Habitat structural complexity of Caribbean coral reefs and its relationships with fish community structure

Abstract

Habitat structural complexity (HSC) is a key component of natural ecosystems. It helps to describe the morphological characteristics of the three-dimensional space that floral and faunal communities exist within. The physical structure can have a profound influence on the associated diversity and functioning of the ecosystem. This thesis is a macroecological study of Caribbean coral reef HSC and how changes in it might influence associated fish communities. An assessment of contemporary reef HSC in 15 countries and its relationship with spatial, environmental and anthropogenic variables, identified the degree and frequency of physical disturbance as significant drivers. There was also a strong link between live coral and HSC indicating that, despite region-wide declines in coral cover, there has not been a decoupling of complexity and coral, and that healthy coral populations facilitate the persistence of structurally complex coral habitats. To examine the relationships between different measures of HSC and their relevance to the fish community, a multidimensional model of HSC was developed. Through this approach it was possible to combine a number of variables which individually indicate different components of reef HSC, and derive compound ‘refuge’ and ‘complexity’ variables which are expected to be of greater relevance to the fish community, and potentially of greater use to understanding macroecological relationships on coral reefs. This approach was used to identify relationships between the Caribbean reef fish functional community structure and the derived refuge variable. Sites with low refuge levels were dominated by generalist fish species and had low levels of functional diversity. Additionally, the biomass of a number of fish functional groups and the size ii structure of groups was found to be related to the refuge characteristics of coral reefs. The examination of fish communities at night revealed a continued relationship with HSC over the entire diel cycle for some functional groups while others varied in their association with reef structure. Nocturnally active groups decoupled from HSC as they moved over flatter areas of reef to feed, while diurnally active groups moved into the reef to shelter. Observations of behaviour and feature preferences identified potential drivers behind the numerical relationships between the fish community and HSC. Some species have strong behavioural associations with HSC or features of reef structure and may suffer as result of habitat complexity declines. However, small non-fisheries target species such as wrasse and damsels, with less direct physical ties to the habitat structure are those most likely to persist after habitat degradation. The thesis highlights the state of and threats to Caribbean HSC and the pivotal role it plays in determining fish community structure.