Some of the most profound effects of climate change on ecological communities are due to alterations in species interactions rather than direct physiological effects of changing environmental conditions. Empirical evidence of historical changes in species interactions within climate-impacted communities is, however, rare and difficult to obtain. Here, we demonstrate the recent disappearance of key habitat-forming kelp forests from a warming tropical-temperate transition zone in eastern Australia. Using a 10-y video dataset encompassing a 0.6°C warming period, we show how herbivory increased as kelp gradually declined and then disappeared. Concurrently, fish communities from sites where kelp was originally abundant but subsequently disappeared became increasingly dominated by tropical herbivores. Feeding assays identified two key tropical/ subtropical herbivores that consumed transplanted kelp within hours at these sites. There was also a distinct increase in the abundance of fishes that consume epilithic algae, and much higher bite rates by this group at sites without kelp, suggesting a key role for these fishes in maintaining reefs in kelp-free states by removing kelp recruits. Changes in kelp abundance showed no direct relationship to seawater temperatures over the decade and were also unrelated to other measured abiotic factors (nutrients and storms). Our results show that warming-mediated increases in fish herbivory pose a significant threat to kelp-dominated ecosystems in Australia and, potentially, globally.climate change | macroalgae | plant-herbivore interactions | range shifts | tropicalization

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Drivers of recruitment in sessile marine organisms are often poorly understood, due to the rapidly changing requirements experienced during early ontogeny. The complex suite of physical, biological, and ecological interactions beginning at larval settlement involves a series of trade‐offs that influence recruitment success. For example, while cryptic settlement within complex microhabitats is a commonly observed phenomenon in sessile marine organisms, it is unclear whether trade‐offs between competition in cryptic refuges and predation on exposed surfaces leads to higher recruitment.To explore the trade‐offs during the early ontogeny of scleractinian corals, we combined field observations with laboratory and field experiments to develop a mechanistic understanding of coral recruitment success. Multiple experiments conducted over 15 months in Palau (Micronesia) allowed a mechanistic approach to study the individual factors involved in recruitment: settlement behavior, growth, competition, and predation, as functions of microhabitat and ontogeny. We finally developed and tested a predictive recruitment model with the broader aim of testing whether our empirical insights explained patterns of coral recruitment and quantifying the relative importance of each trade‐off.Coral settlement was higher in crevices than exposed microhabitats, but post‐settlement bottlenecks differed markedly in the presence (uncaged) and absence (caged) of predators. Incidental predation by herbivores on exposed surfaces at early post‐settlement (<3 mm) stages and targeted predation by corallivores at late post‐settlement (3–10 mm) stages exceeded competition in crevices as major drivers of mortality. In contrast, when fish were excluded, competition with macroalgae and heterotrophic invertebrates intensified mortality, particularly in crevices. As a result, post‐settlement trade‐offs were reversed, and recruitment was more than twofold higher on exposed surfaces than crevices. Once post‐settlement bottlenecks were overcome, survival was higher on exposed surfaces regardless of fish exclusion. However, maximum recruitment occurred in crevices of uncaged treatments, being ninefold higher than caged treatments. Overall, we characterize recruitment success throughout the earliest life‐history stages of corals and uncover some intriguing trade‐offs between growth, competition and predation, highlighting how these change and even reverse during ontogeny and under alternate disturbance regimes.

Ecology Letters (2012) 15: 338-346 ABSTRACT: Successful recruitment in shallow reef ecosystems often involves specific cues that connect planktonic invertebrate larvae with particular crustose coralline algae (CCA) during settlement. While ocean acidification (OA) can reduce larval settlement and the abundance of CCA, the impact of OA on the interactions between planktonic larvae and their preferred settlement substrate are unknown. Here, we demonstrate that CO2 concentrations (800 and 1300 μatm) predicted to occur by the end of this century significantly reduce coral (Acropora millepora) settlement and CCA cover by ≥ 45%. The CCA important for inducing coral settlement (Titanoderma spp., Hydrolithon spp.) were the most deleteriously affected by OA. Surprisingly, the only preferred settlement substrate (Titanoderma) in the experimental controls was avoided by coral larvae as pCO2 increased, and other substrata selected. Our results suggest OA may reduce coral population recovery by reducing coral settlement rates, disrupting larval settlement behaviour, and reducing the availability of the most desirable coralline algal species for successful coral recruitment.