Prey traits linking consumer diversity to ecosystem function remain poorly understood. On tropical coral reefs, herbivores promote coral dominance by suppressing competing macroalgae, but the roles ...of herbivore identity and diversity, macroalgal defenses, and their interactions in affecting reef resilience and function are unclear. We studied adjacent pairs of no-take marine reserves and fished areas on reefs in Fiji and found that protected reefs supported 7-17× greater biomass, 2-3× higher species richness of herbivorous fishes, and 3-11× more live coral cover than did fished reefs. In contrast, macroalgae were 27-61× more abundant and 3-4× more species-rich on fished reefs. When we transplanted seven common macroalgae from fished reefs into reserves they were rapidly consumed, suggesting that rates of herbivory (ecosystem functioning) differed inside vs. outside reserves.
We then video-recorded feeding activity on the same seven macroalgae when transplanted into reserves, and assessed the functional redundancy vs. complementarity of herbivorous fishes consuming these macroalgae. Of 29 species of larger herbivorous fishes on these reefs, only four species accounted for 97% of macroalgal consumption. Two unicornfish consumed a range of brown macroalgae, a parrotfish consumed multiple red algae, and a rabbitfish consumed a green alga, with almost no diet overlap among these groups. The two most chemically rich, allelopathic algae were each consumed by a single, but different, fish species. This striking complementarity resulted from herbivore species differing in their tolerances to macroalgal chemical and structural defenses.
A model of assemblage diet breadth based on our feeding observations predicted that high browser diversity would be required for effective control of macroalgae on Fijian reefs. In support of this model, we observed strong negative relationships between herbivore diversity and macroalgal abundance and diversity across the six study reefs. Our findings indicate that the total diet breadth of the herbivore community and the probability of all macroalgae being removed from reefs by herbivores increases with increasing herbivore diversity, but that a few critical species drive this relationship. Therefore, interactions between algal defenses and herbivore tolerances create an essential role for consumer diversity in the functioning and resilience of coral reefs.
For organisms with discrete life-histories, any mechanism that enhances growth and/or survival at critical life-history transitions may significantly influence adult population size. On coral reefs, ...structurally complex microhabitats enhance the early post-settlement growth and/or survival of fishes and corals; however, the importance of such microhabitats to the early life stages of tropical macroalgae is largely unknown. Here, we investigate the effect of crevices on the recruitment, growth and survival of propagules of a common coral reef macroalga, Sargassum swartzii. We settled S. swartzii propagules onto terracotta settlement tiles that had a series of 3 mm deep crevices evenly spaced on their top (exposed) surface. Recruitment of S. swartzii was 21% greater, but propagules were 18% shorter, 18 d post-settlement within crevices than on adjacent exposed surfaces. Exposing tiles to local fish assemblages for 5 d showed that survival of propagules was 90% greater in crevices than on exposed areas of the tiles on the reef crest, but not on the reef flat. Underwater video footage revealed that few fishes fed from within the crevices (18% of all bites), with the majority of feeding being concentrated on the exposed surface of tiles. Interestingly, small-bodied fishes from the family Blenniidae (predominantly Ecsenius spp.) accounted for the majority of the feeding activity on the tiles and likely contributed to the mortality of propagules. Structurally complex microhabitats, such as crevices, that shelter vulnerable early post-settlement propagules from herbivory may therefore be important for the persistence of macroalgae on coral reefs.
Herbivorous fishes are a critical functional group on coral reefs, and there is a clear need to understand the role and relative importance of individual species in reef processes. While numerous ...studies have quantified the roles of parrotfishes and surgeonfishes on coral reefs, the rabbitfishes (f. Siganidae) have been largely overlooked. Consequently, they are typically viewed as a uniform group of grazing or browsing fishes. Here, we quantify the diet and distribution of rabbitfish assemblages on six reefs spanning the continental shelf in the northern Great Barrier Reef. Our results revealed marked variation in the diet and distribution of rabbitfish species. Analysis of stomach contents identified four distinct groups: browsers of leathery brown macroalgae (
Siganus canaliculatus
,
S. javus
), croppers of red and green macroalgae (
S. argenteus
,
S. corallinus
,
S. doliatus
,
S. spinus
) and mixed feeders of diverse algal material, cyanobacteria, detritus and sediment (
S. lineatus
,
S. punctatissimus
,
S. punctatus
,
S. vulpinus
). Surprisingly, the diet of the fourth group (
S. puellus
) contained very little algal material (22.5 %) and was instead dominated by sponges (69.1 %). Together with this variation in diet, the distribution of rabbitfishes displayed clear cross-shelf variation. Biomass was greatest on inner-shelf reefs (112.7 ± 18.2 kg.ha
−1
), decreasing markedly on mid- (37.8 ± 4.6 kg.ha
−1
) and outer-shelf reefs (9.7 ± 2.2 kg.ha
−1
). This pattern was largely driven by the browsing
S. canaliculatus
that accounted for 50 % of the biomass on inner-shelf reefs, but was absent in mid- and outer-shelf reefs. Mixed feeders, although primarily restricted to the reef slope and back reef habitats, also decreased in abundance and biomass from inshore to offshore, while algal cropping taxa were the dominant group on mid-shelf reefs. These results clearly demonstrate the extent to which diet and distribution vary within the Siganidae and emphasise the importance of examining function on a species-by-species basis.
