Replacing native grazers with livestock influences arthropods to have implications for ecosystem functions and disease

Abstract

Grazing by large mammalian herbivores influences ecosystem structure and functions through its impacts on vegetation and soil, as well as by the influence on other animals such as arthropods. As livestock progressively replace native grazers around the world, it is pertinent to ask whether they have comparable influence over arthropods, or not. We use a replicated landscape-level, long-term grazer-exclusion experiment (14 years) to address how ground-dwelling arthropods respond to such a change in grazing regime where livestock replace native grazers in the cold deserts of the Trans-Himalayan ecosystem of northern India. We analyze spatial and temporal variation in the abundance of 25,604 arthropods sampled using pitfall traps across 2765 trap-days through the duration of the growing season spanning spring, summer, and autumn. These were from 88 operational taxonomic units covering six orders from 33 families (ants, wasps, bees, ticks and mites, spiders, grasshoppers, and beetles). We find that grazer assemblage—whether livestock or native herbivores—had a strong influence on both vegetation and arthropods. Partial redundancy analysis (RDA) showed that 53.6% of the spatial and temporal variation in arthropod communities could be explained by grazing and by grazer assemblage identity, alongside covariation with vegetation composition and soil variables. Structural equation models revealed that grazing and grazer assemblage identity have direct effects on arthropods, as well as indirect effects that are mediated through vegetation. Importantly, spiders (predators) were less abundant under livestock, whereas grasshoppers (leaf eaters) and ticks and mites (parasitic disease vectors) were more abundant, compared with native grazers. Reduction in spiders can fundamentally alter material and energy flow through the cascading effects of losing predators, and an abundance of grasshoppers may even contribute to vegetation degradation that is often associated with livestock. Parallelly, increases in ticks and mites lead to concerns over vector-borne disease that require planned interventions to align animal husbandry with One Health. Thus, losing native grazers to livestock expansion can have wide-ranging repercussions via arthropods. This may not only affect ecosystem structure and functions, but also offer challenges and opportunities to mitigate risks from vector-borne disease.