Morgan S. Pratchett

Phase Shifts, Herbivory, and the Resilience of Coral Reefs to Climate Change

Many coral reefs worldwide have undergone phase shifts to alternate, degraded assemblages because of the combined effects of over-fishing, declining water quality, and the direct and indirect impacts of climate change. Here, we experimentally manipulated the density of large herbivorous fishes to test their influence on the resilience of coral assemblages in the aftermath of regional-scale bleaching in 1998, the largest coral mortality event recorded to date. The experiment was undertaken on the Great Barrier Reef, within a no-fishing reserve where coral abundances and diversity had been sharply reduced by bleaching. In control areas, where fishes were abundant, algal abundance remained low, whereas coral cover almost doubled (to 20%) over a 3 year period, primarily because of recruitment of species that had been locally extirpated by bleaching. In contrast, exclusion of large herbivorous fishes caused a dramatic explosion of macroalgae, which suppressed the fecundity, recruitment, and survival of corals. Consequently, management of fish stocks is a key component in preventing phase shifts and managing reef resilience. Importantly, local stewardship of fishing effort is a tractable goal for conservation of reefs, and this local action can also provide some insurance against larger-scale disturbances such as mass bleaching, which are impractical to manage directly.

Ocean acidification impairs olfactory discrimination and homing ability of a marine fish

The persistence of most coastal marine species depends on larvae finding suitable adult habitat at the end of an offshore dispersive stage that can last weeks or months. We tested the effects that ocean acidification from elevated levels of atmospheric carbon dioxide (CO2) could have on the ability of larvae to detect olfactory cues from adult habitats. Larval clownfish reared in control seawater (pH 8.15) discriminated between a range of cues that could help them locate reef habitat and suitable settlement sites. This discriminatory ability was disrupted when larvae were reared in conditions simulating CO2-induced ocean acidification. Larvae became strongly attracted to olfactory stimuli they normally avoided when reared at levels of ocean pH that could occur ca. 2100 (pH 7.8) and they no longer responded to any olfactory cues when reared at pH levels (pH 7.6) that might be attained later next century on a business-as-usual carbon-dioxide emissions trajectory. If acidification continues unabated, the impairment of sensory ability will reduce population sustainability of many marine species, with potentially profound consequences for marine diversity.

Global warming and recurrent mass bleaching of corals

During 2015–2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

Patterns of recruitment and abundance of corals along the Great Barrier Reef

Different physical and biological processes prevail at different scales. As a consequence, small-scale experiments or local observations provide limited insights into regional or global phenomena. One solution is to incorporate spatial scale explicitly into the experimental and sampling design of field studies, to provide a broader, landscape view of ecology. Here we examine spatial patterns in corals on the Great Barrier Reef, across a spectrum of scales ranging from metres to more than 1,700 km. Our study is unusual because we explore large-scale patterns of a process (recruitment by juveniles) as well as patterns of adult abundance, revealing the relationship between the two. We show that coral-reef assemblages that are similar in terms of abundance may nonetheless show profound differences in dynamics and turnover, with major implications for their ecology, evolution and management.

Global human footprint on the linkage between biodiversity and ecosystem functioning in reef fishes.

A global survey of reef fishes shows that the consequences of biodiversity loss are greater than previously anticipated as ecosystem functioning remained unsaturated with the addition of new species. Additionally, reefs worldwide, particularly those most diverse, are highly vulnerable to human impacts that are widespread and likely to worsen due to ongoing coastal overpopulation.

Recovery of an isolated coral reef system following severe disturbance

Reef Repair Coral reefs suffer mass mortality because of coral bleaching, disease, and tropical storms, but we know much more about when, where, and how rapidly these ecosystems have collapsed than we do about their recovery. Gilmour et al. (p. 69; see the Perspective by Polidoro and Carpenter) studied a highly isolated coral reef before and after a climate-induced mass mortality event that killed 70 to 90% of the reef corals. The initial recovery of coral cover involved growth and survival of remnant colonies, which was followed by increases in larval recruitment. Thus, in the absence of chronic disturbance, even isolated reefs can recover from catastrophic disturbance. Isolated reefs with thriving herbivorous fish populations can recover rapidly after major bleaching events. [Also see Perspective by Polidoro and Carpenter] Coral reef recovery from major disturbance is hypothesized to depend on the arrival of propagules from nearby undisturbed reefs. Therefore, reefs isolated by distance or current patterns are thought to be highly vulnerable to catastrophic disturbance. We found that on an isolated reef system in north Western Australia, coral cover increased from 9% to 44% within 12 years of a coral bleaching event, despite a 94% reduction in larval supply for 6 years after the bleaching. The initial increase in coral cover was the result of high rates of growth and survival of remnant colonies, followed by a rapid increase in juvenile recruitment as colonies matured. We show that isolated reefs can recover from major disturbance, and that the benefits of their isolation from chronic anthropogenic pressures can outweigh the costs of limited connectivity.

