The status and population dynamics of small mammals in broad-leaved andconiferous woodland

T H E STATUS AND POPULATION DYNAMICS OF SMALL M A M M A L S IN BROAD-LEAVED AND CONIFEROUS WOODLAND J. R .

FLOWERDEW

B r o a d - l e a v e d and coniferous w o o d l a n d s p r o v i d e favourable habitats f o r most o f the B r i t i s h small insectivores and rodents. H o w e v e r , the c o m p o s i t i o n of the small m a m m a l Community w i l l depend o n the age and type of the w o o d l a n d , its m a n a g e m e n t , its structure and g r o u n d Vegetation, the geographical d i s t r i b u t i o n of the m a m m a l species and seasonal influences on t h e i r p o p u l a t i o n dynamics.

I n b o t h types of w o o d l a n d short-tailed voles (Microtus agrestis) and harvest mice ( M i c r o m y s minutus) are likely t o be m o r e abundant in the early successional stages as they are associated w i t h grassland and long-stemmed field-layer Vegetation. Y e l l o w - n e c k e d mice (Apodemus flavicollis) and comm o n d o r m i c e ( M u s c a r d i n u s avenellarius) m a i n l y f a v o u r m a t u r e broadleaved w o o d l a n d p r o b a b l y because of their feeding and other specific habitat requirements. T h e o t h e r species - w o o d mouse (Apodemus sylvaticus), bank vole ( C l e t h r i o n o m y s glareolus), c o m m o n shrew (Sorex araneus), pygmy shrew (Sorex minutus), water shrew (Neomys fodiens) and m o l e (Talpa europaea) (not considered in detail) - are all f o u n d i n b o t h types o f w o o d l a n d o f all ages a l t h o u g h the water shrew is very sporadic in its occurrence. T h e d o m i n a n t species i n m a t u r e w o o d l a n d are usually w o o d mice, bank voles, c o m m o n shrews a n d p r o b a b l y moles. Y e l l o w - n e c k e d mice may become d o m i n a n t i n some broad-leaved woodlands w i t h i n their d i s t r i b u t i o n a l ränge and p y g m y shrews, being d i f f i c u l t to sample accurately, may be m o r e a b u n d a n t than c o m m o n l y thought f r o m l i v e - t r a p p i n g studies.

Successional changes in the Vegetation of y o u n g coniferous a n d deciduous w o o d l a n d s lead t o the decline o f the short-tailed vole and harvest mouse p o p u l a t i o n s . A s the canopy closes grassland habitats are lost. T h e bank vole usually increases in density f o l l o w i n g the loss o f the short-tailed v o l e .

H o w e v e r , the bank vole favours open-canopy w o o d l a n d w i t h m u c h g r o u n d cover i n the f o r m of field layer Vegetation such as bracken and b r a m b l e . I n general, the early successional stages of w o o d l a n d s are associated w i t h a diverse vegetational structure and large numbers o f plant species w h i c h Support the most diverse small m a m m a l Community. T h e t i m i n g of changes in the small m a m m a l Community w i l l be l i n k e d t o changes i n the g r o u n d Vegetation as the canopy closes. I n Polish coniferous forest small m a m m a l species n u m b e r peaked at 9 in the eleventh year after f e l l i n g a n d planting. I n managed broad-leaved coppice in southern E n g l a n d the n u m b e r o f species peaked at 8 three years after coppicing. T h e successional changes f o l l o w e d by the depauperate small m a m m a l Community i n I r e l a n d ( w o o d mouse, p y g m y shrew and bank vole, the latter restricted t o the south-west) are in need o f f u r t h e r study.

T h e p o p u l a t i o n dynamics of the small m a m m a l s w i l l affect the relative abundance o f each species in the w o o d l a n d small m a m m a l Community o n a seasonal basis and in some cases between years.

Trans. Suffolk

Nat. Soc. 29 (1993)

T H E STATUS A N D POPULATION DYNAMICS O F SMALL MAMMALS

115

Short-tailed voles show seasonal fiuctuations in numbers in grasslands and in young woodlands so that low numbers in late spring increase to the a u t u m n , there is little change in numbers Over winter and then a decline through the spring. Multi-annual fiuctuations (cyclical changes in numbers) show peaks and troughs at 3 - 4 year intervals. These cycles occur commonly in young conifer plantations and permanent grasslands in northern England and Scotland and may also be present in the dissected grassland and plantation habitats further south. Declines lasting more than a year occur in young and m a t u r e coniferous habitats and although some cases are associated with successional changes in the woodland, others are not.

