The Biology of Mangroves and Seagrasses

300 Pages·2015·12.707 MB·English

by  Peter J. Hogarth

The Biology of Mangroves
and Seagrasses
THE BIOLOGY OF HABITATS SERIES
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physiology, and ecology of the biota in a given habitat, set in the context of the physical environ-
ment. Each book describes practical aspects of working within the habitat, detailing the sorts
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is intended for naturalists, students studying biological or environmental science, those begin-
ning independent research, and professional biologists embarking on research in a new habitat.
The Biology of Streams and Rivers
Paul S. Giller and Björn Malmqvist
The Biology of Soft Shores and Estuaries
Colin Little
The Biology of the Deep Ocean
Peter Herring
The Biology of Lakes and Ponds, 2nd Edition
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Arnold G. van der Valk
The Biology of Peatlands, 2nd Edition
Håkan Rydin and John K. Jeglum
The Biology of African Savannahs, 2nd Edition
Bryan Shorrocks and William Bates
The Biology of Mangroves and Seagrasses, 3rd Edition
Peter J. Hogarth
The Biology
of Mangroves
and Seagrasses
THIRD EDITION
Peter J. Hogarth
Department of Biology, University of York, York, UK
1
The Biology of Mangroves and Seagrasses. Third Edition. Peter J. Hogarth
© Peter J. Hogarth 2015. Published 2015 by Oxford University Press.
1
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First Edition published in 1999
Second Edition published in 2007
Third Edition published in 2015
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Preface
Flowering plants dominate the land, providing nutrition, shelter, and sta-
bility for a host of organisms, and the basis of all terrestrial ecosystems. Of
the hundreds of thousands of species of flowering plants, a mere 100 or so
survive in the sea, about equally divided between mangroves and seagrasses.
Although not rich in species, both mangroves and seagrasses are, like their
terrestrial counterparts, of major ecological importance.
To most people, mangroves call up a picture of a dank and fetid swamp, of
strange-shaped trees growing in foul-smelling mud, inhabited mainly by
mosquitoes and snakes. Mud, methane, and mosquitoes are certainly fea-
tures of mangrove forests—as, sometimes, are snakes. They are not sufficient
to deter mangrove biologists from investigating an ecosystem of great rich-
ness and fascination.
Mangroves are an assortment of tropical and subtropical trees and shrubs
which have adapted to the inhospitable zone between sea and land: the typi-
cal mangrove habitat is a muddy river estuary. Salt water makes it impos-
sible for other terrestrial plants to thrive here, while the fresh water and the
soft substrate are unsuitable for macroalgae, the dominant plants of hard-
bottomed marine habitats. The mangrove trees themselves trap sediment
brought in by river and tide, and help to consolidate the mud in which they
grow. They provide a substrate on which oysters and barnacles can settle, a
habitat for insects, and nesting sites for birds. Most of all, through photo-
synthesis, they supply an energy source for an entire ecosystem comprising
many species of organism. Mangroves are among the most productive and
biologically diverse ecosystems in the world.
Seagrasses, although not true grasses, generally grow in a grass-like way, often
locally dominating their environment in what are known as seagrass mead-
ows. They grow intertidally, like mangroves, but also subtidally to depths of
tens of metres. Like mangroves, too, seagrasses have adapted to conditions of
high salinity and living in soft sediments. They create a habitat, and represent
a food source on which many other organisms depend.
With both mangroves and seagrasses I discuss the adaptations to their chal-
lenging environment, and the communities of organisms that flourish in
and around mangrove forests and seagrass meadows, before turning to more
general questions of evolution, biogeography, and biodiversity.
vi PREfacE
Mangroves and seagrasses are of considerable economic significance. Apart
from the direct collection of mangrove products, many commercially har-
vested species of fish, shrimp, and crab are sustained by mangroves and sea-
grasses, while both mangroves and seagrasses reduce coastal erosion and
protect coastlines against wind and wave action. Unfortunately, the impor-
tance of mangroves and seagrasses is not always appreciated, and recent years
have seen massive degradation and destruction of both habitats, sometimes
deliberate, and in other cases inadvertent. Mangroves and seagrasses are vul-
nerable to climate change—but also, potentially, mitigate its adverse effects.
Conservation, restoration, and sustainable management of these important
resources are therefore essential. The impact of the continuing loss of man-
groves and seagrasses seems almost too obvious to need pointing out. Cas-
sandra was fated to predict the future and to have her predictions ignored;
