Abstract
Invasion by exotic grasses is a severe threat to the integrity of grassland ecosystems all over the world. Because grasslands are typically grazed by livestock and wildlife, the invasion is a community process modulated by herbivory. We hypothesized that the invasion of native South American grasslands by Eragrostis plana Nees, an exotic tussock-forming grass from Africa, could be deterred by grazing if grazers switched dietary preferences and included the invasive grass as a large proportion of their diets. Bos taurus (heifers) and Ovis aries (ewes) grazed plots with varying degrees of invasion by E. plana in a replicated manipulative experiment. Animal positions and species grazed were observed every minute in 45-min grazing session. Proportion of bites and steps in and out of E. plana tussocks were measured and used to calculate several indices of selectivity. Both heifers and ewes exhibited increasing probability of grazing E. plana as the proportion of area covered by tussocks increased, but they behaved differently. In agreement with expectations based on the allometry of dietary preferences and morphology, ewes consumed a low proportion of E. plana, except in areas that had more than 90% E. plana cover. Heifers consumed proportionally more E. plana than ewes. Contrary to our hypothesis, herbivores did not exhibit dietary switching towards the invasive grass. Moreover, they exhibited avoidance of the invasive grass and preference for short-statured native species, both of which should tend to enhance invasion. Unless invasive plants are highly palatable to livestock, the effect of grazing to deter the invasion is limited, due to the inherent avoidance of the invasive grass by the main grazers in the ecosystem, particularly sheep.
Highlights
Threats to the sustainability of grassland production of multiple ecosystem services require that we investigate the factors that drive community dynamics beyond short-term productivity
The ability of invasive plants to become dominant depends on their interactions with native vegetation composition, grazing intensity, herbivore body size, plant productivity and climatic variability [7, 8, 9]
Herbivore selectivity of grazing patches, associated with other disturbances, have resulted in dominance of more tolerant plant species in grassland ecosystems [10, 11]. [12] found predominance of different functional groups depending on grazing intensity, with higher contribution of “conservative” and “capture” strategies in low and high grazing intensities, respectively
Summary
Threats to the sustainability of grassland production of multiple ecosystem services require that we investigate the factors that drive community dynamics beyond short-term productivity. It is necessary to design management strategies with productivity sustained by the control or reversal of invasion by undesirable exotic plants. This requires basic knowledge of factors that determine the ability of invasive species to displace other species and species-specific. The ability of invasive plants to become dominant depends on their interactions with native vegetation composition, grazing intensity, herbivore body size, plant productivity and climatic variability [7, 8, 9]. Herbivore selectivity of grazing patches, associated with other disturbances (e.g. resource availability, environmental factors), have resulted in dominance of more tolerant plant species in grassland ecosystems [10, 11]. Herbivore selectivity of grazing patches, associated with other disturbances (e.g. resource availability, environmental factors), have resulted in dominance of more tolerant plant species in grassland ecosystems [10, 11]. [12] found predominance of different functional groups depending on grazing intensity, with higher contribution of “conservative” (tall grass) and “capture” (short grass) strategies in low and high grazing intensities, respectively
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