The Gray Zone: Relationships between habitat loss and marine diversity and their applications in conservation

Structurally complex habitats are becoming rarer across temperate marine environments: indeed the coastal and marine world is getting flatter. In some cases marine habitats are lost entirely (e.g., wetlands are filled), but in many cases the loss is a gradual transition from a more complex to a less complex habitat (i.e., a change from canopy-forming to turf forming algae). We explore the multiple ways habitat loss affects marine species diversity, and propose a conceptual model that identifies the main interactions and feedbacks between these processes. The loss of habitat structure generally leads to lower abundances (biomasses) and often to declines in species richness. There is often also a suite of colonizing species that prosper from these transitions. These sets of expanding species can amplify the changes to the system, cause variable effects on species richness and other components of diversity, feed back to affect the various components of habitat loss (e.g. maintain new environmental conditions) and prevent the recovery of the system. Less well studied are the effects on between-habitat (beta) diversity and functional diversity. We argue that we need to understand these latter changes to better manage and conserve the structure and function of ecosystems and the diverse services that humans continue to expect from them. Calling for more of the approaches and thinking that John Gray championed we discuss how this work can focus efforts in research, conservation, restoration and management. (C) 2008 Elsevier B.V. All rights reserved.