A Global Perspective on Habitat Disturbance ... - Wiley Online Library

Special Section: Habitat Disturbance and Tropical Rainforest Mammals

A Global Perspective on Habitat Disturbance and Tropical Rainforest Mammals Introduction Tropical rainforests are the richest terrestrial ecosystems on the planet (World Conservation Monitoring Centre 1992; Heywood 1995). They harbor the largest number of species, which participate in complex ecological interactions and processes, and they house valuable economic resources and provide important environmental services on local, regional, and global levels (Constanza et al. 1997; Kremen et al. 2000). The future of tropical rainforests is grim, however, and is closely linked to the current species-extinction crisis (World Conservation Monitoring Centre 1992; Pimm et al. 1995). Many interrelated and complex forces cause tropical deforestation and compromise tropical forest existence, and their importance varies among the different regions (Bawa & Dayanandan 1997; Kaimowitz & Angelsen 1998). These threats often result from an expanding and uncontrolled demand on rainforest resources and from the fragmentation, transformation, and conversion of the forests to other types of land cover. Habitat loss and modification are considered the primary threats to species throughout the world (World Conservation Monitoring Centre 1992; Heywood 1995). In some cases habitat disturbance is an immediate and drastic event—outright conversion—but in others habitat characteristics are altered gradually. Currently, the dominant types of habitat disturbance are directly related to human activities. The effects of some types of habitat disturbance are more pervasive than others, and species may respond in different ways. It is important to understand these processes in order to eliminate or at least mitigate the negative effects of habitat changes. Social considerations are also important. Most tropical rainforest regions are found within the political borders of countries overwhelmed by poverty, weak social institutions, and great demand for natural resources to satisfy local, national, and international markets. The challenge, then, is to learn how to use tropical rainforest resources without hampering their long-term maintenance or the social benefits they provide. The 11 papers in this special section, along with these introductory remarks, provide a global perspective on different types of habitat disturbance and how they affect tropical rainforest mammals. Mammals play an im1574 Conservation Biology, Pages 1574–1579 Volume 14, No. 6, December 2000

portant role in tropical rainforest maintenance and regeneration. They provide vital ecological services in tropical rainforests and are key in the structuring of biological communities through their roles, for instance, in seed predation and dispersal, pollination, folivory, and frugivory, and as top predators (Terborgh 1988, 1992a). Mammals are usually among the favored group of species local people use (e.g., Caldecott 1988; Robinson & Redford 1991; FitzGibbon et al. 1995) for food, pets, and artifacts (e.g., skins, bones, teeth), and they are integral components of the religion and culture of the people who inhabit tropical rainforests. They are also an important economic resource for tropical rainforest inhabitants (Redford 1993; Bodmer et al. 1994; Cuarón 1997, unpublished data). Many mammal species, such as primates and large felids, are charismatic and have been used as flagship or umbrella species to promote the conservation of tropical regions. Thus, they are also important conservation tools. To avoid confounding ecological and management lessons, I decided to limit this special section to tropical rainforests (or, more generally, tropical moist forests) and to ignore more seasonal, dry tropical forests. Nine countries are represented: Equatorial Guinea, Central African Republic, Republic of Congo, Democratic Republic of Congo (formerly Zaire), Malaysia, Brazil, Costa Rica, Guatemala, and Mexico (Fig. 1). They are representative of tropical rainforest regions in westcentral Africa, Southeast Asia, the Brazilian Atlantic forests, the Amazon, and Mesoamerica. The section complements recent efforts to study disturbance and management in tropical forests (e.g., Robinson & Redford 1991; Schelhas & Greenberg 1996; Grieser Johns 1997; Laurance & Bierregard 1997; Fimbel et al. 1999; Laurance 1999; Robinson & Bennett 1999), where mammals, some types of disturbance, and some of the study regions have received little or no attention.