biologists sometimes feel they have a similar role.
The productivity and diversity of these remarkable habitats therefore makes
them of great interest to biologists and of considerable social and economic
value, while degradation and destruction by human activities makes it more
than ever essential to understand their significance. Research has advanced
considerably in recent years, and the time seems right for an attempt to pres-
ent our current understanding of the mangrove and seagrass ecology.
My aim in writing this book is two-fold: to share my own enthusiasm for
these remarkable ecosystems, and to explain how our understanding is
unfolding. Any author depends on the work of others, and I am grateful to
numerous colleagues for their help in various ways. In particular, I should
like to thank Larry Abele, Liz Ashton, Patricia Berjak, Mike Gee, Rony Huys,
Ong Jin Eong, Daphne Osborne, Mohammed Tahir Qureshi, and Di Walker
for their help with this and previous editions. Any errors that remain are, of
course, entirely my own.
Writing books has its pleasures, particularly learning about areas of the
subject with which one was previously not sufficiently familiar. It also has
its disadvantages, and most authors would at some stage agree with the
heartfelt—a nd, in this context, singularly apposite—words of the great Amer-
ican naturalist John James Audubon: ‘God . . . save you the trouble of ever pub-
lishing books on natural science . . . I would rather go without a shirt . . . through
the whole of the Florida swamps in mosquito time than labor as I have . . . with
the pen.’1 For sustaining me throughout the labours with the pen (and for join-
ing me in the Malaysian swamps in mosquito time) I should especially like
to express my gratitude to Sylvia Hogarth, to whom this book is dedicated.
P.J.H.
October 2014
1 Letter to J. Bachman, 1834, quoted by Alice Ford (1957): The Bird Biographies of John James
Audubon (Macmillan, N.Y.), pp. vii–viii.
Contents
1 Mangroves and Seagrasses 1
1.1 Mangroves 2
1.2 Seagrasses 4
2 Mangroves and Their Environment 8
2.1 Adaptations to waterlogged soil 8
2.2 Coping with salt 17
2.3 The cost of survival 23
2.4 Inorganic nutrients 26
2.4.1 Nitrogen 27
2.4.2 Phosphorus 29
2.4.3 Nutrient recycling 29
2.4.4 Are mangroves nutrient limited? 30
2.5 Reproductive adaptations 32
2.5.1 Pollination 32
2.5.2 Vivipary 33
2.5.3 Fecundity and parental investment 36
2.5.4 Dispersal and settlement 37
2.6 Why are mangroves tropical? 41
3 Seagrasses and Their Environment 44
3.1 Growth and structure 44
3.2 Waves, currents, and sediment 46
3.3 Photosynthesis and respiration 47
3.4 Salinity 48
3.5 Nutrients 50
3.6 Reproduction 51
3.7 Propagule dispersal 53
4 Community Structure and Dynamics 55
4.1 Mangroves: form of the forest 55
viii cONTENTs
4.1.1 Species zonation 57
4.1.1.1 Propagule sorting 60
4.1.1.2 Physical gradients 60
4.1.1.3 Plant succession and species interactions 63
4.1.1.4 Geomorphological change 66
4.1.2 How different are mangroves from other forests? 67
4.1.3 Do mangroves create land? Mangroves as ecosystem
engineers 70
4.2 Seagrass meadows 74
5 The Mangrove Community: Terrestrial
Components 80
5.1 Mangrove-associated plants 80
5.2 Animals from the land 81
5.2.1 Insects 82
5.2.1.1 Insect herbivores 82
5.2.1.2 Termites 86
5.2.1.3 Ants 87
5.2.1.4 Mosquitoes and other biting insects 89
5.2.1.5 Synchronously flashing fireflies 90
5.2.1.6 Other insects 91
5.2.2 Spiders 92
5.2.3 Vertebrates 92
5.2.3.1 Amphibians 93
5.2.3.2 Reptiles 94
5.2.3.3 Birds 98
5.2.3.4 Mammals 102
6 The Mangrove Community: Marine Components 107
6.1 Algae 107
6.2 Fauna of mangrove roots 108
6.3 Invertebrates 110
6.3.1 Crustaceans 110
6.3.1.1 Crabs 111
Leaf eating by crabs 112
Are crabs selective feeders? 114
Seedlings 117
Tree-climbing crabs 118
How important are herbivorous crabs? 118
Fiddler crabs 119
The physiology of living in mud 122
6.3.1.2 Other mangrove crustacea 126
6.3.1.3 Crustaceans as ecosystem engineers 127
cONTENTs ix
6.3.2 Molluscs 129
6.3.2.1 Snails 129
6.3.2.2 Bivalves 132
6.4 Meiofauna 132
6.5 Fish 135
7 Seagrass Communities 139
7.1 Epiphytes 139
7.2 Molluscs 141
7.3 Crustaceans 142
7.4 Echinoderms 143
7.5 Fish 144
7.6 Turtles 146
7.7 Marine mammals: dugongs, manatees, and sea otters 147
7.8 Birds 149
8 Measuring and Modelling 151
8.1 Mangroves 151
8.1.1 How to measure a tree 152
8.1.2 Biomass 152
8.1.3 Estimating production 155
8.1.4 What happens to mangrove production? 158
8.1.4.1 Microbial breakdown 159
8.1.4.2 Crabs and snails 161
8.1.4.3 Wood 162
8.1.4.4 The role of sediment bacteria 163
8.1.4.5 The fate of organic particles 164
8.1.4.6 Predators 165
8.1.5 Putting the model together 166
8.2 Seagrasses 168
9 Comparisons and Connections 170
9.1 How distinctive are mangrove and seagrass
communities? 170
9.2 Mangroves and salt marshes 171
9.3 Interactions 172
9.4 Outwelling 173
9.5 The fate of mangrove carbon 175
9.6 Mangroves, seagrasses, and coral reefs 178
9.7 Movement between habitats 179
9.7.1 Larval dispersal and return 179
9.7.2 Commuters 180