The Great Barrier Reef
158
nursery grounds. While the species diversity of mangroves and seagrasses is relatively low compared with
adjacent communities like tropical rainforests and coral
reefs, the diversity of organisms that reside in, or utilise,
mangroves and seagrasses is high. Many fish and shellfish spend all or part of their life cycle in mangroves
and/or seagrasses, including important commercial,
recreational and artisanal fisheries.
The land-sea interface is a dynamic environment,
where subtle natural changes in climate, sea level, sediment and nutrient inputs have dramatic consequences
in the distribution and health of mangroves and seagrasses. Local human disturbances include eutrophication, dredging/filling, overfishing and sedimentation.
The combined pressures of human disturbances and
global climate change have led to mangroves and seagrasses becoming ‘endangered communities’. Small
scale restoration projects have demonstrated the extreme difficulty of scaling up to effective, large scale
restoration. Urgent protective measures need to be implemented to avoid further loss of mangroves and seagrasses and the resulting environmental degradation
of coastal ecosystems of the GBR coast.
N MANGROVES
Overview
Mangroves are a diverse group of predominantly
tropical trees and shrubs growing in the upper half of
the intertidal zone of coastal areas worldwide. They
are well known for their morphological and physiological adaptations for life coping with salt, saturated
soils and regular tidal inundation, notably with specialised attributes like: exposed breathing roots above
ground, extra stem support structures, salt-excreting
leaves, low water potentials and high intracellular salt
concentrations to maintain favourable water relations in saline environments, and viviparous waterdispersed propagules. Mangroves are often mistakenly thought of as a single entity. But, like coral reefs,
mangroves are as functionally diverse and complex as
the range of species, variants and morphotypes
present at particular locations. Also, like coral reefs,
they provide essential structure and habitat for a host
of marine and intertidal species, including residents
among their dense forests and complex roots, and as
visitors with each flooding tide (Box 16.1, Fig. 16.2).
Mangroves are also analogous in function to tropical
rainforests, providing comparable canopy habitat for
birds, mammals and insects. This overlap is reinforced
by ancestral links between these plant habitats. But,
despite the shared features, mangroves include specialist attributes and dedicated resident biota found
nowhere else. Examples include specialist mangrove
forms of the robin, mistletoe bird, mistletoes, grapsid
crabs, molluscs, herbivorous insects, and numerous
floral visitors.
Taxonomy and functional morphology
Mangroves are not a genetic entity, but an ecological
system. Mangrove vegetation includes a range of functional forms, including trees, shrubs, a palm, and a
ground fern. These generally exceed 0.5 metres in
height, and normally grow above mean sea level in the
intertidal zone of marine coastal environments and estuarine margins. Mangrove plants do not come from a
single genetic source, and the only plant families that
are comprised exclusively of mangrove taxa are Avicenniaceae and Sonneratiaceae. Around the world, the
total number of mangrove plants is around 72 taxa
from 21 families, consisting mostly of angiosperms.
Distributed along the GBR World Heritage Area, there
are 39 taxa from 19 families (Fig. 16.1, Table 16.1), representing a significant portion of the worlds’ genetic
variation in mangrove plants, and most of Australia’s.
Some species, like Avicennia marina (Fig. 16.3), Rhizophora
stylosa (Fig. 16.4) and Bruguiera gymnorhiza (Fig. 16.5),
are widespread in the Indo-West Pacific region, while
others, like Aegiceras corniculatum (Fig. 16.6), Ceriops
australis, Bruguiera exaristata, and Diospyros littorea, are
more restricted to the Australasian region. None are
restricted solely to the GBR World Heritage Area. It is
of interest also that Avicennia marina var. eucalyptifolia
(Fig. 16.3) merges with the south-eastern Australian
variety, A. var. australasica towards its southern boundary, south of the Tropic of Capricorn.
