Cascading effects of livestock grazing on insect functional groups associated to flowers in arid lands

Abstract

1. Disturbances affect plant diversity with consequences on other trophic levels, like the associated insect communities. Domestic livestock is a major disturbance affecting flowering plant abundance at the bottom of insect food webs with unpredicted consequences on ecosystem functioning.2. This work evaluated livestock effect on flower-associated insect functional groups (i.e., predators, herbivores and pollinators) hypothesizing that plant damage produced by grazing exerts bottom up effects on insect populations, affecting their interactions. On contrary, livestock might supply food resources for several dipteran pests (e.g., scavengers, parasites or bloodsucking) or benefit predator species by increasing habitat transparency with grazing.3. Seven independent rangelands within the same arid environment and increasing livestock density, were used to test these hypotheses using a regression approach. 4. Insect, herbivore and pollinator diversity decreased with increasing livestock density, and increased with increasing vegetation. Bare soil affected solitary bees (but not all the pollinators) and habitat transparency benefited predators, as expected.Dipteran pests quadratically decreased with increasing livestock density, rejecting the hypothesis of food supply. Apparently, predators reduced the abundance of almost all insect groups. 5. Grazing reduce vegetation positively affecting predators, which negatively affect other insect groups (top-down), potentially altering species complex interactions and ecosystem functioning.

Hippopotamus and livestock grazing near water points: consequences for vegetation cover, plant species richness and composition in African savannas

“…The greater shrub abundance near rivers could also partly be a consequence of higher grazing there. These results partly contrast with findings for xeric environments demonstrating that high livestock stocking rates, densities or intense livestock grazing simplifies and removes vegetation patches and reduces plant cover, abundance, vegetation density and species richness (Tadey, 2006;Pelliza et al, 2021;Tadey, 2023). Our results also partly reinforce those of other studies showing that hippo grazing significantly reduces grassland canopy height but increases the leafiness of common grasses, reduces woody plant abundance and size, and increases the concentrations of several soil elements (McCauley et al, 2018).…”

“…As a result, intermediate grazing intensities that create moderate disturbances can support high levels of diversity in grassland communities (Grime, 1973;Milchunas et al, 1988;Hobbs and Huenneke, 1992). Very high grazing intensity can also reduce diversity of other species groups, such as invertebrates (van Klink et al, 2015;Tadey, 2023) with significant consequences for essential ecosystem functions such as pollination (Tadey, 2023).…”