of species in thee mechanisms, vertical stratificationand
t1 habitatsegregation,resemblealtitudinal
mountains of New Guinea. Thus, in stages 1 through 4 there is neither geographica]
nor altitudinal overlap, in stages 5 through 8a and 8c there is geographical but noh segregation in that closely related
altitudinal overlap, and in stage 8b both geographical and altitudinal overlap. Eackd species become sorted out spatially.
such distributionalpattern representsa "snapshot"of a diSerent stage in speciation and
permits reconstruction of the whole process. Examples of species in each stage are::
Bird species are more stratified
1, Ifrita kowaldi; 2, Climactesis leucop11aea;3, Melidectes nouXuysi and M. princepsi; vertically in the New Guinea forest
poifosoma and P. 1zattamensis;6
4, ..Parotialawesi and P. carolae; 5, Pac11ycep1lalopsis
I' than in forests with fewer bird species
Ael blyo)nis macg}egoriae and A . subala}is; 7, Ducula rufigaste} and D. c11alsonota
but similarvegetationalstructure.Thus,
8a, Eupetes caerulescens, E. castanonotus, and E. Ieucostictus; 8b, Neopsittacus mus
standard bird nets 2 m in height and
schenbroekii and N. pullicauda; 8c, MeZiphagaanaloga and M. orienfalis.
of bird species in the mountains of
l!Jew Guinea evolved out oiTthe lowlands (15).
What is the significance of sharp
altitudinal transistions? If there were
o special behavioral mechanism pro-
ducing such transitions, one would
expect two competing,territorialspecies
with differentaltitudinaladaptationsto
replace each other in an overlap Iband
which would consist of a mosaic of
territories of the two species (2Z). ln
fW
-
-
._
_
,
S
-t
dooo-sts5ssIIsIazIIIIIIIIIIIIIID
SCIENCE, VOL. 179
766
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resting on the ground catch only about
half of the forest species locally present
in New Guinea montane rain forest but
all species in the species-poorrain forest
of New Zealand. Some of the species
in New Guinea forage only in the
canopy; others forage regulalrlyat 4 to
15 m above- the ground but never
descend to 2 m and hence are never
caught in nets; and others forage only
on the ground or up to a height of
2 m. As in the case of altitudinal
ranges, vertical foraging ranges expand
on species-poor mountains or islands
where vertically abutting competitors
are absent (10) .
Some congeners become sorted by
occupying different habitat types, often
to the mutual exclusion of each other.
For example, whereas there is only one
species of barn owl (genus Tyto) in
most parts of the world, New Guinea
has three species: Tyto capensis in
grassland, T. alba in partly wooded
areas, and T. tenebricosa in forest.
Among species sorting by habitat, spa-tial expansion is a frequent response to
the absence of competing congeners
on islands (10, 11). Expansion of New
Guinea second-growth species into
forest on islands is especially common,
since New Guinea forest species are
frequently absent on islands because
of their poor dispersal ability
concentratedtowardthe main branches,
the smaller birds toward the periphery
(Fig. 9). Among congeners sorting by
size in New Guinea, the ratio between
the weights of the larger bird and the
smaller bird is on the average 1.90; it
is never less than 1.33 and never more
than 2.73. Species with similar habits
and with a weight ratio less than 1.33
are too similar to coexist locally (that
is, to share territories)and must segregate spatially.For instance, the cuckooshrikes Coracina tenuirostris and C.
papuensis segregate by habitat on New
Guinea, where their average weights
are 73 grams and 74 grams, respectively, but they often occur together in
the same tree on New Britain, where
their respective weights are 61 g and
101 g. New Guinea has no locally
coexisting pairs of species with similar
habits and with ia weight ratio exceeding 2.73, presumably because a
163
123
_<
/t
Fruit (mm)
/
<L<_
A
7
414
-L
&,
_
jt/
&
20
Nonspatial SegregatingMechallisms
Spatial overlap of closely related
species is possible if they separate on
the basis of time, diet, or foraging
techniques.
Infrequently, closely related bird
species segregate by occupying the
same space at different times of day
or of the year. The kingfisherMelidora
macrorhinais nocturnal,whereas other
kingfishersare diurnal. The south New
Guinea savanna near Merauke is alternately occupied by two marsh hawks,
Circus approximansin the dry season
and C. spilonotus in the wet season.
