1984 hatte ich die Gelegenheit, aufbauend auf meiner 1981 abgeschlossenen Dissertation drei Kapitel im Buch «The Lesser Apes» zu veröffentlichen. Das über 700 Seiten starke Werk war im Anschluss an die internationale Konferenz «The Lesser Apes» entstanden, welche im Juli 1980 auf Schloss Reisensburg bei Ulm durchgeführt worden war und an welcher ein Grossteil der damals aktiven Gibbonforscher teilgenommen hatte. Hier das Kapitel «The Gibbon in Java» im Wortlaut:
© 1984 Markus Kappeler
The Gibbon in Java
(erschienen in: H. Preuschoft et al. «The Lesser Apes. Evolutionary and Behavioural Biology», Edinburgh University Press, 1984)
During four months in early I978 a survey was carried out in Java (Indonesia) to assess the distribution and population size of the Javan silvery gibbon, Hylobates moloch.
Reference to the silvery gibbon's present distribution in Java has been made only by Hoogerwerf (1970) who states that «this gibbon species is said to occur especially in West Java», but «most probably the range also extends to Central Java», and by Chivers (1977b) who mentions that the silvery gibbon «is now mainly restricted to the west half of the island».
Apart from some general statements on the population size of the moloch gibbon (such as «becoming rare» Kloss, 1929; «at the verge of extinction» Marshall et al., 1972), a numerical estimate has been attempted by Chivers (1977b) who mentions a number of 20.000 individuals. Later on in the same paper, however, he questions this estimate and, referring to the reports of other scientists, suggests that «this gibbon species seems to be represented rather by hundreds of animals».
It becomes apparent that both subjects merit further study. Since the Javan silvery gibbon is undoubtedly the rarest and most endangered gibbon taxon, data are needed urgently as a basis for its conservation.
Like the other gibbon taxa (Ellefson, 1967; Chivers, 1974; Tenaza, 1975; Gittins, 1979) the silvery gibbon is a highly specialized forest dweller. The following characteristics of its specialized way of life (for details see Kappeler, 1981b) indicate its habitat requirements:
1) The silvery gibbon is strictly arboreal and therefore requires a forest with a more or less closed canopy.
2) It relies to a great extent on brachiating locomotion. To allow swift movements, the branching pattern of the canopy must not be too dense and should consist of a relatively large number of more or less horizontally growing branches.
3) It feeds mainly on fruit and leaves and lives all the year round in stable home ranges. To ensure sufficient fruit/leaf abundance throughout the year, the forest should not show a marked general periodicity and, especially, no phase of general defoliation. Moreover, the forest should comprise a great diversity of tree species.
These requirements are fully covered only by high, very diverse, evergreen forest, i.e. the forest type described by Whitmore (1975) as «tropical lowland/hill evergreen rain forest on dry land». The silvery gibbon is indeed found to be almost exclusively restricted to this habitat; its distribution essentially coincides with the occurrence of this forest type. (The silvery gibbon is also found to inhabit the lower zones of montane rain forest: this formation seems to cover to some degree the requirements of the species.)
Primary Forest Relicts.
Satellite photographs, taken by Earth Resources Technology Satellite 3 (ERTS or LANDSAT-3, launched by NASA in March 1978), were evaluated to determine the areas in Java that are still covered with original arboreal vegetation. Satellite pictures as a tool in natural resources inventories have already been used for estimates of remaining gibbon habitat in Thailand by Brockelman (1975): their possible application in wildlife management is discussed by Hehuwat (1978).
The photographs revealed that, owing to a tremendous increase in the human population and in land use for urban and agricultural development, the original vegetation of Java has been largely destroyed. Primary forest is now confined to eighty-seven isolated patches of varying size, spread over the whole island.
The Eastern Half of Java.
Twenty-four of these forest patches are situated in the eastern half of Java which is exposed to a pronounced dry season. Consequently, the growth of tropical rain forest is suppressed; the forest in this part of the island shows a general annual defoliation and certainly does not meet the habitat requirements of the gibbon.
