Sequence of Marine Fouling Organisms Found in Succession on Submerged Hard Surfaces
Slime forming organisms Bacteria, diatoms, microalgae, protozoa
Primary fouling organisms Barnacles, hydroids, serpulids, polyzoa
Secondary fouling organisms Mussels, ascidians, sponges, anemones
Adventitious organisms Polynoids, sabeliids, cirratulids, nudibranchs, ostracods, amphipods
Source: Adapted from Crisp, D. J. 1965. Ecology and the Industrial Society. John Wiley & Sons, New York.
Though the intended focus of artificial reefs is the fishes that are attracted to them, they are constructed ecosystems. Aquatic organisms colonize the surface of the artificial reefs. These organisms attach to the surfaces with various adaptations, and are sometimes referred to as "fouling" organisms. "Fouling" is a successional process in which attached organisms colonize a submerged hard surface (Crisp, 1965). The name itself is anthropocentric; the verb "to foul" has negative connotations because the organisms that attach to certain human-produced surfaces, such as pipe outfalls or ship bottoms, can cause significant problems. Table 5.7 provides a sequential listing of typical marine fouling organisms that might colonize an artificial reef in temperate marine waters. Colonization is by natural dispersal of life stages carried by currents, and artificial seeding is practically never necessary. Principles of island biogeography (see Chapter 4 and the discussion earlier in this chapter) have been useful in understanding the development of artificial reef communities because of the role of natural colonization and the insular qualities of reefs themselves (Bohnsack et al., 1991; Molles, 1978; Walsh, 1985). Fishes are attracted to artificial reefs because they provide food, shelter from predators, and sites for orientation and reproduction, i.e., habitat (Bohnsack, 1991). The use of artificial substrates for the scientific monitoring of benthic ecosystems (Cairns, 1982) is related to the topic of artificial reefs because both kinds of structures have similar design considerations. In particular, the materials used for both artificial reefs and scientific monitoring substrates must be similar to natural materials so that attachment by organisms is not inhibited.
The leaders in the use of artificial reefs have been the Americans and the Japanese, but they have taken very different pathways (Stone et al., 1991). In Japan artificial reefs are a highly developed technology that supports commercial fishing (Grove et al., 1994; Mottet, 1985; Yamane, 1989). Records of Japanese artificial reefs date to the 1600s when rock formations were constructed as reefs in shallow waters along the coast. In the present day tens of millions of dollars are spent annually in government supported reef programs on a national scale. Japanese artificial reefs are characterized by sophisticated prefabricated designs. For example, Grove and Sonu (1985) describe 68 different kinds of reef structures and report that more than 100 are in use. Knowledge of fish ecology, life history patterns, and behavior is well developed. In some cases, reefs are designed, constructed, and sited to support particular species, based on this well-developed knowledge base (Nakamura, 1985).
Artificial reef use in the United States differs markedly from the Japanese approach. In the U.S. most artificial reefs are constructed for recreational fishing. They are smaller scale projects supported by local governments or private interest groups such as fishing clubs. Scrap materials are often utilized in designs which are sometimes quite ingenious but still unsophisticated compared with the Japanese models. Artificial reefs were first employed in the U.S. in the 1800s, but usage increased greatly after World War II (Stone, 1985). There are many more freshwater examples in the U.S. than in Japan. Methods for these systems were described as early as Hubbs and Eschmeyer's (1938) important work on fish management in lakes. An interesting development in the U.S. is the use of artificial reefs for mitigation of habitat damage (Foster et al., 1994), as was described earlier in this chapter in relation to wetland restoration. This usage emerged as studies have demonstrated the development of comparable ecosystem structure and function between artificial and natural reef systems.
