| 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.
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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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