Celia A. Harvey[54]
Livestock grazing is one of the most widespread land uses in Central America and is arguably the land use that has had the greatest impact on regional biodiversity. Approximately 26% of the land (13 million ha) is currently under pastures and in many areas pastures are still expanding (Gomez-Pompa ET al., 1993, FAOSTATS, 1997; CCAD, 1998). The gradual transformation of forests to pastures and agricultural lands has had profound ecological impacts in the region, changing the species composition of communities, disrupting ecosystem functions (including nutrient cycling and succession), altering habitat structure, aiding the spread of exotic species, isolating and fragmenting natural habitats, and changing the physical characteristics of both terrestrial and aquatic habitats (Fleischner, 1994; Noss, 1994). These changes, in turn, have often resulted in the reduction of both local and regional biodiversity.
While the negative impacts of the conversion of forests to pasture have been well-documented and publicized, surprisingly little attention has been paid to the biodiversity that is retained within active pastures and within pasture-dominated landscapes. In addition, little is known about the relationships between pasture management and biodiversity conservation. Yet recent studies indicate that a significant portion of the original biodiversity can be maintained within pastures, if they are designed and managed appropriately (Greenberg, 1997; Harvey et al., 2000). One such strategy for maintaining and conserving biodiversity within pasture-dominated landscapes is the promotion of silvopastoral systems, which integrate tree management with cattle production. In addition to producing timber, forage and fruits, providing shade for cattle, and promoting soil conservation and nutrient recycling (Pezo and Ibrahim, 1998), silvopastoral systems provide structures, habitats and resources that may enable the persistence of some plant and animal species within the fragmented landscape, thereby partially mitigating the negative impacts of deforestation and habitat fragmentation.
In this paper, I provide a brief overview of the potential of silvopastoral systems to conserve biodiversity within fragmented landscapes, and present case studies of two common silvopastoral systems (dispersed trees in pastures, and windbreaks) that illustrate the various ways in which silvopastoral systems can help conserve both plant and animal species. I also discuss the ways in which silvopastoral systems can be integrated into conservation programs and identify key gaps in our knowledge of the relations between silvopastoral systems and biodiversity conservation. Because the interest in the conservation of biodiversity within silvopastoral systems is very recent, many of the ideas and information presented here are necessarily of a preliminary nature.
Silvopastoral systems tend to have a high genetic diversity and incorporate a wide variety of tree, shrub and grass species that are deliberately planted or retained by the farmer (‘planned biodiversity’). The tree, shrub and grass components, in turn, provide physical structures, resources and habitats that support additional plant and animal species (‘associated biodiversity’). Rich communities of lianas, mosses, lichens and epiphytic plants often occur on tree branches and trunks, while many forest plant species may establish under the shade of the tree canopies. A wide variety of animals (insects, birds, bats and other mammals) may use the silvopastoral system for food, shelter or protection from predators or adverse weather conditions.
In addition to providing habitats and resources for both plants and animals, silvopastoral systems may also help conserve biodiversity by creating microclimatic and soil conditions that are more favorable to forest species, by acting as ‘stepping stones’ or ‘corridors’ that facilitate animal movement across the agricultural habitats, and by acting as buffer zones around protected or natural areas. Perhaps equally important, silvopastoral systems provide alternative sources of timber, firewood and other wood products, thereby reducing the pressure on the remaining natural forest habitats and their biodiversity.
As the case studies will illustrate, the degree to which silvopastoral systems are able to conserve biodiversity varies greatly among different silvopastoral systems and different locations, and depends (at least in part) on the different design and management of the system.
Case study 1: Windbreaks and the conservation of biodiversity
Windbreaks are commonly established in pastures to reduce wind velocities, provide shelter to cattle, reduce pasture desiccation, and to prevent wind soil erosion. While the primary function of the windbreaks is agronomic, the windbreaks may also an important ecological role as they provide habitats and resources that permit other animals and plants to persist within the pasture habitat. Due to their linear form, windbreaks may also form natural corridors that may allow some animal species to cross the agricultural landscape.