As an aside, restoration of natural reefs is also an important topic in the U.S. In particular, efforts are under way to restore oyster reefs in many coastal areas such as Chesapeake Bay (Leffler, undated). Oyster populations collapsed in the late 1800s and early 1900s due to cumulative impacts including overfishing, disease, and water quality decline. All of these impacts must be dealt with before full recovery is possible, but restoration efforts are being initiated. Techniques for growing oyster reefs are similar to those used for artificial reefs and they have long been known (Brooks 1891). In areas with sufficient current velocities to carry their food source (particulate organic matter), oysters will attach to hard surfaces and grow into self-sustaining reef structures. Old oyster shells are often used as substrate to start new reefs, mimicking the positive feedback that took place on natural oyster reefs. An interesting example of coral reef restoration is the work of Todd Barber of Reef Balls, Inc. (Menduno, 1998). He has developed his own design for artificial substrates which are made of concrete (Figure 5.15). These are called reef balls and they are being used around the world in restoration projects. The hydrodynamic shape of the reef balls facilitates colonization by pelagic larvae of fouling organisms, including corals. General aspects of coral reef restoration with artificial substrates are described by Spieler et al. (2001).
Unlike many other examples of restoration ecology, creation of artificial reef systems requires a significant amount of conventional engineering, including aspects of materials and structural stability along with siting criteria, which is perhaps more closely related to ecological engineering (Sheehy and Vik, 1992). A variety of materials have been used to construct artificial reefs including natural materials (such as brush, quarry rock, and logs), manufactured products (such as poured concrete, fiberglass, and plastic) and scrap (automobile tires and bodies, rubble from construction sites, and scuttled vessels). Considerations in choice of materials include availability, cost, durability, and complexity of surfaces. Because reef materials are submerged and exposed to a number of destructive processes, durability is a critical quality that often determines the life expectancy of the reef structure. Most conventional engineering knowledge used in artificial reef design involves analyses of
stability. For example, Mottet (1985) applied Hudson's formula (see Chapter 3) to evaluate reef stability in relation to wave energy and offered suggestions for similar stability equations in relation to current velocity. Other examples such as calculation of reef block strength are given by Grove et al. (1991) and Sheng (2000).
Siting is a particularly important step in artificial reef development that involves a number of considerations. This requires a knowledge of the energy signature of the site including substrate type, bottom topography, relations to adjacent reefs, and especially current and wave energy. The reef must be exposed to appropriate levels of current energy to advect fouling organism life stages to the reef for colonization, to advect food for fouling organisms, and to attract fishes. There are also features which must be avoided such as interference with navigation, areas used for commercial fishing with nets which might snag on the reef, and sites with very strong tidal currents. Overall, the ideal site would be one with a depth of 30 to 40 m in order to attract large benthic fish species and only a few kilometers offshore in order to facilitate access by fishermen.
Scrap tires are used to construct one of the most common kinds of artificial reef in the U.S. In this type of reef, tires are combined together in various ways to create complex structures that support fouling communities and attract fishes (Figure 5.16). As noted by Candle (1985), "The same tire qualities that are advantageous to motorists, strength, durability and long life are the keys to the advantage of tires as reef-building materials." They are also plentiful, cheap, and easy to handle, process,
and transport to the reef site. However, in order to use scrap tires, they must be purged of air. This is usually accomplished by punching holes in them or splitting them in half. Ballast is also necessary to add stability against wave surge or bottom currents. Tire reefs have been shown to provide effective substrates for aquatic ecosystems (Campos and Gamboa, 1989; Reimers and Brandon, 1994) in both marine and freshwaters. Use of scrap tires for artificial reefs is a good ecological engineering example of turning a waste by-product into a valuable product. Hundreds of millions of scrap tires are produced annually worldwide, creating a disposal problem. This problem is turned into an advantage when tires are used as reefs. Although on a net basis artificial reefs made of scrap tires do require input of money for labor, ballast material, and ship time required in reef placement, a savings is integrated into the project in terms of the disposal fee for landfilling that would otherwise be required. Hushak et al. (1999) provide an analysis of one artificial reef that documents a net surplus income for the overall system, including the local economy.
Exhibit ecosystems are those designed, built, and operated primarily as exhibits for educational purposes. The best examples may be large public aquaria and botanical gardens that represent specific ecosystem types. Exhibit ecosystems require human maintenance but range across a gradient of relative contributions from humans vs.
natural self-sustainability. Although these systems are more or less artificial, they have special value for teaching aspects of ecology to students and to the general public. Significant design ingenuity is often required to help make them appear natural, which is necessary for an optimal education experience. Several examples of exhibit ecosystems are described below.
