Relative roles of herbivory and nutrients in the recruitment of coral-reef seaweeds

The relative effects of and interactions between, bottom-up and top-down processes are fundamental to,population and community structure in both terrestrial and marine systems. These issues are especially critical for seaweed populations on coral reefs, since both bottom-up and top-down factors are suggested as causes of algal invasions during reef degradation. Although algal invasions require the establishment of new recruits, most previous studies of tropical. marine algae have focused on mature stages. We simultaneously manipulated nutrient supply to and herbivory on recruits of two ecologically different species of seaweed on the Great Barrier Reef. We found that herbivory strongly reduced both density and growth of recruits for both species, whereas nutrient supply had minor effects on growth of Lobophora variegata recruits and no detectable effects on Sargassum fissifolium recruits. Notwithstanding the dominance of herbivory. over nutrient effects, herbivory Was not uniform, but varied both between species and among response variables (density and size), and was apparently stronger for nutrient-enriched plants. Our data demonstrate that the relative importance of bottom-up and top-down processes may depend on the species, circumstances, and life-history processes under consideration. These results also emphasize the importance of herbivores to the protection of coral reefs against algal overgrowth

Ecological resilience, climate change, and the Great Barrier Reef

The vulnerability assessments in this volume frequently refer to the resilience of various ecosystem elements in the face of climate change. This chapter provides an introduction to the concept of ecological resilience, and its application as part of a management response to climate change threats. As defined in the glossary, resilience refers to the capacity of a system to absorb shocks, resist dramatic changes in condition, and maintain or recover key functions and processes, without undergoing "phase shifts" to a qualitatively different state (Figure 4.1)32, 72. For example, people who are physically and mentally fit and strong will have good prospect of recovery from disease, injury or trauma: they are resilient. In Figure 4.1, a ball placed at position 1 is dynamically stable: not only will it remain in position, but if pushed in any direction, it will return to its original position; thus the ball in this state is resilient, in that it can absorb shocks and return to a similar condition or state. In contrast, a ball placed at position 2 may be initially stable (it will remain in position if undisturbed) but not dynamically stable: if disturbed, it will move away. Thus the ball at position 2 is not resilient, and disturbances will result in a shift in state. If the ball at position 1 is disturbed to anywhere within the red circle, the ball will return to position 1; however, if disturbed further, the ball may not return, but may move to a new, alternate stable state (eg position 3). This system is resilient to disturbances that push it within the red boundary. However, if external factors decreased the depth of position 1, or lowered the saddle at point 2, then the system's resilience would be reduced. By analogy to coral reef ecosystems, position 1 might be a coral-dominated reef, and position 3 algal dominated. A disturbance such as killing coral that is overgrown by algae would move the reef toward an algal-dominated state; if the reef is resilient, this change would be temporary and natural processes would allow coral to re-establish and recover. If not, the algal dominance might be sufficient to preclude coral regrowth or recruitment, and the reef would change trajectory, moving toward algal dominance. Ecological resilience refers to the capacity of an ecosystem, habitat, population or taxon to withstand, recover from or adapt to impacts and stressors, such as climate change, and retain the same structure, processes and functions³². For example, coral reefs are naturally very dynamic, undergoing constant change and disturbances, but, under natural conditions, they have considerable capacity to recover or maintain key processes and functions in the face of such disturbances or pressures. Tropical storms may cause dramatic damage to coral populations, and hence to the physical habitat structure, with dead coral being overgrown by various forms of algae. This will result in a temporarily changed state, and changes in ecological functions. On a resilient reef, over a period of five to 20 years, the altered state is unstable: coral fragments will regrow, and new corals will settle, grow and gradually replace the algae, restoring the reef to coral dominance, and restoring ecological structure and processes. In contrast, however, if human impacts have undermined that resilience, algal growth may be exacerbated, coral regrowth and colonisation may be suppressed, and the altered state and processes may become stable, causing a long-term "phase shift", or change, to algal dominance

Impact of diurnal temperature fluctuations on larval settlement and growth of the reef coral Pocillopora damicornis

Diurnal fluctuations in seawater temperature are ubiquitous on tropical reef flats. However, the effects of such dynamic temperature variations on the early stages of corals are poorly understood. In this study, we investigated the responses of larvae and new recruits of Pocillopora damicornis to two constant temperature treatments (29 and 31 degrees C) and two diurnally fluctuating treatments (28-31 and 30-33 degrees C with daily means of 29 and 31 degrees C, respectively) simulating the 3 degrees C diel oscillations at 3m depth on the Luhuitou fringing reef (Sanya, China). Results showed that the thermal stress on settlement at 31 degrees C was almost negated by the fluctuating treatment. Further, neither elevated temperature nor temperature fluctuations caused bleaching responses in recruits, while the maximum excitation pressure over photosystem II (PSII) was reduced under fluctuating temperatures. Although early growth and development were highly stimulated at 31 degrees C, oscillations of 3 degrees C had little effects on budding and lateral growth at either mean temperature. Nevertheless, daytime encounters with the maximum temperature of 33 degrees C in fluctuating 31 degrees C elicited a notable reduction in calcification compared to constant 31 degrees C. These results underscore the complexity of the effects caused by diel temperature fluctuations on early stages of corals and suggest that ecologically relevant temperature variability could buffer warming stress on larval settlement and dampen the positive effects of increased temperatures on coral growth

