Rangelands, covering about 40% of the global land area, are crucial for biodiversity. They are impacted by both biotic and abiotic factors, requiring comprehensive understanding for maintaining biodiversity. 

Within this ecosystem, livestock and wild herbivores compete for resources. Livestock often have an advantage due to veterinary care and supplemental feed from locals who rely on them. While competition and pathogen transmission have been extensively studied for 50 years, their impact on wild herbivores population dynamics remains understudied. Long-term monitoring of wild herbivore populations can provide valuable insights into how competition and livestock density affect their population. 

The population dynamics of wild herbivores often exhibit fascinating cycles of rising and falling numbers. These fluctuations are driven by a complex interplay of internal (intrinsic) and external (extrinsic) factors. 

While resource depletion and predation are well-known drivers, the influence of competition from coexisting species is less understood. 

A recent collaborative study by a research team led by Manvi Sharma, Assistant Professor, Ashoka University, and researchers from Nature Conservation Foundation (NCF), Bangalore, investigated the long-term effects of livestock competition on bharal (Pseudois nayaur) populations, a primary prey of the snow leopard, a crucial conservation species. Conducted in the Spiti Valley of the Trans-Himalayas, the research benefited from a long-term bharal monitoring program established in 2010 by the NCF. Researchers focused on two geographically close sites, Kibber and Tabo, with similar environmental characteristics.

The study employed double-observer surveys to estimate bharal abundance in Kibber and Tabo, and also assessed livestock populations in 11 villages across both regions. Sharma says, ​“We asked three questions — 1. How does livestock density impact density-dependent population dynamics of bharal? 2. What is the strength of density dependence in two bharal populations? 3. Does livestock density impact adult survival, offspring recruitment or both parameters in bharal populations?”

In density-dependent dynamics, a population’s growth rate changes with its density. Higher density can lead to resource limitations, slowing growth through reduced birth rates or increased death rates, while lower density can result in higher growth rates due to abundant resources.

Over a decade, bharal populations in both Tabo and Kibber changed significantly. Tabo, with lower livestock density, exhibited negative density dependence: as the bharal population increased, their growth rate decreased, likely driven by fewer offspring being born rather than adult survival rates. This feedback mechanism, which regulates population cycles, was absent in Kibber with higher livestock density, where bharal density fluctuated widely. This suggests that excessive grazing by livestock disrupts bharal population cycles through resource competition. By depleting food sources and degrading habitat, livestock create an environment less suitable for bharal, potentially leading to the lack of a distinct population pattern observed in Kibber. Notably, Tabo’s population exhibited a five-year cycle, while Kibber’s lacked a distinct pattern.

“This study raises fascinating questions,” says Mayank Kohli, INSPIRE Campus Fellow, National Centre for Biological Sciences, Bengaluru, and an expert from the field. 

For instance, is density dependence at the site with fewer livestock driven by changes in bharal fertility rates or offspring survival? 

He adds, ​“Presumably, competition for forage leads to the observed density dependence; however, the site with more livestock has a higher rather than lower density of bharal. This hints at factors like resource quality, fine-scale resource partitioning, and predation that may influence bharal dynamics, and require further investigation.”

This decade-long study reveals that high livestock densities disrupt natural bharal population cycles, highlighting the need for conservation measures to support wild herbivore recovery and maintain ecological balance. It prompts further research into livestock density, resource availability, and bharal population regulation, crucial for informed conservation strategies.