Perhaps the most complex ecosystem on the biosphere is the coral reef. These tropical ecosystems occur in shallow, clean, high-energy waters and have high biodiversity. Two basic approaches have been employed to create exhibits of coral reefs in public aquaria. On the one hand Walter Adey has developed a holistic ecological design method that emphasizes mimicking the energy signature of aquatic ecosystems as a form of modelling. He and his co-workers at the Smithsonian Institution have developed coral reef systems that have been displayed in a number of settings (Luckett et al., 1996). His systems represent a major design advancement because they support representative samples of the high diversity of a coral reef in a sustainable fashion. His first major coral reef exhibit was displayed at the National Museum of Natural History in Washington, DC, starting in 1980 (Miller, 1980; Walton, 1980). This was a 13,000 l (3,430 gal) tank system with more than 200 tropical marine species. One of the most important aspects of Adey's designs is the simple algal turf scrubber system attached to the coral reef aquaria which provides water filtration and oxygenation needed to support the biota (see also Chapter 2). Adey has continued to develop his design approach and the principles are described in his text entitled Dynamic Aquaria (Adey and Loveland, 1998). The largest coral reef models developed with this approach are the 2.5 million l (0.7 million gal) Great Barrier Reef Aquarium in Townsville, Australia, and the 3.4 million liter (0.9 million gal) ocean tank at Biosphere 2 near Tucson, AZ.
At the other extreme are typical coral reef exhibits such as at the National Aquarium in Baltimore, MD (Figure 5.17). This system contains a live fish community characteristic of a coral reef, but it is completely artificial otherwise. Thus, a complex filter system is employed with physical-chemical-biological components to maintain clean water, and fishes are supported by artificial feeding. Most remarkably, the tank is lined with nonliving, plastic corals that provide a quite realistic appearance but no feedback to the reef system. The result is an energy intensive, highly designed, artificial ecosystem which serves the purpose of providing an educational setting for aquarium visitors to learn about coral reefs, but it is mostly nonliving. While both of these extremes are equally valid approaches to the development of exhibit coral reefs, clearly Adey's method involves much more ecological engineering design.
The artificial approach also has been taken in developing tropical rain forest exhibits across the U.S. and in other countries. Rain forests are as complex as coral reefs and, thus, represent similar challenges in terms of exhibit ecosystem design. Most examples are highly artificial, often with plastic plants and rocks along with a few living species. They are, however, interesting systems that attract a great deal of attention from the general public (see, for example, the description of the National Zoo's Amazonia exhibit by Park, 1993). An interesting study would be to survey many of these exhibit rain forests and compare living vs. nonliving components. How much actual ecology is involved in these ecosystems? A similar survey could be made for engineering aspects, which would probably reveal some interesting features that are unique to exhibit ecosystems relative to other ecologically engineered systems.
Another example of these artificial systems is the case of environmental enrichment of zoo exhibits (Ben-Ari, 2001; Markowitz, 1982; Shepherdson et al., 1998). This situation was defined by Shepherdson (1998) as follows:
Environmental enrichment is an animal husbandry principle that seeks to enhance the quality of captive animal care by identifying and providing the environmental stimuli necessary for optimal psychological and physiological well-being. In practice, this covers a multitude of innovative, imaginative, and ingenious techniques, devices, and practices aimed at keeping captive animals occupied, increasing the range and diversity of behavioral opportunities, and providing more stimulating and responsive environments ... On a larger scale, environmental enrichment includes the renovation of an old and sterile concrete exhibit to provide a greater variety of natural substrates and vegetation, or the design of a new exhibit that maximizes behavioral opportunities. The training of animals can also be viewed as an enrichment activity because it engages the animals on a cognative level, allows positive interaction with caretakers, and facilitates routine husbandry activities. Indeed, with correct knowledge, resources, and imagination, caretakers can enrich almost any part of the environment that the captive animal can perceive.
Environmental enrichment attempts to increase the amount of stimulation and complexity of the environment, to reduce stressful stimuli, and to provide for species-appropriate behaviors in captive animals. It is an interesting topic that has engineer ing dimensions (Forthman-Quick, 1984), but is focused primarily at the species level rather than the ecosystem, unlike most of ecological engineering.
