- Shopping Bag ( 0 items )
Ships from: Horcott Rd, Fairford, United Kingdom
Usually ships in 1-2 business days
Ships from: Avenel, NJ
Usually ships in 1-2 business days
Preservation and Management of Biodiversity in Fragmented Landscapes in the Colombian Andes
Gustavo H. Kattan and Humberto Alvarez-López
* * *
Land use in the Colombian Andes has greatly altered natural ecosystems. Up to 85 percent of the area in premontane and montane forests has been modified to some extent (Orejuela 1985), with the result that in most regions natural vegetation remains only in isolated patches. As a consequence, local extinctions of birds have been documented (Kattan et al., 1994), as well as population declines at the regional and national levels (Hilty 1985; Collar and Andrew 1988). Similar trends at the local level presumably occur in other animal groups (e.g., frogs and dung beetles—Kattan 1993; Escobar 1994), although data are scarce or lacking.
Despite this grim picture, the remaining fragments of natural vegetation still harbor a sizable number of species of plants and animals. While it is unquestionable that large areas need to be protected to preserve the bulk of biological diversity, forest fragments may play an important role in the preservation of a significant fraction of the original regional diversity. At the landscape level, forest fragments of different sizes and ages can be incorporated in a management scheme to maximize the persistence of species. In addition to a sheltering function, these forests would provide important environmental services, such as protection of watersheds.
In this chapter we explore the possibilities for preserving the extant biodiversity in landscape units that include highly fragmented forests in the Colombian Andes. We first describe the natural heterogeneity of Andean landscapes, the impact of land-use practices, and how fragmentation may lead to species extinction. Then we discuss how management at the landscape level may provide tools for the minimization of biodiversity losses. Although we rely mostly on examples from the Colombian Andes, the ideas presented here may be applied to similar situations in the tropical Andes and tropical mountains in general.
* * *
The three ranges of the Colombian Andes extend southwest-north and northeast from the Ecuadorean to the Venezuelan borders, in a latitudinal range from about 1 to 11 degrees north of the equator. Mountaintop elevations range from 1,800 meters at low passes to more than 5,000 meters at snow peaks, encompassing six altitudinal belts (Espinal and Montenegro 1963) from sea level to the highest Andean elevations. The lower belts are essentially continuous along the three cordilleras, but the higher ones, mostly above 3,000 meters, frequently become interrupted by topographical irregularities.
Complex patterns of interactions between altitude, temperature, and rainfall result in a correspondingly complex mosaic of plant communities. Above 1,000 meters at least 15 natural life zones (sensu Holdridge 1967; Espinal and Montenegro 1963) have been recognized. These life zones range from premontane thorn woodland in midaltitude inter-Andean valleys affected by rain shadows, to rainforests in the western and eastern foot slopes, to rain tundra adjacent to perpetual snow peaks. Edaphic and topographic factors add to this diversity in the form of an array of transitional life zones and different associations. In addition, as the three ranges of the Colombian Andes are the result of separate orogenic events (González et al. 1989), and the distributions of plants and animals were extensively affected by climatic changes during the Pleistocene (Haffer 1974), historical factors also account for biogeographical features such as complex speciation patterns (Vuilleumier 1986) and high endemism (Terborgh and Winter 1983) and biodiversity (ICBP 1992).
The generally humid mountains generate an extensive, although somewhat irregularly distributed, fluvial network, which in turn provides connectivity among altitudinal belts. Besides, these watercourses, with their associated vegetation, add to the complex interdigitation and irregularity between natural life zones (Espinal and Montenegro 1963).
Although land-use patterns might have initially created a mosaic superimposed on that of natural vegetation, the long-term effect has been one of homogenization of the landscape. While early settlers preferred fertile mountainsides, middle- and high-altitude inter-Andean valleys, and alluvial terraces (Holdridge 1967), demographic growth has forced more intensive land exploitation, and settlement of increasingly steeper slopes and higher and more rainy regions. Therefore, vast regions of the Colombian Andes are presently dominated by pastures and open areas mostly devoid of trees, and sometimes badly eroded. Exceptions to this trend are a few national parks, protected watersheds, some coffee and tree plantations, and the wettest regions of the Pacific and eastern Andean drainages.