Overview The seed of this special section was the symposium on Habitat Disturbance and Tropical Mammals: A Global Perspective, co-convened by Alfredo D. Cuarón and Ruth K. Laidlaw and held during the Seventh Interna-

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Habitat Disturbance and Tropical Rainforest Mammals

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Figure 1. Former (shaded) and present (black) extent of tropical rainforests (adapted with permission from Terborgh 1992b) and approximate location of the study areas of papers in this special section. Numbers indicate the study sites as follows (respective authors in parentheses): 1, Los Tuxtlas, Mexico (Galindo-González et al.); 2, Calakmul, Mexico (Escamilla et al.); 3, northern Guatemala and southern Mexico (Cuarón); 4, Selva Lacandona, Mexico (Medellín et al.); 5, Osa Peninsula, Costa Rica (Carrillo et al.); 6, Eastern Amazon, Brazil (Lopes & Ferrari); 7, Espírito Santo, Brazil (Chiarello); 8, Bioko Island, Equatorial Guinea (Fa et al.); 9, Republic of Congo and Democratic Republic of Congo (Wilkie et al.); 10, Central African Republic (Malcolm & Ray); 11, Peninsular Malaysia (Laidlaw) ( from Diversity and the Tropical Forest by John Terborgh © 1992 by Scientific American Library and used with the permission of W. H. Freeman and Company).

tional Theriological Congress in 1997. We include in this section papers on some of the dominant types of habitat disturbance in tropical rainforests which affect mammal communities: hunting, roads, logging, forest fragmentation, and land-use and land-cover changes. Some of the papers address more than one of these types of disturbance and provide information on how disturbances interact. Although focused on mammals, the papers include substantial information on other vertebrates, particularly those used for food and those with important roles in forest regeneration. Figure 2 illustrates the general scope of this special section. Tropical rainforest and species conservation depend on society’s development trends, locally and globally. Human activities are affected by the landscape context of the site where they take place, but they also determine to a large extent the landscape configuration. Conservation of tropical forest and prospects for its restoration are affected by human disturbance and influenced by what happens to the biota and the ecological interactions in which they participate. The condition of tropical forests and their biota also determine the activities humans can undertake, the benefit they can obtain, and the characteristics of present and future options for social and economic development. If management activities, including research and monitoring, are successful in eliminating or mitigating the negative effects of human activities, our options for the future will be expanded or at least will not be hampered. If management is not effective, the future of tropical rainforests, their

biota, and development options will be compromised. This group of papers emphasizes the situation of tropical rainforest mammals, which can serve as sensors of the rhythms affecting tropical rainforests, but the scheme (Fig. 2) is general and can be used as a framework in the study of disturbance and the conservation of natural ecosystems and their biota. Monitoring Hunting and Protected Areas Hunting is an important activity that affects mammals throughout the tropics. In some cases forests may appear intact, but they may be devoid of game species because of hunting (Redford 1992). This can lead to severe modification of biological communities and ecological interactions, altering the prospects for forest maintenance and regeneration (Dirzo & Miranda 1991; Redford 1992; Wright et al. 2000). Protected areas have been established throughout the world to avoid, among other things, the detrimental effects of resource exploitation, but how effective are they in achieving those objectives? The section opens with four studies of wildlife exploitation and includes three papers that focus on protected areas. Carrillo et al. (2000 [this issue]) compare the abundance of mammals in two Costa Rican protected areas, a national park and a forest reserve, with different hunting restrictions and levels of vigilance. They also compare the abundance of mammals in the national park over a 4-year period. In addition, they present an inexpensive and effective method, based on the use of mammal track

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Figure 2. Scheme of this special section, indicating some of the complex social and biological interactions involved in the conservation of tropical rainforests and mammals and their relationship with habitat disturbance. records and arboreal mammal sightings, for monitoring mammal populations and assessing the effectiveness of management activities. They found important differences in species abundance between the two protected areas and at the national park over time, which they attribute to differential hunting pressure, particularly on game species. Their data suggest that these protected areas are only partially successful in achieving the conservation objectives for which they were established. Alarmingly, their study includes one of the best-protected regions in one of the most robust system of protected areas in the tropics. Social and Biological Determinants of Hunting The ethnic composition and geographical origin of the human population can have important effects on the way Conservation Biology Volume 14, No. 6, December 2000

wildlife is used (Redford & Robinson 1987). The way it is used is also affected by its availability, which in turn depends on the characteristics of the habitats in a region. Escamilla et al. (2000 [this issue]) evaluate game mammal and bird abundance and subsistence-hunting patterns in four villages with different ethnic composition in Calakmul, southern Mexico. They also assess habitat mosaics around these villages. They find that the more traditional human community has, despite a longer settlement history, a lower level of forest disturbance and higher wildlife availability, and that people used more and larger prey. This paper provides a novel perspective on the intersection between the social and biological aspects of tropical forest and wildlife conservation. Although data are few, there is growing concern that the bushmeat trade may be driving some African forest species to extinction ( J. Oates, personal communication).

