The following pages will assist students who are taking part in the fieldwork excursion to Gibberagong Environmental Education Centre. 



Ecology is the scientific study of the relationships between organisms, including humans and their physical environment. Ecologists study these relationships by asking and investigating questions such as:

What lives here (identification)?
How many are there (abundance)?
Where are they found (distribution)?
Why do they live where they are found (relating to abiotic factors)?

During your field trip to Gibberagong EEC as part of the fieldwork component of the Year 11 Biology topic A Local Ecosystem, you will be undertaking a ecological study of the intertidal zone, including the mangroves. During the day, you will be answering these questions.


HSC Biology Syllabus

A Local Ecosystem Student Notes

Gibberagong EEC Excursion Worksheet

Mangrove Study Documentary Part 1

Mangrove Study Documentary Part 2


Alt_Empire Bobbin Head 2

Bobbin Head is located within the west part of Ku-ring-gai Chase National Park and covers part of the Guringai people’s traditional land.

It is approximately 20km north of the city centre, at the base of the bushland valley spanning Cowan Creek a tidal subcatchment of the Hawkesbury River and a typical drowned river valley. Rainwater collected from the surrounding steep sandstone cliffs and plateaus drain into the Hawkesbury River.

Bobbin Head Mangroves

During your fieldwork, you will be studying the intertidal wetland which is an area along a shoreline that is submerged underwater at high tide and is exposed to air during low tide and where deposition of material occurs.

The mangrove forest study site is found on a muddy silt formed by a river deposit located at the southern end of Bobbin Head in and around the Mangrove Boardwalk, a purpose built walkway that cuts through the mangrove ecosystem as part of the longer Gibberagong walking track.


Map of Cowan Creek Mangroves


The term mangroves can be used with reference to:Mangroves

  • the trees themselves
  • a type of forest, and
  • the habitat

They occur within the inter-tidal zone of the Cowan Creek estuary at Bobbin Head.

Bobbin Head (i)This ecosystem is made up of a community of plants and animals interacting with each other and the abiotic environment. The vegetation of the shoreline is dominated by the Grey Mangrove (Avicennia marina) and the River Mangrove (Aegicerus corniculatum). Landward of the mangroves there are small areas of salt marsh and beyond the saltmarsh, stands of Casuarina indicate the upper limits of the high tide mark.

Mangroves are a type of tree that are unique in the ability to grow in intertidal mudflats – a habitat that presents the following conditions:

  • Unstable soils – the wet, sandy sediment do not hold together and are subject to erosion during tides and floods
  • Salt water – living cells usually die in saline environments however mangroves are halophytes (plants tolerant to salt)
  • Soils deprived of oxygen – plants need oxygen for respiration. This oxygen for the roots is normally obtained from the air around the soil particles. However as mangroves grow in waterlogged soil, the air space is flooded with water.
  • Tidal movement of water – every 24 hours, tidal water twice moves in and out of the intertidal zone.


DPI Mangrove Factsheet

DPI Coastal Saltmarsh Factsheet

Field Guide to the Mangroves of Queensland

Guide to the mangroves of Singapore

Plant Adaptations


IMG_1750A population is a group of similar organisms living in a given area as a time. Populations can never be 100% accurately counted; this is because of the difficulty of describing in detail large areas. Also it would be too time-consuming and damaging to the environment. Populations are estimated using sampling techniques. These make an estimate, which is roughly accurate of the population.


Distribution refers to the region where an organism is found. A transect will be used to measure distribution for the mangrove study at Bobbin Head.


Abundance refers to the number of individuals in the area. A 1 metre x 1 metre quadrant will be used to measure abundance for the mangrove study at Bobbin Head

During the field trip, you will be measuring the abundance and distribution of two organisms; Grey Mangroves and crabs.

To learn more about the techniques used during the field trip, refer to the Gibberagong EEC Excursion Worksheet.


Mangrove virtual excursion

Study on correlation between crab holes and abundance

Study on Grey Mangroves distribution and abiotic and biotic factors


The abiotic factors within the mangroves are the limiting factors controlling the growth, abundance and distribution of the Grey Mangrove and crabs. You will be measuring 6 abiotic parameters to understand the relationship between organisms and their habitat.

Soil Moisture

soilSoil moisture is a limiting factor in the growth of plants. Soil moisture that is a consequence of tidal influence is a critical factor in determining the distribution of mangrove and saltmarsh species.

Water logged soils cause a slow rate of oxygen and nutrient replacement, as a result the soils surrounding mangroves are often low in oxygen and nutrients. While salt within tidal waters is toxic to many plant species.