* * *
Negative Effects of Fragmentation in Andean Ecosystems
Although very few studies have addressed the effects of habitat fragmentation in the tropical Andes, the available evidence indicates that fragmentation and current land management practices may result in the local extinction of large numbers of species. The best evidence available is for birds. The existence of historic faunal inventories dating back to 1911 and 1959 provided a rare opportunity to document avian extinctions in the region of San Antonio, a cloud forest site in the western range of the Colombian Andes (Kattan et al. 1994). The area, which was fragmented mostly during the first half of this century, at present constitutes an isolated archipelago of forest patches ranging in size from 10 to 400 hectares in a matrix of small farms and suburban houses. Forty species, or 31 percent of the original 128 forest bird species, were locally extinct in 1990.
An analysis of patterns of extinction at this site revealed two main trends (Kattan et al. 1994). First, species at the limits of their altitudinal or geographic distributions exhibited a high frequency of extinction. Of 29 species for which San Antonio was either the upper or lower limit of their altitudinal distribution, 19 are locally extinct. Second, large fruit-eating birds, such as quetzals and cotingas, were the trophic group most vulnerable to extinction. Out of 18 large frugivores originally present, 12 went locally extinct. Among 13 extinct bird species for which the study area was well within their altitudinal limits, 7 were large frugivores.
While patterns of extinction reveal groups of organisms that are particularly vulnerable, prevention of biodiversity losses requires knowledge of the processes and mechanisms responsible for such losses. The consequences of fragmentation may occur at several levels. First, there are effects on the physical environment (e.g., light incidence, temperature). Second, there are direct biological effects on the distribution and abundance of organisms, sometimes mediated through the physical effects. Third, there may be complex, indirect biological effects on the interactions among species, such as predation and parasitism (Murcia 1995).
One direct, physical consequence of fragmentation is a reduction in habitat heterogeneity, or the deterioration and disappearance of certain microhabitats. Streams, for example, are particularly vulnerable because of the desiccation process that affects isolated forest patches (Lovejoy et al., 1986; Saunders et al., 1991). In addition, in suburban areas in the Colombian Andes, streams frequently are partially or totally piped to supply water for human consumption. This may have catastrophic consequences, not only in causing the local extinction of populations of stream-dependent organisms, but in altering the fragment's hydric dynamics.
The effect of stream disturbance is more evident on amphibians, because of the dependence of many species on water for breeding. Frogs exhibit a wide diversity of reproductive modes, which refers to a combination of egg deposition site (aquatic or terrestrial) and development mode (varying from free-swimming tadpoles to direct development). Diversity of reproductive modes contributes greatly to species richness in Neotropical anuran assemblages (Crump 1982; Kattan 1987). Thus, diversity of breeding microhabitats is an important factor in determining frog species richness.
In the Brazilian Amazon, diversity of breeding microhabitats was a very important factor determining the persistence of frogs in forest remnants (Zimmerman and Bierregaard 1986). In the San Antonio region, preliminary evaluations of frog populations in sites ranging from undisturbed streams to total piping revealed that species richness depends on disturbance level (G. Kattan and C. Murcia, unpublished data). Seven species of stream-breeding frogs were found in an undisturbed stream. In forest patches where water is partially piped, or where water runs free for a short distance (80–200 meters) and then is piped, only one to four species persisted, depending on the amount of water available. At sites where water is piped right at the stream head, no water-breeding frogs persisted.
In contrast to water-breeding frogs, most terrestrial-breeding species were present in all fragments. Frogs with terrestrial reproduction account for 50 percent or more of the species in Andean frog assemblages, in particular the genus Eleutherodactylus, which is very diverse in the tropical Andes (Lynch 1986). These frogs lay terrestrial eggs that undergo direct development (no aquatic larval stage) and are independent of water for reproduction. Thus, reproductive mode will determine the species assemblages in fragmented habitats. The existence of frogs with terrestrial reproduction means that up to 50 percent of the species may persist in small fragments in the tropical Andes, but diversity at the genera and family levels, as well as diversity in reproductive modes, will be drastically reduced.
