term that can be used for at least 70 species of woody plants from 11 families that inhabit tropical intertidal environments (Wang et al. 2003). Depending on the range of salinities you choose to include, many more species could be added. Of course, on a global scale 70 species is not very many and in any given mangrove swamp only one or a few species of mangrove may occur.
Figure 4.12 Mangroves are marine wetlands that occur along many tropical coastlines.
(William Warby/Flickr/CC BY 2.0)
Despite modest levels of plant species diversity, mangrove swamps are important and interesting ecosystems (Kathiresan and Bingham 2001 ; Nagelkerken et al. 2008; Feller et al. 2010). First, they are extremely productive, capturing sunlight and collecting nutrients imported by the tides, and exporting huge amounts of organic matter to the adjacent aquatic ecosystems where they support aquatic food webs and economically valuable fisheries. For some fish species, mangrove swamps are often called nurseries because they provide food and cover for young individuals (Paillon et al. 2014). They also provide a sort of cover for shoreline human communities by creating a buffer against ocean storms and tsunamis. Conversely, they buffer coral‐reef and sea‐grass ecosystems from siltation stemming from inland erosion. Mangrove swamps also provide resources – timber and fuelwood – that sometimes, unfortunately, lead to their overexploitation. Worse than the threat of excessive timber harvesting is the wholesale destruction of mangrove swamps to make room for aquaculture (notably for shrimp production ponds), agriculture, and coastal development (ranging from garbage dumps to high‐rise hotels). Because they occupy a narrow band between the land and the sea, mangrove swamps have never occupied a large total area, and this makes it doubly tragic that so many have been lost. Worldwide, a substantial portion of mangroves have been destroyed, probably leaving far less than 200,000 km2 (Sandilyan and Kathiresan 2012 ; Webb and Friess 2014). This seems particularly short‐sighted given economic studies like one from India that estimated local income of $753/ha/year from natural products tied to mangroves versus $438 from mangroves converted to rice paddies (Chauhan et al. 2017). The mangroves’ value soared to over $5000/ha/year if you add more speculative values such as protection from storms and carbon sequestration.
Summary
The conceptual definition of an ecosystem is straightforward – a group of interacting organisms and their physical environment – but deciding where one ecosystem ends and another begins can be difficult. Evaluating the differences and similarities among many ecosystems and classifying them into different ecosystem types is even more challenging. Despite these difficulties, recognizing and classifying ecosystems are useful exercises for organizing our understanding of the patterns of ecological interactions.
The value of an ecosystem, at a minimum, consists of the sum of all the values of the species that occupy the ecosystem. Beyond this, the instrumental values of ecosystems are primarily based on services: for example, exporting clean water and other economically valuable functions, providing complex models for research and education, and serving as sites for spiritual renewal. Ecosystems have a critical strategic role in conservation because protecting a representative array of ecosystems will protect biodiversity at the species and genetic level to a significant extent. The idea of ecosystems having intrinsic value revolves around an unresolved controversy: to what extent are ecosystems loosely organized collections of species versus highly integrated systems of coevolved species? The closer they are to being highly integrated, the more likely it is that loss of species could lead to ecosystem degradation. In general, ecosystems with high species diversity are likely to be less subject to degradation (more stable), but this is a complex subject. Many factors influence the relative species diversity of different types of ecosystems such as patterns of energy, water, heterogeneity, size, and more. Maintaining ecosystem diversity also requires maintaining the spatial arrangements in which ecosystems occur; in other words, natural landscapes require protection.
FURTHER READING
A 29‐volume series, Ecosystems of the World, published by Elsevier of Amsterdam, is the single most comprehensive treatment available. The Millennium Ecosystem Assessment (millenniumassessment.org) reported on the status of the world’s ecosystems and the services they provide, as well as the basis for conserving them. See cices.eu for a classification of ecosystem services developed by the European Environment Agency. Turner and Gardiner (2015) provide a comprehensive perspective on ecosystems as elements in landscapes, and Groves and Game (2016) cover conservation planning, particularly at that scale. For web‐based information about ecological communities in the western hemisphere, including a classification system, see www.natureserve.org. You can use https://www.worldwildlife.org/biomes to learn more about hundreds of ecoregions, organized by 26 major types across seven biogeographic realms.
TOPICS FOR DISCUSSION
1 In the area where you live, develop a list of 10 types of ecosystems (natural and human‐created). Which types of ecosystems were easiest to define? Which were hardest? Why?
2 Draw a map of the ecological region you inhabit. How did you distinguish it from surrounding regions?
3 What is the rarest type of ecosystem in your region? Have many examples of it been protected?
4 What services are provided by the major types of ecosystems in your region?
5 In one of the major ecosystems near where you live, which “rivets,” if popped out, would be most likely to cause the “plane to crash” (Ehrlich analogy)?
6 What evidence can you cite that supports the idea that ecosystems are just loose collections of species? What evidence refutes the idea? If you do not specifically know of such evidence, how would you design a research program to obtain it?
7 In your region, what type of ecosystem probably has the highest species richness? What type of ecosystem has the greatest ecological value (with its inevitable corollary, how are you defining ecological value in this context)?
CHAPTER 5 Genetic Diversity
The process by which sequences of four simple chemicals – adenine, thymine, cytosine, and guanine – shape a molecule of DNA and, ultimately, all the organisms that comprise the Earth’s biota, is an extraordinary story. It is a story about the foundations of biological diversity because the entire tree of life is rooted in genetic diversity. It can be a rather complex story, and if your recollection of Hardy–Weinberg equilibrium, phenotypes versus genotypes, alleles, diploidy, and so on has rusted a bit, you might find it helpful to review the genetics and evolution sections of an introductory biology textbook before proceeding.
What Is Genetic Diversity?
Charles Darwin articulated long ago in On the Origin of Species the expression of genetic diversity into all forms of biological diversity. Here is his moving description of the “tree of life”:
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever‐branching and beautiful ramifications.
(Darwin
