Evaluating trophic cascades as drivers of regime shifts in different ocean ecosystems

Contrasting management regimes indicative of mesopredator release in temperate coastal fish assemblages

The absence of functional top predators has been proposed as a mechanism acting to shape fish assemblages in temperate marine ecosystems, with cascading effects on lower trophic levels. We explore this scenario by comparing the trophic and functional status of fish assemblages in Norwegian marine national parks, open to fishing, to a nearby coastal seascape that harbors a system of marine protected areas (MPAs) including a no-take zone. Demersal fish assemblages were sampled using fyke nets over three consecutive seasons. Atlantic cod (Gadus morhua) is potentially a dominant top predator in this ecosystem, and historically, this and other gadids have been targeted by the full range of former and present fisheries. In the present study, we find that average body size of the Atlantic cod was significantly larger in the zoned seascape compared to the unprotected areas (mean ± SD: 36.6 cm ± 14.38 vs. 23.4 ± 7.50; p < .001) and that the unprotected seascape was characterized by a higher abundance of mesopredator fish species. These observations are consistent with the hypothesis that the protection of top predators within MPAs aids to control the mesopredator populations and provides empirical support to the notion that the present state of many coastal fish assemblages is driven by mesopredator release linked to functional depletion of large top predators.

Widespread habitat loss and redistribution of marine top predators in a changing ocean

The Northwest Atlantic Ocean and Gulf of Mexico are among the fastest warming ocean regions, a trend that is expected to continue through this century with far-reaching implications for marine ecosystems. We examine the distribution of 12 highly migratory top predator species using predictive models and project expected habitat changes using downscaled climate models. Our models predict widespread losses of suitable habitat for most species, concurrent with substantial northward displacement of core habitats >500 km. These changes include up to >70% loss of suitable habitat area for some commercially and ecologically important species. We also identify predicted hot spots of multi-species habitat loss focused offshore of the U.S. Southeast and Mid-Atlantic coasts. For several species, the predicted changes are already underway, which are likely to have substantial impacts on the efficacy of static regulatory frameworks used to manage highly migratory species. The ongoing and projected effects of climate change highlight the urgent need to adaptively and proactively manage dynamic marine ecosystems.

Stable landings mask irreversible community reorganizations in an overexploited Mediterranean ecosystem

Cumulative human pressures and climate change can induce nonlinear discontinuous dynamics in ecosystems, known as regime shifts. Regime shifts typically imply hysteresis, a lacking or delayed system response when pressures are reverted, which can frustrate restoration efforts. Here, we investigate whether the northern Adriatic Sea fish and macroinvertebrate community, as depicted by commercial fishery landings, has undergone regime shifts over the last 40 years, and the reversibility of such changes. We use a stochastic cusp model to show that, under the interactive effect of fishing pressure and water warming, the community reorganized through discontinuous changes. We found that part of the community has now reached a new stable state, implying that a recovery towards previous baselines might be impossible. Interestingly, total landings remained constant across decades, masking the low resilience of the community. Our study reveals the importance of carefully assessing regime shifts and resilience in marine ecosystems under cumulative pressures and advocates for their inclusion into management.

A review of the knowledge of reef fish in the Southwest Atlantic

Reef environments are rapidly transforming worldwide, and these changes are causing major impacts to the reef ecosystem. Scientific knowledge is strategic for marine conservation and management in these scenarios. Aiming to contribute to this subject, a systematic review from 1967 to 2020 was conducted, in order to identify gaps in studies regarding reef fish species, ecosystem components and processes. Multidisciplinary sciences concerning reef fish have been rising, mainly in the fields of basic biology and ecology. Besides that, phase shifts and ecosystem services were absent terms in the analyzes of co-occurrence. Research in the ethnosciences needs to be increased, and will improve access to local ecological knowledge, which can be used as a tool to address issues in reef environments. Socio-ecological systems are components of this landscape that has had few publications. The participation in the elaboration of public policies can be a new avenue to foster the biodiversity of reef environments.

Food web interactions in a human dominated Mediterranean coastal ecosystem

Mediterranean coastal ecosystems provide various valuable ecosystem goods and services; however, they are vulnerable to ecological degradation due to a dramatic increase in resource use and environmental stress. Disentangling the effects of multiple human interventions on coastal ecosystems requires whole description of food web interactions using quantitative tools. A mass balance Ecopath model has been developed here for Saronikos Gulf, a naturally oligotrophic Mediterranean coastal ecosystem with a long history of human interventions. Our main focus was to describe the structure and functioning of the ecosystem, investigate the trophic interplay among the various compartments of the food web under the impact of mixed multi-gear fisheries, and to quantify resilience related emergent ecosystem properties. To this end, we reviewed a large amount of local and regional biological information which was integrated in 40 functional groups covering all trophic levels, while fishing activities were described with 7 fleets. The model shared characteristics of both productive (e.g., high amount of flows) and oligotrophic systems (e.g., low biomass accumulation) and presented typical features of Mediterranean ecosystem functioning, such as the importance of detritus as an energy source, strong benthic-pelagic coupling and the dominance of the pelagic compartment in terms of total production and consumption. Trophic forcing in the ecosystem of Saronikos Gulf was complex with both top-down and bottom-up drivers being important. Zooplankton was the central nexus between basal resources and higher trophic levels, while top predators such as hake, squids and anglerfish were identified as keystone species presenting a significant overall effect on the food web via direct and indirect trophic interactions. Ecological indicators depicted a moderately complex food-web of a large and immature ecosystem with its strengths in reserve being affected by environmental degradation. Additionally, exploitation indices classified fishing activities in Saronikos Gulf as unsustainable, affecting several target groups, including high trophic level species. However, the morphological and bathymetric complexity of Saronikos Gulf seems to function as a natural ecological reserve for the ecosystem by providing nursery grounds to various species (e.g., hake, small pelagic fishes) and supporting important fish stocks for local fisheries.