Plant diversity within windbreaks and hedges can be considerable, either because a variety of tree and shrub species are intentionally planted or retained by the farmer within the windbreak and/or because many additional plant species colonize the windbreaks once they are established. Plant diversity within windbreaks is usually highest if the windbreaks are fenced to prevent cattle entry and if the understory is left undisturbed (i.e. no herbiciding or manual wedding; personal observation). Most of the plant species that occur within windbreaks are forest edge species that thrive in high light, disturbed conditions, however some forest interior plants may colonize and survive within the windbreaks, especially in wider windbreaks where the microclimatic conditions (higher humidity and more shade) are more favorable (Corbit et al., 1999). For example, windbreaks in Monteverde, Costa Rica (planted with Casuarina equisetifolia, Cupressus lusitanica, Croton niveus, and Montanoa guatemalensis) were colonized by a total of 124 plant species within 5 years of windbreak establishment, of which 90 species were forest tree species typical of the original cloud forest (Harvey, 1999).
Windbreaks may also attract a variety of animals by providing nesting and breeding sites, protection from predators and adverse weather conditions, and supplies of seeds, fruits, nectar and leaves. While there is little information from tropical regions, studies from temperate regions show that a variety of woodland rodents (shrews, mice, and chipmunks), rabbits, squirrels, and other animals frequently inhabit hedgerows and windbreaks, whereas large animals (such as deer) may browse on the vegetation within the windbreaks (Forman and Baudry, 1984).
In the tropics, windbreaks may be particularly important for the conservation of bird species. The windbreaks in Monteverde, Costa Rica (discussed above), are visited by more than 80 bird species, including a variety of frugivores, insectivores and nectivores (DeRosier, 1995). Because different bird species often specialize on a certain strata or level within the windbreak windbreaks that are multi-layered and structurally complex are likely to host a greater diversity of bird species than those that contain a single strata and are structurally uniform (Yahner, 1982). In cases where the windbreaks connect forest patches, some bird species may use the windbreaks as travel lanes or corridors to cross the open pastures (DeRosier, 1995; Haas, 1995). This preference of some bird species to move within the windbreaks may increase the dispersal of forest trees into windbreaks, thereby also increasing the forest regeneration within the windbreaks (Harvey, 2000). For example, windbreaks in Monteverde received 39 times as many tree seeds and twice as many species of tree seeds than adjacent pastures because of higher bird activity within the windbreaks (Harvey, 2000).
Windbreaks may also support a wide variety of insect species (usually much greater than that of neighboring pastures and crops), because they serve as sources of prey, nectar or pollen, and provide protected sites for mating, resting and overwintering. Some of the insects living within the windbreaks are beneficial organisms, such as crop pollinators, predators or parasites of crop pests that overwinter in hedges and spread from the hedges into the fields where they prey on crop pests (Pasek, 1988; Dix et al., 1995), however others may be important agricultural pests. In the USA, for example, windbreaks often contain populations of aphids, cotton boll weevils , and alfalfa weevils that overwinter in the windbreak and then move into the fields in the spring, inflicting considerable damage on nearby crops. (Pasek, 1988). The net effect of the increased insect abundance and diversity within windbreaks on adjacent crops is not yet clear.
Case study 2. Isolated trees in pastures and biodiversity
Another common silvopastoral system that appears to hold considerable potential for biodiversity conservation is the systems of isolated trees within pastures. In most pastures in Central America, some canopy trees and shrubs are usually left standing to provide shade for cattle. These scattered, isolated trees may be relicts of the former forest, or may have regenerated or been planted since the pastures were established. In addition to serving as important sources of fodder, fruits, timber, fuel wood, and shade for livestock, these isolated trees also provide important habitats and resources for biodiversity within the agricultural landscape (Guevara et al., 1998; Harvey and Haber, 1999) and may help promote landscape connectivity for some species.
Although the species richness represented by isolated trees varies greatly among pastures, farms and different ecological regions, the trees often represent a large number of species. Studies in Veracruz, Mexico, for example, found a total of 98 species of isolated trees (belonging to 33 families) occurring in pastures, of which 76 were primary forest species (Guevara et al. 1998), whereas studies of active pastures in Monteverde, Costa Rica found a total 190 species of trees, of which 57% were primary forest species (Harvey and Haber, 1999). Although the densities of isolated trees in pastures are usually quite low (< 25 individuals/ha, compared to densities of 300-500 trees/ha in native forests), the presence of isolated trees helps enhance landscape connectivity by providing additional tree cover within the area and reducing the amount of open area that animals need to cross (Guevara et al., 1998; Harvey and Haber, 1999).
