Spatial patterns in herbivory on a coral reef are influenced by structural complexity but not by algal traits

Abstract

Background: Patterns of herbivory can alter the spatial structure of ecosystems, with important consequences for ecosystem functions and biodiversity. While the factors that drive spatial patterns in herbivory in terrestrial systems are well established, comparatively less is known about what influences the distribution of herbivory in coral reefs.

Methodology and principal findings: We quantified spatial patterns of macroalgal consumption in a cross-section of Ningaloo Reef (Western Australia). We used a combination of descriptive and experimental approaches to assess the influence of multiple macroalgal traits and structural complexity in establishing the observed spatial patterns in macroalgal herbivory, and to identify potential feedback mechanisms between herbivory and macroalgal nutritional quality. Spatial patterns in macroalgal consumption were best explained by differences in structural complexity among habitats. The biomass of herbivorous fish, and rates of herbivory were always greater in the structurally-complex coral-dominated outer reef and reef flat habitats, which were also characterised by high biomass of herbivorous fish, low cover and biomass of macroalgae and the presence of unpalatable algae species. Macroalgal consumption decreased to undetectable levels within 75 m of structurally-complex reef habitat, and algae were most abundant in the structurally-simple lagoon habitats, which were also characterised by the presence of the most palatable algae species. In contrast to terrestrial ecosystems, herbivory patterns were not influenced by the distribution, productivity or nutritional quality of resources (macroalgae), and we found no evidence of a positive feedback between macroalgal consumption and the nitrogen content of algae.

Significance: This study highlights the importance of seascape-scale patterns in structural complexity in determining spatial patterns of macroalgal consumption by fish. Given the importance of herbivory in maintaining the ability of coral reefs to reorganise and retain ecosystem functions following disturbance, structural complexity emerges as a critical feature that is essential for the healthy functioning of these ecosystems.

Figure 1. Seascape patterns in the distribution of herbivory.

Length of Sargassum myriocystum lateral branches consumed per hour (mean ± SE) at lagoon, reef flat and outer reef habitats at each of the experimental sites.

Figure 2. Seascape patterns in the distribution of herbivores, macroalgae, coral cover and rugosity.

Data represent means ± SE of (a) total roving herbivorous fish biomass, (b) total browsing fish biomass, (c) algal cover, (d) algal biomass, (e) coral cover, and (f) rugosity.

Figure 3. Seascape patterns in the distribution of fish and algae assemblages.

Canonical analysis of principal coordinates (CAP) comparing community assemblages of (a) all roving herbivorous fish, (b) all browsing fish, and (c) macroalgae between sites (numbered icons) and habitats (symbols): Triangles facing upwards  =  Lagoon; Triangles facing downwards  =  Outer reef; Squares  =  Reef flat habitat. Data were fourth-root transformed prior to ordination.

Figure 4. Relationships between rates of herbivory, algae cover and rugosity.

(a) Logarithmic relationship between herbivory rates and algal cover. (b) Linear relationship between herbivory rates and rugosity. All variables were averaged for each site.

Figure 5. Experimental test of effects of habitat and herbivory on algal consumption, productivity and chemical composition.

(a) Biomass change and (b) nitrogen and (c) carbon/nitrogen ratio of Lobophora variegata transplanted to reef flat and lagoon habitats in three experimental treatments designed to manipulate access by herbivores (Caged, Open and Partially caged) after 6 weeks. Data pooled across the three sites, bars represent means ± SE.

Figure 6. Effect of proximity to reef on consumption of macroalgae.

Length of Sargassum myriocystum lateral branches consumed after 48 hours (mean ± SE) at increasing distances from the reef flat/lagoon boundary in the three experimental sites.