Architectural Diversity and Ecosystem Engineering

The framework of modern reefs is generated by scleractinian corals, with a significant contribution from coralline algae and early diagenetic cements. Architecturally similar structures, at least at a gross scale, have been built by microbial communities, sponges and archaeocyathids, tabulate and rugose corals, stomatoporoids, bryozoans, brachiopods, and rudist bivalves. Reefs are a specific example of the provisioning of architectural diversity, which can provide a positive feedback on biodiversity. Such ecosystem engineering allows species to modify the environment in ways that can affect, either positively or negatively, resource availability for other species (Jones et al., 1997). A related concept is niche construction, in which species modify their own environment in a way that influences the fitness of the population and, through ecological inheritance, the fitness of subsequent generations (Odling-Smee et al., 2003). Although ecosystem engineering can be recognized in the fossil record, identifying niche construction requires an understanding of selection pressures that is generally more difficult for paleontologists. Both niche construction and ecosystem engineering are currently the subject of considerable investigation and appear to have significant implications for macroevolution (Erwin, 2008).

Reef ecosystems provide a clear example where the loss of the 3D complexity of the reef has a strong negative impact on biodiversity. Kiessling (2005) showed that over million-year periods high biodiversity on reefs is related to stability, as measured by the density of skeletal organisms, the style of reef building, and the types of biotic reefs. Some mass extinction events destroy this buffering from environmental fluctuations. The composition and consequent fabric of reefs has undergone considerable variation during the 543 Ma of the Phanerozoic (Wood, 1999; Kiessling, 2002). The structure of Early Cambrian to Early Ordovician reefs was

180 / Douglas H. Erwin dominantly microbial. From the Middle Ordovician radiation through the Late Devonian mass extinctions, stromatoporoids (coralline sponges) and corals were the primary reef builders, with important contributions from other sponges in the early part of the interval. Latest Devonian through Early Permian reefs are often described as ''mud mounds'' because of the absence of abundant framework builders in these primarily algal and microbial systems. In the Early to Middle Permian, between five and seven different reef types have been described with sponges, brachio-pods, corals, and bryozoans being prominent components of different types. Scleractinan corals become the major reef builders in the Late Tri-assic, with significant contributions from bivalves during some intervals. Indeed, post-Aptian Cretaceous reefs were built largely by rudist bivalves. Cenozoic reefs were constructed by scleractinian corals and coralline red algae. These gross patterns obscure Phanerozoic trends of changing ecology, including higher nutrient requirements toward the recent (Kiessling, 2002). An important issue for further exploration is the extent to which these different reef types were ecosystem builders that enhanced the diversity of other groups. For example, the phylloid algal mounds of the Lower Permian of West Texas apparently were so dense that they excluded many other organisms (Toomey, 1976), whereas later scleractinian reefs appear to have enhanced diversity. On land, trees and forests often provide a similar architectural structure to reefs in the ocean.

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