Sean R. Connolly

Projecting coral reef futures under global warming and ocean acidification

Many physiological responses in present-day coral reefs to climate change are interpreted as consistent with the imminent disappearance of modern reefs globally because of annual mass bleaching events, carbonate dissolution, and insufficient time for substantial evolutionary responses. Emerging evidence for variability in the coral calcification response to acidification, geographical variation in bleaching susceptibility and recovery, responses to past climate change, and potential rates of adaptation to rapid warming supports an alternative scenario in which reef degradation occurs with greater temporal and spatial heterogeneity than current projections suggest. Reducing uncertainty in projecting coral reef futures requires improved understanding of past responses to rapid climate change; physiological responses to interacting factors, such as temperature, acidification, and nutrients; and the costs and constraints imposed by acclimation and adaptation.

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.

A LATITUDINAL GRADIENT IN RECRUITMENT OF INTERTIDAL INVERTEBRATES IN THE NORTHEAST PACIFIC OCEAN

A latitudinal gradient in the recruitment rates of intertidal mussels and barnacles was detected in the Northeast Pacific during 1996 and 1997. This gradient was approximately a stepcline: annual recruitment, on average, was 1-2 orders of magnitude higher in central and northern Oregon than in central and northern California. In contrast to the regional differences, large-scale gradients in recruitment within California were small: correlations of recruitment with latitude were weak, and, in all but one case, statistically insignificant. Nonetheless, trends in the data suggest that recruitment within central and northern California was highest between San Francisco and Monterey Bay, where larvae may be retained more nearshore than to the north or south. If so, apparently conflicting claims about latitudinal gradients in recruitment within California can be reconciled. The large scale transition in recruitment rates supports the hypothesis that a marked shift in the intensity of upwelling near Cape Blanco in southern Oregon is a major cause of a coincident transition in community structure. Stronger upwelling (and thus offshore flow) to the south has been hypothesized to transport larvae further offshore and thereby reduce larval supply to nearshore benthic communities. This study confirms that the predicted differences in recruitment exist, and that these differences are large. Preliminary calculations indicate that regional differences in offshore flow are likely to make a larger contribution to the recruitment transitions than several other plausible causes. In addition, recruitment transitions are larger, more abrupt, and more consistent across species than corresponding shifts in percentage cover, which favor competitive dominants. This supports model predictions that competition for space is more intense where recruitment is high. However, the absence of strong, large-scale recruitment gradients within California suggests that mesoscale processes are relatively more important than latitudinal trends in upwelling as determinants of community structure patterns at smaller scales.

Ongoing Collapse of Coral-Reef Shark Populations

Marine ecosystems are suffering severe depletion of apex predators worldwide; shark declines are principally due to conservative life-histories and fisheries overexploitation. On coral reefs, sharks are strongly interacting apex predators and play a key role in maintaining healthy reef ecosystems. Despite increasing fishing pressure, reef shark catches are rarely subject to specific limits, with management approaches typically depending upon no-take marine reserves to maintain populations. Here, we reveal that this approach is failing by documenting an ongoing collapse in two of the most abundant reef shark species on the Great Barrier Reef (Australia). We find an order of magnitude fewer sharks on fished reefs compared to no-entry management zones that encompass only 1% of reefs. No-take zones, which are more difficult to enforce than no-entry zones, offer almost no protection for shark populations. Population viability models of whitetip and gray reef sharks project ongoing steep declines in abundance of 7% and 17% per annum, respectively. These findings indicate that current management of no-take areas is inadequate for protecting reef sharks, even in one of the worlds most-well-managed reef ecosystems. Further steps are urgently required for protecting this critical functional group from ecological extinction.

Connectivity, biodiversity conservation and the design of marine reserve networks for coral reefs

Networks of no-take reserves are important for protecting coral reef biodiversity from climate change and other human impacts. Ensuring that reserve populations are connected to each other and non-reserve populations by larval dispersal allows for recovery from disturbance and is a key aspect of resilience. In general, connectivity between reserves should increase as the distance between them decreases. However, enhancing connectivity may often tradeoff against a network’s ability to representatively sample the system’s natural variability. This “representation” objective is typically measured in terms of species richness or diversity of habitats, but has other important elements (e.g., minimizing the risk that multiple reserves will be impacted by catastrophic events). Such representation objectives tend to be better achieved as reserves become more widely spaced. Thus, optimizing the location, size and spacing of reserves requires both an understanding of larval dispersal and explicit consideration of how well the network represents the broader system; indeed the lack of an integrated theory for optimizing tradeoffs between connectivity and representation objectives has inhibited the incorporation of connectivity into reserve selection algorithms. This article addresses these issues by (1) updating general recommendations for the location, size and spacing of reserves based on emerging data on larval dispersal in corals and reef fishes, and on considerations for maintaining genetic diversity; (2) using a spatial analysis of the Great Barrier Reef Marine Park to examine potential tradeoffs between connectivity and representation of biodiversity and (3) describing a framework for incorporating environmental fluctuations into the conceptualization of the tradeoff between connectivity and representation, and that expresses both in a common, demographically meaningful currency, thus making optimization possible.

