Supported by:

The role of small mammals in grassland degradation in China: Defining the problem
by Dr Roger Pech¹, Dr Tony Arthur², Dr Lyn Hinds³ and Professor Shi Dazhao⁴

Small mammals play a central role in the ecological processes that sustain grassland ecosystems.  Their burrows, up to thousands per ha, structure the soil profile and, with foraging activities, influence nutrient cycling and the composition of the plant community.  They also provide refugia and nest sites for organisms including invertebrates, amphibians and birds.  Small mammals are primary prey for many terrestrial and avian predators and hence a key component of biodiversity.  However, in many parts of the world rodents and other small mammals are also agricultural pests (Stenseth et al. 2003).  In China they are thought to contribute to the degradation of grasslands (Fan et al., 1999; Zhang et al, 1999; Zhong et al. 1999) where they compete with livestock for scarce forage resources, contribute to the loss of palatable plant species and promote soil erosion through burrowing activity.  Also, some species of small mammals are reservoirs for zoonoses such as bubonic plague.

There are many signs that degradation of the grasslands of northern and western China is a serious problem.  These include widespread soil erosion (evidenced by an increasing frequency of dust storms blowing over eastern China in spring), dominance of plant communities by unpalatable species, increasing frequency of outbreaks of small mammals, insufficient forage for livestock and reliance on supplementary feeding during winter, declining productivity per household in nomadic communities, and implementation of national programs to control species of small mammals in grasslands.  The key question is: are small mammals a cause, or just a sign, of degradation?

Brandt’s vole (Microtus brandti) is endemic to the grasslands of central Inner Mongolia, eastern Mongolia and adjacent parts of Russia.  Zhang et al. (2003) compiled a history of outbreaks of Brandt’s voles in Inner Mongolia over the last 50 years and found that outbreaks have increased in frequency since 1970 from 1 every 7 years to 1 every 3 years. A statistically significant link between outbreaks and years with high monthly averages of the Southern Oscillation Index suggests that climatic factors might be the underlying cause.  But the population dynamics of Brandt’s voles indicate that changes in the management of grasslands are the primary cause of the higher frequency of outbreaks.  The substantial increase in livestock numbers over the last 50 years appears to have increased the number of years when the balance between grazing and plant growth results in rapidly increasing populations of Brandt’s vole.

The grasslands of the Qinghai-Tibet Plateau support about 10 million semi-nomadic herders whose per capita income is one of the lowest in the Asia-Pacific region. As well as supporting livestock production, the grasslands are the headwaters of Asia’s major river systems and are a refuge for wildlife.  Over 90% of these grasslands are degraded to some extent, leading to loss of local productivity and significant down-stream effects. The primary cause of this degradation is probably overgrazing by livestock but grazing and burrowing by small mammals are considered to be significant contributing factors.  The major pest species is the plateau pika (Ochotona curzoniae), which occupies 80% of the total area of infestation by small mammals. However, Tibetan herders have mixed attitudes on whether or not pikas are a recent problem, whether damage can be attributed solely to pikas and whether or not control programs are effective or necessary (Beimatsho, unpublished data).

Evidence from Inner Mongolia and Tibet shows that although current control techniques can result in short-term reductions in the abundance of pest mammals, in general the high rate of recovery of these species usually results in a return to pre-control levels in one to two years (Shi et al., 2002; Pech et al., unpublished data).  There are exceptions; for example, in parts of Qinghai broad scale control appears to have been sufficiently effective to produce sustained low densities of pikas (Lai and Smith 2002).  However, control has also been associated with negative effects on biodiversity.  Lai and Smith (2002) observed the disappearance of several dependent bird species, and direct impacts of control programs on non-target species, including livestock and people, have been reported in Mongolia (Zahler et al. 2004).  Based on current knowledge, there is limited scope for improving cost efficiencies by applying control programs only in years when outbreaks are likely to occur (Davis et al. 2003), none of the current techniques are species-specific and none include concepts of ecologically-based management (Pech et al., 2003; Hinds et al., 2004).

At present there are conflicting views and a paucity of knowledge on the relative importance of processes leading to degradation of the grassland ecosystems of China.  In the long term, a failure to understand the real causes of the problem could result in inappropriate policies, ineffective solutions, and perverse consequences of removing keystone species (Smith and Foggin, 1999; Sinclair and Byrom, 2006).  This presents a real danger of irreversible environmental damage due to delays in correctly diagnosing the problem.

Acknowledgements

Most of the material for this paper was collected during projects funded by ACIAR and AusAID in Inner Mongolia and Tibet.

References

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¹Manaaki Whenua – Landcare Research

²CSIRO Sustainable Ecosystems

³CSIRO Entomology

⁴College of Agriculture and Biotechnology, China Agriculture University