Differences in body size provide the
commonest means by which closely
related species can take the same type
of food in the same space at the same
time (15). Larger birds can take larger
food items than can smaIler birds, but
smallerlDirdscan perch on more slender
branches than can larger birds. One
can frequently see a bird foraging out
along a branch up to the point where
the branch begins to bend under its
weight. In a tree occupied by birds of
many species the larger birds are often
802
slAf-.,Lz'>
t40
Fig. 9. Schematic representation of niche
reIations among the eight species of
Ptilislopus and Ducula fruit pigeons in
New Guinea lowland rain forest. On the
right is a fruit of a certain diameter (in
millimeters), and on the left are pigeons
of different weights (in grams) arranged
along a branch. Each pigeon weighs approximately 1.5 times the next pigeon.
Each fruit tree attracts up to four consecutive members of this size sequen-ce.
Trees with increasingly large fruits attract
increasingly large pigeons. In a given tree
the smaller pigeons are preferentially distributed on the smaller, more peripheral
branches. The pigeons having the weights
indicated are: 49 g, Ptilinopus nanus; 76
g, P. pulc11ellus;123 g, P. supesbus; 163
g, P. orl1atus; 245 g, P. pe11atus; 414 g,
Ducula sulSga*ter;592 g, D. zoeae; 802 g,
D. pinon.
medium-sized bird of relative weight
+/2.73 = 1.65 can coexist successfully
with both the large species and with
the small species. Thus one finds a
sequence of three or more species
rather than just two species of such
different sizes. For example, the eight
fruit pigeons of the genera Ptilinopus
and Ducula coexisting in the lowland
forests of New Guinea form a graded
size sequence over a 16-fold range in
weight (Fig. 9).
May and MacArthur (6) predicted
on theoretical grounds that sprecies
segregatingalong a single niche dimension in a Ructuatingenvironment must
maintain a certain minimum niche difference. This minimum spacing seems
in fact to have been reached in nature
by those bird species that segregate
according to size. Thus, on Pacific
islands with 30 to 50 bird species,
the ratio between the weights of pairs
of birds segregatingby size is approximately 4, but this ratio has already
been compressed to 2 on islands with
100 species. On New Guinea (513
species) the average value of this ratio
is compressedno further, and the extra
species are accommodated by expanding the size sequences to smaller or
larger birds or else by finer subdivision
of space or foraging techniques.
Similarly sized species that take the
same food may overlap spatiallyif they
harvest the food in different ways. For
instance,small insectivorousbirds differ
in tactics according to whether insects
are caught in midair by sallying, are
pounced on and plucked off surfaces,
are gleaned off surfaces, pried out of
bark, taken from flowers, or are extracted from epiphytes and accumulations of dead leaves. Species with a
given type of strategy further differ in
the ratio between traveling time and
stationary time, in the frequency of
movements, and in their average rate
of travel (23). Thus, the montane
Rycatchers Pachycephala modesta and
Poecilodryas albonotata differ in that
the former remains perched for an
average of 2 seconds, the latter for
an average of 30 seconds between
moves; and in that the former travels
1 m, and the latter 12 m, per move.
Pachycephala modesta could be described as a quick and cursory searcherz
Poecilodryas albonotata as a slow and
seI>ctive searcher.
Finally, relatedspecies may segregate
by different diets. For example, the
whistler Pachycephala leucostigma eats
mainly fruit while other whistlers eat
.
maln y lnsects.
23 FEBRUARY 1973
767
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Some General Features of Competition increase linearly or even more rapidly
with species diversity,so that an island
Despite the abundant distributional with few species also has a low density
evidence for competition between New of individuals (11). However, on the
Guinea bird>;species, one rarely sees Pearl Islands oS Panama, total populaa member of one territorial species tion densities of birds are even higher
fightinga memberof anotherterritorial than on the Panama mainland l(25).
species. Once territoriesare established, Part of the difference between these
fighting simply does not pay: the win- two sets of results may depend on
ner as well as the loser may be injured, whether the island colonists are as
or both combatants may attract the well adapted to the available habitat
attention of a predator. Even among as are the mainland species they remigrant North American thrushes that place. Also, the low populationdensities
must reestablish their territories each on the New Guinea satellite islands,
spring, interspecificchases and aggres- and on old isolated islands such as
sive behavior disappear within a week Madagascar (26) and New Zealand,
after arrival on the breeding grounds may reflectgenetic deteriorationin their
(24). In tropicalrain forest, where many isolated populations, because of small
bird species are permanently resident gene pools, reduced intraspecific and
and are relativelylong-lived, and where interspecific competition, and (frevisibility inside the forest is poor, quently)short populationsurvivaltimes
neighboring individuals may confine to extinction (1.1).
their aggressive behavior for years to
songs and calls a.nd simply learn to
Conclusions
avoid each others foraging space.
