PhD Studentships: Reef Structural Complexity and the Dwindling Habitat for Diverse Caribbean Fish Communities

Charlie Dryden

Newcastle University

Supervisor(s): Nick Polunin & Steve Newman

Scleractinian corals are ‘ecosystem engineers’, providing most of the foundations of the coral reef ecosystem, specifically creating a three-dimensional physical habitat and micro-climatic conditions for a plethora of species and ecosystem services. Corals act as a refuge from predators, provide habitat surfaces for prey and offer nesting sites for brooding species. Threats to the existence of coral reefs such as climate-related bleaching, diseases, nutrient susceptibility and fishing-related impacts, have created an urgent need to more fully understand the role corals, and the habitat they create, play in supporting the diverse and abundant coral reef communities.

Early work on coral reef degradation focussed largely on phase shifts from coral to algal dominated states and effects on community structure. This rather ignored the role of the physical structure of the reef sustained by corals. Structural complexity has been correlated with higher levels of diversity in both terrestrial and marine habitats, including coral reefs. However, these findings relied upon simplistic habitat measures and broad community metrics. It is necessary for this relationship to be examined in greater detail to identify which aspects of structural complexity are important to specific components of the community. Related to this, there is scarcely anything known about how mobile species interact with the reef framework. Yet the utilisation of space on reefs by fish is key to predicting how degradation will affect the ecosystem and the humans that rely on them. Such understanding will offer insight into how species, trophic groups and size classes react to loss of habitat structure.

The Caribbean has been undergoing continued losses of structurally complex Acropora spp. and Montastraea spp. of coral since the late 1970s. Stress-tolerant corals that form smaller and less complex colonies, such as Porites spp. and Agaricia spp. have now become relatively more abundant and the consequences of this shift are scarcely known. While the coral cover of Caribbean reefs has been declining for 40 years, changes in fish community structure were negligible until 10 years ago. These changes in the Caribbean fish communities are thus unlikely to be exclusively linked to live coral-cover loss. Unlike their Indo-Pacific counterparts, no Caribbean reef fish are obligately dependent on living corals for food or refuge, therefore decline in reef fish communities appears to more closely relate to generic effects of the loss of reef structure.

Temporal trends in Caribbean reef complexity and community structure have been explored through sparse existing data however, there is at present no methodologically-constrained information on spatial trends in Caribbean regional complexity, yet this is crucial for understanding the current status of reefs, the extent of ongoing changes, and the implications for environmental managers.

This study will examine the concept of structural complexity in natural systems and detail the spatial patterns of reef complexity across 10 Caribbean countries. It will then focus on the relationship between complexity and the fish community and the behavioural interactions between fish and the reef structure.

Marine Science and Technology
Newcastle University NE1 7RU