The role of silvopastoral systems in the rehabilitation of Andean stream habitats

J Chará and E Murgueitio

Centre for Research on Sustainable Agricultural Production Systems (CIPAV),
Carrera 2 Oeste 11-54, Cali, Colombia
julian@cipav.org.co

Abstract

This paper briefly describes the history of cattle occupation in the Andean region of Colombia, explains the environmental impacts of this activity with particular reference to aquatic environments and suggests some remediation practices based on silvopastoral systems.

The Andean region of Colombia is characterised by its biodiversity and the large number of lotic habitats. It concentrates the majority of the urban and rural population of the country and supports an important agricultural activity. Extensive pastures occupy most of the land area and, although studies of the impact of cattle on water environments are scarce, some research done by CIPAV has demonstrated that cattle grazing in the Andean region of Colombia negatively affects water environments, mainly by reducing the quality of the physical habitat in streams and by increasing organic matter and reducing oxygen concentration in the water.

However, there are alternatives for reducing damage by cattle which include the protection of springs and water courses, reduction of the effect of diffuse pollution and the transformation of homogeneous pastures into silvopastoral systems. With these practices, cattle grazing can contribute to improve water quality and regulate the hydrological cycle at micro-watershed level with benefits for the environment and local population.

Key Words: Cattle, environmental impact, pastures, tropical streams, watersheds.

Introduction

Colombia, located in the north west of South America, is the only Andean and South American country with coasts in the Pacific and Atlantic Oceans. It extends over both sides of the Ecuator and the Andes and borders with Venezuela and Brazil in the east, Peru and Ecuador in the south and Panama in the northwest.

The area of the country is 1 141 748 Km² distributed in four well defined regions: The Pacific region extending between the Andes and the Pacific Ocean; the Andean region in the middle of the country; the Atlantic region in the north between the Andes and the Caribbean Sea; and the Eastern flat lands (that belong to the Orinoco and Amazon basins) at the east of the Andean region (IGAC 2002). The population of the country was estimated at 42 299 301 according to the 1993 census, 71% of which live in urban areas and the remaining 29% in the rural sector (DANE 1998).

Colombia is one of the most biodiverse countries in the world, second only to Brazil that is seven times larger. In less than 1% of the land, Colombia has more than 10% of world species. In relation with the number of species in the world, it is estimated that Colombia has 15% of the orchids, 20% of the birds, 7% of the mammals, 15% of the primates, 6% of the reptiles, 10% of the amphibians and 20% of the butterflies in a relatively small area (McNeely et al 1990; Instituto Alexander von Humboldt 2003).

Among the natural regions of the country, the Andean zone is the most populated and developed. Within this region that has a harsh topography, at least 12 million hectares are devoted to pastures with important negative effects on the soil and aquatic environment. However, this negative impact can be reduced with the implementation of several strategies based on silvopastoral systems.

Here we review some of the most important negative impacts of cattle grazing on streams of the Andean region and present some strategies based on silvopastoral systems that can be used to reduce those impacts.

Andean Region in Colombia

The Andes is the world's longest range of continuous high mountains along a line and the second highest after the Himalayas. They extend along the entire West Coast of South America, from Tierra del Fuego to Colombia and Venezuela for a distance of 7,200 km, forming a mountainous range of over 3,000m high with important upland basins on the top in some regions such as Bolivia and Peru.

Once the Andes enter Colombia, they split into three mountainous ranges or "cordilleras" oriented in a south-north direction, giving origin to two intermountain valleys. The three cordilleras with altitudes that can be as high as 5000 m. provide a geographical variability that combined with climatic factors and geological heterogeneity, create a high diversity of ecosystems and species (Murgueitio and Calle 1999).

The Andean region of Colombia covers 278 000 km², taking the 500m contour line as the lower limit. This region is the most populated and intensively exploited of the country: In 24.5% of the area it holds over 66% of the population, the most important agricultural and industrial centres and the three largest cities of the country (Etter and Wyngaarden 2000).

Due to its topography, the Andean region is very rich in lotic environments. In northern South America, it is estimated that at least 10⁷ km of low-order streams exist, as well as extensive floodplains (Lewis et al 1995). In Colombia alone, it is estimated that the amount of small watersheds is about 742 000. In addition to this, Colombia has 24 237 km of rivers and 2 680 000 ha of lakes, lagoons and swamps (Leyva 1998).

Cattle production in Tropical America

Although there is evidence that indigenous communities domesticated animal species such as llama Lama glama, alpaca Lama pacos and guinera pigs Cavia porcellus, the majority of domestic animals in the neotropics were brought by the Spaniards more than five centuries ago (Patiño 1970). Cattle farming started in Latin America taking advantage of savannah ecosystems present in the Caribbean region, the Orinoco and the Argentinean Pampas. It was then expanded gradually to other ecosystems with the deforestation of dry and humid forests.