Large‐bodied fish are critical for sustaining coral reef fisheries, but little is known about the vulnerability of these fish to global warming. This study examined the effects of elevated ...temperatures on the movement and activity patterns of the common coral trout Plectropomus leopardus (Serranidae), which is an important fishery species in tropical Australia and throughout the Indo West‐Pacific. Adult fish were collected from two locations on Australia's Great Barrier Reef (23°S and 14°S) and maintained at one of four temperatures (24, 27, 30, 33 °C). Following >4 weeks acclimation, the spontaneous swimming speeds and activity patterns of individuals were recorded over a period of 12 days. At 24–27 °C, spontaneous swimming speeds of common coral trout were 0.43–0.45 body lengths per second (bls⁻¹), but dropped sharply to 0.29 bls⁻¹ at 30 °C and 0.25 bls⁻¹ at 33 °C. Concurrently, individuals spent 9.3–10.6% of their time resting motionless on the bottom at 24–27 °C, but this behaviour increased to 14.0% at 30 °C and 20.0% of the time at 33 °C (mean ± SE). The impact of temperature was greatest for smaller individuals (<45 cm TL), showing significant changes to swimming speeds across every temperature tested, while medium (45–55 cm TL) and large individuals (>55 cm TL) were first affected by 30 °C and 33 °C, respectively. Importantly, there was some indication that populations can adapt to elevated temperature if presented with adequate time, as the high‐latitude population decreased significantly in swimming speeds at both 30 °C and 33 °C, while the low‐latitude population only showed significant reductions at 33 °C. Given that movement and activity patterns of large mobile species are directly related to prey encounter rates, ability to capture prey and avoid predators, any reductions in activity patterns are likely to reduce overall foraging and energy intake, limit the energy available for growth and reproduction, and affect the fitness and survival of individuals and populations.
Herbivorous fishes are a key functional group on coral reefs. These fishes are central to the capacity of reefs to resist phase shifts and regenerate after disturbance. Despite this importance few ...studies have quantified the direct impact of these fishes on coral reefs. In this study the roles of parrotfishes, a ubiquitous group of herbivorous fishes, were examined on reefs in the northern Great Barrier Reef. The distribution of 24 species of parrotfish was quantified on three reefs in each of three cross-shelf regions. Functional roles (grazing, erosion, coral predation and sediment reworking) were calculated as the product of fish density, bite area or volume, bite rate, and the proportion of bites taken from various substrata. Inner-shelf reefs supported high densities but low biomass of parrotfishes, with high rates of grazing and sediment reworking. In contrast, outer-shelf reefs were characterised by low densities and high biomass of parrotfish, with high rates of erosion and coral predation. Mid-shelf reefs displayed moderate levels of all roles examined. The majority of this variation in functional roles was attributable to just two species. Despite being rare,
Bolbometopon muricatum
, the largest parrotfish species, accounted for 87.5% of the erosion and 99.5% of the coral predation on outer-shelf reefs.
B. muricatum
displayed little evidence of selectivity of feeding, with most substrata being consumed in proportion to their availability. In contrast, the high density of
Scarus rivulatus
accounted for over 70% of the total grazing and sediment reworking on inner-shelf reefs. This marked variation in the roles of parrotfishes across the continental shelf suggests that each shelf system is shaped by fundamentally different processes.