Java can roughly be divided into two climatic halves at approximately longitude 11O° east (Fig. 4.1). The western half generally has a weak, albeit nowhere marked, dry season; the eastern half has a pronounced dry season (concerning reasons for the climatic division of Java, see Oldeman, 1975).
Accordingly, the original tropical vegetation of Java consists of two distinct main types: the rain forest in the west and the monsoon forest in the east (Van Steenis and Schippers-Lammertse, 1965).
In otherwise seasonally dry East Java, small enclaves of ever-wet rain forest do exist on the southern and south-eastern slopes of the high mountains (Fig. 4.1). These «wet islands» (Van Steenis, 1972) result from the fact that even the dry south-east trade winds, which are responsible for the seasonal drought in the lowlands, give off rain by condensation at higher altitudes. There is no evidence, however, that in recent times they have been inhabited by gibbons: they seem to lie too far inside the drought area (some 150 km and more) to allow colonization by the animals from the west.
Thus, contrary to the opinion of Tenaza (1975), who supposes that human activities have exterminated the gibbon from eastern Java, the absence of the gibbon from this part of the island is caused primarily by climatic factors.
The Western Half of Java.
In the western half of Java, natural vegetation is restricted almost exclusively to hydrological forest reserves capping summits and ridges - sometimes not below 1500-2000 metres - and a number of generally small nature reserves in the lowlands. Some unprotected original vegetation still persists in a few sparsely inhabited, almost inaccessible areas in the southern part of West Java (Fig. 4.1).
All areas in the western half of Java that are still covered with original rain forest are listed in Table 4.1, arranged from west to east. Each such area represents either one separate forest patch or several patches not too far from each other: the number of forest patches in each area is indicated. Forty areas with sixty-three isolated forests remain in this half of the island.
Nearly all of these remaining sixty-three isolated patches of original rain forest were visited. Each forest patch was visited at least once, and the occurrence of gibbons was checked by either sighting the animals or noting their characteristic calls (Kappeler, 1981b). The method was supplemented by interviewing local people (mainly hunters and collectors of forest produce). If gibbons were found to occur in the part of a patch that was visited, the whole forest patch was assumed to be inhabited by the animals. The correctness of this assumption was tested several times by visiting other parts of the patch.
To provide some idea of the perimeter of the silvery gibbon's present geographic range, all forests found still harbouring the species are represented in the map in Fig. 4.1 as black patches.
Out of the total of sixty-three forest patches, thirty-two are certainly, and three unvisited ones possibly, inhabited by gibbons. The range lies between latitude 6-8° south and longitude 105-110° east. It extends from the westernmost tip of the Javan mainland through the southern half of West Java into the western part of Central Java.
Reference to localities occupied by the silvery gibbon has been made by Forbes (1894), Haberlandt (1910), Docters Van Leeuwen (1933), Sody (1949), Satmoko (1961), Schenkel and Schenkel-Hulliger (1969), Hoogerwerf (1970), Groves (1972) and Wind and Van der Zon (1976). All statements based on the direct observations of these authors could be verified. Some statements of Sody and Groves, based on labels of museum skins, mention as localities the names of human settlements, but lack more accurate information concerning the origin of the animals.
Twenty-eight of the total of sixty-three forest patches are not inhabited by the silvery gibbon. Probable explanations for the absence of the species from these forests are the following:
Areas 10, 27, 31, 32, 39, 40 and 22, 23, 25, 37 (twelve forest patches): The absence of the gibbons from areas 10, 27, 31, 32, 39 and 40 is the consequence of the total destruction - or severe degradation - of the rain forest below the altitudinal limit of the gibbon's occurrence.
The silvery gibbon has been found to occur from sea-level up to an altitude of 1400-1600 metres. The absence of gibbons higher up results from altitudinal vegetation changes. At approximately 1000 metres in Java, the tropical hill rain forest changes into lower montane rain forest, which in turn gives way to the upper montane rain forest at approximately 2400 metres (Van Steenis, 1972). Differences between these three forest formations concern
1) forest structure and physiognomy: the height of the canopy de creases with increasing altitude. Higher up, the trees are more slender in appearance with a much denser branch system; the branches are generally inclined vertically and are laden with epiphytes.
