Small mammals called plateau pikas are often treated as pests for competing with livestock for food, but research from China shows they can improve the quality of vegetation for grazing yaks.
The study, published previously as a Reviewed Preprint in eLife and appearing today as the final Version of Record, is described by eLife's editors as important. They say the findings, based on convincing evidence, provide new insights on reciprocal relationships between small mammals and large herbivores, and will be of interest to livestock and rangeland managers.
Pikas, voles, prairie dogs, pocket gophers, and other small herbivores are both consumers and ecosystem engineers in many grassland areas. Through their feeding, clipping, and burrowing behaviours, they profoundly alter vegetation and soil properties, with potential effects on large herbivores that co-exist with them.
"In rangelands and other less productive ecosystems, small mammals are often considered pests because their population outbreaks can lead to competition with livestock for food. However, it remains unclear whether their competitive effects on livestock diminish or even help livestock growth when their numbers are at lower, non-outbreak levels," says co-first author Zhiwei Zhong, Professor of Ecology at the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, China. "Livestock grazing currently uses around 77% of global agricultural land and sustains billions of people worldwide. A critical assessment of small mammals' impacts on livestock production is therefore urgently needed to guide management for both production and biodiversity conservation."
For their study, the team assessed how a moderate population density of plateau pikas – as defined by approximately 200 active burrows per hectare – affects yak performance in the alpine ecosystem of the Qinghai–Tibetan Plateau. They hypothesised that, at moderate densities, pikas would improve vegetation for yaks by suppressing tall poisonous plants (forbs) called Stellera, thereby increasing the abundance and nutritional quality of palatable grasses and sedges during the growing season in June–August.
To test these predictions, they first conducted two field surveys to examine how diets differ between pikas and yaks, as well as the associations among pika density, Stellera abundance, and yak grazing activity. They then performed a manipulative field experiment using 150x150m fenced enclosures on the Qinghai–Tibetan Plateau to test the interactive effects of pikas and Stellera on yak body growth and the underlying mechanisms driving these effects.
The field surveys showed that pikas and yaks have distinct diets: pikas very frequently clipped – but did not eat – Stellera plants and fed mostly on other forbs, while yaks consumed greater proportions of grasses and sedges as expected.
"By selectively clipping tall poisonous Stellera forbs, pikas reduced the cover of these plants by two-thirds, which in turn increased the availability and protein content of grasses and sedges. By improving the composition of vegetation in this way, the moderate presence of pikas facilitated weight gains in yaks," explains co-first author Bingbo Ni, an environmental researcher at the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences.
The team also found that pikas helped improve foraging efficiency in yaks, as measured by the yaks' bite rate (the number of bites taken on plants per hour) and bites–step ratio (the number of bites taken on plants per step). During the manipulative field experiment, when pikas were absent, the team saw that the yaks' sedge bite rate and sedge bites per step decreased by 32% and 46%, respectively, compared to when pikas were present. Grass bite rate and grass bites per step similarly decreased by 29% and 43%, respectively, compared to when pikas were present. These decreases in foraging efficiency in the absence of pikas can be attributed to the increase in the cover of the poisonous Stellera.
"Our study is the first to show that maintaining a moderate density of pikas can substantially enhance livestock growth by suppressing Stellera growth, reconciling debates on the effects of pikas on yak production through changing vegetation properties. These observations shed light on how small and large mamamlian herbivores co-exist in a complex way," Zhong adds.
The authors say their work raises questions that warrant further investigation. First, they examined pika–yak interactions during the summer period only, when food resources are most abundant. Whether or not these effects weaken or even become more competitive under more stressful conditions – for example, when foraging becomes limited during autumn or winter – remains to be tested. Additionally, their study site was located at approximately 3,200m elevation, which is relatively low by Qinghai-Tibetan Plateau standards. Further studies would be useful to understand whether similar facilitation occurs at different elevations where other unpalatable species replace Stellera as the problematic plant. Finally, it remains unclear whether similar relationships occur between pikas and other livestock species in the area, such as domestic sheep and goats.
"Our work lays the groundwork for understanding more about the complex relationships that exist between small mammals and large herbivores," concludes senior author Zhibin Zhang, Professor at the School of Ecology, Hainan University, Haikou, China. "As our conclusions are supported by experimental and observational evidence across realistic contexts, they are applicable beyond the Qinghai-Tibetan Plateau ecosystem and to rangeland management elsewhere. We hope the insights will help inform conversations and future studies concerning biodiversity and ecosystem functioning worldwide."
Accompanying multimedia for this study are available to view and download here .
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