Unexpected Ecological Resilience in Bornean Orangutans and Implications for Pulp and Paper Plantation Management

Ecological studies of orangutans have almost exclusively focused on populations living in primary or selectively logged rainforest. The response of orangutans to severe habitat degradation remains therefore poorly understood. Most experts assume that viable populations cannot survive outside undisturbed or slightly disturbed forests. This is a concern because nearly 75% of all orangutans live outside protected areas, where degradation of natural forests is likely to occur, or where these are replaced by planted forests. To improve our understanding of orangutan survival in highly altered forest habitats, we conducted population density surveys in two pulp and paper plantation concessions in East Kalimantan, Indonesia. These plantations consist of areas planted with fast-growing exotics intermixed with stands of highly degraded forests and scrublands. Our rapid surveys indicate unexpectedly high orangutan densities in plantation landscapes dominated by Acacia spp., although it remains unclear whether such landscapes can maintain long-term viable populations. These findings indicate the need to better understand how plantation-dominated landscapes can potentially be incorporated into orangutan conservation planning. Although we emphasize that plantations have less value for overall biodiversity conservation than natural forests, they could potentially boost the chances of orangutan survival. Our findings are based on a relatively short study and various methodological issues need to be addressed, but they suggest that orangutans may be more ecologically flexible than previously thought.     

Habitat use of Bornean Orangutans (<Emphasis Type="Italic">Pongo pygmaeus morio</Emphasis>) in an Industrial Forestry Plantation in East Kalimantan,Indonesia

Many primates now live in anthropogenic landscapes dominated by human activity such as agriculture. Conserving primates in such contexts requires detailed information about habitat use, including landscape features that may influence population viability. We studied Northeast Bornean orangutan (Pongo pygmaeus morio) habitat use in a forestry plantation in East Kalimantan, Indonesia. We conducted camera trapping and nest surveys at 13 locations across three habitat types in the plantation (planted acacia stands, planted eucalyptus stands, and secondary forest patches left uncut or allowed to regenerate) September 2012–March 2013, and calculated four measures of orangutan abundance for each location (independent photo captures/100 camera trap days, or RAI₂; nest encounter rate; nest density; and orangutan density). Orangutans are relatively common in the plantation; they used all three habitat types and exhibited a higher RAI₂ than 70% of other mammal species detected. A logistic regression found that proximity to natural forest areas best predicted orangutan abundance calculated using camera trap data (RAI₂) but that habitat type combined with distance to natural forest best predicted orangutan abundance calculated using nest counts. This suggests that orangutans use planted areas for movement and feeding, but rely on patches of natural forest for resting and access to key resources. Our study and others indicate that orangutans can coexist with some human activities if provided with sufficient access to natural forest. However, we must conduct further research to facilitate effective conservation planning, including gathering additional details about habitat and resource use and possible long-term population impacts.     

Aerial surveys give new estimates for orangutans in Sabah, Malaysia

Great apes are threatened with extinction, but precise information about the distribution and size of most populations is currently lacking. We conducted orangutan nest counts in the Malaysian state of Sabah (North Borneo), using a combination of ground and helicopter surveys, and provided a way to estimate the current distribution and size of the populations living throughout the entire state. We show that the number of nests detected during aerial surveys is directly related to the estimated true animal density and that a helicopter is an efficient tool to provide robust estimates of orangutan numbers. Our results reveal that with a total estimated population size of about 11,000 individuals, Sabah is one of the main strongholds for orangutans in North Borneo. More than 60% of orangutans living in the state occur outside protected areas, in production forests that have been through several rounds of logging extraction and are still exploited for timber. The role of exploited forests clearly merits further investigation for orangutan conservation in Sabah.     