2) floristic composition: tree species diversity as well as biomass decrease markedly (Richards, 1952; Whitmore, 1975).
Thus, the structure of the trees, which renders brachiation extremely difficult, the relative floristic poverty and therefore poor supply and diversity of food, and the low temperatures at night might be the main factors limiting the altitudinal extension of the gibbon's range.
In the areas 22, 23, 25 and 37, a few remnant groups have managed to survive just at the altitudinal limit, but marginal habitat conditions and human impact make their situation extremely precarious.
Areas 3, 16, 33 (five forest patches): These are not inhabited by gibbons owing to unsuitable vegetation composition; a permanent or periodical high water table suppresses the growth of high diversified climax forest. Area 3 is covered mainly with freshwater swamp vegetation, areas 16 and 33 with mangrove forest. For both these formations very low tree species diversity and locally pure monotypic stands are characteristic (Whitmore, 1975).
Area 1 (one forest patch): This area has a tropical climate - the highest altitude being 480 metres - and, since it is a well-protected nature reserve, has not been exposed to recent clearing activities. Nevertheless it lacks a continuous high climax forest and, accordingly, a closed canopy.
As outlined in some detail by Schenkel and Schenkel-Hulliger (1969) and by Hoogerwerf (1970), besides local areas with climax rain forest, extensive stretches of mangrove forest, swamp vegetation, monotypic stands on dry land (e.g. bamboo), regrowth rain forest (as a result of former cultivation and the tidal wave following the Krakatau eruption) and other vegetation types are encountered. These formations do not fulfil the habitat requirements of the gibbon.
Hoogerwerf (1970) states that although the main part of the peninsula «is assumed to have mainly a primary vegetation, this is rarely evidenced by the plants growing there» and discusses the phenomenon.
The few groups of gibbons that inhabit a patch of mixed lowland rain forest in the extreme south-east of the peninsula are border groups of the gibbon population from the adjoining area 2.
Areas 4, 5, 15, 29 (seven forest patches): In these areas, the rain forest patches below the 1500 metres contour - partly isolated, partly as spurs of greater forests - are not inhabited by gibbons although they probably would provide suitable habitat for them.
Indeed, it can be shown, on the basis of reliable information by local inhabitants, that gibbons formerly occurred there but have disappeared in recent times or are presently on the verge of extinction. This is attributable chiefly to hunting: the gibbon population of area 5 has suffered mainly from the influence of the local Badui people, a tribe which hunts primates for food. Areas 4, 15 and 29 (the latter being an official hunting park) are afflicted by weekend sport hunters from the towns.
The silvery gibbon has been protected by law in Java since 1924, when the first Game Hunting Ordinance came into operation. The legal provisions for the protection of wild animals under the 1909 Ordinance were not valid for the silvery gibbon. Not until later was its protection considered necessary when deforestation, its attractiveness as a pet and its value for medical research put it in danger of extermination (Hoogerwerf, 1970). In 1931, together with all other gibbon species living in Indonesia, the silvery gibbon was included in the list of strictly protected animals of the Animal Protection Ordinance 1931 and Regulations (Martodiardjo, 1975).
Law enforcement, however, has never been very strict. Up to the present day, hunting gibbons for food and for sport, and keeping gibbons as pets (Figs 4.2 and 4.3) occur in nearly all areas visited. This may reduce local population levels around human settlements or along roads. In the case of small remaining patches of primary forest, or of marginal habitat conditions, hunting can easily result in the extermination of the local gibbon population.
Areas 34, 35, 36 (three forest patches): No definite explanation has been found for the gibbon's absence from the rain forest in these three areas. Interviews with local inhabitants gave the impression that the silvery gibbon has never - or at least not within living memory - occurred there. A possible explanation could be that today's forest does not represent virgin forest, but old regrowth forest. The gibbon might then have been eliminated from these places without having the possibilitv to reimmigrate.
In general, gibbons are rarely found in secondary tropical rain forest. Two main explanations can be quoted for this:
1) In Java, secondary forest successions are mostly of relatively low quality (for reasons see Whitmore, 1975), and can therefore not be used by the gibbon.
