Diet Switching by Mammalian Herbivores in Response to Exotic Grass Invasion

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.

Defoliation Dynamics on Grazing Horizons in Pastures Intercropped by Panicum maximum, Brachiaria brizantha, and Brachiaria decumbens

“…However, this factor did not prevent all grasses from being grazed during the period of occupation of the paddocks, because from the first to the fourth horizon, the defoliations in all cultivars were verified (Table 2), being evident that the population density of tillers does not compromise defoliation. On the other hand, the vertical distribution of grasses and structural components (Figure 2) can be a determinant factor in the defoliation pattern (Romera et al, 2012;Bremm et al, 2016). Therefore, the vertical distribution of the grasses allied to the grazing intensity influenced the defoliation pattern differently in the intercropping.…”

Disentangling by additive partitioning the effects of invasive species on the functional structure of communities

“…aerenchyma and starch in the parenchymatic pith; Favaretto et al, 2015). Moreover, invasion generates a positive feedback because E. plana is avoided by livestock due to its high fibre content while native species are overgrazed, which accelerates the expansion process (Bremm et al, 2012(Bremm et al, , 2016Scheffer-Basso et al, 2016). Consequently, E. plana invasion has been associated with the reduction of native species richness and cover Dresseno et al, 2018), and the loss of forage palatability for livestock grazing (Reis & Coelho, 2000;Medeiros & Focht, 2007;Medeiros et al, 2009).…”

The importance of nutrient hotspots for grazing ungulates in a Miombo ecosystem, Tanzania

“…Various mammalian grazers such as the wildebeest (Connochaetes taurinus) are attracted by short green grass of high nutrient content [18][19][20]. While the importance of grazing lawns for herbivores has been demonstrated [15,21,22] few experiments exist that quantified the extent to which these lawns attract grazers in nutrient poor areas [8].…”