Relative importance of coral cover, habitat complexity and diversity in determining the structure of reef fish communities

Abstract

The structure of coral reef habitat has a pronounced influence on the diversity, composition and abundance of reef-associated fishes. However, the particular features of the habitat that are most critical are not always known. Coral habitats can vary in many characteristics, notably live coral cover, topographic complexity and coral diversity, but the relative effects of these habitat characteristics are often not distinguished. Here, we investigate the strength of the relationships between these habitat features and local fish diversity, abundance and community structure in the lagoon of Lizard Island, Great Barrier Reef. In a spatial comparison using sixty-six 2m(2) quadrats, fish species richness, total abundance and community structure were examined in relation to a wide range of habitat variables, including topographic complexity, habitat diversity, coral diversity, coral species richness, hard coral cover, branching coral cover and the cover of corymbose corals. Fish species richness and total abundance were strongly associated with coral species richness and cover, but only weakly associated with topographic complexity. Regression tree analysis showed that coral species richness accounted for most of the variation in fish species richness (63.6%), while hard coral cover explained more variation in total fish abundance (17.4%), than any other variable. In contrast, topographic complexity accounted for little spatial variation in reef fish assemblages. In degrading coral reef environments, the potential effects of loss of coral cover and topographic complexity are often emphasized, but these findings suggest that reduced coral biodiversity may ultimately have an equal, or greater, impact on reef-associated fish communities.

Figure 1. Relationships between fish species richness and habitat variables in the lagoon of Lizard Island, GBR.

All relationships were statistically significant (p<0.05).

Figure 2. Relationships between log fish abundance and habitat variables in the lagoon of Lizard Island, GBR.

Lines were only fitted to the statistically significant relationships (p<0.05).

Figure 3. Relationships between habitat variables in the lagoon of Lizard Island, GBR.

All relationships were statistically significant (p<0.05).

Figure 4. Regression tree analysis of the fish species richness at Lizard Island, QLD, Australia.

The explanatory variables were: site, topographic complexity, habitat diversity, coral species richness, coral diversity, percent hard coral cover, branching coral cover and corymbose coral cover. For each of the four terminal nodes the distribution of the observed values of fish species richness is shown in a histogram. Each node is labeled with the mean rating and the number of observations in a group (in parentheses). The tree explained 70.5% of the total variability in the data. The first and second splits based on coral species richness explained 56.4% and 7.2% respectively, the third split based on percent hard coral cover explained additional 6.9%.

Figure 5. Regression tree analysis of the log fish abundance at Lizard Island, QLD, Australia.

The explanatory variables were: site, topographic complexity, habitat diversity, coral species richness, coral diversity, percent hard coral cover, branching coral cover and plate coral cover. For each of the four terminal nodes the distribution of the observed values of log fish abundance is shown in a histogram. Each node is labeled with the mean rating and the number of observations in a group (in parentheses). The tree explained 34.1% of the total variability in the data. The first split based on percent hard coral cover explained 17.4%, second split based on coral species richness explained 8.1% and the third split based on site explained additional 8.6%.

Figure 6. Fish community structure.

Constrained distance-based redundancy analysis plot of fish community fitted to significant predictor variables (hard coral cover, habitat complexity, coral species richness, branching coral cover) identified using forward DistLM selection procedure and AIC selection criteria (Table 4). The relative influence of fitted predictor variables is indicated by the length of vector overlays. Each black point represent a single sample. a. Fitted predictor variables. Significant variable are identified with an asterisk (*) b. Fish species vectors.