It’s Not Just Conflict That Motivates Killing of Orangutans

We investigated why orangutans are being killed in Kalimantan, Indonesia, and the role of conflict in these killings. Based on an analysis of interview data from over 5,000 respondents in over 450 villages, we also assessed the socio-ecological factors associated with conflict and non-conflict killings. Most respondents never kill orangutans. Those who reported having personally killed an orangutan primarily did so for non-conflict reasons; for example, 56% of these respondents said that the reason they had killed an orangutan was to eat it. Of the conflict-related reasons for killing, the most common reasons orangutans were killed was fear of orangutans or in self-defence. A similar pattern was evident among reports of orangutan killing by other people in the villages. Regression analyses indicated that religion and the percentage of intact forest around villages were the strongest socio-ecological predictors of whether orangutans were killed for conflict or non-conflict related reasons. Our data indicate that between 44,170 and 66,570 orangutans were killed in Kalimantan within the respondents’ active hunting lifetimes: between 12,690 and 29,024 for conflict reasons (95%CI) and between 26,361 and 41,688 for non-conflict reasons (95% CI). These findings confirm that habitat protection alone will not ensure the survival of orangutans in Indonesian Borneo, and that effective reduction of orangutan killings is urgently needed.     

Orangutan Nesting Behavior in Disturbed Forest of Sabah,Malaysia: Implications for Nest Census

High concentrations of orangutans remain in the multiple-use forests of the Lower Kinabatangan, Sabah, Malaysia. Compared to primary forest, the habitat is highly fragmented, characterized by a low tree density (332 stems/ha), small tree size (83.6% of trees are <20 m high), low basal area (18 m²/ha), abundance of canopy gaps and high level of soil disturbance. The forest structure and composition influence orangutan nesting patterns, and thus directly influence the results of nest surveys used to determine orangutan population size. In logged forests, tall and large trees are the preferred nesting sites of orangutans. The scarcity of suitable nesting sites in the logged-over forests of Kinabatangan, could partly explain the lower daily rate of nest construction (r = 1.00) versus those of other orangutan populations. The nest decay rate t recorded at the study site (average ± SD = 202 ± 151 days) strongly depends on the species of tree in which a nest is built. Our results illustrate that the nest-related parameters used for orangutan censuses fluctuate among habitat types and emphasize the need to determine specific values of r for specific orangutan populations and of t for different tree species in order to achieve accurate analysis of census data.     

A simple alternative to line transects of nests for estimating orangutan densities

We conducted a validation of the line transect technique to estimate densities of orangutan (Pongo pygmaeus) nests in a Bornean swamp forest, and compared these results with density estimates based on nest counts in plots and on female home ranges. First, we examined the accuracy of the line transect method. We found that the densities based on a pass in both directions of two experienced pairs of observers was 27% below a combined sample based on transect walks by eight pairs of observers, suggesting that regular line-transect densities may seriously underestimate true densities. Second, we compared these results with those obtained by nest counts in 0.2-ha plots. This method produced an estimated 15.24 nests/ha, as compared to 10.0 and 10.9, respectively, by two experienced pairs of observers who walked a line transect in both directions. Third, we estimated orangutan densities based on female home range size and overlap and the proportion of females in the population, which produced a density of 4.25–4.5 individuals/km² . Converting nest densities into orangutan densities, using locally estimated parameters for nest production rate and proportion of nest builders in the population, we found that density estimates based on the line transect results of the most experienced pairs on a double pass were 2.82 and 3.08 orangutans/km², based on the combined line transect data are 4.04, and based on plot counts are 4.30. In this swamp forest, plot counts therefore give more accurate estimates than do line transects. We recommend that this new method be evaluated in other forest types as well.     

Are Montane Forests Demographic Sinks for Bornean White‐bearded Gibbons Hylobates albibarbis?