The corallivorous crown-of-thorns starfish (COTS),
Acanthaster planci
, is recognised as a major cause of coral reef degradation throughout much of the Pacific Ocean. However, the effects of COTS on ...the high diversity reefs in Indonesia have been largely overlooked. In 2007, high densities of COTS were observed in two regions of Indonesia: Aceh and Halmahera. Densities of COTS ranged from 0 to 52 starfish 2,000 m
2
across 24 sites in Aceh and from 0 to 18 starfish 2,000 m
2
at 10 sites in Halmahera. Mortality rates of
Acropora
spp. were very high at affected sites: over 50 % of colonies had been killed at seven of the 16 affected sites. A review of historical sources going back to 1969 suggests that COTS have damaged many reefs throughout Indonesia, including much activity within the Indonesian section of the Coral Triangle. Furthermore, the data suggest that COTS activity has increased rapidly since 2000. Very little of this activity has been reported in the primary literature, and there is a general lack of awareness in Indonesia of COTS as a potential cause of reef degradation. This lack of awareness, combined with limited monitoring efforts, means that damage caused by COTS is often attributed to other causes, such as destructive fishing, bleaching or tsunami. COTS are clearly a major source of coral mortality in Indonesia of which scientists and government need to be more cognizant.
The Red Sea has long been recognized as a region of high biodiversity and endemism. Despite this diversity and early history of scientific work, our understanding of the ecology of coral reefs in the ...Red Sea has lagged behind that of other large coral reef systems. We carried out a quantitative assessment of ISI-listed research published from the Red Sea in eight specific topics (apex predators, connectivity, coral bleaching, coral reproductive biology, herbivory, marine protected areas, non-coral invertebrates and reef-associated bacteria) and compared the amount of research conducted in the Red Sea to that from Australia’s Great Barrier Reef (GBR) and the Caribbean. On average, for these eight topics, the Red Sea had 1/6th the amount of research compared to the GBR and about 1/8th the amount of the Caribbean. Further, more than 50 % of the published research from the Red Sea originated from the Gulf of Aqaba, a small area (<2 % of the area of the Red Sea) in the far northern Red Sea. We summarize the general state of knowledge in these eight topics and highlight the areas of future research priorities for the Red Sea region. Notably, data that could inform science-based management approaches are badly lacking in most Red Sea countries. The Red Sea, as a geologically “young” sea located in one of the warmest regions of the world, has the potential to provide insight into pressing topics such as speciation processes as well as the capacity of reef systems and organisms to adapt to global climate change. As one of the world’s most biodiverse coral reef regions, the Red Sea may yet have a significant role to play in our understanding of coral reef ecology at a global scale.
We explore the role of specialization in supporting species coexistence in high-diversity ecosystems. Using a novel ordination-based method to quantify specialist and generalist feeding structures ...and diets we examined the relationship between morphology and diet in 120 wrasses and parrotfishes from the Great Barrier Reef. We find that wrasses, despite their morphological diversity, exhibit weak links between morphology and diet and that specialist morphologies do not necessarily equate to specialized diets. The dominant pattern shows extensive overlap in morphology (functional morphospace occupation) among trophic groups; fish with a given morphology may have a number of feeding modes. Such trophic versatility may lay the foundation for both the origins and maintenance of high biodiversity on coral reefs.
Climate-induced coral bleaching represents the foremost threat to coral assemblages globally, however bleaching susceptibility varies among and within coral taxa. We compared bleaching susceptibility ...among 10 coral morpho-taxa and two colony size classes relative to reef-scale bleaching severity at 33 reefs across the Great Barrier Reef and Coral Sea Marine Parks in February–March 2020. Colony size and bleaching severity caused the hierarchy of bleaching susceptibility among taxa to change considerably. Notably, massive Porites shifted from being among the least likely taxa to exhibit bleaching, to among the most susceptible as overall bleaching severity increased. Juvenile corals (≤5 cm diameter) were generally more resistant to bleaching, except for Montipora and Pocillopora colonies, which were more likely to bleach than adults (>5 cm). These findings suggest that colony size and reef-scale bleaching severity are important determinants of bleaching susceptibility among taxa and provide insights into possible shifts in the structure of coral assemblages caused by bleaching events.
•The bleaching likelihood for 10 coral taxa in two colony size classes, at five bleaching severity levels was investigated.•Moderate levels of bleaching severity produced the most pronounced differential susceptibilities among taxa.•Massive Porites shifted to become one of the most susceptible taxa as bleaching severity increased.•Coral colony size influenced bleaching susceptibility but this was dependent on taxonomic identity and bleaching severity.•Juvenile Montipora and Pocillopora were more likely to bleach than adult congeners, contrary to other taxa modelled.
PDF
