Utilisation of Molinia caerulea Tussocks by Overwintering Beetles at Epping Forest 119
(1998) states that in damp areas insects in hibernacula such as tussocks are at a distinct advantage
being raised above water level.
Utilising grass tussocks is by no means unique to boggy areas but if, as suggested here, the difference
between beetle densities within and outside tussocks is particularly high in such areas due to water-
logged ground, then one would expect density in tussocks to be lower in areas that are drier and/or
have other suitable hibernacula. Site 1 would constitute such an area; the water table was lower, and
logs and stumps provided abundant alternative hibernacula. Sampling the logs and intervening soil
would reveal whether the hypothesis is correct. It is consistent with a study of marsh carabids. where
beetle density in tussocks was greatest near the open water and reduced dramatically in the tussocks
of the drier parts (Murdoch 1966).
Management implications
Molinia is a deciduous grass. The litter from these tussocks tends to form a dense mat of vegetation
stopping light from penetrating to the seed bank and inhibiting growth of Heather in its pioneer
phase (Gimingham 1992). For this reason, and with the objective of regenerating the specialist
lowland heath habitat and associated communities, Molinia tussocks arc in some areas being removed
where Calluna vulgaris, Erica, tetralix and associated sedges and other heathland species are being
out-competed. Although currently not practised at the sites sampled, grazing and turf-scraping arc
being employed to aid heath regeneration in other similar parts of the Forest, for example Long
Running (Dagley & Samuels 1999).
As previously discussed, the Molinia tussocks are probably the main (if not only) available
overwintering habitats for wetland beetles, and indeed many other invertebrate groups at Deer-
shelter Plain. Removal of a significantly large number of tussocks could deplete the invertebrate
populations of the area. Indeed recent agricultural research (e.g. Dennis et al 1994) suggests that the
localised promotion of grass tussocks in management techniques increases arthropod density.
Further work
It has already been noted that the sampling of whole tussocks would be preferable and that sampling
between tussocks and in other hibernacula would be required to confirm the hypotheses presented
above.
With regard to the hypotheses offered in section 3, site specific information on timing of overwintering
and flooding would be desirable. Remains of carabids in flooded core sections would suggest mortality
by drowning and support the third hypothesis. If remains were not found, discriminating between
the flight and swimming hypotheses would be best achieved by a combination of observational and
experimental work. The capabilities of flight and swimming at different temperatures and across
different distances could be investigated in the laboratory, as could any species' aversions to crossing
water. Once identified the most likely hypotheses could be tested by field observations in different
areas, by the artificial flooding of tussocks at different times and by comparing the effects on beetle
distribution with those predicted by the hypothesis.
If the exclusion of carabids was found to be common to many seasonally flooded habitats the
distribution of this family may be used as an indicator of localised drying. A strong significant
negative correlation between numbers of flooded tussocks and numbers of tussocks successfully
utilised by overwintering carabids found over a large range of sites with a high sample size would
support this.
Essex Naturalist (New Series) 17 (2000)