I estimated habitat-specific population densities for a population of Bornean white-bearded gibbons Hylobates albibarbis inhabiting seven distinct forest types at Gunung Palung National Park, West Kalimantan, Indonesia. Population densities in montane forests (0.44 individuals/km²) were almost ten times lower than those in the next best habitat (upland granite forest; 4.2 individuals/km²) and far lower than those in lowland forest types. Demographic data on 33 gibbon groups living across the seven forest types showed that reproduction was substantially depressed in montane forests compared to high-quality lowland habitats. A simple model suggests that montane forests are demographic sinks for gibbons at Gunung Palung. Follow-up data from observations of montane groups 5 yr after the initial observation period support this result. As high-quality lowland forests (source habitat for gibbons) are being disproportionately lost in and around Gunung Palung National Park due to illegal logging and conversion to oil palm plantations, an increasing percentage of the remaining forest in the park comprises sink habitat for gibbons. This result has disquieting implications for the long-term viability of gibbon populations at Gunung Palung. In addition, as montane forests are generally low-quality habitat for most rainforest vertebrates, and since lowland forests are being lost at alarming rates across the tropics, source-sink population dynamics similar to those I describe here may characterize populations of other tropical vertebrate species.     

Bird Community Changes in Response to Single and Repeated Fires in a Lowland Tropical Rainforest of Eastern Borneo

Our current understanding of bird community responses to tropical forest fires is limited and strongly geographically biased towards South America. Here we used the circular plot method to carry out complete bird inventories in undisturbed, once burned (1998) and twice burned forests (1983 and 1998) in East Kalimantan (Indonesia). Additionally, environmental variables were measured within a 25 m radius of each plot. Three years after fire the number of birds and bird species were similar for undisturbed and burned forests, but species diversity and turnover were significantly lower in the burned forests. The bird species composition also differed significantly between undisturbed and burned forests, with a strong decline of closed forest preferring bird species accompanied by a strong increase in degraded forest preferring species in burned forests. These differences were strongly related to differences in environmental conditions such as shifts in vegetation cover and layering and differences in ground and understorey vegetation structure. We also found significant shifts in body mass distribution, foraging height and feeding guilds between the bird communities in unburned and burned forests. Surprisingly, repeated burning did not lead to increasing impoverishment of the avifauna, and both once and twice burned forests still contained most of the bird species that were also present in undisturbed forest, even though their densities were considerably lowered.     

Forest Structure and Support Availability Influence Orangutan Locomotion in Sumatra and Borneo

The influence of habitat structure and support availability on support use is an important aspect of understanding locomotor behavior in arboreal primates. We compared habitat structure and support availability in three orangutan study sites—two on Sumatra (Pongo abelii) in the dry‐lowland forest of Ketambe and peat swamp forest of Suaq Balimbing, and one on Borneo (Pongo pygmaeus wurmbii) in the disturbed peat swamp forest of Sabangau—to better understand orangutan habitat use. Our analysis revealed vast differences in tree and liana density between the three sites. Sabangau had a much higher overall tree density, although both Sumatran sites had a higher density of larger trees. The two peat swamp forests were more similar to each other than to Ketambe, particularly with regard to support availability. Ketambe had a wider variety of supports of different sizes and types, and a higher density of larger lianas than the two peat swamps. Orangutans in all three sites did not differ substantially in terms of their preferred supports, although Sumatran orangutans had a strong tendency to use lianas, not observed in Sabangau. Differences in observed frequencies of locomotor behavior suggest the homogeneous structure of Sabangau limits the locomotor repertoire of orangutans, with high frequencies of fewer behaviors, whereas the wider range of supports in Ketambe appears to have facilitated a more varied locomotor repertoire. There were no differences among age‐sex classes in the use of arboreal pathways in Suaq Balimbing, where orangutans selected larger trees than were typically available. This was less apparent in Sabangau, where orangutans generally used trees in relation to their environmental abundance, reflecting the homogeneous nature of disturbed peat swamp forest. These results demonstrate that forest architecture has an important influence on orangutan locomotion, which may become increasingly important as the structure of orangutan habitat continues to be altered through human disturbance. Am. J. Primatol. 74:1128‐1142, 2012. © 2012 Wiley Periodicals, Inc.     