Equipment – Fingers and discussion


All members of your group pick up some soil

Use the scale of 0 is very dry, 10 is very wet to determine the moisture level

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Soil Temperature

Soil temperature is critical in the germination of seeds and the rate and duration of plant growth

Equipment – probe thermometer


Insert the thermometer probe five centimetres into the soil.

Turn on the the thermometer.

Measure and record the temperature in degrees Celsius.

Repeat this procedure three times and record the average.

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Air Temperature

Air temperature is a limiting factor upon plant and animal species. None more so than mangroves. In the cooler temperate southern locations of Australia only two species of mangroves occur, the grey mangrove and the river mangrove. While in the warmer northern parts of Australia the number of mangrove species is close to thirty.

Air humidity is important for the sustainability of mangrove forest ecosystems. Air humidity influence the amount of mangrove litter in forms of leaves, twigs and other biomass, it is a food source for the aquatic biota and the nutrients released determining productivity of waters. Logging and opening of mangrove forests impacted on decreasing air humidity, increasing evaporation and encourages the occurrence of soil moisture deficit and drought (Hatcher. et al., 2012; Yan. et al., 2007)

Equipment – thermometer (temperature) and hygrometer (humidity) – one piece of equipment will measure both


Turn on the thermometer/hygrometer

Hold one meter above the ground.

Measure and record the temperature in degrees Celsius and humidity as a %.

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kestrelWind speed

Wind speed and direction affects the temperature and humidity of an area. Wind affects creates small waves in the estuarine environment and also contributes to the dispersal of propagules in water currents.

Equipment – anemometer (part of a kestrel instrument)


Turn on the kestrel.

Push right arrow button until a wind blowing icon appears.

Measure and record wind speed in km/h.

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Light Intensity

lightmeterLike other plants mangroves convert sunlight energy into organic matter through the process of photosynthesis. Sunlight intensity and duration influences the levels of photosynthetic productivity.

Equipment – A Light Meter (unit of measurement = lux)


Remove the cover from the light meter.

Hold the light meter sensor horizontally one meter above the ground.

Turn on the light meter. Note if reading from the 20000 Lux scale the reading is to be multiplied by 10 and if reading from the 50000 scale multiply the reading by 100.

Record the measurement in Lux.

Repeat the procedure three times and record the average sunlight intensity.

Turn the light meter off when the measurements have been made.


Biotic (living) interactions between organisms are also factors that affect distribution and abundance organisms within the Mangroves. These interactions between species can be classified as either trophic, competitive or symbiotic.


The trophic level of an organism is the position it occupies in the food chain. Energy captured by plants as carbon compounds is lost at each trophic level through respiration. This reduces the biomass (living matter) at each trophic level.

Leaf litter in the mangroves is the main source of carbon available to consumers. Much of this litter is shredded and consumed by crabs which along with faeces from crabs is broken down by bacteria and fungi. This decomposed material is rich in protein, food for fish, worms, molluscs, crustaceans and zooplankton.

With each outgoing tide large quantities of nutrient is relocated to the estuary to be consumed by various organisms.

Producers – Examples include phytoplankton, algae, Avicennia marina, Aegicerus corniculatum

Primary consumers – Examples include zooplankton, shrimp, molluscs, crabs, worms, nippers, amphipods, mullet

Secondary consumers – Examples include bream, blackfish, white faced heron

Tertiary consumers – Examples include sea eagle, pelican, cormorant


Ecological competition is the interaction between two or more organisms, or groups of organisms for the same resource.


Symbiosis is two different species living together. These include:

Parasitism – one organism benefits and the other is disadvantaged

Example: Plants galls in Avicennia marina

Mutualism – both organisms benefit

Example: Lichen is a mutualistic relationship between algae and fungi

Commensalism – one organisms benefits, the other is unaffected

Example: Sydney rock oysters attach to the base of mangroves

Alleopathy – one organism produces one or more biochemicals that influence the growth, survival and reproduction of other organsisms

Example: Casurina glauca (Swamp She-oak) restricts the growth of other trees and shrubs


An Ecosystem Under Threat

Due to human activity, the current loss rate for mangrove forests is higher than the loss of inland tropical forests and coal reefs (source: www.ouramazingplanet.com/mangrove-forest-inventory-shows-steep- declines-0456/). In fact it is believed that 35% of mangrove forests were lost from 1980 to 2000 and today there are approx. 137,760 square kilometres of mangroves found on the Earth.

Even today, there is a long held view that mangroves are an unsightly place that are only suitable for the dumping of waste or to be redeveloped to enable prospective residents the opportunity for water views.

It is becoming more obvious however the mangrove ecosystems are extremely important for both the environment and the economic sustainability of many parts of society.


Impacts on Mangroves from Sea Level Rise