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Switching wildlife for livestock can foster harmful pests

Greater monitoring of the risk of diseases spread by insects is needed in environments where animals and people coexist, say researchers after studying the effect of livestock on spiders in the Himalayas.

Researchers found that replacing wild animals with domestic livestock can affect spiders and other small predators that balance the ecology by feeding on blood-sucking ticks and mites and leaf-eating grasshoppers.

Their , published in Ecological Applications, was conducted in Kibber village, in the Spiti region of northern India and spanned a 14-year period. It followed the fate of ants, wasps, bees, ticks, mites, spiders, grasshoppers, beetles and other arthropods in fenced and control plots.

Arthropods are a group of creatures with jointed legs and a hard outer shell called an exoskeleton.

Ticks and mites were more abundant in plots where livestock were grazing compared with areas left to wildlife, according to the analysis.

Ticks spread disease among both wild and domestic animals, affecting more than 80% of the world's cattle, says the study. They also present a threat to humans.

Wildlife in the study area included mixed herds of bharal or blue sheep, wild yak, ibex, and wild ass while the livestock included cattle, horses, donkeys, goats and sheep.

Areas with livestock showed an abundance of leaf-eating grasshoppers and a depletion of the spiders that prey on them.

This means that livestock expansion at the cost of native wildlife has implications for the ecosystem, due to the different impacts on arthropods, says Sumanta Bagchi, corresponding author of the study and associate professor at the Center for Ecological Sciences, at the Indian Institute of Science Bangalore.

"These differences offer challenges and opportunities for improved land management and for animal husbandry," says Bagchi.

An abundance of grasshoppers can result in the degradation of vegetation used to graze animals, while proliferation of ticks and mites raises the risk of vector-borne diseases, he explains.

One Health approach

These impacts call for planned interventions, says Bagchi, aligned to the UN's One Health approach, which aims to balance the heath of people, animals and ecosystems and control zoonoses—diseases that spread between animals and humans.

He says the findings highlight the need for monitoring and surveillance of vector-borne disease risks in ecosystems where there is a close interface between wild animals, domestic livestock and humans.

Spiders and other arthropods perform ecological functions such as nutrient cycling, pollination and seed dispersal, as well as preying on pests like grasshoppers and ticks. Grazing animals can affect arthropods directly by reducing their food resources, as well as indirectly by altering vegetation.

"As predators, spiders influence material and energy flow in the same way as the wolves in a forest and sharks in the ocean," the study says.

"Thus, human land use can have not only well-known cascading effects via large-bodied predators, but also through smaller predators such as spiders."

Krishna Gopal Saxena, a fellow at India's National Academy of Agricultural Sciences and specialist in natural resource management, tells SciDev.Net: "No study, including the present one, has been able to conclusively say why spider populations decline under livestock or which particular types are affected—and there are many. There are too many variables, such as the seasons, species and actual locations."

Bagchi admits that data from the study offers no explanation for the phenomenon of dwindling spiders and insect predators or the proliferation of grasshoppers in areas where livestock graze. He suggests that changes in vegetation could affect the ability of spiders to catch their prey.

Saxena says it is common to see wildlife such as blue sheep grazing alongside livestock in the Spiti area. "Disease cross-transmission between livestock and wild grazing mammals is not unknown and there are concerns not only over the spread of ticks but also gastro-intestinal worms," he adds.

"Clearly, further investigations are needed to discover the exact mechanisms that link livestock, wildlife, vegetation, pests and predatory arthropods such as spiders in the interests of public health and ecology conservation."