Apes in space: saving an imperilled orangutan population in Sumatra

Deforestation rates in Sumatra are amongst the highest in the tropics. Lowland forests, which support the highest densities of orangutans, are particularly vulnerable to clearance and fragmentation because they are highly accessible. Consequently, many orangutans will, in the future, live in strictly or partially isolated populations. Whilst orangutans have been extensively studied in primary forests, their response to living in human-dominated landscapes remains poorly known, despite it being essential for their future management. Here, we focus on an isolated group of critically endangered Sumatran orangutans (Pongo abelii) that co-exist with farmers in a mixed agroforest system consisting of degraded natural forest, smallholder (predominantly rubber) farms and oil palm plantations. Over 24 months we conducted the first ever spatial assessment of orangutan habitat use in the human-transformed landscape of Batang Serangan, North Sumatra. From 1,204 independent crop-raiding incidents recorded, orangutans showed strong foraging preference for mixed farmland/degraded forest habitat over oil palm patches. The core home range areas of the eight adult orangutans encompassed only 14% of the available study area. Monthly home range sizes averaged 423 ha (±139, SD) for males, and 131 ± 46 ha for females, and were positively influenced by wild and cultivated fruit presence, and by crop consumption. The average daily distance travelled was similar for both adult males (868 m ± 350, SD) and females (866 m ± 195), but increased when orangutans raided crops. These findings show that orangutans can survive, demographically, in certain types of degraded landscapes, foraging on a mixture of crops and wild fruits. However, the poor quality habitat offered to orangutans by oil palm plantations, in terms of low food availability and as a barrier to female movements, is cause for concern since this is the land use type that is most rapidly replacing the preferred forest habitat across both Sumatran and Bornean orangutan ranges.     

Ape Abundance and Habitat Use in the Goualougo Triangle, Republic of Congo

Chimpanzee (Pan troglodytes troglodytes) and western gorilla (Gorilla gorilla gorilla) populations in central Africa are rapidly declining as a result of disease epidemics, commercial bushmeat hunting, and habitat destruction. Our main objective was to estimate the absolute abundance and habitat utilization of chimpanzees and gorillas in the intact forests of the Goualougo Triangle in the Republic of Congo, and in an adjacent area in which selective logging will take place in the near future. The estimates provide a unique baseline for apes inhabiting an undisturbed environment. A second objective was to compare estimates of abundance and patterns of habitat utilization generated by different techniques: 1) distance sampling of individual ape nests and nest sites along line transects, 2) direct observations of apes during reconnaissance surveys, and 3) observations of ape traces during reconnaissance surveys. We completed a total of 222 km of line transect surveys in 4 sampling areas, resulting in overall density estimates of 1.53 chimpanzees/km² and 2.34 gorillas/km² from nest sites. We generated a density estimate of 2.23 chimpanzees/km² from direct observations during reconnaissance surveys of a semihabituated community in 1 of the 4 sampling areas. Habitat use profiles that nest surveys depicted on transects differed from those of direct observations and traces we encountered on reconnaissance surveys. We found the highest overall abundance of chimpanzee nests in monodominant Gilbertiodendron forest, whereas our direct observations showed that chimpanzees preferred mixed species forest. Transects that traversed the core area of the community range had the highest encounter rates of chimpanzee nests and nest sites. Gorilla nests on transects showed a preference only for mixed species forest with an open canopy, but direct observations and traces on reconnaissance surveys clearly indicated that gorillas use several habitat types. We conclude by evaluating the precision of these nest surveys and our ability to detect future trends in ape densities in the Goualougo Triangle.