In addition to its direct consequences, fragmentation may alter interactions among species. Studies in the temperate zone have shown that processes such as bird nest predation and brood parasitism by brown-headed cowbirds (Molothrus ater) are important factors in the decline of bird populations in small forest patches (Robinson 1992; Paton 1994). Rates of nest predation and brood parasitism are frequently elevated in small patches, when compared to large fragments or continuous forest. This frequently occurs as a consequence of an edge effect. In small patches, there is a large perimeter-to-area ratio, which makes the patch vulnerable to invasion by organisms from the matrix or to ecological changes caused by proximity to the edge.
Data on the effects of fragmentation on these ecological processes are sorely lacking in the tropics. Preliminary data suggest that parasitism by shiny cowbirds (M. bonariensis) is not a problem in cloud forest fragments (G. Kattan, unpublished data). However, a study on predation of artificial nests revealed that fragmentation may be an important factor in the extinction of some understory birds in Andean cloud forest (Arango 1991). This study, however, did not find support for an edge effect. Instead, circumstantial evidence suggested that the elevated rates of predation may result from the absence of medium-sized mammalian predators in the small fragments and a consequent abundance of small predators (Arango 1991).
Besides affecting predation rates on bird nests, changes in mammal assemblages caused by fragmentation (Malcolm 1988; Fonseca and Robinson 1990) may also have indirect consequences on other organisms. Dung beetles (Scarabaeidae) are highly sensitive to deforestation and fragmentation (Klein 1989; Escobar 1994). The species composition and activity patterns in cleared areas are drastically different from those in forests (Escobar 1994), probably reflecting parallel trends in mammals. If large and medium-sized mammals disappear from fragmented areas, dung beetles will follow a similar fate. This, in turn, may have repercussions in the entire ecosystem, as dung beetles may play a major role in secondary seed dispersal and control of pests dispersed in dung (Doube and Moola, 1988; Estrada and Coates-Estrada, 1991).
Frugivory is another potentially vulnerable process that may have important consequences for the preservation of montane forests. Large fruit-eating birds have emerged as vulnerable in several studies (Willis 1979; Kattan 1992; Kattan et al., 1994). There is increasing evidence that many frugivorous birds migrate altitudinally (Loiselle and Blake 1991; Levey and Stiles 1992). Most frugivorous birds may require continuous habitat along altitudinal gradients because fruit availability is variable in time and space, and tracking these resources involves seasonal movements that cover large areas. Forest fragmentation severs the connection between foraging areas and may severely restrict access to a year-round food supply. Because large proportions of plants in tropical montane forests have bird-dispersed seeds, extinction of fruit-eating birds may cause the disruption of ecological processes that pervade the entire ecosystem (Howe 1984; Terborgh 1986).
* * *
What Can Be Preserved in Fragmented Forests?
Reduction of continuous forests to small, isolated patches has severe consequences for biological diversity. It is a fact, however, that in the Colombian Andes, except for a few national parks and some remote areas, only fragments remain of the natural vegetation. What can we expect to preserve in such a situation? In this section we evaluate the biological diversity that remains in forest fragments and the contribution that these fragments can make to the preservation of biodiversity in multiple-use landscapes.