Species that colonize species-poor
The concepts by which MacArthur
islands, where they are freed of competition from close relatives, often and Wilson have transformed the
broaden the spatial parameters of the science of ecology in the past decade,
niche by immedia.telyoccupyinga wider and the results of ecological studies
range of habitats, altitudes, or vertical such as mine on New Guinea bird
foraging positions. However, the col- communities, have implications for
onists rarely expand their diets or conservation policies. For example,
range of foraging techniquesuntil after primary tropical rain forest, the most
relatively long periods of time on an species-rich and ecologically complex
evolutionary scale (11). This combina.- habitat on earth, has for millions of
tion of spatial elasticity with tactical years served as the ultimate evoluand dietary conservatismreRectspartly tionary source of the world's dominant
the degree of genetic programming plant and animal groups. Throughout
underlying the stereotyped foraging the tropics today, the rain forests are
strategies of birds, and partly the being destroyed at a rate such that
economics of feeding. Given a certain little will be left in a few decades.
set of inflexible tactics, the economics When the rain forests have been reof energy yield and energy expenditure duced to isolated tracts separated by
in foraging dictate the diet but leave open country, the distributionof oblito the individualthe decision about the gate rain forest species will come to
space in which the tactics can be applied resemble bird distributions on New
Guinea land-bridgeislands after severprofitably(3, chap. 3).
How does the combined population ing of the land bridges.The smaller the
density of all bird species on an island tract, the more rapidly will forest
compare with the combined population species tend to disappear and be redensity on a mainland?That is, istands placed by the widespread secondhave fewer species than mainla.nds,but growth species that least need protecthe types of competitive release we tion (13). This ominous- process is
have examined mean that the island illustrated by Barro Colorado Island,
colonists frequentlyhave broaderniches a former hill in Panama that became
and higher populationdensitiesthan on an island when construction of the
the ma.inland.How well does competi- Panama Canal flooded surrounding
tive release compensate on islands for valleys to create Gatun Lake. In the
the population densities of missing succeeding 60 years several forest bird
mainland species? Studies on this species have already disappearedfrom
problem of density compensationhave Barro Colorado and been unable to
provided conSictingresults. In the New recolonize across the short intervening
Guinea area, total population densities water gap from the forest on the nearby
in similar habitats on different islands shore of G;atunLake.
768
The consequences of the speciesarea relation (Fig. 1) should be taken
into considerationduring the planning
of tropical rain forest parks (13). In a
geographical area that is relatively
homogeneouswith regardto the fauna,
one large park would be preferable to
an equivalent area in the form of
several smaller parks. Continuousnonforest strips through the park (for
example, wide highway swaths) would
convert one rain forest "island" into
two half-size islands and should be
avoided. If other considerationsrequire
that an area be divided into several
small parks, connecting them by forest
corridors might significantly improve
their conservation function at little
further cost in land withdrawn from
development.
Modern ecological studies may also
be relevant to the understiandingof
human populations. For instance,
during a long period of human evolution there appear to have been not one
but two coexistent hominid lines in
Africa, the AustralopithecusrobustusA . boisei l("Zin
janthropus') line, which
became extinct, and the A ustralopithecusafricanus-A. habilisline, which
led to Homo sapiens (27). The need to
maintain niche differences between
these lines must have provided one of
the most important selective pressures
on the ancestors of modern man in
the late Pliocene and early Pleistocene.