2) Destruction of original forest implies the complete extermination of the resident gibbon population. To allow recolonization, the regrowth forest must be connected with forest still inhabited by gibbons. If such a connecting link is missing, even patches of old, primary-like regrowth rain forest (e.g. the nature reserve Pangandaran, Halder, 1976) are not in habited by the silvery gibbon.
It is virtually impossible to count all the individuals of the Javan gibbon population. Population numbers must be estimated by extrapolation, based on (1) the geographic area inhabited by the animals, and (2) the density of their population.
Thus, in this section, the estimation of the population numbers of the silvery gibbon is preceded by the determination of the forest area inhabited by the species and by the estimation of its population density.
The Area Inhabited by the Silvery Gibbon.
The forest patches inhabited by the silvery gibbon (according to the previous section) were projected from the satellite photographs onto a topographical map (scale 1:250.000), and their sizes were measured. The sizes of the parts above 1500 metres (where no gibbons were found) were measured separately.
The thirty-two forest patches (in seventeen areas) which the gibbon is known to occupy are listed in Table 4.2, as well as the three forest patches (in two areas) where it probably occurs. Indicated (for each area) are (1) the size of the whole forested land, and (2) the size of the forested land below 1500 metres.
If no major gaps in the carpet of the forest patches were detected either in the field or on the satellite pictures, it was assumed that the whole forested area (up to 1500 metres) was inhabited by the gibbon. The total inhabitable surface of all thirty-two forest patches amounts to some 1400 km2.
Estimation of Population Density.
The density of the gibbon population in Java was estimated by means of an acoustic census method. Similar methods have already been applied by Chivers (1974) in Malaya and by Brockelman et al. (1977) in Thailand.
The basic idea of this method is that the number of resident individuals can be estimated from the number of gibbon calls heard in a given area during a certain time period.
The procedure in the field was for one observer to remain at one and the same place in the forest during a specific time period each day and to note all female song bouts (see Kappeler 1981b). On the basis of his experience, the observer estimated the maximum distance from which he could hear females singing. This distance might vary - according to local topography and wind conditions - between 500 and 1500 metres. Thus the size of the acoustically sampled area could be determined. The next step was to calculate for every census place and day the density of female song bouts.
Next, these densities were «translated» into local population densities. For this translation, information was required concerning (1) the correlation between the number of song bouts (heard on one day) and the number of resident females, and (2) the proportion of resident females to resident individuals. This information was available from a fifteen-month study, performed at Turalak in the nature reserve Ujung Kulon/Gunung Honje (West Java) in 1975/76.
The census technique is described in detail, and the biases introduced by it discussed, in Kappeler (1981a).
Without doubt the density of the gibbon population in Java is not constant, but subject to variation from place to place. It is influenced (1) by variations in habitat quality, i.e. variations in climate, soil and vegetation, and the human impact on the latter, as well as (2) by the varying extent of hunting, capture and other human disturbance.
An accurate determination of the population numbers based on density estimates would therefore require estimations of (1) density in each forest patch, in order to take into account geographic variations, and (2) density in each distinct habitat type, in order to take into account variations within the tropical rain-forest patches.
However, for the following reasons these postulates could not be fulfilled during the 1978 survey:
1) There is little information available about the nature and occurrence of plant-sociological differences within the tropical rain-forest formation. The only means of distinguishing between different habitat types within this formation is offered by the study of Van Steenis (1972) on altitudinal vegetation zones, the extent of which can be more or less accurately determined with the aid of topographic maps (lowland zone, 0-500 metres; hill zone, 500-1000 metres; montane zone, above 1000 metres).
2) Owing to lack of time, it was not possible to perform censuses in all forest patches. Moreover, only 1-3 censuses could be performed for each place visited. The number of samples was not sufficient to determine a reliable density figure for any of the census places or forest patches.
Consequently, the only way to evaluate the census data collected was to pool the individual one-day density estimates from the various places, to classify them with respect to the altitudinal zones, and to determine a mean density for each zone. This procedure assumes that the population density, within each altitudinal vegetation zone, is similar throughout the range of the silvery gibbon, and that the individual density estimates represent samples of the standard density per zone.
