SECTION THREE - ORANGUTAN DEMOGRAPHY AND POPULATION AND HABITAT VIABILITY ANALYSIS Introduction

What is the probability that a species will survive given a certain degree of protection and a measurable set of environmental circumstances? How many individuals must be conserved, and how large and ecologically diverse an area must be committed? Answers to these complex questions can be obtained in part through the methods of population biology and the mathematics of Population and Habitat Viability Analysis (PHVA). It is now recognized that endangered species, whose numbers are increasing, must be studied as metapopulations, comprised of core and satellite populations (Primack, 1993). Core popu• lations are crucial because they are the primary source of migrants for satellite populations, whose sizes may fluctuate and periodically crash. Destruction of core populations and/or the elimination of dispersal routes between the core and satellite popUlations thus have a profound impact on the satellites. Shaffer (1981) described a species' minimum viable population size as the number of individuals required to yield a 99% probability of survival for at least 1000 years. He noted, however, that this calculation is complicated by several factors. The likelihood of extinction in both the short and the long term is influenced by the complex interactions among a species' morphology, physiology, behavior, genetics, distri• bution, habitat, and demography. Conservation biologists further point out that there is a tendency for small populations to decline toward extinction, i.e., extinction vortices (Pri• mack, 1993). Environmental and demographic variability coupled with a loss of genetic diversity is the cause of this downward spiral in small popUlations. A strategy to oppose extinction vortices is the development of population and habitat management procedures that minimize the effects of small population size and habitat fragmentation, degradation, and isolation. These plans must meet the species' requirements for survival in both typical and exceptional years, i.e., when primary food resources are abundant and when they are unavailable. The Conservation Breeding Specialist Group (CBSG) of the International Union for the Conservation of Nature (IUCN) developed PHVAs to meet these diverse requirements, yielding separate conservation plans tailored for indi• vidual species and their ecological conditions (e.g., tigers, gibbons and orangutans). The IUCN cooperates with host countries in developing PHVA workshops. The involvement of government officials, indigenous scientists, and interested lay people insures that recom• mendations are meaningful and applicable to the host country. Section 3 contains the results of the first PHVA Workshop for Orangutans held in Medan, Sumatra in 1993.

79 80 Introduction to Section Three

Soemarna and his co-workers provide the background for the Workshop which was jointly organized by the CBSG of the mCN and the Indonesian Directorate of Forest Protection and Nature Conservation. They succinctly describe the characteristics of orangu• tans that make them particularly susceptible to extinction (e.g., a relatively long interbirth interval), many of which are elaborated upon in subsequent papers in this section. Faust and his colleagues emphasize that different habitats and microhabitats currently occupied by orangutans support different numbers of animals. Through the use of a Geographic Information System (GIS), they analyze the likely ramifications of implement• ing various policies, such as building a road that bisects a reserve, and thereby, threatens the dispersal routes of orangutans in the area. Other variables that are examined with GIS are the effects of deforestation and changes in the boundaries of the Gunung Leuser National Park, North Sumatra. Leighton and his co-authors describe the influence that environmental variation (e.g., drought, loss of key food species) exerts on the future of orangutan populations, particularly noting the impact oflosing animals of different age/sex classes. Through the use of VORTEX modelling, they estimate the changes in population size contingent upon differing rates of reproduction and mortality. van Shaik and his collaborators propose that the orangutans inhabiting the Gunung Leuser National Park are the most important core population in Sumatra. This population is vital in terms of its overall size, likelihood of continued protection, and as a major source for dispersal of orangutans to other areas. Rijksen and his colleagues describe the isolated orangutan populations found in the fragmented forests of Borneo. Using results from the PHVA workshop, they describe the impact of population isolation on the survival of Bornean orangutans and the alarming decline in this subspecies which has continued unabated in recent years. The PHVA workshops culminate with the development of models and guidelines for the host countries. Soemarna and his co-authors summarize the results of the PHVA and present the conclusions that follow from their deliberations. The result is a list of concrete recommendations to government officials of both Sumatra and Borneo which illuminate the probable outcome of various economic and environmental decisions. One noteworthy recommendation is that the supplementation of natural populations will not enhance the population viability. The most critical problem which the governments must resolve is the loss and degradation of orangutan habitat. A final paper in Section 3 which was not a part of the PHVA workshop is that of van Schaik and colleagues. These investigators used line transects of nests to estimate the geographic distribution and densities of orangutans in a wide range of habitats in Sumatra. They describe the relationship between habitat type and population density and then present preliminary evidence regarding the importance of floodplain forests for the conservation of orangutans more widely distributed.

REFERENCES

Primack, R. B., 1993, Essentials o/Conservation Biology, Sunderland, MA: Sinauer Associates Inc. Shaffer, M. L., 1981, Minimum population sizes for species conservation. Bioscience 31: 131-134.