Ecological consequences of major hydrodynamic disturbances on coral reefs

A recent tsunami and an apparent increase in the frequency of severe tropical storms underscore the need to understand and predict the ecological consequences of major hydrodynamic disturbances. Reef corals provide the habitat structure that sustains the high biodiversity of tropical reefs, and thus provide the foundation for the ecosystem goods and services that are critical to many tropical societies. Here we integrate predictions from oceanographic models with engineering theory, to predict the dislodgement of benthic reef corals during hydrodynamic disturbances. This generalizes earlier work, by incorporating colonies of any shape and by explicitly examining the effects of hydrodynamic gradients on coral assemblage structure. A field test shows that this model accurately predicts changes in the mechanical vulnerability of coral colonies, and thus their size and shape, with distance from the reef crest. This work provides a general framework for understanding and predicting the effects of hydrodynamic disturbances on coral reef communities; such disturbances have a major role in determining species zonation and coexistence on coral reefs, and are critical determinants of how coral assemblages will respond to changes in the frequency and intensity of tropical storms associated with a changing climate.

Coral reef diversity refutes the neutral theory of biodiversity

The global decline of coral reefs highlights the need to understand the mechanisms that regulate community structure and sustain biodiversity in these systems. The neutral theory, which assumes that individuals are demographically identical regardless of species, seeks to explain ubiquitous features of community structure and biodiversity patterns. Here we present a test of neutral-theory predictions with the use of an extensive species-level data set of Indo-Pacific coral communities. We show that coral assemblages differ markedly from neutral-model predictions for patterns of community similarity and the relative abundance of species. Within local communities, neutral models do not fit relative abundance distributions as well as the classical log-normal distribution. Relative abundances of species across local communities also differ markedly from neutral-theory predictions: coral communities exhibit community similarity values that are far more variable, and lower on average, than the neutral theory can produce. Empirical community similarities deviate from the neutral model in a direction opposite to that predicted in previous critiques of the neutral theory. Instead, our results support spatio-temporal environmental stochasticity as a major driver of diversity patterns on coral reefs.

A Latitudinal Gradient in Northeast Pacific Intertidal Community Structure: Evidence for an Oceanographically Based Synthesis of Marine Community Theory

Intertidal systems have been models for the study of the roles of competition, predation, and disturbance in determining community structure. These systems exhibit considerable regional variability in percentage cover and in the strength of interspecific interactions, which may be due largely to effects of varying larval supply. In Oregon and Washington, experimental studies of space allocation among sessile invertebrates have emphasized the role of benthic processes such as competition and predation. In contrast, studies in central California have emphasized the importance of larval supply. In this article, we identify a gradient in percentage cover in the middle and upper intertidal zone that is consistent with an oceanographically based explanation for these differences: percentage cover of mussels and barnacles is much higher in Oregon, where nearshore circulation promotes high recruitment, than in California, where strong offshore currents inhibit recruitment. A mathematical model incorporating larval transport and interspecific competition for space offers an explanation for the one violation of the hypothesis—higher percentage cover of Chthamalus spp. in California. The findings illustrate that attempts to synthesize regional differences in community structure and dynamics can benefit from considering both the benthic adult and pelagic larval environments.

INDO‐PACIFIC BIODIVERSITY OF CORAL REEFS: DEVIATIONS FROM A MID‐DOMAIN MODEL

Understanding the nature and causes of global gradients in species richness is a perennial ecological problem, and recent work has highlighted the need to assess these gradients relative to an appropriate statistical expectation. This paper examines latitudinal and longitudinal gradients in species richnesses of corals and reef fishes in the Indo-Pacific domain and compares them with gradients predicted by a mid-domain model in which geographic domains are located at random between the latitudinal and longitudinal boundaries of this region. We test for significant differences between observed and predicted species-richness patterns, and we identify regions that are enriched or depauperate in species, relative to expectation. In addition, we move beyond previous mid-domain analyses by directly comparing observed spatial distributions of geographic ranges with those predicted by a mid-domain model. This comparison indicates precisely how species-richness anomalies are produced by nonrandomness in the distribution of species ranges. For both corals and fishes, large and statistically significant differences exist between observed latitudinal and longitudinal species-richness gradients and those predicted by mid-domain models. Longitudinally, species richness is markedly higher than predicted along the African coast and, to a lesser extent, within the Indo-Australian Archipelago (IAA), and it is markedly lower than expected in the eastern Pacific. Latitudinally, species richness becomes increasingly higher than predicted as one moves from the equator to the tropical margins; then it becomes sharply lower than predicted beyond the tropics. Unexpectedly, differences between observed and predicted spatial distributions of range endpoints and midpoints reveal a pattern of nonrandomness that is highly congruent with the hypothesis that gyres in the Indian and Pacific Oceans, with the IAA forming a porous boundary between them, have a major influence on Indo-Pacific species-richness patterns. Our analyses indicate that the perspective offered by a focus on explaining nonrandomness in the location of geographic ranges (rather than explaining why species numbers vary in space) is likely to dramatically alter our assessments of alternative explanations for global species-richness gradients.

THEORY OF MARINE COMMUNITIES: COMPETITION, PREDATION, AND RECRUITMENT‐DEPENDENT INTERACTION STRENGTH

Of the marine animals that spend their adult lives inhabiting benthic communities, most have a planktonic larval phase. In this paper, we derive the relationship between the physical oceanographic processes that transport these larvae and the strength of species interactions in the benthic habitat. We review a model of hierarchical competition for space between two species with planktonic larvae and develop a model for predator–prey dynamics in which prey are space-limited. Lotka-Volterra approximations to these models are developed. The approximations provide per capita interaction strength (the effect of an individual of one species on the per capita growth rate of another) and population interaction strength (the effect of a population of one species on the per capita growth rate of another) as functions of parameters in the original model. Per capita and population interaction strengths of dominant competitors on subordinates decrease in magnitude as offshore advection of larvae increases. The per cap...