Revisiting “Success” and “Failure” of Marine Protected Areas: A Conservation Scientist Perspective

Marine protected areas (MPAs) form the cornerstone of marine conservation. Identifying which factors contribute to their success or failure is crucial considering the international conservation targets for 2020 and the limited funds generally available for marine conservation. We identified common factors of success and/or failure of MPA effectiveness using peer-reviewed publications and first-hand expert knowledge for 27 case studies around the world. We found that stakeholder engagement was considered to be the most important factor affecting MPA success, and equally, its absence, was the most important factor influencing failure. Conversely, while some factors were identified as critical for success, their absence was not considered a driver of failure, and vice versa. This mismatch provided the impetus for considering these factors more critically. Bearing in mind that most MPAs have multiple objectives, including non-biological, this highlights the need for the development and adoption of standardized effectiveness metrics, besides biological considerations, to measure factors contributing to the success or failure of MPAs to reach their objectives. Considering our conclusions, we suggest the development of specific protocols for the assessment of stakeholder engagement, the role of leadership, the capacity of enforcement and compliance with MPAs objectives. Moreover, factors defining the success and failure of MPAs should be assessed not only by technical experts and the relevant authorities, but also by other stakeholder groups whose compliance is critical for the successful functioning of an MPA. These factors should be considered along with appropriate ecological, social, and economic data and then incorporated into adaptive management to improve MPA effectiveness

Doom and Boom on a Resilient Reef: Climate Change, Algal Overgrowth and Coral Recovery

Background: Coral reefs around the world are experiencing large-scale degradation, largely due to global climate change, overfishing, diseases and eutrophication. Climate change models suggest increasing frequency and severity of warming-induced coral bleaching events, with consequent increases in coral mortality and algal overgrowth. Critically, the recovery of damaged reefs will depend on the reversibility of seaweed blooms, generally considered to depend on grazing of the seaweed, and replenishment of corals by larvae that successfully recruit to damaged reefs. These processes usually take years to decades to bring a reef back to coral dominance

Towards a collaborative research: A case study on linking science to farmers' perceptions and knowledge on Arabica coffee pests and diseases and its management

The scientific community has recognized the importance of integrating farmer's perceptions and knowledge (FPK) for the development of sustainable pest and disease management strategies. However, the knowledge gap between indigenous and scientific knowledge still contributes to misidentification of plant health constraints and poor adoption of management solutions. This is particularly the case in the context of smallholder farming in developing countries. In this paper, we present a case study on coffee production in Uganda, a sector depending mostly on smallholder farming facing a simultaneous and increasing number of socio-ecological pressures. The objectives of this study were (i) to examine and relate FPK on Arabica Coffee Pests and Diseases (CPaD) to altitude and the vegetation structure of the production systems; (ii) to contrast results with perceptions from experts and (iii) to compare results with field observations, in order to identify constraints for improving the information flow between scientists and farmers. Data were acquired by means of interviews and workshops. One hundred and fifty farmer households managing coffee either at sun exposure, under shade trees or inter-cropped with bananas and spread across an altitudinal gradient were selected. Field sampling of the two most important CPaD was conducted on a subset of 34 plots. The study revealed the following findings: (i) Perceptions on CPaD with respect to their distribution across altitudes and perceived impact are partially concordant among farmers, experts and field observations (ii) There are discrepancies among farmers and experts regarding management practices and the development of CPaD issues of the previous years. (iii) Field observations comparing CPaD in different altitudes and production systems indicate ambiguity of the role of shade trees. According to the locality-specific variability in CPaD pressure as well as in FPK, the importance of developing spatially variable and relevant CPaD control practices is proposed. (Résumé d'auteur

Contrasting effects of turf algae on corals: massive Porites spp. are unaffected by mixed-species turfs, but killed by the red alga Anotrichium tenue

