When resilience is not enough: 2022 extreme marine heatwave threatens climatic refugia for a habitat-forming Mediterranean octocoral.

Climate change is impacting ecosystems worldwide, and the Mediterranean Sea is no exception. Extreme climatic events, such as marine heat waves (MHWs), are increasing in frequency, extent and intensity during the last decades, which has been associated with an increase in mass mortality events for multiple species. Coralligenous assemblages, where the octocoral Paramuricea clavata lives, are strongly affected by MHWs. The Medes Islands Marine Reserve (NW Mediterranean) was considered a climate refugia for P. clavata, as their populations were showing some resilience to these changing conditions. In this study, we assessed the impacts of the MHWs that occurred between 2016 and 2022 in seven shallow populations of the octocoral P. clavata from a Mediterranean Marine Protected Area. The years that the mortality rates increased significantly were associated with the ones with strong MHWs, 2022 being the one with higher mortalities. In 2022, with 50 MHW days, the proportion of total affected colonies was almost 70%, with a proportion of the injured surface of almost 40%, reaching levels never attained in our study site since the monitoring was started. We also found spatial variability between the monitored populations. Whereas few of them showed low levels of mortality, others lost around 75% of their biomass. The significant impacts documented here raise concerns about the future of shallow P. clavata populations across the Mediterranean, suggesting that the resilience of this species may not be maintained to sustain these populations face the ongoing warming trends.

El canvi climatic està impactant els ecosistemes arreu del planeta, i el Mar Mediterrani no n'és una excepció. Els esdeveniments climàtics extrems, com ara les onades de calor marines, estan augmentant en freqüència, extensió i intensitat en les darreres dècades, i estan sent associades a un increment dels esdeveniments de mortalitat massiva de múltiples espècies. El coral·ligen, on hi viu l'octocorall Paramuricea clavata, està altament afectat per les onades de calor marines. La Reserva Marina de les Illes Medes (NO del Mediterrani) es considerava un refugi climàtic per aquesta espècie, degut a que les seves poblacions mostraven certa resiliència a les condicions canviants. En aquest estudi hem avaluat els impactes de les onades de calor marines succeïdes entre els anys 2016 i 2022 a set poblacions someres de l'octocorall P. clavata, en una Àrea Marina Protegida del Mediterrani. Els anys en els quals les taxes de mortalitat van incrementar significativament s'associen amb els anys amb fortes onades de calor marines, sent el 2022 l'any amb la mortalitat més elevada. Al 2022, amb 50 dies d'onada de calor, la proporció total de colònies afectades va ser prop del 70%, amb un percentatge de superfície afectada de gairebé el 40%, arribant a valors mai observats en el lloc d'estudi des de que es va iniciar el seguiment d'aquestes poblacions. També hem observat variabilitat espacial entre les poblacions mostrejades. Mentre que algunes d'elles han mostrat poca mortalitat, altres han perdut al voltant del 75% de la seva biomassa. Els impactes documentats en aquest estudi mostren un futur preocupant de les poblacions someres de P. clavata arreu del Mediterrani, i això suggereix que la resiliència d'aquesta espècie podria no ser suficient per mantenir les seves poblacions en l'escenari d'escalfament que es preveu.

Mediterranean octocoral populations exposed to marine heatwaves are less resilient to disturbances.

The effects of climate change are now more pervasive than ever. Marine ecosystems have been particularly impacted by climate change, with marine heatwaves (MHWs) being a strong driver of mass mortality events. Even in the most optimistic greenhouse gas emission scenarios, MHWs will continue to increase in frequency, intensity and duration. For this reason, understanding the resilience of marine species to the increase of MHWs is crucial to predicting their viability under future climatic conditions. In this study, we explored the consequences of MHWs on the resilience (the ability of a population to resist and recover after a disturbance) of a Mediterranean key octocoral species, Paramuricea clavata, to further disturbances to their population structure. To quantify P. clavata's capacity to resist and recover from future disturbances, we used demographic information collected from 1999 to 2022, from two different sites in the NW Mediterranean Sea to calculate the transient dynamics of their populations. Our results showed that the differences in the dynamics of populations exposed and those not exposed to MHWs were driven mostly by differences in mean survivorship and growth. We also showed that after MHWs P. clavata populations had lower resistance and slower rates of recovery than those not exposed to MHWs. Populations exposed to MHWs had lower resistance elasticity to most demographic processes compared to unexposed populations. In contrast, the only demographic process showing some differences when comparing the speed of recovery elasticity values between populations exposed and unexposed to MHWs was stasis. Finally, under scenarios of increasing frequency of MHWs, the extinction of P. clavata populations will accelerate and their capacity to resist and recover after further disturbances will be hampered. Overall, these findings confirm that future climatic conditions will make octocoral populations even more vulnerable to further disturbances. These results highlight the importance of limiting local impacts on marine ecosystems to dampen the consequences of climate change.