Twenty-eight censuses were performed in twelve different forest patches between the western and eastern limits of the silvery gibbon's range in which the species had previously been found. For each census place, between 0.8 and 4 km2 of forested land were inspected for acoustic evidence of gibbons: altogether more than 70 km2 were surveyed.
In Table 4.3, the mean population density estimate is indicated separately for each altitudinal vegetation zone. Density is highest in lowland rain forest with 4-13 individuals/km2. It decreases with increasing altitude (2-7 individuals/km2 in hill and 1-3 in lower montane forest) and reaches zero around 1500 metres.
It must be stressed that these density estimates are of limited reliability and have a low level of accuracy due to the various assumptions that had to be made to allow an evaluation of the census data (Kappeler, 1981a). Since it is not possible to determine or estimate the error produced by these assumptions, error margins cannot be given.
Despite these deficiencies, the results are presented here because it is believed that even rough estimates of population numbers are useful.
Estimates of the population numbers of the silvery gibbon were obtained by multiplying the density scores determined for the census areas by the corresponding total areas of forest inhabited by the species.
In Table 4.3, the population numbers calculated for each altitudinal zone are listed. The total of the three zones amounts to 2400-7900 gibbons. This total population is spread out over thirty-two isolated forest patches of varying size, the largest of which may contain about 600-1800 individuals.
Chances of Survival
The gibbon habitat still existing in Java consists of a number of small, isolated patches which are scattered over the western half of the island. Accordingly, the remaining gibbon population is discontinuous; the taxon only occurs in small, isolated local remnant populations and lacks a continuous gene pool with gene flow between populations.
There is no likelihood of a fundamental improvement in the silvery gibbon's prospects, since the destruction of its habitat is largely irreversible. Any eventual population growth, under optimum circumstances, cannot possibly lead to the colonization of new areas and the reconnection of currently isolated population fragments.
In fact, the circumstances are far from optimal. Although the majority of forest patches inhabited by the silvery gibbon are included in existing or designated nature and forest reserves, effective protection measures are mainly lacking. Habitat destruction is continuing, so that virtually all forest patches, regardless of their legal status, are shrinking in area by several per cent per year. Moreover, since the animals (although legally protected) are still exposed to hunting and capture, populations are being progressively reduced.
The only management strategies that will save the silvery gibbon for the future are (1) immediate and efficient protection of the remaining habitat patches from destruction, and (2) strictly enforced protection of the animals themselves from hunting and capture.
During 4 months in 1978, a survey was carried out in Java with the aims (1) to determine the distribution and (2) to estimate the population size of the Javan Silvery Gibbon (Hylobates lar moloch). The procedure in the field and the findings are presented in this paper; they can be summarized as follows:
The Javan Silvery Gibbon is a highly specialized forest dweller which depends almost exclusively on tropical lowland/hill rain forest on dry land. Its distribution therefore essentially coincides with the occurrence of this forest type.
Satellite photographs were evaluated to determine the extent of primary forest in Java. They revealed that - due to the tremendous increase of the human population and consequently of land use for urban and agricultural development - primary forest is now confined to 87 isolated patches of varying size, spread over the whole island.
24 of them are situated in the eastern half of Java which is exposed to a pronounced dry season. Consequently, the forest in this part of the island shows a general annual defoliation and certainly does not meet the habitat requirements of the gibbon.
The 63 forest patches in the western half of Java were nearly all visited, and gibbons were found to occur - up to an altitude of approximately 1500 meters a.s. - in 32 of them. Possible explanations for the absence of the gibbon from the other forest patches in this half of the island as well as from the forest parts above 1500 meters are given.
The total area inhabited by the subspecies is estimated at around 1400 sq.km.
An acoustic censusing method was applied to estimate the densities of the gibbon populations. The census technique is described in detail, and the biases introduced by it discussed.
28 censuses were performed in 12 different forest patches inhabited by the gibbon; the mean density estimate was highest in lowland rain forest (4-13 individuals/sq.km), while it decreased with increasing altitude (2-7 in hill and 1-3 in lower montane rain forest) and reached zero around 1500 m a.s.