At a much larger scale are the restored tall grass prairies of the midwestern U.S. Although it is somehow unfair to call these systems exhibits since they range in size from less than one to hundreds of hectares, the restored prairies are still a small part of the landscape and their primary function is in education. They are not artificial in the same way as exhibit rain forests but they require controlled burns by humans for their maintenance. Most restored prairies are park-like with interpretative trails and associated displays.
In the pre-Colombian vegetation of the U.S., the tall grass prairie (5 to 8 ft or 1.5 to 2.4 m in height) bordered the temperate forests to the east. It occupied a zone stretching from Illinois and Minnesota in the north to Texas in the south. In this zone a dynamic relationship occurred between forests and grasslands mediated by shade competition which favored trees and fire resistance which favored grasses and forbs. To the west, zones of midgrass (2 to 4 ft or 0.6 to 1.2 m in height) and short grass prairie (0.5 to 1.5 ft or 0.2 to 0.5 m in height) extended across the Great Plains to the Rocky Mountains, completing the vast grassland biome or biotic region. All of these natural grasslands were eventually replaced by crop agriculture and rangeland as human development proceeded through the 1800s, leaving only scattered prairie remnants in small plots of land such as along railroad and highway rights-of-way and in unmaintained cemetaries. A movement to restore prairies began slowly in the 1930s and continues to the present time throughout the grassland biome. The prairie remnants were the seed sources for these original restorations but now nurseries have taken over this role. The oldest and best-known restored prairies are in the tall grass region, especially in southern Wisconsin and in northern Illinois. The first prairie restoration occurred at the University of Wisconsin Arboretum in Madison, WI, and was conducted by the famous conservationist Aldo Leopold, starting in the 1930s (Meine, 1999). This prairie was subsequently named after the Wisconsin plant ecologist John T. Curtis who applied a scientific approach to developing restoration techniques there. In fact, Curtis seems to have been able to develop the first scientific evidence for the importance of fire in maintaining prairie ecosystems through his research on restoration methods (Curtis and Partch, 1948). The Curtis Prairie is a lowland system with deep organic soils and a diversity of over 300 native prairie plant species (Cottam, 1987). The Greene Prairie, which is an upland system, was later added to the Wisconsin Arboretum. Restoration of this prairie was carried out by H. C. Greene, starting in the 1940s with collaboration from Curtis (Greene and Curtis, 1953). Long-term studies of both of these prairies have been made by several academic generations of Wisconsin ecologists and these studies have provided a simple, reliable technology for restoration. The basic procedure is to (1) clear and plow the soil of the site which is to be restored, (2) plant a mix of grass and forb seed, and (3) keep the area free of woody and non-native weeds with periodic, controlled burns. This is, of course, a rather simple procedure, but it requires attention to scheduling of planting and burning, and to matching seed mixes to soil types. Of particular interest is the need for fire, which represents a
disturbance input in the restoration's energy signature. Figure 5.18 is an overview model of a prairie ecosystem. Fire is depicted with a consumer group symbol since it actually consumes biomass, similar to a herbivore. The storage of fire is composed of the concentration of high temperatures from combustion, which exists only for a short time period. Fire was initiated in the natural prairie by lightning, but controlled burns by humans are a form of technology in which fire is used as a tool. Controlled burns are usually implemented in the spring or fall to clear away dead vegetation and to kill plant species lacking fire adaptation. Native prairie species survive fires by having living portions below ground whose growth can actually be stimulated by burning, though details of fire adaptation are still not completely worked out (Anderson, 1982). Dominant grass species in most tall grass prairie restorations are little bluestem (Schizachyrium scoparius), big bluestem (Andropogon gerardi), switch grass (Panicum virgatum), and indiangrass (Sorghastrum nutans), along with a variety of non-grass, forb species such as asters (Aster sp.) and sunflowers (Helian-thus sp.). Other historically important tall grass prairie restorations are the Schulenberg prairie at the Morton Arboretum (Schulenberg, 1969) and the Fermi Laboratory prairies which even have a small herd of buffalo (Thomsen, 1982). Both of these restorations are located in the Chicago region of northern Illinois. A popular account of tall grass prairie restoration is given by Berger (1985) in his Chapter 8, and technical references are given by Kurtz (2001), Packard and Mutel (1997b), and Shirley (1994).
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