Initially, cattle production was based on the animals brought from Spain that became adapted to South America. The introduction of more specialised European cattle breeds, such as Shorthorn, Aberdeen Angus, Holstein, Charolais and Brown Swiss, among others, only took place towards the end of the nineteenth century. In the XX century, the introduction of African species of grass, such as Hyparrhenia rufa, Melinis minutiflora, Brachiaria spp. and Panicum maximum was very important for the transformation of millions of hectares due to their rusticity and high production of seeds. The introduction of Zebu cattle at the end of the nineteenth century and beginning of the twentieth was also of great importance for the transformation of areas with more extreme climate conditions. The use of fire was a common tool, both for establishing pastures and for controlling weeds (Parsons 1972). In altitudes above 2000 m., where a milder climate allowed the production with specialised European breeds, the introduction of kikuyo grass Pennisetum clandestinum around 1920 was very important for starting dairy production to supply the growing urban demand (Parsons 1972).

The pasture-based cattle farming has modified dramatically the rural landscapes at a continental scale and has been recognised as a process with huge environmental and social impacts (Bennett and Hoffmann 1992). In Latin America and the Caribbean region, there are currently more than 602 million hectares occupied by permanent pastures, more than 33% of the region and 11% of agricultural lands in the world (FAO 2002). The growing degradation of soils and pastures is associated with low efficiency of production, biodiversity loss and emissions of global warming gases (Kaimowitz 1996).

In Colombia, grazing areas are dominated by cattle with around 26 million animals. The highest number is located in the Atlantic region followed by the Andean zone that holds 6 690 101 cattle, 30.9% of the total (Ministerio de Agricultura y Desarrollo Rural 2001).

Influence of land use on streams

As streams originate and transect through terrestrial environments, they are a manifestation of the landscapes they drain, and their basic characteristics are controlled at catchment level (Hynes 1975). The influence of landscape operates at different spatial scales. For example, hydrologic regime, sediment delivery, water chemistry and channel morphology have been demonstrated to be the product of regional climate, geology and vegetation that operate at a broad scale (Allan and Johnson 1997).

Vegetative cover and land use act at an intermediate scale influencing sediment and nutrient inputs to the streams (Richards and Host 1994; Richards et al 1996). At local scale, the streamside vegetation directly influences critical processes such as provision of organic matter and shade to the streams, and bank stability (Osborne and Kovacic 1993; Corkum 1999). The forests that originally dominated the landscape in the Andean region played an important role by protecting the soil and reducing the speed at which water runoff reached the stream channel, thus regulating the discharge.

Among the spatial scales of influence, the riparian zone has a disproportionate importance (relative to its land area) on running waters because of its immediate effects on the transport of water, nutrients and sediments. It acts as a natural filter reducing the organic, nutrient and sediment load reaching the stream (Winterbourn and Townsend 1991; Osborne and Kovacic 1993). In headwater streams, riparian vegetation is the principal source of energy for the water ecosystem providing a large amount of allochthonous detritus via leaf shedding (Vannote et al 1980). It restricts penetration of light to the streams and reduces the fluctuation in temperature (Zalewski and Frankiewicz 1998), and contributes to habitat diversity providing defining elements of stream habitat such as woody debris and shoreline protection (Richards et al 1996). Due to these factors, the manipulation of riparian buffer strips is a common tool in the restoration of stream and river ecosystems (Osborne and Kovacic 1993; Corkum 1999).

The influence of human alteration of landscape on stream ecosystems has been widely demonstrated (Rothrock et al 1998; McFarland and Hauck 1999). Anthropogenic landscape disturbances such as row crop agriculture, deforestation and grazing shift the structural and functional relationships among the landscape elements and the stability of the stream physical environment (Schlosser 1991). The main influences of landscape modification are the increase of sediment and nutrient delivery to the streams (Allan and Johnson 1997) and hydrological deregulation of watersheds (Etter and Wyngaarden 2000). At a local scale, the most important mechanisms include alterations of the terrestrial to aquatic energy transfers (Osborne and Kovacic 1993), reduction of the amount of woody debris entering the stream and direct modification of stream channel (Allan 1995).

For small streams, the reduction of the amount of woody debris entering the channel due to deforestation, agriculture and grazing reduces the depth, substrate, and current diversity associated with pool and lateral habitat development (Chará 2004). Additionally, the destruction of riparian vegetation reduces the terrestrial to aquatic energy transfers (allochthonous) while increasing the in-stream (autochthonous) energy production (Winterbourn and Townsend 1991). Consequently, land use activities result in significant alteration in the population and community dynamics of stream biota (Schlosser 1991; Rothrock et al 1998).

