Influence of grazing intensity on patterns and structuring processes in plant-pollinator networks in a subtropical grassland

Understanding how disturbances influence interactions networks is a
central but still poorly explored issue in ecology and management. To test how the structure of plant-pollinator
networks and the structuring importance of morphological traits, phenological
overlap and relative abundance are influenced by grazing in a subtropical
grassland community.Twelve sampling plots were allocated in order to cover a grazing
gradient ranging from overgrazed to ungrazed sites. For each sampling plot, we
created a quantitative matrix containing all observed pair wise insect–plant
interactions and described morphology, phenology and abundances of each species.
We then fitted series of models to test the influence of grazing intensity on
metrics describing networks structure; and used probabilistic matrices, maximum
likelihood analyses and models selection to investigate the determinants of frequencies
of interactions. Grazing intensity influenced connectance, specialization and
interaction evenness, while number of species and links, nestedness and modularity
were more stable. Species abundance was the most important determinant of
frequencies of interaction regardless the grazing intensity. In contrast to northern hemisphere pollination networks,
the studied subtropical plant-pollinator networks and their structuring
processes were remarkably consistent along the gradient of grazing intensity.
We argue that the dominance of Asteraceae species – that are selectively
avoided by cattle – play a crucial role in attracting a wide range of
pollinators and thereby structuring plant-pollinator interactions, conferring
resistance to these networks.