Wildlife Australia - Spring 2017

Mangroves occupy around 15,300 ha of Moreton Bay and are

among the region’s most crucial ecosystems. They are most

extensive and diverse in the southern region and islands, dominated

by Avicennia marina in relatively open forests crowded with pencil-

like breathing roots. Its leaves are notably light and flexible, dancing

with the wind to reveal pale undersides, and an unusual, cold-

tolerant variety (var. australasica) is found in south-eastern Australia.

It has been suggested that mangroves now penetrate further

upriver, but historical observations made by Watson in 1928 record

the presence of Avicennia marina in upper reaches of the Brisbane

River early this century. So, while the dredging and opening of some

rivers for navigation may have facilitated great mangrove growth,

this may not have affected the upstream extent.

Moreton Bay’s mangroves have faced significant change over

the past century, especially since European colonisation in 1824.

Industrial and urban expansion, as well as the natural impacts of

severe storms and localised sea level rises, have damaged mangrove

species. Recent investigations have sought to clarify and quantify

changes in tidal wetlands to enable better-informed management

of these valued but threatened ecosystems.

A history of neglect

My team and I have identified and quantified the chief drivers

of change in the tidal wetlands of Moreton Bay over the last

two centuries. Losses along the Brisbane River reaches began

shortly after European settlement, and over the last century, the

major change has been loss of mangrove habitat to landfill and

reclamation. Some 200 ha were affected by 1946. By 2002, the

impact had become more severe. More than half (52%) of tidal

wetlands remaining in 1946 had been removed, amounting to

around 1516 ha lost. Port and airport development drove these

changes, in conjunction with expansion of industrial facilities

towards the mouth of the Brisbane River. Significant losses have

also been recorded around canal estate developments on Bribie

Island. In recent decades, observations of a decline in diversity and

biomass relating to bottom-feeding fauna suggest that the health

of mangrove ecosystems in the port area has been seriously altered.

Prior to 1860, minor impacts occurred from light cutting and

root trampling to make walking tracks, by both Indigenous people

and early European arrivals. Based on aerial photographs and

population numbers, even in 1946 the impact was moderate,

Catching crabs

Healthy mangrove forests have few leaves lying around – not

because they’re washed away, but because crab ecosystem

engineers busily remove them at low tide when we’re not

looking. When people clamber through mangroves, all is quiet

except for the occasional audible claw-click from a crab hiding

in a burrow. But if you tie a leaf to a string fastened to a low

root and leave it for an hour, you’re in for a real surprise. In

healthy mangrove stands, you’ll return to find your string pulled

tight and your leaf down a tiny burrow. We have estimated that

crabs take 50%–80% of fallen leaves, their nutrients cycling back

through the trees and making them grow bigger than they would

otherwise. As an experiment, we collected all of the crabs in an

array of plots, only to find that the trees suffered reduced growth

within days! Clearly, these small crabs make a huge difference

to how mangroves grow, selectively eating certain species

of mangrove seeds and propagules and thus influencing the

species assemblages and structure of the mature forests. That is

powerful ecosystem engineering!

An abusive relationship?

Much as mangroves are a vital part of maintaining healthy

marine ecosystems, our relationship with them has been

dismissive at best and abusive at worst. Where healthy

mangrove-lined estuaries traditionally offered critical dampening

of land runoff, protecting nearby reefs from the worst of the

damage, in recent decades these bastions of coastal buffering

are succumbing to the unrelenting pressure of expanding

human populations. Mangroves are often at the edge of their

physiological limits, so disturbances can have severe impacts

and can be valuable indicators of environmental change.

Variations in the extent, distribution and relative ‘health’ of

mangroves may be symptomatic of larger-scale processes.

Mangroves and tidal wetlands are essential, highly productive

and beneficial natural coastal environments. However, along

with their dependant biota, they’re under serious threat. In

populated areas, key coastal rivers have become little more than

drains that transport mud and effluent downstream to settle

in estuarine reaches, on shallow embayments, and on near-

shore coral reefs. The mangroves that remain are becoming less

biodiverse, less functional remnants. But it’s not all gloom and

doom. MangroveWatch (www.mangrovewatch.org.au) specifically

facilitates partnerships between community volunteers and

scientists to raise awareness and to rigorously monitor mangrove

habitat biodiversity and health. Citizen scientists have readily

adopted this program. Working with local community groups

such as Wildlife Queensland, individuals can help monitor

mangrove habitats and form their own lasting ‘love affairs’ with

these under-rated protectors of our coastal environments.

MORETON BAY

MANGROVES NEED

PROTECTION

Eight mangrove species occur in Moreton Bay and

are variously tolerant to tidal submersion and

salinity. Only three genera (Avicennia, Excoecaria,

and Aegiceras) generally occupy rivers. Photo: Mario

Vecchi. Inset: Mangrove crab, Norm Duke.

When treated with respect,

individual mangroves may live

for 700 years or more.

32 | Wildlife Australia | SPRING 2017