118 Utilisation of Molinia caerulea Tussocks by Overwintering Beetles at Epping Forest
therefore, avoid drowning when the water rises, although the constantly low number of individuals
from both families in bottom core sections does not support this.
It was generally observed that the height of water level attained in the tussocks was considerably
higher than that of the surrounding open water, due to the sponge like nature of the dense vegetation.
Although carabids can survive a degree of inundation, (Evans et al. (1971) recorded A. fuliginosum
surviving complete immersion for up to 17 days) long periods would cause mortality (Thiele 1977).
Freezing may also need to be avoided.
The carabids recorded here, like many species of the family probably exhibit an obligatory diapause.
That is to say that once in hibernation they exhibit little or no activity, Dawson, (1965) for example
found no carabids in pitfall traps in the winter months. T. hypnorum (the most abundant of staphylinids)
is also an overwintering species (eg. Dennis et al. 1994) but may be able to exit hibernation in
response to certain triggers, to avoid unfavourable conditions or to exploit favourable ones. This
type of hibernation is consistent with personal observations of T. hypnorum, the only species observed
to become active whilst sampling, especially when disturbed by the coring process. It may be that T.
hypnorum and other staphylinids are able to induce activity when inundated and move to higher
parts of the tussock. Luff (pers comm.) believes this explanation to be possible and suggests it as an
area of further work.
If this hypotheses is correct it would be expected that whole, or parts of carabids would be present in
the core samples from site 3. Post-extraction core samples were examined for beetles from site 1, in
order to evaluate the efficiency of the extraction method. With only 2 individuals being found in 12
cores, the method was deemed suitable and further examination was not carried out. In hindsight this
was a regrettable oversight.
It is noted that T. dermestoides and Ceuthoryhncus spp. were also present in the area but absent from
the flooded tussocks. The lack of information on these species prevents them from inclusion in the
above hypotheses.
Whatever the cause, this phenomenon does not appear to be universal to many habitats, as Luff (pers
comm.) has found carabids in flooded tussocks and under bark in a flooded woodland. Exclusion or
otherwise is likely to vary with species and habitat and especially with time of entry to overwintering
state and timing of water level change. Temperature, photoperiod and rainfall, therefore, will be
important influencing factors.
The significance of tussocks in areas of high water table
Without sampling the soil and vegetation between tussocks one cannot be positive, but it is likely
that overwintering beetle densities in the unflooded tussocks of Deer-shelter Plain are considerably
greater than densities of inter-tussock regions. This assumption is consistent with results published
by Dennis et al. (1994), who found arthropod abundance to be consistently higher in tussocks than
between, and those of Dawson (1965), who recorded 4 - 8 times as many carabids in grass tussocks
in winter than summer.
It is likely that the concentration of beetles in tussocks during winter hibernation is partly caused by
the surrounding soil and vegetation being unsuitable due to the high water table. Dawson (1965)
also believed that carabids were util ising Molinia tussocks to avoid waterlogged soil, and Hammond
Essex Naturalist (New Series) 17 (2000)