In addition to the floristic diversity that the trees themselves represent, many isolated trees that are remnants of the original forest retain rich communities of epiphytes on their branches and trunks (Williams et al., 1995). Studies in Veracruz, Mexico, for example, found a total of 58 species of vascular epiphytes and hemiepiphytes (representing 37% of the total epiphytic flora in the region) occurring on 38 isolated forest trees in pastures and epiphyte densities similar to that found on undisturbed forest trees (Hietz-Seifert et al., 1996). Interestingly, whereas the isolated forest trees in pastures contained rich and diverse epiphytic populations, the isolated trees that are planted or that regenerate naturally within the pastures appear to be colonized very slowly by epiphytes A total of only 14 epiphyte species were found on 45 planted isolated trees in pastures (25 Cedrela odorata and 20 Citrus trees) compared to the 58 epiphytic species found on isolated forest trees growing in the same pastures (Hietz-Seifert et al., 1996).
Another way in which isolated trees may help promote the floristic diversity within pastures is by facilitating natural forest regeneration. Isolated trees in pastures function as foci for seed dispersal and forest regeneration, as they attract birds and other animals that defecate or drop seeds(Guevara and Laborde, 1993). As a result of high seed input and favorable microclimatic conditions below the tree canopy, the diversity and abundance of seedlings beneath isolated tree canopies is often very high. For example, a study in Veracruz, Mexico found that 193 plant species (109 woody, 84 herbaceous) established under 50 isolated trees in pastures, whereas only 42 species established in open pastures (Guevara et al., 1992). Similarly, a study by Otero-Arnaiz et al. (1999) found a total of 134 plant species (45 families) underneath isolated trees in pastures, of which 38% of the species were primary species.
Isolated trees in pastures and other agricultural habitats may also provide valuable resources and habitats for a variety of animals, including both resident and migrant bird species (Lynch 1989a, 1989b, Saab and Petit, 1992; Naranjo, 1992). For example, recently- abandoned pastures in Belize that contained isolated shrubs and trees harbored a total 39 bird species whereas pastures that were actively grazed and had few shrubs or trees present contained only 15 bird species (Saab and Petit, 1992). Many of the isolated trees in pastures provide fruits for visiting birds. Of the trees found within the pastures of Monteverde, Costa Rica, 94% of all of the trees are known to provide fruits for birds, bats or other animals, and many of the most common species (e.g. Acnistus arborescens, Citharexylum costaricensis, Ficus pertusa, Hampea appendiculata and Sapium glandulosum) each provide food sources for more than 20 species of birds (Harvey and Haber, 1999). Similarly, 55% of the isolated trees present in pastures in Chiapas, Mexico, were fleshy-fruited (Otero-Arnaiz et al., 1999) and presumably provide food for visiting bird and animal species. A final way in which isolated trees can help conserve bird diversity is by helping conserve landscape connectivity and facilitating bird movement across open areas. Birds travelling across pastures and fields routinely fly from one isolated tree to the next, using the isolated trees as ‘stepping stones’ to cross the agricultural landscape (Laborde, 1996).
In Central America, pastures with dispersed trees and other remnant native vegetation appear to play a critical role not only in the conservation of resident bird species, but also in the conservation of the many species that migrate to Central America during the winter months of the northern hemisphere. Numerous studies suggest that a subset of these migrant species are capable of using pastures and agricultural fields and that pastures with isolated trees or other native vegetation are better habitats than open, heavily grazed pastures that lack perch sites (Lynch, 1989; Powell et al., 1989). For example, studies in pastures in the Yucatan Peninsula of Mexico found a total of 17 migrant bird species (Lynch, 1989), whereas studies in the lowland Atlantic habitats of Costa Rica found 12 migrant birds species in pastures and agricultural fields (Powell et al., 1989). The use of pastures by migrant bird species varies greatly among species with some species such as the Yellow-throated warbler and the Rose-beaked grosbeak occurring commonly in pastures, while other species such Kentucky Warbler and Wood thrush occurring only very rarely in these open pasture habitats (Lynch, 1989b). However, even small patches of dispersed trees are capable of sheltering 'forest' migrants though at lower densities than those in forests (Lynch, 1989b).
Although there is still limited information about the role of silvopastoral systems in biodiversity conservation, silvopastoral systems appear to offer a promising avenue for the conservation of biodiversity in fragmented, human-dominated landscapes and at the very least offer an alternative to pasture monocultures they usually replace. The incorporation of trees into livestock grazing systems provides habitats and resources that are otherwise lacking within the landscape and may serve as stepping stones or corridors for animals dispersing between forest remnants, thereby making pastures suitable for some plant and animal species. In addition, by providing alternative sources of wood, timber and fence posts within the landscape, silvopastoral systems may reduce the pressure on adjacent forests for these products, and thereby lessen the pressure on forests and the biodiversity within them.
