Case studies: community-level effects of habitat construction
Reef-building corals, and habitat deconstruction due to their decline
A dramatic example of individual nicheconstructing activity that gives rise to an entire ecological community is the classic case of reef-building corals. The main builders of undersea reefs are colonial cnidarians in the order Scleractinia, also known as "stony corals." These tiny, sessile animals secrete calcium carbonate to create huge three-dimensional structures that provide a porous, topographically complex habitat for astonishingly diverse marine communities. The coral polyps themselves comprise the habitat for symbiotic photosynthetic dino- flagellates called zooxanthellae, which inhabit cells of the animal's endoderm. This coral-phytoplankton symbiosis extends from shallow, well-illuminated waters to waters well over 100 m deep, where it is evidently limited more by cold temperatures than by low light (Muscatine et al. 1991; Moberg and Folke 1999 and references therein). Indeed, the system accommodates depth-related changes in irra- diance through phenotypic adjustments at several levels: both the form of host coral colonies and the tissue morphology and behavior of individual coral animals change in apparently adaptive ways when light availability is reduced (Muscatine 1990 and references therein), while their zooxanthellae inhabitants alter pigment content and other biochemical features related to light harvesting (e.g., Chang et al. 1983).
These one-celled photosynthetic symbionts are major primary producers of tropical reef communities: zooxanthellae use the plentiful carbon dioxide dissolved in seawater, as well as that produced by host coral respiration, to fix an estimated 0.5-5.0 kg of carbon per square meter per year (Muscatine 1990 and references therein). Only a small fraction of the fixed carbon is used for growth and respiration of the symbionts themselves. Instead, up to 97% of this abundant organic carbon is translocated to the animal host in the form of sugars, lipids, and other compounds that "subsidize" host growth and respiration (Muscatine 1990; Moberg and Folke 1999). Ultimately, approximately half of the photosynthetically fixed carbon is either incorporated as carbonate into the skeletal matrix of the reef or released into the water (Muscatine 1990; Moberg and Folke 1999). This symbiotically derived carbon is thus key to the corals' two habitat-constructing impacts: calcification that builds the physical reef structure, and primary productivity for the reef community.
The resulting coral reefs give rise to highly productive and diverse ecosystems. For instance, although this habitat covers less than 0.5% of the ocean floor, almost one-third of all marine fish species are believed to occur on coral reefs (Moberg and Folke 1999; see also Messmer et al. 2011). Along with a characteristic rainbow of associated fish and other vertebrates such as sea turtles and marine mammals that visit to graze and hunt, reef communities include a rich and diverse invertebrate fauna of sponges, echinoderms, mollusks, crustaceans, and polychaete worms. Primary producers in coral reefs include diatoms and other planktonic microalgae (in addition to the zooxanthellae described above), as well as green and red macroalgae. Among the latter are calcium carbonate-producing crustose coralline algae that also contribute to physical reef formation.
Like other complex ecological communities, this system self-organizes based on dynamic interactions among primary producers, herbivores, predators, competitors, and facilitators. Corals maintain their dominance or codominance in the reef community by virtue of several such interactions. For instance, the intense competition for space (and hence light) between corals and macroalgae is mediated at several trophic levels (Mumby et al. 2006; Powell et al. 2014; and references therein). By preferentially grazing on macroalgae, herbivores such as sea urchins, order Echinoidea, can prevent reef-building corals (with their photosynthetic symbionts) from being overgrown and outcompeted. As a result, these algal grazers help to maintain a coral-based community, as do predators of coral-eating animals (although some direct feeding by predators on coral is needed as a distribution vector for zooxanthellae; Moberg and Folke 1999). Conversely, predators that reduce herbivore abundance (e.g., fish that eat sea urchins), or invertebrates that release algae from grazing by providing an alternative food source for generalist fish (e.g., sponges), may promote the dominance of algae over corals (Gonzalez-Rivero et al. 2011). Marine reserves that aim to restore large predators must be managed so as to avoid overpredation on populations of grazers, if the coral reefs they encompass are to persist (Mumby et al. 2006). Outside of reserves, overfishing of herbivorous fish can have the same result, leading to local replacement of corals by algae (Anthony et al. 2011; Pratchett et al. 2011).
Along with grazing herbivores, the ecological dominance of corals is strongly influenced by sponges, phylum Porifera, which are ancient metazoan animals that comprise a major part of the reef fauna. Sponges compete with corals and other sessile animals for space and food resources, while themselves providing food for fish, turtles, and starfish (Powell et al. 2014). Sponges can be fierce competitors with corals, often displacing them through direct, antagonistic interactions. "Excavating" sponges such as Cliona delitrix (from the Latin delitor, meaning "obliterator") establish themselves in the reef by boring deeply into the calcium carbonate matrix and then killing adjacent coral tissues by releasing allelochemicals directly from sponge cells to living coral cells (Chaves-Fonnegra and Zea 2007; Gonzalez-Rivero et al. 2011).