The widespread group of species mentioned above
are also ubiquitous mangrove representatives commonly found throughout the GBRWHA. They are commonly located on many mainland islands, sand cays
158
nursery grounds. While the species diversity of mangroves and seagrasses is relatively low compared with
adjacent communities like tropical rainforests and coral
reefs, the diversity of organisms that reside in, or utilise,
mangroves and seagrasses is high. Many fish and shellfish spend all or part of their life cycle in mangroves
and/or seagrasses, including important commercial,
recreational and artisanal fisheries.
The land-sea interface is a dynamic environment,
where subtle natural changes in climate, sea level, sediment and nutrient inputs have dramatic consequences
in the distribution and health of mangroves and seagrasses. Local human disturbances include eutrophication, dredging/filling, overfishing and sedimentation.
The combined pressures of human disturbances and
global climate change have led to mangroves and seagrasses becoming ‘endangered communities’. Small
scale restoration projects have demonstrated the extreme difficulty of scaling up to effective, large scale
restoration. Urgent protective measures need to be implemented to avoid further loss of mangroves and seagrasses and the resulting environmental degradation
of coastal ecosystems of the GBR coast.
N MANGROVES
Overview
Mangroves are a diverse group of predominantly
tropical trees and shrubs growing in the upper half of
the intertidal zone of coastal areas worldwide. They
are well known for their morphological and physiological adaptations for life coping with salt, saturated
soils and regular tidal inundation, notably with specialised attributes like: exposed breathing roots above
ground, extra stem support structures, salt-excreting
leaves, low water potentials and high intracellular salt
concentrations to maintain favourable water relations in saline environments, and viviparous waterdispersed propagules. Mangroves are often mistakenly thought of as a single entity. But, like coral reefs,
mangroves are as functionally diverse and complex as
the range of species, variants and morphotypes
present at particular locations. Also, like coral reefs,
they provide essential structure and habitat for a host
of marine and intertidal species, including residents
among their dense forests and complex roots, and as
visitors with each flooding tide (Box 16.1, Fig. 16.2).
Mangroves are also analogous in function to tropical
rainforests, providing comparable canopy habitat for
birds, mammals and insects. This overlap is reinforced
by ancestral links between these plant habitats. But,
despite the shared features, mangroves include specialist attributes and dedicated resident biota found
nowhere else. Examples include specialist mangrove
forms of the robin, mistletoe bird, mistletoes, grapsid
crabs, molluscs, herbivorous insects, and numerous
floral visitors.
Taxonomy and functional morphology
Mangroves are not a genetic entity, but an ecological
system. Mangrove vegetation includes a range of functional forms, including trees, shrubs, a palm, and a
ground fern. These generally exceed 0.5 metres in
height, and normally grow above mean sea level in the
intertidal zone of marine coastal environments and estuarine margins. Mangrove plants do not come from a
single genetic source, and the only plant families that
are comprised exclusively of mangrove taxa are Avicenniaceae and Sonneratiaceae. Around the world, the
total number of mangrove plants is around 72 taxa
from 21 families, consisting mostly of angiosperms.
Distributed along the GBR World Heritage Area, there
are 39 taxa from 19 families (Fig. 16.1, Table 16.1), representing a significant portion of the worlds’ genetic
variation in mangrove plants, and most of Australia’s.
Some species, like Avicennia marina (Fig. 16.3), Rhizophora
stylosa (Fig. 16.4) and Bruguiera gymnorhiza (Fig. 16.5),
are widespread in the Indo-West Pacific region, while
others, like Aegiceras corniculatum (Fig. 16.6), Ceriops
australis, Bruguiera exaristata, and Diospyros littorea, are
more restricted to the Australasian region. None are
restricted solely to the GBR World Heritage Area. It is
of interest also that Avicennia marina var. eucalyptifolia
(Fig. 16.3) merges with the south-eastern Australian
variety, A. var. australasica towards its southern boundary, south of the Tropic of Capricorn.
The widespread group of species mentioned above
are also ubiquitous mangrove representatives commonly found throughout the GBRWHA. They are commonly located on many mainland islands, sand cays