Trampled underfoot
Wendy Laursen

We rarely stop to consider what is underfoot when we walk around, but there is a world down there that needs to be considered if we want to protect our wild places.

Mangroves are trees and shrubs that grow in the intertidal zone, the region between high and low tide marks, of estuaries and sheltered shores. They provide habitat for a range of terrestrial, intertidal and aquatic organisms including commercially harvested fish, crustaceans and oysters.

These mangrove forests are the home to a variety of organisms.

My research is centred on the Clyde River, 275km south of Sydney. The mangrove habitats support local fisheries and provide access to the river for people engaged in aquatic activities such as oyster farming, fishing, boating, and camping. One popular fishing location has been set up for ecotourism with a boardwalk through the mangroves and sign-posted walking tracks.

The aim of the study is to analyse the effects of human trampling on the mangrove habitats in the Clyde River area. This analysis involves the development of a computer simulation model that estimates the amount of damage sustained by particular trampling intensities and the time taken for the habitat to recover. Trampling intensity is a measure of how many people walk over a certain section of ground, which has implications for how much damage is caused.

The model examines three components of the habitat: pneumatophores, macroalgae and crabs. Pneumatophores are vertical exposed roots that assist the mangrove trees in gas exchange. Macroalgae are multicellular algae that grow on the pneumatophores. They are about one centimetre in length with tiny branches and leaf blades. A variety of crabs burrow in the soft ground around the pneumatophores.

Imagine what this habitat would be like to one of its inhabitants, such as a small crab:

• The pneumatophores would seem like trees.
• The macroalgae would seem like vines and creepers.
• The fish moving in with the tide would seem like packs of hunters.
• Humans walking through would seem like bulldozers, and
• Our clearing and infilling activities would have effects on a cataclysmic scale.

Recreational ecology aims to develop an understanding of the vulnerability of ecosystems to human traffic and to characterise their responses to it. A review of the environmental impacts of recreation in Australia has called for management plans to be based on estimates of carrying capacity.⁹ Carrying capacity is the level of trampling that can be sustained before significant damage occurs in the biological community.

Field studies involving deliberate trampling would need to examine a large number of different trampling intensities to arrive at an accurate determination of carrying capacity. Usually a small number of arbitrarily chosen treatments are undertaken to minimise damage caused by the research itself, and, as a consequence, carrying capacity has been loosely defined.

Simulation modelling overcomes this limitation. Not only can it be used to investigate the effects of human trampling before, it has the potential to provide predictive power for a wide range of treatments and initial conditions, and provide long-term forecasts.

The model has been designed as a web-based application. Three processes are undertaken within the model: trampling damage, recovery, and the gathering of statistics.

The processing has been modularised so the model can be applied to other habitats or treatments in the future, for example, vehicular or animal trampling in terrestrial systems. It could also be rescaled to examine environmental impacts such as oil spills or habitat fragmentation.

The principles underlying the model processing were derived independently of any previous trampling information. I conducted fieldwork to determine the spatial arrangement of organisms within the habitat and their specific responses to trampling. This information was then combined with published growth rates.

Initially the model was used to produce results that could be compared with traditional field studies to confirm its validity. It has been successfully verified against my own field study and results obtained from two other field studies that are underway around Sydney.⁸

The model is now being used to make predictions about the effects of trampling for different sites and for a range of treatments beyond those of previous studies. It will be used to investigate trampling intensities that are relevant to habitat management such as carrying capacity and the formation of paths when the habitat is virtually destroyed and further trampling has little effect.

The results obtained so far demonstrate the forecasting potential of the model and its ability to provide results for a variety of scenarios. For coastal towns, such as Batemans Bay, the modelling provides a tool for managing the effects of recreation and aquaculture on mangrove habitats.

Wendy Laursen is a PhD student at UNSW studying how trampling affects the plants and animals that live on the muddy floor of mangrove forests. Her supervisor is Associate Professor Paul Adam of the School of Biological Earth and Environmental Sciences at the University of New South Wales. .

Glossary

Estuary: The part of the river that is met by tides.


References

1. Adam, P. (1994). Saltmarsh and Mangrove. (In) Australian Vegetation (Ed.
Groves, R. H.). Cambridge University Press. p395-435.

2. Cole, D. N., (1995). Experimental trampling of vegetation. 1. Relationship
between trampling intensity and vegetation response. Journal of Applied Ecology 32: 203-214.

3. Eckrich, C. E., and Holmquist, J. G., (2000). Trampling in a seagrass
assemblage: direct effects, response of associated fauna, and the role of
substrate characteristics. Marine Ecology Progress Series 201: 190-209.

4. Keough, M. J., and Quinn, G. P. (1998). Effects of periodic disturbances from trampling on rocky intertidal algal beds. Ecological Applications 8: 141-161.

5. King, R. J., (1995). Mangrove macroalgae: a review (Presidential Address 1993 Linnean Society of NSW). Proceedings of the Linnean Society of NSW. 115: 3-13.

6. King, R. J., and Puttock, C. F., (1994). Macroalgae associated with mangroves in Australia: (Rhodophyta). Botanica Marina 37: 181-191.

7. King, R. J., and Wheeler, M. D., (1985). Composition and geographic
distribution of mangrove macroalgal communities in New South Wales. Proceedings of the Linnean Society of N.S.W. 108: 97-117.

8. Lasiak, T., (2002). Response of mangrove biota to human trampling. (In) Centre for Research on Ecological Impacts of Coastal Cities Annual Report 2002. University of Sydney. p 8.

9. Sun, D., and Walsh, D. (1998). Review of studies on environmental impacts of recreation and tourism in Australia. Journal of Environmental Management 533: 323-338.

10. Underwood, A. J., and M. G. Chapman, (1995). Coastal Marine Ecology of Temperate Australia. (UNSW Press: Australia).

11. West, R. J., (1989). Mangroves. (NSW Agriculture and Fisheries: Australia).

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