The extent to which silvopastoral systems can help harbor biodiversity appears to depend both on the design and management of the system[55]. Actions that may enhance biodiversity within silvopastoral systems include: maximizing the structural and floristic diversity of the silvopastoral system (by planting a variety of plant species and life forms); including native plant species that provide resources and habitats for wildlife year-round (e.g., trees that produce abundant nectar, flowers, pollen or fruits); retaining a diverse and dense understory within the silvopastoral system; allowing epiphytes, lianas and parasitic plants to grow on the trees; minimizing the use of agrochemicals and fertilizers (replacing them with biological controls, cultural practices and integrated pest management); minimizing the extraction, harvesting and management of the system; positioning silvopastoral in such a way that they enhance landscape connectivity (e.g., connecting them to forest patches, or placing them near streams), and minimizing cattle damage to the tree component.
Although silvopastoral systems clearly provide a better conservation alternative to the extensive, open pasture monocultures they usually replace, it is important to keep in mind that silvopastoral systems can only maintain conserve a subset of the original biodiversity within the landscape, regardless of how well designed or managed they are. Silvopastoral systems will generally tend to favor the conservation of species that require tree cover, but can adapt to disturbed areas and to human disturbance; only rarely will they be able to support forest specialists or species that require large tracts of extensive forest. However, silvopastoral species do harbor many plant and animal species that are of interest to conservationists (e.g. migrating birds).
In order to promote the conservation of biodiversity within the pasture-dominated landscapes of Costa Rica, a landscape-level conservation strategy is needed. This strategy, in addition to promoting silvopastoral systems, should also include the establishment of protected areas, the conservation of forest fragments and other remnant vegetation, the integration of trees into crop fields, and the reforestation or natural regeneration of degraded lands. In order to be effective, this conservation strategy must be specifically tailored to the ecological and socioeconomic conditions of a given region, and ensure that both conservation and production goals are fulfilled.
Our ability to promote and use silvopastoral systems as conservation tools is still heavily limited by a lack of information about the ecological relations between silvopastoral systems and biodiversity, the factors that affect the degree to which silvopastoral systems can help maintain biodiversity, the effects of management practices on biodiversity, an understanding of how farmers interact, perceive and use the biodiversity within their silvopastoral systems, and the tradeoffs or synergisms between designing and managing silvopastoral systems for production vs. conservation goals.
Key research question include:
1. To what degree do individual species depend upon silvopastoral as habitats, resources and corridors? Which species are benefited by the presence of silvopastoral systems? Which species are affected negatively? How does the survivorship of plant and animal species within silvopastoral systems compare to that of other habitats?
2. What factors influence the biodiversity present within different silvopastoral systems?
3. How does the surrounding landscape (e.g. degree of forest fragmentation, level of connectivity) influence the level of biodiversity within silvopastoral systems?
4. How do different management schemes affect the biodiversity present within silvopastoral systems? How could these management schemes be changed to minimize the effects on biodiversity?
5. How can the biodiversity present in silvopastoral systems be used to improve farm production and sustainability?
6. How do farmer attitudes and perceptions of biodiversity influence the conservation of biodiversity on their farms? And what factors influences these attitudes?
7. What are the social and economic costs (and benefits) of maintaining high levels of biodiversity in silvopastoral systems and farms?
8. What are the tradeoffs (or synergisms) between designing and managing silvopastoral systems for conservation versus productive goals?
9. To what extent do laws and agricultural policies negatively affect farm biodiversity?
A more detailed understanding of these relationships and longer-term studies of the biodiversity within silvopastoral systems will hopefully permit us to more effectively design and manage silvopastoral systems such that they both enhance farm productivity, while retaining as much biodiversity as possible.
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[54] Area de Cuecas y
Sistemas Agroforestales (ACSAF), Centro Agronómico Tropical de
Investigación y Enseñanza, Turrialba, Costa Rica [55] The socioeconomic context is also likely to have an important effect on the degree to which silvopastoral systems conserve biodiversity, but there is little information on how the social, cultural and economic conditions of livestock production systems influence the biodiversity retained within silvopastoral systems. |