Estimating Orangutan Densities Using the Standing Crop and Marked Nest Count Methods: Lessons Learned for Conservation

Reliable estimates of great ape abundance are needed to assess distribution, monitor population status, evaluate conservation tactics, and identify priority populations for conservation. Rather than using direct counts, surveyors often count ape nests. The standing crop nest count (SCNC) method converts the standing stock of nests into animal densities using a set of parameters, including nest decay rate. Nest decay rates vary greatly over space and time, and it takes months to calculate a site-specific value. The marked nest count (MNC) method circumvents this issue and only counts new nests produced during a defined period. We compared orangutan densities calculated by the two methods using data from studies in Sumatra and Kalimantan, Indonesia. We show how animal densities calculated using nest counts should be cautiously interpreted when used to make decisions about management or budget allocation. Even with site-specific decay rates, short studies using the SCNC method may not accurately reflect the current population unless conducted at a scale sufficient to include wide-ranging orangutan movement. Density estimates from short studies using the MNC method were affected by small sample sizes and by orangutan movement. To produce reliable results, the MNC method may require a similar amount of effort as the SCNC method. We suggest a reduced reliance on the traditional line transect surveys in favor of feasible alternative methods when absolute abundance numbers are not necessary or when site-specific nest decay rates are not known. Given funding constraints, aerial surveys, reconnaissance walks, and interview techniques may be more cost-effective means of accomplishing some survey goals.     

Structure,composition and diversity of plant communities in FSC-certified,selectively logged forests of different ages compared to primary rain forest

The impact of logging on plant communities was studied in forest that has been logged selectively 1, 5 and 10 years previously (following a certified procedure): diversity was compared with that of primary rain forest in the Berau region of East Kalimantan, Indonesia. Four sets of 20 transects located within an area of 6 ha were sampled for all trees, saplings and seedlings, and records were made of topographic position, structure, composition and species diversity. There was a high level of floristic similarity between primary forests at the study sites compared to primary forest elsewhere in Kalimantan. The impact of logging is therefore likely to be the most important factor determining any differences between the plant communities of the selectively logged and primary forest sites. We found differences in species composition and abundance of most plants between selectively logged and primary forest. Overall, stem densities of trees in the primary forest were higher than in the three selectively logged forest sites. Stem densities of saplings were equivalent in all four forests. Seedling stem densities were higher in the forest site logged 10 years previously than in primary forest. Our results showed that the forests logged selectively under certified regimes still have a high plant diversity, possibly indicating that biodiversity values may be conserved by following certification procedures.     

Fur-Rubbing as a Form of Self-Medication in <Emphasis Type="Italic">Pongo pygmaeus</Emphasis>

Orangutans exhibit fur-rubbing possibly for medicinal use. I hypothesize that they use a species of Commelina, an uncommon herb in the peat-swamp forests of Central Kalimantan, Borneo, as either an antibacterial or anti-inflammatory agent. In Central Kalimantan, local indigenous people use the same species as an external medication to treat their arms after a stroke, for muscular pain, and for sore bones and swellings. Thus, the possible convergence of human and orangutan use of Commelina may indicate that orangutans are using it for a similar purpose.     

Orangutan diet,range, and activity at Tanjung Puting,Central Borneo

Previously, wild orangutan feeding and ranging behaviors have been described only from populations in hilly or mountainous regions. The Tanjung Puting study focuses on an orangutan population in a swampy lowland area near sea level. Tanjung Puting also differs from other areas in the virtual absence of large figs, which are significant orangutan food sources elsewhere. During a 4-year period and 6804 hr of observation, focal orangutans were recorded in 11,338 foraging bouts accounting for 3805 hr. Composition and phenology of the forest habitat were documented. The orangutans were predominantly frugivorous, with fruit-eating accounting for 61% of the foraging time. However, the overall variety in their diet was remarkable; 317 different food types have been identified, including fungus, insects, and honey. Orang-utans were strongly opportunistic foragers, with the composition of their diet varying markedly from month to month. During most months orangutans fed on a complex mix of fruit, leaves, bark, insects, and small vines. During some months fruit was not the major component of the diet. All orangutans foraged in both the dry-ground mixed dipterocarp forest and the peatswamp forest habitats found in their ranges. Adult males and females utilized different proportions of certain resources in their diets. Prime adult males also ranged further per day and spent more time on the ground than prime adult females. At Tanjung Puting contact with other orangutans usually increased a focal orangutan’s day length, day range, and amount of time spent moving. This suggests that foraging alone maximized each orangutan’s foraging returns by minimizing the day range traveled. Orangutan solitariness is the result of a large body size and of a predominantly frugivorous and opportunistic diet.     