However, though agriculture in general affects negatively the streams, several practices may help to reduce such impacts and contribute to an improvement of water currents draining those landscapes.

Environmental impact of cattle grazing

The environmental impact of cattle grazing may be produced at different levels such as soil, biodiversity, atmosphere and water, among others. In the neotropics there are few studies about the impact of livestock on the environment (Murgueitio 2003). Some of the most important negative impacts of cattle grazing systems on water environments are described as follows:

Deforestation

Forest clearing and burning in order to establish pastures gives origin to the most important negative impacts produced by cattle grazing at global level. For this reason, pastures have been considered internationally as a threat to the tropical forest (Kaimowitz 1996). The destruction of the forest cover has obvious negative effects on water quality and quantity: The replacement of heterogeneous multi-strata forest by homogeneous pastures allows a higher impact of rain on the soil and reduces its water retention capacity. Furthermore, soils in pasturelands tend to compact reducing the infiltration and augmenting runoff speed which increases erosion and the possibility of spates after rain events (Chará 2004). Due to the low infiltration, soils cannot accumulate water and during dry seasons, the amount of water in streams is drastically reduced. These factors not only deteriorate the soil but affect the stability of water environments. Additionally, the destruction of forest cover in riparian zones eliminates the shade which increases water temperature and evaporation, stimulates algal growth and changes the trophic structure of the aquatic environment. It also reduces the supply of allochthonous energy and affects the integrity of the channel.

Erosion

Erosion is probably the most common type of soil degradation in the world. Its impact is high, particularly in Asia, Africa and South America where it averages 30 to 40 tonnes of soil ha^-1 year^-1 (FAO 1996). In Colombia, 90 392 661 ha are affected by erosion. The main effects of erosion on water bodies are the increase in turbidity, the sedimentation of pools and the embedding of coarse substrates which reduces habitat diversity. Sedimentation is also harmful since it may cover important food items for fish and macroinvertebrates.

Diffuse pollution

This type of pollution occurs when different substances harmful for the aquatic environment are deposited in the catchment area and are then carried with the water runoff to the streams. In pastures the most important sources of diffuse pollution are the pesticides, fertilisers and cattle faeces (Chará 2004). Substances such as herbicides and insecticides, among others, affect directly the aquatic biota by their toxicity and some fertilisers act either as toxic compounds or by increasing the growth of algae, thus changing the trophic structure of the environment. Cattle faeces are a source of nutrients, organic matter and pathogens. When organic matter reaches the water, the concentration of oxygen available for the biota is reduced.

Physical impact of cattle on aquatic environments

Since in most cases cattle have free access to water currents, it is common that the integrity of the channel and margins are physically damaged and disturbed. Cattle grazing is known to cause negative effects on aquatic habitats due to degradation of bank soils and vegetation which affects channel morphology, water chemistry and habitat diversity in streams (Sovell et al 2000; Weigel et al 2000).

Effect of agriculture on the Andean region of Colombia

After the introduction of cattle by the Spaniards in the 16^th century, the Andean region of Colombia has suffered a process in which large areas of sloping terrain have been transformed into extensive grazing lands (Etter and Wyngaarden 2000). These modifications have had negative effects such as landscape homogenisation, erosion and hydrological deregulation of watersheds. At present it is considered that 70% of the forest cover in the region has been transformed and that 80% of that cleared land is occupied by pastures (Etter and Wyngaarden 2000; Calle et al 2002).

One of the main problems with pastures is that they occupy steep areas not suitable for this activity which increases its impact. In the Valle del Cauca province, for example, the area devoted to pastures is 450 000 ha when only 43 680 ha are considered adequate for this activity (CVC 1998). Recently, due to the decrease in international coffee prices, an important proportion of the coffee-growing areas have been transformed into pastures at altitudes ranging between 1200 and 1800 m. (C. Mejía, personal communication November 2001). As a result of human activities, the region is suffering an important deterioration of its environment, with loss of biodiversity and degradation of soil and water resources (Sadeghian et al 1999; Murgueitio and Calle 1999).

At the level of intensity that cattle grazing is practised in the region, it produces an important negative environmental impact. Sadeghian et al (1999) studied the effects of coffee and cattle grazing on soil properties in Quindío province and found that the most intensive production methods affected the physical, chemical and biotic characteristics of soil, reducing porosity and microbial activity. In other latitudes, it has also been demonstrated that cattle grazing compacts soil, reduces infiltration and increases runoff (Weigel et al 2000).

Chará (2003), in a study comparing the characteristics of low-order streams draining pastures and forests in Quindío, Colombia, found that streams in pasture catchments had lower habitat quality and differed in water quality parameters and macroinvertebrate assemblages. Using the habitat quality score proposed by Barbour et al (1999) (see Table 1), it was found that characteristics such as pool diversity, channel sinuosity, bank protection and riparian vegetation were critically affected in streams under the influence of grazing. The alteration of channels either intentionally by human activity or caused by cattle, and its consequences for the diversity of habitats, were the most significant for producing lowest scores in grazing areas.