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Fa et al. (2000 [this issue]) provide longitudinal data on the use of bushmeat on Bioko Island, Equatorial Guinea. They documented wildlife use by recording animal carcasses arriving at the main market in Malabo, the capital city. Their data show a change in wildlife use patterns over time, which may be explained by a reduction in the number of hunters, shifts in hunting techniques, variation in consumer preferences, and, perhaps most important, species depletion. The demise of large species is of particular concern.

Effects of Roads Roads are important for the transportation of people and goods and are therefore critical for the social and economical development of regions and nations. They also bring about important ecological effects, some of which are of conservation concern (Forman & Alexander 1998; Trombulak & Frissell 2000). Wilkie et al. (2000 [this issue]) deal with the conservation and economic dilemmas confronted by nations and governments when developing or reconstructing road systems in wilderness areas, illustrated with information from the Republic of Congo and the Democratic Republic of Congo. Wilkie et al. found that the development of road systems, often by logging companies, increases bushmeat hunting. Roads provide hunters with greater access to wildlife populations and lower costs to hunters of transporting bushmeat to markets. There are lessons in this paper for governments, donor agencies, and conservation organizations. The conservation message is particularly important for the Congo Basin, one of the largest remaining tropical rainforest areas, but it has general relevance for any wilderness area threatened by the expansion of road networks. Logging is an activity of great economic significance in many tropical regions (Grieser Johns 1997). Logging concessions require the construction of roads for access to timber and for the extraction of forest products. Extensive networks of extraction routes are often created, and they are responsible for direct and incidental forest damage. Malcolm and Ray (2000 [this issue]) compare three different types of timber extraction routes with unlogged forest in the Central African Republic, assessing how they affect forest structure and the composition and diversity of small-mammal and tree communities. They also consider time since timber extraction. They find changes in mammal abundance, vegetation structure, and tree diversity to be highly intercorrelated, and that type of road more than time since logging is the main source of variation in rodent community and vegetation parameters. Their data suggest that roads can continue affecting forest regeneration two decades after they have been established. The authors argue that canopy damage may be the most important measure of disturbance and that rodent communities can be used as in-


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dicators of ecological change at the local and regional levels. Forest Fragmentation Logging and other human activities often cause forest fragmentation. The size of a forest area is important in determining community composition, but to what extent do management activities affect the way these communities are structured? Laidlaw (2000 [this issue]) examines the effects of habitat disturbance on mammal species richness in Malaysian forest reserves. She considers several types of disturbance: selective logging, forest fragmentation, conversion of land to plantations, and establishment of farm bush. She gives particular attention to the conservation contribution of virgin jungle reserves (VJRs), small unlogged protected areas established within logged forest reserves that have been conceived as forestry research control sites and nature reserves. Laidlaw confirms the area of forest largely determines mammal community composition, and finds that VJRs show consistently higher species richness than adjacent logged or converted areas. Virgin jungle reserves can play an important management role, but longterm maintenance of species richness depends on the conservation of large forested areas. One of the consequences of forest fragmentation is the subdivision of populations, which reduces their conservation prospects because they may be subject to detrimental demographic, environmental, and genetic factors (Saunders et al. 1991; Caughley 1994). Chiarello (2000 [this issue]) assesses the differences in population density and size of the five most abundant forest-restricted large-mammal species in Brazilian Atlantic Forest fragments. Chiarello considers a conservation-relevant range of forest-patch sizes (approximately 200–20,000 ha). An alarming conclusion of the study is that only a small proportion of large-mammal species would be able to sustain long-term viable populations in even the largest fragments. Also, few reserves in the entire Brazilian Atlantic Forest are large enough to sustain viable populations of the assessed species, not to mention scarcer species such as top predators. Aggressive management actions are urgently needed to conserve the unique biological diversity of the highly threatened Brazilian Atlantic Forest. Colonization of Frontier Areas Virtually all tropical wilderness areas are threatened by human encroachment. We can learn from past experiences in some areas to avoid negative trends in other areas. Lopes and Ferrari (2000 [this issue]) present new and important data on the effects of colonization on mammal abundance and diversity in a poorly known area of the eastern Brazilian Amazon. They surveyed five sites representing a disturbance gradient, including difConservation Biology Volume 14, No. 6, December 2000

