Ronald H. Fortney
 
 

Cattle Grazing and Sustainable Plant Diversity in the Pantanal: What Do We Know? What Do We Need to Know?
 
 

(Excerpts from the full paper presented in the "uncorrected, advance proof" of The Pantanal of Brazil, Bolivia and Paraguay, Hudson MacArthur Publishers, copyright 2000 by Waterland Research Institute.)

Many ranchers, conservationists and politicians claim cattle ranching has saved the Pantanal from uncontrolled growth. Conversely, others view ranching as causing significant degradation. Generally, both sides acknowledge that each point of view has merit; in other words, ranching has contributed to keeping the Pantanal together as a functional ecosystem, and ranching activities have had some negative impacts on the environment. Considering the future of the Pantanal ecosystem raises several questions, including how do we determine the short- and long-term ecological impacts of sustained cattle grazing and associated activities on the Pantanal ecosystem? 

In this paper I will focus on the need to develop a model or paradigm for predicting ecological impacts associated with cattle grazing in the Pantanal. One way of approaching this problem is to ask two related questions: What do we know and what do we need to know about the impacts of cattle grazing on plant diversity in the Pantanal? I want to mention first that it is not the intent of this paper to deride ranching or to hold ranching up as a savior. Rather, it is only to examine the current knowledge of the Pantanal ecosystem, evaluate existing paradigms on the environmental impacts of grazing and suggest recommendations for future studies. 

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The effects of grazing by large ungulates are often cited in ecological literature. The following list summarizes the frequently referenced impacts (Milichunas et al. 1988, Cox 1993, Stohlgren et al. 1998, Smith 1996):

· Compaction of soil 
· Trampling of vegetation 
· Over-enrichment through excrements 
· Elimination of native species (plants and animals) 
· Restricted penetration of floodwaters into soils 
· Invasion of noxious weeds 


These impacts have been linked directly to degradation of habitat structure (Smith 1996). The degradation of habitats, according to Perevolotsky and Seligman (1998), can be expressed in terms of reduced species diversity, primary and secondary productivity, and general utility of a habitat to society. 

Are all of these impacts associated with grazing in the Pantanal? We might assume so, but without accurate records or descriptions of the physical environment and biota in the Pantanal before European colonization, we really do not know. In other words, without this benchmark or reference, it is difficult to judge current conditions relative to the pre-colonization period. Illustrating this lack of preciseness is Figure 1, which is a generic representation of the general state of the Pantanal ecosystem depicting several theoretical trends for ecosystem health since the late 1700s. 

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To have the tools needed to gauge the impacts of sustained grazing, we need to understand the basic physical and biological environmental conditions in the Pantanal. The Pantanal lies at the head the Paraguay River Basin, nearly at the geographic center of South America. The environmental conditions are straightforward in some respects, and in others very complex. The Pantanal has a characteristic wet-dry annual climate in which flood conditions occur during and after an extended summer rainy period (Hamilton et al. 1996). Typically, following the wet summer, near desert-like conditions extend through the winter period. The effect is a sustained pulsing hydroperiod that results in annually fluctuating floodwaters that predictably range from one-to-four meters deep (Junk and da Silva 1995). Topography strongly influences the flooding regimes, since elevations gradually rise from west to east causing an inundation pattern where the northern and eastern sections have shorter and more shallow inundation patterns. The Pantanal is further characterized by such geomorphological landscape features as cordilheriras (irregular) and capaos (circular) hillocks and by fresh and saline lakes. Reflecting this complexity in land forms, hydrology and edaphic conditions are unique floristic compositions and vegetation distribution patterns. 

I believe we can agree that neo-European settlers have greatly modified the vegetation patterns in many portions of the Pantanal. Numerous changes have resulted from the large-scale removal of woody vegetation to create open fields (Wilcox 1992). This was accomplished by cutting and slashing and by the use of fire. Although the vegetation has been modified, it is still a complex mosaic of plant communities, with grassland communities being the most common. The vegetation formations include the cerrado-type scrub-shrub thickets and forests, palm savannas, gallery forests, and wet meadows and marshes. All of these vegetation types have been subject to some level of grazing pressure.

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Ranching activities within the Pantanal watershed has a nearly 250-year history (Wilcox, 1992). This time period is typical for many areas in the Western world, but is notably short by Old World yardsticks. In the Mediterranean Basin, for example, intensive grazing by several species of domesticated animals has occurred for more than 8,000 years. 

According to Wilcox (1992), cattle were introduced into the Pantanal about the middle of the 18th century in the north near Cuiabá, with ranching activities spreading to other areas of this low basin. However, the Pantanal has not been uniformly grazed. Because of the climate and differences in hydrologic cycles, the drier eastern and northern sections have generally received the most ranching activities. 

In the 1890s, the largest ranches ranged between 100,000 and 400,000 hectares. By the 1970s, 70 percent of all properties ranged between 1,000 and 10,000 hectares. Conversely, since the mid 1800s the number of cattle grazed in the Pantanal has increased. In 1920, a Brazilian census estimated there were about 700,000 cattle in the Pantanal. By the early 1970s, the number had increased to over 5 million. Since that time, because of market conditions and periods of extreme flooding, the number has decreased somewhat (Cox 1992). Therefore, considering cattle ranching throughout its history in the Pantanal, there have been two general trends: more and smaller ranches and increasing numbers of cattle. 

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A related issue is carrying capacity. What is the carrying capacity for the Pantanal? Just as the environmental conditions vary throughout the Pantanal, so does the capacity to sustain grazing activity. The number of cattle per hectare increased during the 1900s, reaching about 3 per hectare (Wilcox 1992). While this number may seem small considering the size of the Pantanal, Wilcox questions the capacity of the Pantanal to sustain such pressure; he particularly questions whether native grasses could withstand the grazing pressure. At this point, I believe the carrying capacity is an unknown and unquantified measurement for much of the Pantanal. 

Ranching and the use of fire as a management tool go hand in hand. This is especially true for the Pantanal with its distinct wet and dry season (Prance and Schaller 1982, Wilcox 1992). The use of fire has a long history throughout the Pantanal. Although fire has some desirable effects, it can cause significant negative impacts if used persistently in an area. Among the most important for the Pantanal are the impoverishment and hardening of the soil, gradual displacement of indigenous plant species by fire-hardy grasses and woody species, and long-term loss of soil nutrients (Hobbs et al. 1991, Cox 1993, Wilcox 1992, Smith 1996). The long-term impacts of the widespread and repetitive use of fire even in areas like the Pantanal, where we suspect fire occurred naturally on some regularity, is not known. The impacts of fire, therefore, are an important issue to explore.

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The studies referenced here suggest that the paradigms for predicting ecological impacts of grazing in the temperate regions are being challenged. Further, any paradigm developed for temperate areas have limited applicability for the Pantanal because of climatic differences on a broad regional scale and the uniqueness of the hydroperiods, soils and vegetation assemblages on a narrow ecological scale. Therefore, it would be a misapplication to use models developed for temperate systems in the Pantanal. In view of these differences and the lack of tested quantitative data on the ecological impacts of cattle grazing in the Pantanal, I suggest that there is no paradigm established for predicting or modeling the impacts of grazing in the Pantanal. Further, one or more paradigms are needed if we are to have a predicable yardstick for measuring the ecological impacts of sustained grazing on biodiversity and other ecosystem attributes. Any model developed must assume that grazing will continue as a major activity in the Pantanal. Therefore, the model must have a management component, as well as natural functional criteria.

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