Thus, any attemptto understandhuman
evolution must confront the problemof
what these ecological segregating
mechanismswere. To what extent were
contemporaneous species of the two
lines separated by habitat, by diet, by
size difference, or by foraging technique, and were their local spatial distributions broadly overlapping or else
sharpenedby behavioralinteractionsas
in the case of the Craterosceliswarblers
of Fig. 6? To take another example,
there are striking parallels between the
present distributionsof human populations and of bird populations on the
islands of Vitiaz and Dampier straits
between New !Guineaand New Britain.
Some of these islands were sterilized
by cataclysmic volcanic explosions
within the last several centuries. The
birds that recolonizedthese islandshave
been characterizedas coastal and smallisland specialists of high reproductive
potential, high dispersal powers, and
low competitive ability, unlike the geographically closer, competitively superior, slowly dispersing, and breeding
birds of mainlandNew Guinea.(10, 11,
13). It remains to be seen whether
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SCIENCE, VOL. 179
15.
the people of the Vitiaz-Dampier
islands, the Polynesians, and other
humanpopulationsthat colonize insular
or unstableh;abitatsalso have distinctive
poptllationecologies.
6.
7.
8.
References and Notes
1. A. R. Wallace, The Geographical Distribution
of Animals (Macmillan, London, 1876).
2. E. Mayr, Systematics and the Origin oJ
,Species (Columbia Ulliv. Press, New York,
1942); Arlimal Species and Eltolution (Harvard
Univ. Press Cambridge, Mass., 1963).
3. R. H. MacArthur, Geographical Ecology
(Harper & Row, New York, 1972).
4, - -- and E. O. Wilson, The T}teory ot
[sland Biogeogra phy (Princeton Univ. Press,
Prunceton, N.J., 1967).
5. Is Elfolution 17, 373 (1963); R. H. MacArthur and R. Levins, Proc. Nat, Acad. Sci.
U.,S.A. 51, 1207 (1964); R. -H. MacArthur, Biol.
Rev. Cambridge 40, 510 (1965)-- , H.
Recherfi M. Cody, Amer. Natur 100, 319
(1966?; R. H. MacArthtlr and E. Pianka,
H*
9.
10.
11.
12.
13.
14.
- Avifauna
of the
Eastern
Highlands
ibid., p. 603; E. O. Wilson, Evolution 13,
122 (1959); Amer. Natur. 95, 169 (1961);
and D. S. Simberloff, Ecology 50, 267
(1969).
R. M. May and R. H. MacArthur, Proc. Nat.
Acad. Sci. U.S.A. 69, 1109 (1972).
E. O. Wilson, Psyche 65, 26 (1958).
J. M. Diamond, A mer. Mus. Nol . No. 2284
(1967); Explorers J. 46,
210(1968);
and J. W. Terborgh, Auk 85, 62 (1968); J. M.
Diamond, Amer. Mtes. Nos!. No. 2362 (1969);
J. Terborgh and J. M. Diamond, Wilson Bull.
82, 29 (1970).
J. M Diamond, Proc. Nat. Acad. Sci. U.S.A.
64, 57 (1969).
-,
ibid. 67, 529 (1970).
,
ibid., p. 1715.
-, ibid. 68, 2742 (1971).
,
ibid. 69, 3199 (1972).
,
Science 151, 1102 (1966).
ot New Guinea (Nuttall Ornithological Club
Cambridge, Mass., 1972).
16. F. W. Preston, Ecology 43, 185 (1962).
17. J. Terborgh and J. Faaborg, Atlk, in press.
18. R. F. Fl}nt, Glacial and Pleistocene Geolog>
(Wiley, New York, 1957).
19. F. Vuilleumier, Amer, Natxr. 104, 373 (1970);
J H. 13rown ihid. 105, 467 (1971).
e
umanlzlng
the
Earth
Rene J. Dubos
How gray and drab, unappealingand which were most Rourishingin antiqunsignificant,our planet would be with- uity are now among the poorest in the
out the radiance of life. If it wereCPnot world. Some of their most famous cities
covered with living organisms the sur- have been abandoned;lands which were
face of the earth would resemble that once fertile are now barren deserts.
of the moon. Its colorful and diversified
Disease, warfare,and civil s;trifehave
appearance is largely the creation of certainly played important roles in the
microbes, plants, and animals which collapse of ancient civilizations;but the
endlessly transform its inanimate rocks primarycause was probablythe damage
and gases into an immense variety of caused to the quality of the soil and to
organic substances. Man augments still water supplies by poor ecological pracfurther this diversification by altering tices. Similarly today, the environment
the physical characiterIstics
of the land, is being spoiled in many parts of the
changing the distribution of living world by agricultural misuse or overthings, and adding human order and use, by industrial poisoning, and of
fantasy to the ecologtcal determinism course by wars.
of nature.