Bank voles commonly show a seasonal pattern of population dynamics with a peak in autumn and a trough in spring although there may be occasional reverses with increases from winter to spring or later, followed by a decline. T h e winter-spring increase is likely to be associated with heavy masting of broad-leaved trees and possibly winter breeding and may continue to the next winter. Populations in coniferous plantations may depend on immigration more than breeding during population increases.

Wood mice commonly fluctuate from 1-40/ha but increase to 130-200/ha after a heavy mast crop in broad-leaved woodland. Coniferous woodlands are likely to support lower densities (a Scots pine plantation at Thetford forest had a mean density of 12/ha over a two year period), but exceptions are possible. Grazing leads to relatively low densities in areas of the New Forest and numbers are low with high numbers only appearing after heavy masting in sessile oak woodlands in North Wales. T h e distribution of wood mice in mixed coniferous and broad-leaved woodland has been shown to be positively correlated with the distribution of tree seeds. Numbers show regulär annual fiuctuations with a peak in autumn/winter and a decline through the spring; the increase after the Start of breeding in late spring may be slow, giving the impression of a stable period, but this usually gathers m o m e n t u m in late summer/autumn to reach a peak again. As with the bank vole, heavy masting in broad-leaved woodland has marked effects on dynamics so that increases f r o m one winter to the next and winter breeding are recorded; numbers may also stay at a contant high density and then decline in s u m m e r .

Yellow-necked mice are restricted to the south and south-east and the Welsh borders. They favour deciduous woodlands with little ground cover, dense cover between 1 and 5m and with tree species providing high-energy fruits such as Yew. Population densities may equal those of wood mice but more usually they are found at low densities with centres of population dispersed throughout the woodland. There is again a strong influence o f m a s t crops on population dynamics but the annual fiuctuations are slightly different from wood mice. N u m b e r s reach a peak in late autumn or early winter and decrease slowly through winter and spring. T h e increase to peak numbers occurs gradually through the late spring and summer from the start of the breeding season. Studies in the vicinity of high density deciduous woodland populations in Gloucestershire indicate that numbers are relatively low in grassland and conifer plantations and only moderate in secondary deciduous woodland.

Harvest mice are found in tall Vegetation in grassland, reed beds and Trans. Suffolk Nat. Soc. 29 (1993)

116

Suffolk Natural History, Vol. 29

woodland edge. They occur from Edinburgh south but show only isolated populations in northern England. Numbers are extremely variable in favourable habitats such as rough grassland; from 0 in early and mid-summer to circa 250/ha in late summer have been recorded. Up to four generations may breed in one summer. In young conifer plantations highest numbers are found in late summer to winter with a peak in the autumn. After coppicing in deciduous woodland numbers reached a peak in thefirstyear and declined to less than 1% of the small mammal Community by 10 years. Live-trapping studies may not be representative of true fluctuations as much above-ground movement occurs in summer.

Common dormice are widespread but patchy, mainly in deciduous woodlands from mid-Wales, the midlands and Suffolk southwards. Further north they are scarce. Deciduous woodland with scrub growth and species such as bramble are favoured. They may be found on the edges of coniferous plantations if deciduous shrubs are present. Their population dynamics are not well known but recent studies estimate mean density at 8-10/ha in prime habitat. In Europe numbers increase after mast crops to cause damage to trees. Numbers probably peak in autumn just before hibernation, and decline through winter. Longer rotation (older) coppice of 15-20 years is favoured over younger stands and relict areas.

Common shrews are widespread in Britain and are common in most habitats except arable land and grazed pasture. Densities reach 33/ha in old larch/spruce woodland and slightly less in deciduous woodland in late summer. Young successional stages of woodland and grassland populations may reach 42-67/ha. The peak in late summer is followed by a decline ove winter to the spring and then an increase following the Start of breeding.

Trapping may be less efficient in winter.

Pygmy shrews are widespread in Britain and Ireland and usually favour habitats with plenty of ground cover. They are usually less abundant than the common shrew. The dynamics are similar to those of the common shrew with peaks of 11-12/ha in grassland with shrub growth.

The water shrew is widespread in Britain but localised in northern Scotland and occurs sporadically in woodland and grassland habitats. In a larch plantation low peak numbers were observed in September.

Small insectivores and rodents make up similar communities in coniferous and deciduous woodlands at similar successional stages if the more restricted and rare species are ignored. Current conservation advice such as the coppicing of broad-leaved woodland is likely to increase the diversity of small mammal communities in the short-term in existing habitats. In addition, the planting of new woodland is being encouraged on arable land; farm woodland initiatives promote broad-leaved and to some extent coniferous plantations and so the available woodland habitats for small mammals will, hopefully, increase in future years.

Dr. J. R. Flowerdew, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ.

Trans. Suffolk Nat. Soc. 29 (1993)