Trees in the ecosystem pt II: Trees & molluscs

Whilst the presence of herbivorous slugs and snails within forest ecosystems has been far from extensively researched in the past – usually because of their low density within forests (Emberton et al., 1996), though high density hotspots may exist in ideal habitat conditions provided by trees that are practicably inaccessible (Cameron & Pokryszko, 2005) – there has been an increased focus on surveying for their presence because of their sometimes unexplained high mortality rates (Hadfield & Miller, 1989), because of deforestation and woodland degradation (Kappes et al., 2009; Schilthuizen et al., 2005), and because molluscs may act as an indicator of ancient woodland (Alexander, 2011). Such research has demonstrated how individual (and groups of) trees are critical for the survival of molluscs, with large expanses of woodland cover (at times, over 1,000ha) being necessary to sustain diverse and healthy populations. This may be because molluscs cannot travel with any degree of pace by themselves, and thus the standard concept of fragmentation and its associated isolation effects will not apply to such species so steadfastly (Kappes et al., 2009). However, more open wooded landscapes, such as wood pastures, can also support molluscs, and notably when such wood pastures have a higher canopy cover; either due to abandonment or a reduction in grazing intensity.

The stumps of trees, or other habitats containing deadwood (including veteran trees and coarse woody debris), may be of great value for molluscs – even, at least in the short-term, where their provision is due to thinning or felling of stands. For example, stumps generated through management regimes can act as an attractive resting and hibernation spot for molluscs (Fondo & Martens, 1998), though where deadwood may exist within floodplain areas, land molluscs may take a preference to standing deadwood in order to avoid flood waters (Kappes et al., 2014). However, the openness created by felling operations, as well as its associated disturbances, will detract from the quality of the landscape (principally through the reduction in humidity), and therefore sheltered and undisturbed deadwood sites are of particular importance to many terrestrial molluscs (Rancka et al., 2015; Remm & Lõhmus, 2016). Thus, the presence of deadwood (such as coarse woody debris, though also standing deadwood) within a damp woodland setting may be highly beneficial for molluscs with regards to resting (Kappes, 2005) – particularly during advanced stages of decay (Stokland et al., 2012). Deadwood may also be beneficial for predaceous snails, where the remains of other organisms can be digested, or living organisms can be predated upon (Kappes et al., 2006).

Sites such as this (New Forest, UK) could be very beneficial for molluscs and perhaps most notably as the wood succumbs to white rotting fungi.

However, it is not always the trees themselves that provide molluscs with the ability to travel between isolated patches, or even within the same woodland patch, but other species the woodlands attract – deer and wild boar are but two examples. Molluscs may ‘use’ these mammals to travel vast distances – intentionally or not – thereby enabling for effective dispersal of offspring (Bruinderink et al., 2003). This is particularly critical in terms of genetic diversity, as populations of mollusc may be very similar on a genetic level even across wide distances (Hillis et al., 1991). In addition, slugs may feed upon the sporophores of macrofungi and slime moulds, which are themselves harboured upon a wood substrate or supported by the presence of trees (Keller & Snell, 2002; Rathcke, 1985).

Curiously, it is not just terrestrial land molluscs that benefit from deadwood (Lorion et al., 2009). Deep sea bivalve molluscs may, for instance, bore holes into and lay their eggs within deadwood that has been washed down from rivers and into the ocean, where it has then sunk to a depth of up to 500m (Tyler et al., 2007), or perhaps been provided by a sunken vessel. In addition to this, riverine molluscs may also be drawn to deadwood and be found in particular abundance where a river runs through woodland (Thorp & Belong, 1998). Furthermore, driftwood may transport molluscs across many kilometres of ocean and to new shores, where the molluscs on board can then begin to colonise the new landscape. Not only this, but sunken driftwood harbouring estuarine molluscs has also been linked to such estuarine molluscs becoming fully adapted to marine environments, to depths of 135m (Kano et al., 2013). Interestingly, this research suggests that the versatility of molluscs is not fully understood.

Marine bivalve molluscs may also utilise wood, harvested by humans and then used to construct naval vessels (ships), for transport across vast tracts of ocean. The shipworm (Teredo navalis) is a fantastic example of a marine mollusc that disperses itself via this process, and its destructive presence for such ships it colonised by boring into led to, particularly in the centuries gone by, the hulls of ships being dressed in copper (Grave, 1928). This – and other – shipworms (known as teredo worms), would also colonise upon sunken deadwood (from ships), and other man-made aquatic wooden structures, such as bridges, piers and groynes, causing sometimes irreparable damage (Britton, 1875; Nordstrom et al., 2007; Thompson, 1830).

Damage caused by Teredo navalis to a branch of a tree. Source: Wikimedia.

References

Alexander, K. (2011) A Survey of Ancient Woodland Indicator Molluscs in selected sites on the Isle of Man. [Online] Available at: http://www.manxwt.org.uk/sites/default/files/files/wfom_ancientwoodlandmolluscsurvey2011.pdf

Britton, T. (1875) A Treatise on the Origin, Progress, Prevention, and Cure of Dry Rot in Timber: With Remarks on the Means of Preserving Wood from Destruction by Sea Worms, Beetles, Ants, Etc. UK: E. & F. N. Spon.

Bruinderink, G, van der Sluis, T., Lammertsma, D., Opdam, P., & Pouwels, R. (2003) Designing a coherent ecological network for large mammals in northwestern Europe. Conservation Biology. 17 (2). p549-557.

Cameron, R. & Pokryszko, B. (2005) Estimating the species richness and composition of land mollusc communities: problems, consequences and practical advice. Journal of Conchology. 38 (5). p529-548.

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Fondo, E. & Martens, E. (1998) Effects of mangrove deforestation on macrofaunal densities, Gazi Bay, Kenya. Mangroves and Salt Marshes. 2 (2). p75-83.

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Hadfield, M. & Miller, S. (1989) Demographic studies on Hawaii’s endangered tree snails: Partulina proxima. Pacific Science. 43 (1). p1-16.

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Lorion, J., Duperron, S., Gros, O., Cruaud, C., & Samadi, S. (2009) Several deep-sea mussels and their associated symbionts are able to live both on wood and on whale falls. Proceedings of the Royal Society of London B: Biological Sciences. 276 (1654). p177-185.

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Remm, L. & Lõhmus, A. (2016) Semi-naturally managed forests support diverse land snail assemblages in Estonia. Forest Ecology and Management. 363 (1). p159-168.

Schilthuizen, M., Liew, T., Elahan, B., & Lackman‐Ancrenaz, I. (2005) Effects of karst forest degradation on pulmonate and prosobranch land snail communities in Sabah, Malaysian Borneo. Conservation Biology. 19 (3). p949-954.

Stokland, J., Siitonen, J., & Jonsson, B. (2012) Biodiversity in Dead Wood. UK: Cambridge University Press.

Thompson, W. (1830) Observations on the Teredo navalis and Limnoria terebrans, as at Present Existing in Certain Localities of the British Islands. Abstracts of the Papers Printed in the Philosophical Transactions of the Royal Society of London. 3 (1830-1837). p291-292.

Thorp, J. & Delong, M. (1998) In situ experiments on predatory regulation of a bivalve mollusc (Dreissena polymorpha) in the Mississippi and Ohio Rivers. Freshwater Biology. 39 (4). p649-661.

Tyler, P., Young, C., & Dove, F. (2007) Settlement, growth and reproduction in the deep-sea wood-boring bivalve mollusc Xylophaga depalmai. Marine Ecology Progress Series. 343 (1). p151-159.