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ferent degrees of forest fragmentation, logging and clearance, and hunting pressure, and they identify several patterns in species abundance, biomass, and diversity. Their data, which complements an extensive data set from western Amazonia (Peres 2000), show the pervasive effects of anthropogenic disturbance in frontier areas and point out the direction in which the more pristine regions of the Amazon could go if similar development trends are followed.

Biotic Indicators of Disturbance Sometimes the different degrees of disturbance in some areas are not evident, so we need ways to objectively detect these differences in order to design conservation strategies and assess their effectiveness. Medellín et al. (2000 [this issue]) document the effects of habitat disturbance on a tropical bat community at the Selva Lacandona, southern Mexico, and they argue that this group of mammals can be good indicators of habitat disturbance in tropical rainforests. They sampled bat communities and vegetation parameters in active cornfields, oldfields, cacao plantations, and rainforest, and they use a fuzzy set technique to define a classification of habitatdisturbance gradients. Their data show that bats respond readily and sensitively to habitat disturbance. Bat diversity is related in a predictable way to the structural complexity and composition of the vegetation. The approach of Medellín et al. could be useful in assessing subtle differences in degree of disturbance among similar habitats or sites with no obvious differences in terms of disturbance (e.g., oldfields vs. cacao plantations).

Land-Cover Changes Land-cover changes are central to many of the most pressing environmental problems of our time (Meyer & Turner 1994; Vitousek 1994) and are a main cause of species extinction. Cuarón (2000 [this issue]) assesses the effects of land-cover changes on 54 nonvolant mammal species in a 2.7-million ha area that covers part of Guatemala and of the Mexican states of Campeche, Tabasco, and Chiapas, including two protected areas in the latter state. He uses data derived from remote sensing of land-cover change, field and bibliographic information on mammal habitat requirements, Markov models, and elasticity analyses to assess the effects of changes in mammal habitat availability. He also evaluates the consequences for mammals of the regional variability of land-cover changes and the effects of variations in management and development trends. His approach is useful for assessing the effects of changes in species habitat availability and survival perspectives for a substantial part of a biota in large areas, even in the context of limited information.


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Frugivores as Restoration Facilitators There are areas where tropical forests have been degraded to such an extent that the emphasis of management must switch to restoration. Wildlife, through the ecological interactions in which they participate, can assist in this process. Galindo-González et al. (2000 [this issue]) document the important role of bats and birds in the regeneration and restoration of former tropical rainforest sites in Los Tuxtlas, Mexico, which have been replaced by pastures. They assess seed dispersal by the remaining frugivorous bat and bird fauna from rainforests to isolated trees in pastures. In the absence of cattle grazing, isolated trees can function as nuclei of forest regeneration in pastures. Through seed dispersal, bats and birds may connect landscape elements and may play an important role in the maintenance, recovery, and enhancement of plant diversity in fragmented landscapes. Major challenges to ensuring the survival of tropical mammals and rainforests lie ahead. I hope that this special section, and especially its management recommendations, will serve as a framework for and will stimulate further work on these important conservation and research issues.

Acknowledgments I am most grateful for the enthusiastic participation of all contributors to the special section and for the generous support of the many reviewers of the papers. G. Meffe provided invaluable help and encouragement as I prepared this section. The papers benefited immensely from the expert editorial advice of E. Main. I thank M. Martínez-Ramos for support and useful discussions; P. Carton de Grammont and H. Ferreira for help preparing the figures; and P. Balvanera, J. Benítez-Malvido, and O. Chassin for comments on the manuscript. The symposium from which this section originated was financially supported by Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO) and the editorial process of this special section by Instituto de Ecología, Universidad Nacional Autonoma de Mexico and grants to individual authors. Alfredo D. Cuarón Guest Editor Departamento de Ecología de los Recursos Naturales, Instituto de Ecología, Universidad Nacional Autónoma de México, Apartado postal 27-3 (Xangari), Morelia, Michoacán, 58089 México, email [email protected]

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