Kelp Forests: Catastrophes, Resilience, and Management

Resilient kelp forests provide foundation habitat for marine ecosystems and are indicators of the ecosystems’ sustainable natural capital. Loss of resilience and imperfectly reversible catastrophic shifts from kelp forests to urchin barrens, due to pollution or loss of a top predator, are part of an ecological tipping point phenomenon, and involve a loss in sustainable natural capital. Management controls to prevent or reverse these shifts and losses are classified in a number of ways. Systemic controls eliminate the cause of the problem. Symptomatic controls use leverage points for more direct control of the populations affected, urchin harvesting or culling, or kelp enhancement. There is a distinction between ongoing structural (press) controls versus temporary or intermittent perturbation (pulse) controls, and one between shift preventing versus shift reversing or restorative controls. Adaptive management and the options it creates both focus on reductions in uncertainty and control policies with the flexibility to take advantage of those reductions. The various management distinctions are most easily understood by modeling the predator-urchin-kelp marine ecosystem. This paper develops a mathematical model of the ecosystem that has the potential for two different catastrophic shifts between equilibria. Pulse disturbances, originating from exogenous abiotic factors or population dynamics elsewhere in the metacommunity, can activate shifts. A measure of probabilistic resilience is developed and used as part of an assessment of the ecosystem’s sustainable stock of natural capital. With perturbation outcomes clustered around the originating equilibrium, hysteresis is activated, resulting imperfect reversibility of catastrophic shifts, and a loss in natural capital. The difficulty of reversing a shift from kelp forest to urchin barren, with an associated loss in sustainable natural capital, is an example. Management controls are modeled. I find that systemic and symptomatic, and press and pulse, controls can be complementary. Restorative controls tend to be more difficult or costly than preventative ones. Adaptive management, favoring flexible, often preventative, controls, creates option value, lowering control costs and/or losses in sustainable natural capital. Two cases are used to illustrate, Tasmania, Australia and Haida Gwaii, Canada.

Distribution of Sprattus sprattus phalericus (Risso, 1827) and zooplankton near the Black Sea redoxcline

Understanding what environmental drivers influence marine predator-prey relationships can be key to managing and protecting ecosystems, especially in the face of future climate change risks. This is especially important in environments such as the Black Sea, where strong biogeochemical gradients can drive marine habitat partitioning and ecological interactions. We used underwater video recordings in the north-eastern Black Sea in November 2013 to observe the distribution and behaviour of the Black Sea sprat (Sprattus sprattus phalericus, Risso 1827) and its zooplankton prey. Video recordings have shown that the Black Sea sprat S. sprattus phalericus tolerates severely hypoxic waters near the redoxcline. The school was distributed in the 33-96 m layer [oxygen concentration (O₂ ) 277-84 μmol L^-1 ]. Some individuals were observed to leave the school and descended 20 m deeper for foraging on copepods in the 119-123 m layer (O₂ 12-10 μmol L^-1 ). Zooplankton appeared concentrated on the upper boundary of the suboxic zone (O₂  < 10 μmol L^-1 ). No zooplankton were observed below O₂ 6-7 μmol L^-1 (128 m). Understanding the ability of this species to tolerate low oxygen waters is crucial to predicting future responses to natural and anthropogenic changes in hypoxia.

Effects of stochasticity and social norms on complex dynamics of fisheries

Recreational fishing is a highly socioecological process. Although recreational fisheries are self-regulating and resilient, changing anthropogenic pressure drives these fisheries to overharvest and collapse. Here, we evaluate the effect of demographic and environmental stochasticity for a social-ecological two-species fish model. In the presence of noise, we find that an increase in harvesting rate drives a critical transition from high-yield-low-price fisheries to low-yield-high-price fisheries. To calculate stochastic trajectories for demographic noise, we derive the master equation corresponding to the model and perform a Monte Carlo simulation. Moreover, the analysis of the probabilistic potential and mean first-passage time reveals the resilience of alternative steady states. We also describe the efficacy of a few generic indicators in forecasting sudden transitions. Furthermore, we show that incorporating social norms on the model allows a moderate fish density to maintain despite higher harvesting rates. Overall, our study highlights the occurrence of critical transitions in a stochastic social-ecological model and suggests ways to mitigate them.

