Órla McLaughlin

Study site

The Gearagh is a mixed deciduous alluvial woodland in the floodplain of the River Lee. It is one of the last semi-natural forested floodplains in Europe and is unique in Ireland (Brown, 1997). The Gearagh formerly extended approximately 5.5 km in length (Cross and Kelly, 2003), but as a result of a hydroelectric scheme, most of the woodland was felled and flooded in the 1950's (White, 1985). Approximately 200 hectares remains (Harper, et al., 1997). The remaining area, in the upper reaches, is a complicated, braided river system, composed of up to thirteen channels (Harper, et al., 1997; Cross and Kelly, 2003). Tree roots stabilize the main channels and the width of each channel varies from 1-7 metres.
The islands within the river system were formed as a result of debris dams. Trees that were felled or uprooted and left in the river enable the build up of debris. Overtime, this build-up resulted in the island system now visible in the Gearagh.

Before the destruction of the Gearagh, it was a prominent botanical site, and had been visited by several renowned botanists including Praeger, Braun-Blanquet, Tüxen, and Tansley (Brown, 1997). It is noted for its unique floristic composition. It has a closed canopy of Quercus robur (Pendunculate Oak), Fraxinus excelsior (Ash), and Betula pubescens (Birch) (O'Reilly, 1955; White, 1985). It contains an understorey (small trees, shrubs and bushes) of Salix aurita (Willow), Ilex aquifolium (Holly), and Alnus glutinosa (Alder) (White, 1985). Ground flora includes Allium ursium (Ramsons), which are rare in Ireland (Rose, 1978), Euphorbia hyberna (Irish Spurge), and Circaea lutetiana (Enchanter's Nightshade) (White, 1985).

The Gearagh is situated on Old Red Sandstone sediments, an Upper Palaeozoic rock (Pracht, 1997). This is an acidic type rock, which can have major consequence on the soil type, and the soil biota. However there are outcrops of shale and limestone in the Gearagh region, which has a neutralizing effect on the pH. The Gearagh has a thick layer of organic matter, and brown podzols and glacial till.
The stream itself has been shown to contain a high variation in invertebrate composition (Brown, 1997). The Gearagh is protected as a nature reserve, Biogenetic reserve, a Ramsar Wetland site and is owned by ESB (Electrical Supply Board) and managed by the National Parks and Wildlife Service. Because of its dynamic nature tree falls often occur in the Gearagh. This creates an ideal habitat for otters (Lutra lutra), an Annex II species on the European Habitats Directive.
   

My PhD.

  
Food web structure has been widely studied but until recently food webs have been poorly resolved. Despite previous examination of both the floristic and stream invertebrate compositions of the Gearagh, there is no data on the terrestrial invertebrate communities of the system.

My research is concerned with resolving the food web structure for the terrestrial invertebrates found on a subset of the islands within the Gearagh. The food web will be resolved by identifying all terrestrial invertebrates found in pitfall samples to species level, examining the gut contents of the invertebrates and by conducting feeding trials

Currently, I am examining literature dealing with terrestrial invertebrates and their ecology. However, there is very little data explicitly detailing the diet of common terrestrial invertebrates. This lack of information on the basic ecology of the types of invertebrates found in our backyards shows the importance of food web studies, particularly in terrestrial habitats. I will also be surveying the vegetation of the islands to resolve the bottom of the food web. Body weight/length ratios will be established for each species. Using this data, predator-prey body size ratios will be determined to parameterise a food web model for the system. I will also look at how body size affects the per capita strength of species interactions within the food web (Emmerson and Raffaelli, 2004).
Once the food web structure has been resolved, I will perform a sensitivity analysis using Lotka-Volterra community dynamics to highlight key species or functional groups within the system. Sensitivity analysis has been primarily used to determine the effect of life history changes to population behaviour (Hodgson and Townley 2004).
My aim is to use sensitivity analyses to look at the effect of small perturbations on ecosystem processes and properties (e.g. stability). Using this information, I aim to perturb the islands either by species removal (species identified by theoretical analysis) or by denuding the islands of terrestrial invertebrates and examine the recolonisation rate and resulting food web structure. I am also examining ecosystem functioning on the islands (e.g. primary productivity and decomposition). I would like to relate the resolved food web structure to the ecosystem processes and see how perturbations of the terrestrial invertebrates will affect primary productivity and decomposition rates.

References:
1. Brown, A. G. (1997) Biogeomorphology and diversity in multiple-channel river systems, Global Ecology and Biogeography Letters, Volume 6, pp. 179-185.
2. Cross, J. R., and Kelly, D. L. (2003) Wetland woods, In Wetlands of Ireland, Distribution, ecology, uses and economic value, by Otte, M. L. (Ed.), University College Dublin Press, Dublin.
3. Emmerson, M.C. and Raffaelli, D. (2004) Predator-prey body size, interaction strength and the stability of a real food web, Journal of Animal Ecology, 73, pp. 399-409.lackwell Publishing, Ltd.
4. Harper, D., Mekotova, J., Hulme, S., White, J, Hall, J. (1997) Habitat heterogeneity and aquatic invertebrate diversity in floodplain forests, Global Ecology and Biogeography Letters, Volume 6, pp. 275-285.
5. Hodgson, D.J. and Townley, S (2004) Linking management changes to population dynamic responses: the transfer function of a projection matrix perturbation, Journal of Applied Ecology, 41, pp. 1155-1161.
6. O'Reilly, H. (1955) Survey of The Gearagh, an area of wet woodland on the River Lee, near Macroom, Co. Cork, The Irish Naturalists' Journal, pp. 279-286.
7. Pracht, M. (1997) Geology of Kerry-Cork, A geological description to accompany the bedrock geology 1:100,000 scale map series, sheet 21, Kerry-Cork, Geological Survey of Ireland.
8. Rose, F. (1978) Wild Flowers, Clairemont Books, London.
9. White, J. (1985) The Gearagh Woodland, Co. Cork, The Irish Naturalists' Journal, Volume 21, Number 9, pp. 391-396.