From the amount of forested land harbouring gibbons and the population density scores, a total estimate of 2400 to 7900 individuals was obtained for Java's gibbon population. Accordingly, H. l. moloch must be regarded as the rarest gibbon taxon.
Gibbons are illegally hunted in most regions for food, the pet market or for fun («sport»), and deforestation is continuing. Therefore, the only management strategies that might save viable populations of the Silvery Gibbon for the future are immediate and vigorous protection of (1) the remaining habitat patches from further destruction, and (2) the animals themselves from hunting and capture.
1918 wurde auf der Insel Java (Indonesien) während vier Monaten eine Untersuchung über Verbreitung und Bestand des Javanischen Silbergibbons (Hylobates lar moloch) durchgeführt. Das Vorgehen im Feld und die Befunde der Untersuchung sind in der vorliegenden Arbeit aufgezeichnet; sie können folgendermassen zusammengefasst werden:
Der Javanische Silbergibbon ist ein hochspezialisierter Baumbewohner; seine Lebensbedürfnisse werden praktisch nur von tropischem, auf trockenem Boden stehendem Tiefland/Hügel-Regenwald gedeckt. Seine Verbreitung stimmt deshalb weitgehend mit dem Vorkommen dieses Waldtyps überein.
Um Kenntnis über die Ausdehnung des Primärwalds auf Java zu erlangen, wurden Satelliten-Aufnahmen ausgewertet. Es zeigte sich, dass - infolge des enormen Anwachsens der javanischen Bevölkerung und der entsprechend starken Ausweitung der landwirtschaftlich genutzten Bodenfläche - der Primärwald heute bis auf 87 isolierte, über die ganze Insel verstreute Stücke unterschiedlicher Grösse gerodet ist.
24 dieser Waldstücke liegen in der östlichen Hälfte Java's, wo zur Jahresmitte eine ausgeprägte Trockenzeit herrscht. In diesem Teil der Insel gedeiht kein Regenwald, sondern Monsunwald; letzterer genügt den Habitatsansprüchen des Gibbon nicht.
Fast alle der 63, in der westlichen Hälfte Java's noch vorhandenen Waldstücke wurden aufgesucht. In 32 von ihnen wurden - bis auf eine Höhe von ca. 1500 m ü.M. - Gibbons gefunden. Mögliche Gründe für das Fehlen des Gibbons in den übrigen 31 Waldgebieten in diesem Teil der Insel - sowie in den Waldzonen oberhalb 1500 m - sind aufgeführt.
Die Gesamtfläche des Gebiets, in welchem heute noch Gibbons vorkommen, beträgt rund 1400 km2.
Mittels einer akustischen Zensusmethode wurden die Dichten der Gibbon-Restpopulationen geschätzt. Das Verfahren ist ausführlich beschrieben; seine Mängel werden besprochen.
In 12 von Gibbons bewohnten Waldstücken wurden auf verschiedenen Höhenstufen insgesamt 28 Dichteschätzungen vorgenommen. Die mittlere Dichte war im Tiefland-Regenwald am höchsten (4-13 Individuen/km2); mit zunehmender Höhe nahm sie ab (2-7 im Hügel- und 1-3 im Berg-Regenwald) und erreichte bei ca. 1500 m den Wert Null.
Wendet man diese für die Zensusgebiete ermittelten Dichtewerte an auf die Gesamtfläche des bewaldeten Gebiets, welches Gibbons beherbergt, so erhält man eine Schätzung von 2400 bis 7900 Individuen für die Grösse der Gibbonpopulation auf Java. Demzufolge ist H. l. moloch als das seltenste Gibbontaxon anzusprechen.
Gibbons werden in den meisten Gebieten illegal gejagt - für den Tierhandel, die Ernährung oder zum Vergnügen (Sport). Und die Rodung des Regenwalds geht weiter. Die einzigen Massnahmen, durch welche lebensfähige Populationen des Silbergibbons für die Nachwelt erhalten werden könnten, sind deshalb sofortiger und strikter Schutz (1) der verbliebenen Habitatsstücke vor weiterer Zerstörung und (2) der Tiere selbst vor Jagd und Fang.