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 (β) 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.