Competition between corals and algae is an important process on coral reefs, especially during reef degradation, when abundant corals are often overgrown by benthic macroalgae. Despite the widespread assumption that macroalgae are able to out-compete corals for space, there have been very few experimental studies testing the nature of this interaction. This study compared the effects of a filamentous red alga, Anotrichium tenue, with those of mixed-species, filamentous algal turfs on massive Porites spp. corals on inshore reefs of the central Great Barrier Reef, Australia. We compared mortality of coral tissue in plots with A. tenue naturally present on live coral tissue, plots in which A tenue was naturally present but experimentally removed, and plots where mixed algal turfs were naturally present but A. tenue was not. The results indicate that A tenue killed coral tissue by active overgrowth. Removing the alga removed the effect. In contrast, general, mixed-species algal turfs did not cause any mortality of coral tissue. We suggest that 2 particular traits of A. tenue may facilitate its effects on the corals. First, unlike most: filamentous turf species present, it was able to overgrow live coral tissue, perhaps due to allelochemical effects. Second, individual algal filaments trap relatively large amounts of mucus from the corals and of sediment, apparently increasing the damage to underlying coral tissue. Surveys indicated that A. tenue primarily affected massive Ponies spp., that overgrowth effects were not site-specific, but that occurrence of infected corals was not widespread. In particular, distribution patterns were not consistent with an effect of terrestrial runoff. This study provides evidence of an exceptionally lethal effect on corals by a single species of filamentous alga, and emphasizes the species-specific nature of coral-algal competitive outcomes, even within a functional group

Effects of competition and herbivory on interactions between a hard coral and a brown alga

Despite widespread acceptance of the negative effects of macroalgae on corals, very few studies have experimentally tested the competitive nature of the interaction, and most have ignored the potential effects of corals on algae. We report the effects of herbivory and competition on the growth of the branching scleractinian coral Porites cylindrica Dana and the creeping foliose brown alga Lobophora variegata (Lamouroux) Womersley, on an inshore fringing reef of the central Great Barrier Reef. L. variegata overgrows branches of P. cylindrica from the base up, forming a distinct boundary between the alga and the coral tissue. The experiment used exclusion cages to test for effects of herbivores, and removal of algae and coral tissue, at their interaction boundary, to test for inhibition of the competitors by each other. Comparisons of coral branches with the algae present or removed showed that the presence and overgrowth of the alga caused significant coral tissue mortality. Comparisons of branches with coral tissue unmanipulated or damaged showed that the coral inhibited the overgrowth by L. variegata, but that the algae were markedly superior competitors. Importantly, reduced herbivory resulted in faster algal growth and consequent overgrowth and mortality of coral tissue, demonstrating the critical importance of herbivory to the outcome of the competitive interaction

Coral-algal competition: macroalgae with different properties have different effects on corals

Competition between hard corals and macroalgae is a key ecological process on coral reefs, especially during reef degradation, which often involves a 'phase-shift' from coral- to alga-dominated reefs. However, there are relatively few published studies exploring the variability in this interaction. This paper expands the range of documented coral-algal interactions by comparing the mechanisms and outcomes of interactions involving 3 different algal species, as well as general, mixed algal turfs. Mixed filamentous turfs had relatively minor effects on corals. However, the turfing filamentous red alga Corallophila huysmansii provided a dramatic exception to this pattern, being able to settle on, overgrow and kill live coral tissue, perhaps due to allelochemical production by the alga, although this was not directly demonstrated. The larger filamentous alga Chlorodesmis fastigiata ('Turtle Weed'), which is conspicuous and abundant on Indo-Pacific reefs, caused polyp retraction but had little other noticeable effect on coral tissue. A corticated red alga Hypnea pannosa, frequently observed living within colonies of the branching coral Porites cylindrica, did not have a major impact on underlying coral tissue, even over a period of 1 yr, apparently because its relatively translucent and porous thallus structure does not strongly inhibit coral tissue functions. Together, the results demonstrate the considerable potential variability in both the process and outcome of coral-algal competition. This variability can be effectively interpreted in terms of the limited number of mechanisms by which algae can affect corals, with these mechanisms depending largely on the properties (physical, biological, chemical) of the algae. Given the central importance of coral-algal competition to the process of coral reef phase-shifts, understanding the variability and complexity in such competition will have important implications for the prediction and consequences of such phase-shifts

The effects of nutrients and herbivory on competition between a hard coral (Porites cylindrica) and a brown alga (Lobophora variegata)

Coral reef degradation often involves a phase shift from coral- to macroalgal-dominated reefs. Declining levels of herbivory or increasing supply of nutrients have both been suggested as a cause of increased algal abundance and consequent competitive overgrowth of corals. However, explicit demonstration of the processes involved and their relative strengths requires simultaneous tests of all three factors: competition, herbivory, and nutrient effects. We experimentally tested the factorial effects of nutrients and herbivory on the competitive interaction between a brown alga Lobophora variegata and a scleractinian coral Porites cylindrica. The results of the experiment show that coral tissue mortality was strongly enhanced by the presence of the competitor (L. variegata), and this effect was significantly greater when herbivores were excluded. In contrast, the coral growth (skeletal extension) of P. cylindrica was not significantly affected by any treatments. The addition of nutrients did not have a significant effect on corals overall, but had a small effect on algal growth and consequent coral tissue mortality when herbivores were excluded. The factorial combination of treatments in this experiment allows interpretation of the causal relationships between each factor, demonstrating that nutrient effects on algal growth only led to competitive effects on corals when herbivory was insufficient to consume excess algal growth and that both herbivore and nutrient effects on corals were dependent on the strength and outcome of the competitive interaction between corals and algae