Small Mammals

Case study: Harvest mouse

Harvest mouse Micromyms minutus is a Biodiversity Action Plan species that is thought to have declined by 71% over the past 18 years due to changes in agricultural practice, habitat loss and fragmentation.

Whilst its name suggests an association with cereal crops, ECON's research (which started back in the early 1990s) shows that Harvest mouse is actually very scarce in modern cereal fields. This is due to the short time that modern winter-sown cereals are in the ground, with crops being harvested during the harvest mouse breeding period. It is also exacerbated by spraying because it reduces the abundance of invertebrates, which are important summer food for the mice. Another major factor is the decline in suitable luxuriant hedgerows for harvest mice to head for once the crop is cut. Without quality habitat nearby (i.e. unmanaged hedgerows) survival would be poor which would affect the number of mice available to breed in the following season.

Whilst it was known that Harvest mouse occurs in wetlands on the continent, ECON's surveys carried out in the early 1990s showed that this is also the case in the UK, with harvest mouse ubiquitous in the wetlands of the Norfolk Broads in all fen habitats. To provide some context, harvest mice occurred in 83% of unmanaged wet and rough meadows, 35% of sites under cereals, 25% of managed grasslands and 20% of orchards, waste grounds and overgrown habitats in urban habitats.

In fact, it was during these surveys that ECON developed some pioneering techniques to better survey and monitor Harvest mice. Earlier surveys carried out in the late 1970s relied on counting the characteristic spherical nests. However, since a single female will build several nests throughout the breeding season, this does not provide any indication of absolute abundance. Since harvest mice nest in the stalk zone, they are underrepresented in ground traps. ECON therefore designed a trap (the Jordan trap) which could be attached to a cane and set in the stalk zone. ECON also used hair tubes, which are single lengths of plastic tube that have sticky tape on the inside roof of the tube. Any animals passing through leaves a sample of hair attached to the tape. This can generally be identified to species level using a variety of microscopy techniques and established keys. Hair tubes are a useful technique in their own right, as they do not need to be checked the following morning, but can be left out for a week, and can therefore be more effective in detecting and sampling small mammals which are present in low densities (though this only gives a measure of relative abundance).

This extensive survey work revealed reed beds and sedge beds to be the most favoured habitat for Harvest mouse. For this habitat to persist over time it must be managed otherwise succession occurs and the fen will move towards wet woodland. However, research in a reedbed subject to different management regimes (cutting, burning and unmanaged) revealed strong short-term management effects upon patterns of use by Harvest mice. Unsurprisingly, Harvest mice were not found in the areas which were cut and burned immediately after management. However, even during the following winter period, a strong preference for the unmanaged areas was still apparent. This preference is most likely related to the presence of a thick litter layer, which offers mice shelter during the winter period.

The timing of management can also be critical to harvest mice, and this is best understood with reference to their population dynamics. Harvest mice have what may be described as 'boom and bust' population dynamics. The females are quick to become sexually mature, and since gestation is only 17-19 days females can very conceivably have three litters in a season. The average litter size is 5. Thus large numbers of juveniles can be recruited into the adult population in the autumn. By contrast mortality in the winter can be extremely high and by the spring there can be very few individuals left. Thus it is easy to envisage that drastic habitat management when numbers are very low in late winter could even in some situations lead to local extinctions, especially if habitat is already fragmented.

Indeed, in all habitats populations of Harvest Mice have become increasingly fragmented. As mentioned previously, within arable habitat, unmanaged hedgerows are necessary to provide refuges for harvest mice once the crop is cut. Even within reedbeds, habitat is being lost as areas are drained. For example, between 1940 and 1981 >750,000ha were drained or re-drained. Unfortunately drainage converts marshy pastures to arable land, which is far less valuable for Harvest mice.

So whilst effort is required to manage reedbeds to ensure that the habitat persists, the timing of management should always be considered so that it is not carried out when Harvest mouse numbers are lowest and the population is most vulnerable. Also, refuge areas must always be provided, and practices such as mowing should be undertaken on a rotational basis ensuring that suitable habitat is left available for mice.


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