It is a remarkable fact that our understanding of the habitats of terrestrial molluscs has advanced little since Boycott’s (1934) exemplary account. The term ‘habitat’ effectively means ‘where they live’ but is more often used to refer to vegetation types or other landscape or land-use features. Is the habitat of a particular species ‘woodland’ or the very precise conditions which occur very locally within woodland? Are permanently moist conditions more important than closed canopy?
Tony Wardhaugh (2011) has written a very interesting and useful account of the actual habitat of Spermodea lamellata. It is exclusively a ground-living species, living in deep leaf litter (generally of at least 10 cm or more) overlying deep, soft, loamy soil. The sites remain moist throughout the year but are rarely very wet. It is not found in shallow leaf-litter overlying drier or compacted soils, nor in the rapidly-decaying leaf litter from tree species such as ash, sycamore or hazel. It therefore has a high dependence on undisturbed ancient semi-natural woodland, although is tolerant of partial clear-felling and replanting in managed woodland. This is precisely the sort of information that we need to know if we are to successfully promote its conservation.
figure 1: an uncut compartment in St John’s Wood, County Rosscommon.
I have recently been carrying out a detailed survey of the invertebrates in St John’s Wood Nature Reserve in County Roscommon, Ireland. This ancient semi-natural woodland occupies 110 ha and has developed over Carboniferous Limestone overlain by sandy clay, with acid soils in places (Rackham, 1995). It has had a long, albeit erratic, history of exploitation as coppice with standards. This is a two-storey woodland management system where among cut trees or ‘coppice’ some trees are left to grow as larger size timber, called ‘standards’. The National Parks and Wildlife Service have recently reinstated this style of management in a few compartments. Most remains in non-intervention management for the time being (figure 1). Part of the brief for the study was to assess the impacts of current woodland management practices, such as coppicing, ride creation or non-intervention, on invertebrate assemblages.
At the start of the project I drew up a list of the more desirable molluscs which might be expected, in order to make sure that I carried out survey techniques which might increase the chances of detecting them if present. The selection was broadly chosen from species known to have a preference for native woodlands and which are included in the Irish Red List (Byrne et al., 2009). S. lamellata was therefore on my list but was not found. Reading Tony Wardhaugh’s account suggests one key reason why it was not found – no pockets of deep leaf litter were encountered, although patches may have been overlooked. Despite lying over limestone, the soils in the least disturbed parts of the wood are actually as much as 9 cm deep and remained permanently moist throughout the field season. Suction sampling here revealed a large population of Leiostyla anglica in the leaf litter but that species was not found anywhere else in the wood. The deeper moister soils and the relatively thin leaf litter were sufficient for L. anglica but not for S. lamellata. Leaf litter depth was not measured but was probably no more than 5 cm deep.
The habitat requirements of L. anglica are known not to confine it to ancient woodland across much of Ireland, but have these requirements been properly defined? Byrne et al. (2009) state that it is common in wet, shaded habitats on neutral to base-rich soils, although they admit that towards the west coast it occupies acid coastal heath and can be found around gorse in rough pasture. However this is about vegetation and land-use alone, whereas what we really need to know is how these relate to key habitat requirements in terms of humidity regimes and stability. The St John’s Wood population appears to be very restricted in distribution but is this due to the relatively undisturbed nature of those areas? They are still of a coppice with standards structure and have clearly been actively exploited in the past. Maybe soil depth is important here, helping to maintain permanently moist conditions despite the underlying limestone?
figure 2: Z. subrufescens on hazel re-growth in St John’s Wood.
The other two Irish Red List species found in St John’s Wood were very different in their distribution. Zenobiella subrufescens (figure 2) was present more or less throughout the woodland, being mainly found amongst the ground vegetation, on the foliage of shrubs and even into the lower canopy of trees. Only empty shells were found in the leaf litter. Its presence above soil level might suggest that it would be intolerant of active woodland management, but it was found to be common on hazel re-growth in the recently cutover coppice compartments (figure 3). It clearly has the capacity to rapidly re-colonise cleared areas. Byrne et al. (2009) merely describe it as an old woodland relict species threatened by habitat destruction. But the St John’s Wood coppices demonstrate that habitat destruction is not the issue – rather the permanent loss which might be brought about by grubbing the wood out completely. This may seem a fine point but the presence of Z. subrufescens in this reserve might have been used to justify non-intervention, whereas active coppice cutting has proved beneficial to a wide range of the other invertebrates in the wood, and has no detrimental impact on the Zenobiella population as a whole. The wood also has a past history of livestock grazing, so Zenobiella is also tolerant of a certain amount of grazing – a fact which needs to be appreciated.
figure 3: hazel re-growth in cut-over compartment of St John’s Wood.
The third Red List species found was Limax cinereoniger (figure 4). Interestingly this was only encountered in two of the most recently cut-over compartments. The reasons for this are not clear and may relate more to increased light levels and the improved ability of the surveyor to see the slugs! But the opening of the canopy actually stimulates the development of ground vegetation and so may actually increase soil moisture and humidity at ground level. Byrne et al. (2009) state that this species is found in old, minimally disturbed, broadleaf woodlands or on cliffs with relic woodland vegetation on western coasts. Again, this might be used as a case against restoration of active management in St John’s Wood, but – as I found – this species is certainly tolerant of coppice cutting, if not favoured by it. It too is tolerant of certain levels of grazing.
figure 4: Limax cinereoniger in a cut-over compartment of St John’s Wood.
This account is not intended as a criticism of the Irish Red List – this has provided a much-needed framework for promoting mollusc conservation in Ireland. My intention is to draw the attention of mollusc conservationists in general to the need to provide precise and carefully worded accounts of the habitat requirements of molluscs, to be realistic about the potential responses to changes in land management, and not to fall into the trap of using vegetation and land-use terms loosely. My examples come from a particular vegetation type with a history of exploitation by people, but the principles are clearly applicable to all situations, to all mollusc habitats.
References
Boycott, A.E. (1934) The habitats of land molluscs in Britain. Journal of Ecology 22: 1-38.
Byrne, A., Moorkens, E.A., Anderson, R., Killeen, I.J. & Regan, E.C. (2009) Ireland Red List No. 2 – Non-Marine Molluscs. National Parks and Wildlife Service, Department of the Environment, Heritage and Local Government, Dublin, Ireland.
Wardhaugh, A.A. (2011)The Scarborough Snail and what it has to tell us about ancient semi-natural woodland. British Wildlife 22: 176–183.