The roles of dimensionality, canopies and complexity in ecosystem monitoring

Canopies are common among autotrophs, increasing their access to light and thereby increasing competitive abilities. If viewed from above canopies may conceal objects beneath them creating a 'canopy effect'. Due to complexities in collecting 3-dimensional data, most ecosystem monitoring programmes reduce dimensionality when sampling, resorting to planar views. The resultant 'canopy effects' may bias data interpretation, particularly following disturbances. Canopy effects are especially relevant on coral reefs where coral cover is often used to evaluate and communicate ecosystem health. We show that canopies hide benthic components including massive corals and algal turfs, and as planar views are almost ubiquitously used to monitor disturbances, the loss of vulnerable canopy-forming corals may bias findings by presenting pre-existing benthic components as an altered system. Our reliance on planar views in monitoring ecosystems, especially coral cover on reefs, needs to be reassessed if we are to better understand the ecological consequences of ever more frequent disturbances.     

The contribution of small individuals to density-body size relationships: examination of energetic equivalence in reef fishes

A key relationship in ecology is that between density and body size, with the emphasis placed on energetic rules constraining the abundance of larger organisms below that of smaller organisms. Most studies have focused upon the density-body size relationship at the species level. However, energy is gathered at an individual level. We therefore examined this relationship in a coral reef fish assemblage, focusing on individuals. Using a comprehensive data set, with over 14,000 observations we found that the relationship between local density and adult body size differs from the linear relationship predicted by the energetic equivalence rule. However, excluding the smallest size classes, the relationship between body size and individual abundance for intermediate to larger fish did not depart from the predicted –0.75. Unlike plants and intermediate to large reef fishes, the smallest fishes appear to have constraints that may reflect different patterns of resource acquisition.     

Spatial ecology of the steephead parrotfish (Chlorurus microrhinos): an evaluation using acoustic telemetry

Herbivory and other ecosystem processes are widely accepted as important factors in maintaining coral reef resilience. While the spatial scales over which these processes occur have been evaluated, the spatial ecology of individual taxa responsible for shaping these processes is almost entirely unknown. This study combined acoustic telemetry and ecological assessments to evaluate the movement patterns and feeding range of a functionally important coral reef fish, Chlorurus microrhinos (f. Labridae). The diurnal home range and feeding areas of C. microrhinos, on Orpheus Island, Great Barrier Reef, were quantified using active acoustic telemetry. The average diurnal home range of C. microrhinos was 7,830 m² ± 940 (SE). Core areas of activity (50% kernel utilization distributions) were relatively small, encompassing approximately 22% of an individual’s home range (1,690 m² ± 220). Core areas exhibited greater topographic complexity. C. microrhinos may select these areas because of decreased predation risk. Feeding intensities were not homogenous throughout the home range. Core areas were found to have a greater number of feeding scars and are thus exposed to increased bioerosion and algal removal by C. microrhinos. While important in shaping key ecosystem processes, the ecosystem impact of individual C. microrhinos in Pioneer Bay appears to be restricted to small areas within a narrow band along the reef crest.     

Coral recovery may not herald the return of fishes on damaged coral reefs

The dynamic nature of coral reefs offers a rare opportunity to examine the response of ecosystems to disruption due to climate change. In 1998, the Great Barrier Reef experienced widespread coral bleaching and mortality. As a result, cryptobenthic fish assemblages underwent a dramatic phase-shift. Thirteen years, and up to 96 fish generations later, the cryptobenthic fish assemblage has not returned to its pre-bleach configuration. This is despite coral abundances returning to, or exceeding, pre-bleach values. The post-bleach fish assemblage exhibits no evidence of recovery. If these short-lived fish species are a model for their longer-lived counterparts, they suggest that (1) the full effects of the 1998 bleaching event on long-lived fish populations have yet to be seen, (2) it may take decades, or more, before recovery or regeneration of these long-lived species will begin, and (3) fish assemblages may not recover to their previous composition despite the return of corals.     

Regional Endothermy in a Coral Reef Fish?

Although a few pelagic species exhibit regional endothermy, most fish are regarded as ectotherms. However, we document significant regional endothermy in a benthic reef fish. Individual steephead parrotfish, Chlorurus microrhinos (Labridae, formerly Scaridae) were tagged and their internal temperatures were monitored for a 24 h period using active acoustic telemetry. At night, on the reef, C. microrhinos were found to maintain a consistent average peritoneal cavity temperature 0.16 ± 0.005 °C (SE) warmer than ambient. Diurnal internal temperatures were highly variable for individuals monitored on the reef, while in tank-based trials, peritoneal cavity temperatures tracked environmental temperatures. The mechanisms responsible for a departure of the peritoneal cavity temperature from environmental temperature occurred in C. microrhinos are not yet understood. However, the diet and behavior of the species suggests that heat in the peritoneal cavity may result primarily from endogenous thermogenesis coupled with physiological heat retention mechanisms. The presence of limited endothermy in C. microrhinos indicates that a degree of uncertainty may exist in the manner that reef fish respond to their thermal environment. At the very least, they do not always appear to respond to environmental temperatures as neutral thermal vessels and do display limited, but significant, visceral warming.     

Comparative efficiency of clove oil and rotenone for sampling tropical reef fish assemblages

A quantitative examination of the reef fish assemblage at Orpheus Island, Great Barrier Reef, contrasting clove oil and rotenone, sampled 365 individuals of 47 species with clove oil v. 536 individuals of 53 species with rotenone. Number of species and individuals were not found to differ significantly between the two techniques, largely due to variation among samples. Neopomacentrus bankieri (Pomacentridae) and Eviota queenslandica (Gobiidae) were the most dominant in the samples using either technique. Although the samples appeared to be comparable, only 31 species (45%) in eight families were common to both techniques. Fishes often recovered before collection when using clove oil and were not driven out of the reef during induction to anaesthesia. Although the samples collected with clove oil approximate the results obtained using rotenone, enclosed rotenone stations are the preferred method for providing relatively complete quantitative samples.     

Fish predation on sea urchins on the Great Barrier Reef

Predators are important for regulating adult sea urchin densities. Here, we employ remote underwater video cameras to record diurnal predation on tethered sea urchins at Lizard Island on the Great Barrier Reef (GBR). We identified four fish predators of adult sea urchins (Balistoides viridescens, Balistapus undulatus, Lethrinus atkinsoni and Choerodon schoenleinii). Predator activity appeared to be site-specific. Balistoides viridescens and B. undulatus (f: Balistidae) were the two most important predators of Echinometra mathaei with the former handling E. mathaei significantly faster (mean 0.7?min) than B. undulatus (5.2?min). Balistoides viridescens also successfully preyed on 70?% of detections, while C. schoenleinii, B. undulatus and L. atkinsoni preyed on just 33, 17 and <1?%, respectively. Additionally, B. viridescens were behaviourally dominant among predator species and were observed as aggressors in 30 encounters with B. undulatus and 8 encounters with L. atkinsoni. In only one encounter was B. viridescens the recipient of any aggression (from B. undulatus). In terms of relative vulnerability, of the three sea urchin species examined, E. mathaei were more vulnerable to predation than Diadema setosum or Echinothrix calamaris, with mean handling times of 1.2, 4.8 and 10.3?min, respectively. Balistoides viridescens and B. undulatus both appear to be able to play an important role as predators of sea urchins on the relatively intact coral reefs of Lizard Island. However, B. viridescens emerge as the most efficient predator in terms of handling speed and the proportion of detections preyed upon. They were also the behaviourally dominant predator. This preliminary study of the predators of sea urchins on the GBR highlights the potential significance of relatively scarce but functionally important species.