Regarding water quality parameters, the streams draining livestock areas had higher Biochemical Oxygen Demand (BOD), and total and faecal coliforms (Chará 2003). This was presumably caused by the manure deposited directly on the pastures that contributed organic matter and pathogens to the streams via runoff  (Table 2). The amount of nutrients, however, was not increased in grazing areas.

Aquatic macroinvertebrate assemblages were also negatively affected in streams draining pasture areas since the most sensible taxa were reduced in number in these catchments. The study also showed that streams in pasture catchments that had a minimum protection of the riparian corridor had better habitat and macroinvertebrate parameters than those with free access of cattle to the channel.

Silvopastoral systems as a tool for reducing negative impact on streams

Although cattle grazing is known by its negative impacts on the environment, it may act as an activity that helps in restoring degraded ecosystems and provide environmental services with different strategies (Calle et al 2002). One of the strategies that can be used to reduce the impact of this activity is the implementation of silvopastoral systems that provide environmental services at global and local level such as carbon sequestration, conservation of biodiversity and improvement of water quality and quantity (Murgueitio 1999; Ibrahim and Camargo 2001).

The type of silvopastoral system to be implemented depends on the topography, type of soil and the presence of strategic areas for water, soil or biodiversity conservation. In a process of zonification of the farm, there will be areas to release due to their fragility or importance for biodiversity or water conservation, areas where grazing should be avoided but that can be used to produce forages and areas of pastures with low and high density of trees. This approach implies an improvement and intensification of production in some areas of the farm (Murgueitio and Ibrahim 2001).

Some of the silvopastoral arrangements that can be used in farms and their benefits for the water are described as follows:

Protection of springs and water courses

Though the protected areas of the farm cannot be considered silvopastoral systems since cattle are not allowed in these areas, they are included here due to their importance for the protection of the aquatic resources and because they make part of an integral approach where the production of the released areas is replaced by an intensification in production in other areas of the farm.

It is relatively well known by farmers and technicians that the increase of tree cover around springs is very important for the protection of water sources. However, the protection of water courses such as streams and rivers is not normally considered as beneficial for the water. The establishment of riparian corridors contributes greatly to the conservation of the integrity of water habitats since they act as buffers between the catchment area and the stream, retaining excess of sediments and nutrients, reducing runoff speed, providing energy and enhancing habitat diversity (Osborne and Kovacic 1993).

The establishment of riparian corridors should be complemented with the enrichment of these areas with native tree species that protect stream banks, increase shade and provide elements to diversify the physical habitat. It is also necessary to provide drinking devices that eliminate the need of direct access of cattle to the streams.

A study carried out in La Vieja river catchment in Colombia by (Pedraza and Chará 2004), demonstrated that when headwater streams draining pastures were protected with riparian corridors containing trees and shrubs, they had better habitat quality (according to the diversity of inorganic substrates) and a significantly higher percentage (p=0.037) of the combined taxa Ephemeroptera-Plecoptera-Trichoptera, a group of aquatic insects sensible to perturbation (Figure 1 and Table 3).

Increment of tree cover in productive systems

From the point of view of water protection, the increase of trees in the productive systems seeks to restore partially the functions of the forests that previously covered pasture areas. Of particular importance is their function in reducing the impact of rainfall, and protecting and improving the structure of the soil which increases its water retention capacity. In steep areas, the trees associated with pastures have an additional benefit since they contribute to reduce the erosion with their root system. The variety of species is very important since the different lengths of the root systems help to retain the soil more effectively (Rivera 2002; Calle 2003). In addition to this, the use of legume trees reduces the need of nitrogen fertilisation avoiding the pollution caused by the application of nitrogen to the pastures.

The increment of trees in pastures and productive systems can be obtained in different arrangements such as protein and energy banks (cut and carry systems), live fences, wind barriers, land-slides control and low and high density silvopastures (Calle et al 2002; Rivera 2002). Protein and energy banks can be established in steep areas where direct access of cattle is not recommended because this increases erosion. The introduction of different forage trees in the banks creates agro-ecosystems that resemble the forest with regards to water protection. Live fences, wind barriers and trees in pastures help to protect the soil, reduce runoff speed and have also an indirect positive effect on water since they provide firewood, posts, forage and other products thus reducing the destruction of native forests.

With these practices, livestock grazing can contribute to improve the quality of water in streams and to regulate the hydrological cycle. This is beneficial for both the aquatic environment and the different users of water at local and regional level.

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Received 7 July 2004; Accepted 3 December 2004; Published 1 February 2005

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