Beyond supporting this ecologically rich marine biome, coral reefs play a role in building two associated types of ecological community. Because reefs physically buffer the coastline from storms, waves, and ocean currents, they create lagoons and sedimentary areas that (over geological time) facilitate the development of mangrove forests and seagrass beds (Moberg and Folke 1999). Reefs are functionally interwoven with these two adjoining habitats; all three interact to bind sediment and to exchange nutrients. These ecosystem processes are influenced in complex ways by individual feeding and migrating behavior (e.g., Acosta and Butler 1997). For instance, grazing sea urchins and herbivorous fish that move from reefs to seagrasses affect community dynamics at the seagrass beds (in part by weeding out large algae) but may migrate back to deposit their nitrogenous wastes at the reef. Migrant reef animals may also use the grass beds as protected breeding, spawning, or nursery grounds. These links between coral reefs and associated marine communities ultimately lead to the export of nutrients, organic materials, and plankton to surrounding waters (Moberg and Folke 1999).
Because of the far-reaching impacts of coral reefs in supporting marine biodiversity and ecosystem function, the well-documented decline and loss of these ecosystems comprise a particularly troubling aspect of contemporary environmental change (references in T. Hughes et al. 2010; Powell et al. 2014). That decline, along with those of seagrass and mangrove systems, exemplifies how the loss of major habitat-constructing organisms (e.g., scleractinian corals, marine grasses, and mangrove trees) can result in habitat degradation and loss of structural complexity, both of which lead to reduced community biodiversity (Messmer et al. 2011; Pratchett et al. 2014).
A number of natural and anthropogenic factors, including disease outbreaks, hurricanes and other severe storms, coastal development (with resulting fragmentation, eutrophication, and sedimentation), marine pollution, and mining of reefs for building materials, can contribute to the decline and loss of colonial reef-forming corals (Powell et al. 2014). Global climate change is a particular concern, because high water temperatures disrupt the symbiotic relationship between corals and their photosynthetic symbionts. Periodic increases in seawater temperature (e.g., of 1.0° C above normal local maxima) cause corals to expel their zooxan- thellae, a phenomenon known (for the loss of pigment color) as coral bleaching. Such "bleaching" has been increasing in frequency and geographic extent for the past three decades (Pratchett et al. 2011 and references therein; see Figure 6.5). The loss of the photosynthetic symbionts changes the corals' metabolic function and nutrition, leading to prolonged stress, reduced vigor, and eventually mortality; loss of the mutualism also changes the calcium flux in the reef system (Moberg and Folke 1999). Coral bleaching can also occur in response to heavy metal contamination or increased coastal runoff due to forest clear-cutting (Moberg and Folke 1999), a poignant reminder that environmental management decisions in inland cities or forests can disrupt cellular processes in marine organisms critical to entire marine communities.
Figure 6.5 Reef-building corals collectively produce structurally complex underwater habitats that support diverse communities. The coral polyps themselves provide habitat for unicellular photosynthetic symbionts, zooxanthellae, which are major primary producers. Increased seawater temperature, pollutants, and other stresses can cause corals to expel these pigmented symbionts, causing the coral to appear "bleached" and, eventually, to die. This photograph of a reef in Hawai'i shows its three-dimensional complexity; the dark-colored corals retain living symbionts, while the "bleached" coral (top right) has lost its zooxanthellae. Photograph by Raphael Ritson-William, courtesy of Ruth Gates. For the color image, see Plate 19.
Environmental changes can also destabilize the ecological interactions that allow reef-forming corals to remain dominant in reef communities and that consequently serve to help maintain the diversity of habitats corals create. As noted above, overfishing or loss of reef habitat can cause population declines in the herbivorous fishes that regulate macroalgal populations so as to insure suitable sites for the settlement of coral polyps; a reduced abundance of these fishes can accelerate the process of reef habitat degradation by rendering structural reef renewal more difficult (Pratchett et al. 2011, 2014). Increased ocean acidification due to higher atmospheric carbon dioxide levels is predicted to reduce rates of calcification in corals (Anthony et al. 2011), with consequences for corals' competitive success as well as their collective habitat-forming activities. Physical and biotic disruptions to reef ecology can interact to dramatically alter these diverse systems. One pristine, remote Caribbean reef was transformed over a period of just 40 months from a diverse community codominated by corals and macroalgae to a depauperate system dominated entirely by algae, because of a combination of three distinct events: mass coral bleaching (likely due
to stresses such as increased temperature and pollution), physical damage to the reef structure by a hurricane, and a pathogen outbreak that decimated populations of an herbivorous (algae-eating) sea urchin (Ostrander et al. 2000).
Disruptions to the reef habitat are accompanied by a reduced abundance and diversity of coral reef-associated fish species and invertebrates, with specific changes contingent upon each species' "resilience" to coral loss (Messmer et al. 2011 and references therein). For instance, reef fish that specialize on particular coral species for food, habitat, or both are likely to be steeply reduced in number, while generalist fish may increasingly come to dominate the altered reef community (Pratchett et al. 2014). The general outcome of these environmental stresses and associated trophic and competitive disruptions is a profound change in habitat-constructing activities, as corals are replaced as dominant ecological actors by either sponges or macroalgae, both of which give rise to less structurally and functionally complex habitats and less biologically diverse communities (Gonzalez-Rivero et al. 2011; Pratchett et al. 2011; Powell et al. 2014; additional references in T. Hughes et al. 2010).