Abstract. The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them. We apply such an approach to 30-60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.

Coral reefs world-wide are threatened by escalating local and global impacts, and some impacted reefs have shifted from coral dominance to a state dominated by macroalgae. Therefore, there is a growing need to understand the processes that affect the capacity of these ecosystems to return to coral dominance following disturbances, including those that prevent the establishment of persistent stands of macroalgae. Unlike many reefs in the Caribbean, over the last several decades, reefs around the Indo-Pacific island of Moorea, French Polynesia have consistently returned to coral dominance following major perturbations without shifting to a macroalgae-dominated state. Here, we present evidence of a rapid increase in populations of herbivorous fishes following the most recent perturbation, and show that grazing by these herbivores has prevented the establishment of macroalgae following near complete loss of coral on offshore reefs. Importantly, we found the positive response of herbivorous fishes to increased benthic primary productivity associated with coral loss was driven largely by parrotfishes that initially recruit to stable nursery habitat within the lagoons before moving to offshore reefs later in life. These results underscore the importance of connectivity between the lagoon and offshore reefs for preventing the establishment of macroalgae following disturbances, and indicate that protecting nearshore nursery habitat of herbivorous fishes is critical for maintaining reef resilience.

The long‐standing interest in density dependence in demographic rates of organisms stems from its influence in bounding population fluctuations and in shaping spatial patterns of abundance. Despite growing evidence that early mortality of marine reef fishes can be density dependent and can involve predation, the underlying biological mechanisms have not as yet been fully explored in any system. Here we examine the causes of density‐dependent juvenile mortality for two tropical damselfishes, Dascyllus flavicaudus and D. trimaculatus. These species shelter in branching corals or anemones, and they feed on plankton above their microhabitats during the day. Field experiments confirmed that density‐dependent juvenile mortality of both Dascyllus species arose from predation and that most of the density‐dependent loss could be attributed to small‐bodied, resident piscivores (e.g., sandperch, squirrelfish) rather than larger, transient species (e.g., jacks). Over the diel cycle, mortality was strongly density dependent during the dark when damselfish were sheltering but not during daylight when fish were actively foraging. Infrared video recordings revealed the species of predators responsible for most losses and indicated that most predatory events occurred from late twilight to early night, when damselfishes were in shelters and not feeding. Individuals were most at risk when located near or just outside the perimeter of a shelter. The proportion of a cohort in the riskiest areas of a microhabitat increased with density. The cause of the increased fraction of individuals at risk with increasing density was intraspecific interactions among sheltering fish jostling for space in the safest regions; this resulted in the displacement of less aggressive individuals to riskier locations. Thus, density‐dependent mortality in both damselfishes arose from interference competition for refuge space from crepuscular and nocturnal predators.

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The power of any test of an environmental impact is simultaneously constrained by (1) the variability of the data, (2) the magnitude of the putative impact, and (3) the number of independent sampling events. In the context of the Before-After-Control-Impact design with Paired sampling (BACIP), the variability of interest is the temporal variation in the estimated differences in a parameter (e.g., population density) between two unperturbed sites. The challenges in designing a BACIP study are to choose appropriate parameters to measure and to determine the adequate number and timing of sampling events. Two types of studies that are commonly conducted can provide useful information in designing a BACIP study. These are (1) long-term studies that provide estimates of the natural temporal and spatial variability of environmental parameters and (2) spatial surveys around already-perturbed areas ("After-only" studies) that can suggest the magnitude of impacts.Here we use data from a long-term study and an After-only study to illustrate their potential contributions to the design ofBACIP studies. The long-term study of parameters sampled at two undisturbed sites yielded estimates of natural temporal variability. Betweensite differences in chemical-physical parameters (e.g., elemental concentration) and in individual-based biological parameters (e.g., body size) were quite consistent through time, while differences in population-based parameters (e.g., density) were more variable. Serial correlation in the time series of differences was relatively small and did not appear to vary among the parameter groups. The After-only study yielded estimates of the magnitude of impacts through comparison of sites near and distant from a point-source discharge. The estimated magnitude of effects was greatest for population-based parameters and least for chemical-physical parameters, which tended to balance the statistical power associated with these two parameter groups. Individual-based parameters were intermediate in estimates of effect size. Thus, the ratio of effect size to variability was greatest for individualbased parameters and least for population and chemical-physical parameters.The results suggest that relatively few of the population and chemical-physical parameters could provide adequate power given the time constraints of most studies. This indicates that greater emphasis on individual-based parameters is needed in field assessments of environmental impacts. It will be critical to develop and test predictive models that link these impacts with effects on populations.