How populations and species respond to modified environmental conditions is critical to their persistence both now and into the future, particularly given the increasing pace of environmental change. The process of adaptation to novel environmental conditions can occur via two mechanisms: (1) the expression of phenotypic plasticity (the ability of one genotype to express varying phenotypes when exposed to different environmental conditions), and (2) evolution via selection for particular phenotypes, resulting in the modification of genetic variation in the population. Plasticity, because it acts at the level of the individual, is often hailed as a rapid-response mechanism that will enable organisms to adapt and survive in our rapidly changing world. But plasticity can also retard adaptation by shifting the distribution of phenotypes in the population, shielding it from natural selection. In addition to which, not all plastic responses are adaptive—now well-documented in cases of ecological traps. In this theme issue, we aim to present a considered view of plasticity and the role it could play in facilitating or hindering adaption to environmental change. This introduction provides a re-examination of our current understanding of the role of phenotypic plasticity in adaptation and sets the theme issue's contributions in their broader context. Four key themes emerge: the need to measure plasticity across both space and time; the importance of the past in predicting the future; the importance of the link between plasticity and sexual selection; and the need to understand more about the nature of selection on plasticity itself. We conclude by advocating the need for cross-disciplinary collaborations to settle the question of whether plasticity will promote or retard species' rates of adaptation to ever-more stressful environmental conditions. This article is part of the theme issue ‘The role of plasticity in phenotypic adaptation to rapid environmental change’.

Despite the widely accepted importance of herbivory as a determinant of reef benthic community structure, few studies have examined the relative contributions of individual species to ecosystem processes at the whole reef scale. This study quantifies the grazing impact of individual species of roving herbivorous fishes across an inner shelf fringing reef at Orpheus Island, Great Barrier Reef, Australia. Estimates of roving herbivore impact based on dawn to dusk observations of feeding rates, measurement of bite sizes and relative abundance revealed that the Orpheus Island system was dominated by 3 species: Scarus rivulatus, Chlorurus microrhinos and Siganus doliatus. The estimated impact of all 3 species varied significantly across the reef depth gradient, with the rate of disturbance peaking at the crest and decreasing significantly down the slope and across the reef flat. The estimated species-specific disturbance levels suggested that during the course of a single month 104% of a square metre area of the reef crest is grazed by S. rivulatus while 40% is subject to grazing by C. microrhinos. A total of 26 cm 3 of algal material is removed from the same area by S. doliatus. Overall, there was a 240-fold decrease in grazing activity across the reef flat from that at the crest. The pattern of grazing impact of the numerically dominant siganid and scarid fishes was negatively correlated with the distribution of macroalgae across the same reef gradient. The results of the current study provide support for the hypothesis that algal community structure is shaped by levels of herbivory. KEY WORDS: Coral reef · Herbivore disturbance · Ecosystem impact · Macroalgae · Fishes · ResilienceResale or republication not permitted without written consent of the publisher

ABSTRACT. Around the world, the decreasing health of coral reef ecosystems has highlighted the need to better understand the processes of reef degradation. The development of more sensitive tools, which complement traditional methods of monitoring coral reefs, may reveal earlier signs of degradation and provide an opportunity for pre-emptive responses. We identify new, sensitive metrics of ecosystem processes and benthic composition that allow us to quantify subtle, yet destabilizing, changes in the ecosystem state of an inshore coral reef on the Great Barrier Reef. Following severe climatic disturbances over the period 2011-2012, the herbivorous reef fish community of the reef did not change in terms of biomass or functional groups present. However, fish-based ecosystem processes showed marked changes, with grazing by herbivorous fishes declining by over 90%. On the benthos, algal turf lengths in the epilithic algal matrix increased more than 50% while benthic sediment loads increased 37-fold. The profound changes in processes, despite no visible change in ecosystem state, i.e., no shift to macroalgal dominance, suggest that although the reef has not undergone a visible regime-shift, the ecosystem is highly unstable, and may sit on an ecological knife-edge. Sensitive, process-based metrics of ecosystem state, such as grazing or browsing rates thus appear to be effective in detecting subtle signs of degradation and may be critical in identifying ecosystems at risk for the future.

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