The primary purpose of the recent
Many of man's interventionsinto na- United Nations Conference on the Huture have, of course, been catastrophic. man Environment, held in Stockholm
History is replete with ecological dis- in June 1972, was to formulate gIobal
asters caused by agriculturaland indus- approaches to the correction and pretrial mismanagement. The countries vention of the environmental defects
resultingfrom man's mismanagementof
The author is a professor emeritus of the
the earth. I shall not discuss the techniRockefeller University, New York 10021. This
article is the text of the B. Y. Morrison MemoriaI
cal aspectsof-these problems,but rather
lecture, sponsored by the Agricultural Research
Service of the U.S. Department of Agriculture.
shall try to look beyond them and preThe lecture was delivered at the annual meeting
sent facts suggesting that man can acof the AAXS, 29 December 1972, in Washington
D.C. Reprints may be obtained from Dr. Robert
tually improve on nature. In my opinNelson, Public lnformation Office, Agrlcultural
ion, the human use of natural resources
Research Service, U.S. Department of Agriculture
Washington, D.C. 20250.
and of technology is compatible with
2(). D. Lack, Ecol*geial Isolation in Birds (Harvard Univ. Press, Cambridge,Mass., 1971).
21. J. W. Terborgh,Ecology 52, 23 (1971).
22. M. L. Cody, ibid. 51, 455 (1970).
23.
; --I
s Anzer. Ncxtltr. 102>t07 (19683.
24. D. H. MorsefiWilson Bull. 83, 57 (1971).
25. R. H. MacArthur, J. M. Diamond, J. R.
Karr, Ecology 53, 330 (1972).
26. J. R. Karr, personalcommunication.
27. D. Pilbeam, The A scent of Man (Macmillan,
New Yorks 1972); L. S. B. Leakey, Nalure
209, 1279 (1966), F. C. Howell, ibid. 223,
1234 (1969); W. Schaffer, Amer. Natllr. 102,
559 ( l 968).
Z8. l thank R. H. MacArthurand J. W. Terborgh,
for stimulatingdiscussions;the National Geographic Society, Explorers Club, American
Philosophical Society, Chapman Fund, and
Sanford Trust of the American Museum of
NaturalHistory, and Alpha Helix New Guinea
Programof the National Science Foundation,
for support; M. Cody, A. Grinnell, S.
Kaufman-Diamond,S. Krasne, and G. Szabo,
for criticismof the manuscript;D. Amador,for
permission to use facilities of the American
Museum of Nattlral History; and more residents of New Guinea than can be mentioned
hy name. tor makinvficldworkpossible.
ecological health, and can indeed bring
out potentialitiesof the earth which remain unexpressed in the state of wilderness.
The disastrousecological consequences of many past and present human
activities point to the need for greater
knowledge and respect of natural laws.
This view is succinctly expressed by
Barry Commoner in his fourth law of
ecology: "Nature knows best." I shall
first discuss the limitations of this law.
When left undisturbed, all environments tend toward an equilibriumstate
called the climax or mature state by
ecologists. Under equilibrium conditions, the wastes of nature are constantly being recycled in the ecosystem,
which becomes thereby more or less
self-perpetuating. In a natural forest,
for example, acorns fall to the ground
and are eaten by squirrels,which in turn
may be eaten by foxes or other predators; the dead leaves and branches the
excrements of animaIs, are utilized by
microbes, which return their constituents to the soil in the form of humus
and mineral nutrients. More vegetation
grows out of the recycled materials,
thus assuring the maintenance c)f the
ecosystem.
When applied to such equilibrated
systems,the phrase "Natureknows best'
is justified, but is in fact little more
than a tautology. As used in this
phrasesthe word nature simply denotes
a state of affairs spontaneouslybrought
about by evolutionary adaptation resulting from feedbacks which generate
a coherent system. There are no problems in undisturbed nature; there are
only solutions, precisely because the
equilibrium state is an adaptive state.
23 FEBRUARY 1973
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