Transition probabilities help identify putative drivers of community change in complex systems

Understanding the role of larger-scale processes in modulating the assembly, structure, and dynamics of communities is critical for forecasting the effects of climate-change and managing ecosystems. Developing this comprehensive perspective is difficult though, because species interactions are complex, interdependent, and dynamic through space and time. Typically, experiments focus on tractable subsets of interactions that will be most critical to investigate and explain shifts in communities, but qualitatively base these choices on experience, natural history, and theory. One quantitative approach to identify the putative forces regulating communities, without reducing system complexity, is estimating transition probabilities among species occupying space (i.e., multispecies Markov chain models). Although not mechanistic, these models estimate the relative frequency and importance of ecological pathways in community assembly and dynamics, and can serve as a framework to identify how pathways change across large scales and which are most important to investigate further. Here, we demonstrate this method in the Gulf of Maine (GOM) intertidal zone, where research has largely focused on the local-scale processes that influence communities, while the mechanisms responsible for more regional shifts in communities are less clear. Transition probabilities of faunal elements were quantified bimonthly for ~2.5 yr in local intertidal communities at three replicate sites in the southern, mid-coast, and northern GOM. Transitions related to mortality, colonization, and replacement by mussels, barnacles, red algae, and encrusting corallines differed regionally, suggesting specific pathways related to consumer pressure and recruitment vary across the GOM with shifting intertidal community structure. Combined with species abundance data and insights from previous research, we develop and evaluate the pathways by which communities likely change in the GOM. Species interactions in local communities can be complex, and this complexity should be incorporated into hypothesis building, experiments, theory, interpretations, and forecasts in ecology. Such a comprehensive approach will be critical to understand how regional shifts in local interactions can drive large-scale community change.

Regime shifts and resilience in China's coastal ecosystems

Regime shift often results in large, abrupt, and persistent changes in the provision of ecosystem services and can therefore have significant impacts on human wellbeing. Understanding regime shifts has profound implications for ecosystem recovery and management. China's coastal ecosystems have experienced substantial deterioration within the past decades, at a scale and speed the world has never seen before. Yet, information about this coastal ecosystem change from a dynamics perspective is quite limited. In this review, I synthesize existing information on coastal ecosystem regime shifts in China and discuss their interactions and cascading effects. The accumulation of regime shifts in China's coastal ecosystems suggests that the desired system resilience has been profoundly eroded, increasing the potential of abrupt shifts to undesirable states at a larger scale, especially given multiple escalating pressures. Policy and management strategies need to incorporate resilience approaches in order to cope with future challenges and avoid major losses in China's coastal ecosystem services.

A holistic view of marine regime shifts

Understanding marine regime shifts is important not only for ecology but also for developing marine management that assures the provision of ecosystem services to humanity. While regime shift theory is well developed, there is still no common understanding on drivers, mechanisms and characteristic of abrupt changes in real marine ecosystems. Based on contributions to the present theme issue, we highlight some general issues that need to be overcome for developing a more comprehensive understanding of marine ecosystem regime shifts. We find a great divide between benthic reef and pelagic ocean systems in how regime shift theory is linked to observed abrupt changes. Furthermore, we suggest that the long-lasting discussion on the prevalence of top-down trophic or bottom-up physical drivers in inducing regime shifts may be overcome by taking into consideration the synergistic interactions of multiple stressors, and the special characteristics of different ecosystem types. We present a framework for the holistic investigation of marine regime shifts that considers multiple exogenous drivers that interact with endogenous mechanisms to cause abrupt, catastrophic change. This framework takes into account the time-delayed synergies of these stressors, which erode the resilience of the ecosystem and eventually enable the crossing of ecological thresholds. Finally, considering that increased pressures in the marine environment are predicted by the current climate change assessments, in order to avoid major losses of ecosystem services, we suggest that marine management approaches should incorporate knowledge on environmental thresholds and develop tools that consider regime shift dynamics and characteristics. This grand challenge can only be achieved through a holistic view of marine ecosystem dynamics as evidenced by this theme issue.

The importance of within-system spatial variation in drivers of marine ecosystem regime shifts

Comparative analyses of the dynamics of exploited marine ecosystems have led to differing hypotheses regarding the primary causes of observed regime shifts, while many ecosystems have apparently not undergone regime shifts. These varied responses may be partly explained by the decade-old recognition that within-system spatial heterogeneity in key climate and anthropogenic drivers may be important, as recent theoretical examinations have concluded that spatial heterogeneity in environmental characteristics may diminish the tendency for regime shifts. Here, we synthesize recent, empirical within-system spatio-temporal analyses of some temperate and subarctic large marine ecosystems in which regime shifts have (and have not) occurred. Examples from the Baltic Sea, Black Sea, Bengula Current, North Sea, Barents Sea and Eastern Scotian Shelf reveal the largely neglected importance of considering spatial variability in key biotic and abiotic influences and species movements in the context of evaluating and predicting regime shifts. We highlight both the importance of understanding the scale-dependent spatial dynamics of climate influences and key predator–prey interactions to unravel the dynamics of regime shifts, and the utility of spatial downscaling of proposed mechanisms (as evident in the North Sea and Barents Sea) as a means of evaluating hypotheses originally derived from among-system comparisons.