Sublethal effects of coral bleaching on an obligate coral feeding butterflyfish

Coral bleaching is a significant and increasingly prevalent source of coral mortality, representing one of the most severe and widespread disturbances affecting coral reef ecosystems (Hoegh-Guldberg 1999; Pockley 2000). In the last few years (mostly since 1998), major episodes of coral bleaching have occurred on many coral reefs throughout the world, killing 20–80% of zooxanthellate corals (including both scleractinians and alcyonaceans) across expansive reef areas (e.g., Great Barrier Reef, Baird and Marshall 1998; Japan, Shibuno et al. 1999; eastern Pacific, Glynn et al. 2001; Caribbean, Ostrander et al. 2000). In addition to killing zooxanthellate corals, severe large-scale bleaching events may cause significant declines in the abundance of coral reef fishes, particularly among reef fish species that depend on live coral for food or shelter (Shibuno et al. 1999; Kokita and Nakazono 2001; Adjeroud et al. 2002).

Detecting regional variation using meta-analysis and large-scale sampling: latitudinal patterns in recruitment

Regional-scale variation of recruitment by marine organisms may reflect geographic patterns in adult stock sizes or fecundities, large-scale hydrodynamic features that influence the transport of larvae (e.g., currents, upwelling), and patterns of early mortality. In turn, recruitment may play a vital role in determining patterns of adult abundance and community structure, from local to biogeographic scales. We examined spatial variation in recruitment by corals at a regional scale, along 3300 km of the tropical and subtropical coast of eastern Australia (10°–31° S). We used two complementary approaches: (1) a metaanalysis of 21 different studies undertaken over a 16-yr period, each of which was generally conducted at a single reef, and (2) a large-scale sampling effort in which recruitment was measured in two years on 33 reefs arrayed along the length of the Great Barrier Reef (GBR). Our goal is to compare the emergent large-scale picture derived from many small-scale studies with patterns revealed by shorter-term regional sampling. The two approaches show very similar large-scale patterns. Recruitment by spawning corals (mainly acroporids) was highest in the central GBR and declined steadily with increasing latitude by up to more than 20-fold. A smaller decline occurred on the northern GBR between Australian and Papua New Guinea. Recruitment by brooding corals (mostly pocilloporids) was greatest in the northern GBR and also declined to the south. The latitudinal decline in brooders was three-to fivefold, i.e., not as great as for spawners. Consequently, the proportion of brooded recruits increased to the south, and they generally exceeded spawners on the southern GBR and on isolated subtropical reefs at higher latitudes. Our meta-analysis shows that fully half of the variation in the ratio of spawners to brooders is attributable to one of 11 variables that we extracted from the published studies: the month when the recruitment panels were deployed. This result suggests that the intensity and timing of spawning have a crucial impact on large-scale patterns of recruitment. Elsewhere, we tested this hypothesis in the field, and confirmed that regional variation in recruitment by spawning acroporid corals was driven by spatial and temporal variation in the extent of mass spawning. Together, large-scale sampling and meta-analyses provide a powerful, combined approach for investigating large-scale patterns and the mechanisms underlying them.

Coral reef fish smell leaves to find island homes

Recent studies have shown that some coral reef fish larvae return to natal reefs, while others disperse to distant reefs. However, the sensory mechanisms used to find settlement sites are poorly understood. One hypothesis is that larvae use olfactory cues to navigate home or find other suitable reef habitats. Here we show a strong association between the clownfish Amphiprion percula and coral reefs surrounding offshore islands in Papua New Guinea. Host anemones and A. percula are particularly abundant in shallow water beneath overhanging rainforest vegetation. A series of experiments were carried out using paired-choice flumes to evaluate the potential role of water-borne olfactory cues in finding islands. Recently settled A. percula exhibited strong preferences for: (i) water from reefs with islands over water from reefs without islands; (ii) water collected near islands over water collected offshore; and (iii) water treated with either anemones or leaves from rainforest vegetation. Laboratory reared-juveniles exhibited the same positive response to anemones and rainforest vegetation, suggesting that olfactory preferences are innate rather than learned. We hypothesize that A. percula use a suite of olfactory stimuli to locate vegetated islands, which may explain the high levels of self-recruitment on island reefs. This previously unrecognized link between coral reefs and island vegetation argues for the integrated management of these pristine tropical habitats.

Evolutionary history of the butterflyfishes (f: Chaetodontidae) and the rise of coral feeding fishes

Of the 5000 fish species on coral reefs, corals dominate the diet of just 41 species. Most (61%) belong to a single family, the butterflyfishes (Chaetodontidae). We examine the evolutionary origins of chaetodontid corallivory using a new molecular phylogeny incorporating all 11 genera. A 1759‐bp sequence of nuclear (S7I1 and ETS2) and mitochondrial (cytochrome b) data yielded a fully resolved tree with strong support for all major nodes. A chronogram, constructed using Bayesian inference with multiple parametric priors, and recent ecological data reveal that corallivory has arisen at least five times over a period of 12 Ma, from 15.7 to 3 Ma. A move onto coral reefs in the Miocene foreshadowed rapid cladogenesis within Chaetodon and the origins of corallivory, coinciding with a global reorganization of coral reefs and the expansion of fast‐growing corals. This historical association underpins the sensitivity of specific butterflyfish clades to global coral decline.