Fire survival of lowland tropical rain forest trees in relation to stem diameter and topographic position

The objective of this study was to relate patterns in forest structure, tree species diversity, and tree species composition to stem diameters and topography in unburned, once burned and twice burned lowland dipterocarp rain forests in East Kalimantan, Indonesia. To do this four unburned old growth forests were compared with three forests that burned once (1997/1998) and three forests that burned twice (1982/1983 and 1997/1998). Fire resulted in a strong reduction of climax tree density which was negatively related to tree diameter. However, a disproportionate reduction in small diameter understorey climax tree species occurred only after repeated fires. Climax tree species in both burned forest types were most common in swamps, river valleys and on lower slopes, while their density was much lower on places higher along hillsides. In unburned forest the opposite was observed, with climax tree density increasing steadily from swamp and river valleys to upper slopes and ridges. In contrast to climax trees, pioneer trees were abundant throughout the burned forest, with highest numbers on hill sides and ridges. Our results indicate that both diameter and topographic position of trees strongly affect their fire survival chances in tropical lowland forests.     

The nature and evolution of intelligence in orangutans (Pongo pygmaeus)

Orangutans share many intellectual qualities with African great apes and humans, likely because of their recent common ancestry. They may also show unique intellectual adaptations because of their long evolutionary divergence from the African lineage. This paper assesses orangutan intelligence in light of this evolutionary history. Evidence derives from observations of juvenile ex-captive orangutans reintroduced to free forest life by the Wanariset Orangutan Reintroduction Project, East Kalimantan, Indonesia. The intellectual qualities shared by great apes and humans point to a distinct “great ape” intelligence with hierarchization as a pivotal cognitive mechanism. Evolutionary reconstructions jibe with this view and suggest that technically difficult foods may have been key selection pressures. Orangutans should then show hierarchical intelligence when obtaining difficult foods. Evidence on ex-captive orangutans' techniques for processing difficult foods concurs. Intellectual qualities distinct to orangutans may owe to arboreal travel pressures; in particular arboreality may aggravate foraging problems. Evidence confirms that ex-captive orangutans' techniques for accessing difficult foods located arboreally are intellectually complex—i.e. they show hierarchization. These findings suggest other factors probably important to understanding great ape and orangutan forms of intelligence and their evolutionary origins.     

Evidence for altitudinal migration of forest birds between montane Eastern Arc and lowland forests in East Africa

Burgess, N.D. & Mlingwa, C.O.F. 2000. Evidence for altitudinal migration of forest birds between montane Eastern Arc and lowland forests in East Africa. Ostrich 71 (1 & 2): 184–190.