We reviewed available studies on bird communities in the Colombian Andes to explore the relationship between forest resident species richness and forest area. Although these studies are highly heterogeneous in terms of types of vegetation sampled and sampling efforts, the analysis revealed some trends. We classified areas in three categories, according to fragment size. Sites with more than 1,000 hectares of forest, with high connectivity and habitat heterogeneity, were defined as large. These were all protected sites contiguous to large extensions of forest (several thousand hectares), and we assumed they contained a full complement of species (Renjifo and Andrade 1987; Orejuela and Cantillo 1990; Naranjo 1994; H. Alvarez-López and G. Kattan, unpublished data). Fragments that ranged between 100 and 600 hectares, isolated in a matrix of agricultural and pasture lands, were classified as medium (Orejuela et al., 1979a; Ridgely and Gaulin 1980; Cuadros 1988; Mondragón 1989; Kattan et al., 1994), while those that ranged between 10 and 50 hectares were defined as small (Orejuela et al., 1979b; Orejuela and Cantillo 1982; Orejuela et al., 1982; Johnels and Cuadros 1986; Corredor 1989).
There were, on average, 144 forest bird species in areas with large expanses of continuous forest and high habitat heterogeneity. Species richness decreased markedly with area (figure 1-1). The mean number of species in small fragments was 25 percent of the mean number of species found in continuous forest areas, while on average 60 percent of the species persisted in medium-sized fragments. The number of species of fruit-eating birds followed a parallel trend. A mean of 16.5 species of large frugivores can be expected in large areas of forest. This number decreases to 7.4 species in medium and 2.4 in small fragments (figure 1-1).
Because there is great concern about the effect that accelerated deforestation of Neotropical habitats may have on migratory birds (Hagan and Johnston 1992), we also looked at the numbers of forest migrants recorded in each of the fragment size categories and found no difference in the number of species as a function of fragment size (figure 1-1). Variability in the number of species, however, was greater in small- and medium-sized fragments. This result suggests that forest migrants are using small- and medium-sized fragments, at least as temporary habitats, as well as large, continuous forests. Some species may even be selecting disturbed habitats, as no migrants were recorded at a continuous forest site in the Pacific lowlands (Orejuela et al., 1980), while six species were recorded at a disturbed site nearby (Hilty 1980).
Excerpted from Forest Patches in Tropical Landscapes by John Schelhas, Russell Greenberg. Copyright © 1996 Island Press. Excerpted by permission of ISLAND PRESS.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
|Introduction: The Value of Forest Patches|
|Ch. 1||Preservation and Management of Biodiversity in Fragmented Landscapes in the Colombian Andes||3|
|Ch. 2||Forest Fragmentation and the Pollination of Neotropical Plants||19|
|Ch. 3||The Consequences of Prolonged Fragmentation: Lessons from Tropical Gallery Forests||37|
|Ch. 4||Managed Forest Patches and the Diversity of Birds in Southern Mexico||59|
|Ch. 5||Arthropod Diversity in Forest Patches and Agroecosystems of Tropical Landscapes||91|
|Ch. 6||Hunting Wildlife in Forest Patches: An Ephemeral Resource||111|
|Ch. 7||The Ecological Importance of Forest Remnants in an Eastern Amazonian Frontier Landscape||133|
|Ch. 8||Biology and Conservation of Forest Fragments in the Brazilian Atlantic Moist Forest||151|
|Ch. 9||The Importance of Forest Fragments to the Maintenance of Regional Biodiversity in Costa Rica||168|
|Ch. 10||Islands in an Ever-Changing Sea: The Ecological and Socioeconomic Dynamics of Amazonian Rainforest Fragments||187|
|Ch. 11||Modification of Tropical Forest Patches for Wildlife Protection and Community Conservation in Belize||205|
|Ch. 12||Forest Use and Ownership: Patterns, Issues, and Recommendations||233|
|Ch. 13||Land-Use Choice and Forest Patches in Costa Rica||258|
|Ch. 14||Reading Colonist Landscapes: Social Interpretations of Tropical Forest Patches in an Amazonian Agricultural Frontier||285|
|Ch. 15||Sacred Groves in Africa: Forest Patches in Transition||300|
|Ch. 16||Managing Forest Remnants and Forest Gardens in Peru and Indonesia||327|
|Ch. 17||Timber Management of Forest Patches in Guatemala||343|
|Ch. 18||Community Restoration of Forests in India||366|
|Ch. 19||Challenges in Promoting Forest Patches in Rural Development Efforts||381|