Grazing dramatically alters the vegetation structure and, among the different vegetation types, grasslands are the most affected by this activity (Mc Naughton, 1985). Grazing produces changes in the architecture of the canopy, the composition of species, spatial heterogeneity and diversity (Sala et al., 1986; Chaneton and Facelli, 1991). In addition, herbivory may alter the functional properties of grasslands in terms of the energy flow and nutrient circulation in the ecosystem (McNaughton, 1985; Detling, 1987; McNaughton et al., 1988). The disturbance of grassland species by herbivory is caused directly by the removal of leaves and other photosynthetic organs; indirectly, grazing provokes environmental alterations (e.g. alterations in the water balance and soil thermal regime, in the ligth environment, and in nutrient availability) (Deregibus et al., 1985; Detling, 1987; Lavado et al., 1996). Grazing also alters the competition among neighboring plants (Harper, 1977; Crawley, 1986). Selective herbivory determines the defoliation of some individuals and not others; the selective impact of grazing on dominant species may provoke the release of resources that can be exploited by the neighboring species which are less preferred by herbivores. Thus, grazing may help less competitive species and contribute to the preservation of diversity among the vegetal community (Harper, 1977). Nevertheless, when preferential herbivory affects the less competitive species, grazing may cause a decrease in the species diversity (Crawley, 1986).
In the Flooding Pampa grasslands, the relatively recent introduction of large domestic herbivores has provoked an increase in the abundance and diversity of the native dicots species, as well as the invasion of exotic dicots (Sala et al., 1986; Rusch and Oesterheld, 1997).
Grazing represents a major constraint in grasslands, primarily through its effect on biomass loss rates (McNaughton 1985) and plant regeneration patterns (Bullock et al. 1995). In the Flooding Pampa, continuous grazing by cattle maintains the grassland standing biomass at low levels and may also limit above-ground primary productivity (Rusch and Oesterheld 1997). These effects are related to changes in life form and species composition promoted by selective herbivory and gap disturbance (Sala et al.1986, Rusch and Oesterheld 1997).Typically, in the argentine pampas grazed grasslands become dominated by low-growing perennial forbs at the expense of more palatable and productive caespitose grasses and sedges (Sala et al., 1986; Facelli et al. 1989).
The species that form the Flooding Pampa grasslands may be divided in two functional groups (Wilson and Roxburgh, 1994; Insausti et al., 1999): dicots and gramineous. The later species, along with other families (Cyperaceous and Juncaceous, etc.) of similar structure, are called graminoids (Insausti et al., 1999). There are differential responses to the disturbances that may be attributed to those two functional groups and there are clear evidences that both grazing and floods affect the grassland dicots and graminoids in different ways (Sala et al., 1986; Insausti and Soriano, 1987; Chaneton et al., 1988; Rusch and Oesterheld, 1997; Insausti et al., 1999). Nevertheless, the fact that certain species have similar responses to a particular disturbance does not mean that other important functional features are homogeneous (Lavorel et al., 1997). For example, grazing-tolerant species may differ in flooding response patterns and, in turn, flooding-tolerant species may differ according to their seasonal pattern and phenology (Elmquist et al., 2003). The capacity of a plant species to recover from a disturbance depends on this "response diversity" within the functional groups (Walker et al., 1999; Luck et al., 2003).
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