In this paper we assess the evidence for altitudinal movements of forest birds from the montane forests of the Eastern Arc mountains of East Africa to nearby lowland forest patches. For 34 montane species, including all the Eastern Arc endemics except Banded Green Sunbird Anthreptes rubritorques there is no evidence that they undertake seasonal movements to lower altitudes. An additional 26 montane species, of somewhat wider distribution, have been recorded at low (<500 m) altitudes during the cold/dry season (June to September). Most records of these montane birds at lower altitudes are from sites adjoining montane forest areas, although a few records are from lowland coastal forests at 100–240 km distance from montane areas. Only five of the 26 species (White-chested Alethe Alethe fulleborni, White-starred Forest Robin Pogonocichla stellata, Orange Ground Thrush Zoothera gurneyi, Evergreen Forest Warbler Bradypterus mariae and Barred Long-tailed Cuckoo Cercococcyx montanus) are regularly and commonly reported in the lowlands. They are also found in the lowlands in small numbers during the warm/wet season (October to February), when they may breed. The abundance of at least four, and probably more, of the forest birds with a more widespread distribution in the lowland and montane forests of East Africa declines greatly at high altitudes from the onset of the cold/wet season (February) and only increases again at the start of the warm/wet season (September). It is not known how far these species move as they cannot be easily separated from resident populations in lowland forests, and there are no ringing recoveries in different forests. Altitudinal migration of a proportion of the Eastern Arc avifauna is the most likely explanation for available data, although source-sink metapopulation theories may be helpful to explain the distributions of some species. As the movement of forest birds from the Eastern Arc to the lowland forests does not involve the rare endemics, they are of lower conservation concern, but the presence of montane and lowland forest may be important for the long-term survival of some more widely distributed forest species.     

Quantifying killing of orangutans and human-orangutan conflict in Kalimantan, Indonesia

Human-orangutan conflict and hunting are thought to pose a serious threat to orangutan existence in Kalimantan, the Indonesian part of Borneo. No data existed prior to the present study to substantiate these threats. We investigated the rates, spatial distribution and causes of conflict and hunting through an interview-based survey in the orangutan's range in Kalimantan, Indonesia. Between April 2008 and September 2009, we interviewed 6983 respondents in 687 villages to obtain socio-economic information, assess knowledge of local wildlife in general and orangutan encounters specifically, and to query respondents about their knowledge on orangutan conflicts and killing, and relevant laws. This survey revealed estimated killing rates of between 750 and 1800 animals killed in the last year, and between 1950 and 3100 animals killed per year on average within the lifetime of the survey respondents. These killing rates are higher than previously thought and are high enough to pose a serious threat to the continued existence of orangutans in Kalimantan. Importantly, the study contributes to our understanding of the spatial variation in threats, and the underlying causes of those threats, which can be used to facilitate the development of targeted conservation management.     

El Niño droughts and their effects on tree species composition and diversity in tropical rain forests

In this study I investigated the effects of the extreme, 1997/98 El Niño related drought on tree mortality and understorey light conditions of logged and unlogged tropical rain forest in the Indonesian province of East Kalimantan (Borneo). My objectives were to test (1) whether drought had a significant effect on tree mortality and understorey light conditions, (2) whether this effect was greater in logged than in undisturbed forest, (3) if the expected change in tree mortality and light conditions had an effect on Macaranga pioneer seedling and sapling densities, and (4) which (a)biotic factors influenced tree mortality during the drought. The 1997/1998 drought led to an additional tree mortality of 11.2, 18.1, and 22.7% in undisturbed, old logged and recently logged forest, respectively. Mortality was highest in logged forests, due to extremely high mortality of pioneer Macaranga trees (65.4%). Canopy openness was significantly higher during the drought than during the non-drought year (6.0, 8.6 and 10.4 vs 3.7, 3.8 and 3.7 in undisturbed, old logged and recently logged forest, respectively) and was positively correlated with the number of dead standing trees. The increase in light in the understorey was accompanied by a 30 to 300-fold increase in pioneer Macaranga seedling densities. Factors affecting tree mortality during drought were (1) tree species successional status, (2) tree size, and (3) tree location with respect to soil moisture. Tree density and basal area per surface unit had no influence on tree mortality during drought. The results of this study show that extreme droughts, such as those associated with El Niño events, can affect the tree species composition and diversity of tropical forests in two ways: (1) by disproportionate mortality of certain tree species groups and tree size classes, and (2) by changing the light environment in the forest understorey, thereby affecting the recruitment and growth conditions of small and immature trees.