Els efectes del canvi climàtic són més presents que mai. Els ecosistemes marins s'han vist especialment afectats pel canvi climàtic, i les onades de calor marines (MHW) són un fort motor d'esdeveniments de mortalitat en massa. Fins i tot en els escenaris d'emissió de gasos d'efecte hivernacle més optimistes, les MHWs continuaran augmentant en freqüència, intensitat i durada. Per aquest motiu, entendre la resiliència de les espècies marines davant l'augment de les MHWs és crucial per predir la seva viabilitat en condicions climàtiques futures. En aquest estudi hem explorat les conseqüències de les MHWs sobre la resiliència (la capacitat d'una població de resistir i recuperar­se després d'una pertorbació) d'una espècie d'octocorall clau en el Mar Mediterràni, Paramuricea clavata, a pertorbacions en la seva estructura poblacional. Per quantificar la capacitat de P. clavata de resistir i recuperar­se de pertorbacions futures, hem utilitzat informació demogràfica recollida des del 1999 fins al 2022, de dos localitats diferents al NW del mar Mediterrani, per calcular la dinàmica transitòria de les seves poblacions. Els nostres resultats van mostrar que les diferències en la dinàmica de les poblacions exposades i no exposades a MHWs es deuen, principalment, a diferències en la taxa mitjana de supervivència i creixement. També vam demostrar que després de MHWs, les poblacions de P. clavata tenien menor resistència i taxes de recuperació més lentes que les que no estaven exposades a MHWs. Les poblacions exposades a MHWs tenien una elasticitat de resistència menor en la majoria dels processos demogràfics en comparació amb les poblacions no exposades. En canvi, l'únic procés demogràfic que mostra algunes diferències en comparar els valors d'elasticitat de la velocitat de recuperació entre poblacions exposades i no exposades a MHWs va ser l'estasi. Finalment, en escenaris d'augment de la freqüència dels MHW, l'extinció de les poblacions de P. clavata s'accelerarà i la seva capacitat de resistir i recuperar­se després de noves pertorbacions es veurà obstaculitzada. En general, els nostres resultats confirmen que les condicions climàtiques futures faran que les poblacions d'octocoralls siguin encara més vulnerables a més pertorbacions. Aquests resultats posen de manifest la importància de limitar els impactes locals sobre els ecosistemes marins per esmorteir les conseqüències del canvi climàtic.

Life history mediates the trade-offs among different components of demographic resilience.

Accelerating rates of biodiversity loss underscore the need to understand how species achieve resilience-the ability to resist and recover from a/biotic disturbances. Yet, the factors determining the resilience of species remain poorly understood, due to disagreements on its definition and the lack of large-scale analyses. Here, we investigate how the life history of 910 natural populations of animals and plants predicts their intrinsic ability to be resilient. We show that demographic resilience can be achieved through different combinations of compensation, resistance and recovery after a disturbance. We demonstrate that these resilience components are highly correlated with life history traits related to the species' pace of life and reproductive strategy. Species with longer generation times require longer recovery times post-disturbance, whilst those with greater reproductive capacity have greater resistance and compensation. Our findings highlight the key role of life history traits to understand species resilience, improving our ability to predict how natural populations cope with disturbance regimes.

Community structure determines the predictability of population collapse.

Early warning signals (EWS) are phenomenological tools that have been proposed as predictors of the collapse of biological systems. Although a growing body of work has shown the utility of EWS based on either statistics derived from abundance data or shifts in phenotypic traits such as body size, so far this work has largely focused on single species populations. However, to predict reliably the future state of ecological systems, which inherently could consist of multiple species, understanding how reliable such signals are in a community context is critical. Here, reconciling quantitative trait evolution and Lotka-Volterra equations, which allow us to track both abundance and mean traits, we simulate the collapse of populations embedded in mutualistic and multi-trophic predator-prey communities. Using these simulations and warning signals derived from both population- and community-level data, we showed the utility of abundance-based EWS, as well as metrics derived from stability-landscape theory (e.g. width and depth of the basin of attraction), were fundamentally linked. Thus, the depth and width of such stability-landscape curves could be used to identify which species should exhibit the strongest EWS of collapse. The probability a species displays both trait and abundance-based EWS was dependent on its position in a community, with some species able to act as indicator species. In addition, our results also demonstrated that in general trait-based EWS were less reliable in comparison with abundance-based EWS in forecasting species collapses in our simulated communities. Furthermore, community-level abundance-based EWS were fairly reliable in comparison with their species-level counterparts in forecasting species-level collapses. Our study suggests a holistic framework that combines abundance-based EWS and metrics derived from stability-landscape theory that may help in forecasting species loss in a community context.

Effect of Environmental Change Distribution on Artificial Life Simulations.

It is already well known that environmental variation has a big effect on real evolution, and similar effects have been found in evolutionary artificial life simulations. In particular, a lot of research has been carried out on how the various evolutionary outcomes depend on the noise distributions representing the environmental changes, and how important it is for models to use inverse power-law distributions with the right noise colour. However, there are two distinct factors of relevance-the average total magnitude of change per unit time and the distribution of individual change magnitudes-and misleading results may emerge if those factors are not properly separated. This article makes use of an existing agent-based artificial life modeling framework to explore this issue using models previously tried and tested for other purposes. It begins by demonstrating how the total magnitude and distribution effects can easily be confused, and goes on to show how it is possible to untangle the influence of these interacting factors by using correlation-based normalization. It then presents a series of simulation results demonstrating that interesting dependencies on the noise distribution remain after separating those factors, but many effects involving the noise colour of inverse power-law distributions disappear, and very similar results arise across restricted-range white-noise distributions. The average total magnitude of change per unit time is found to have a substantial effect on the simulation outcomes, but the distribution of individual changes has very little effect. A robust counterexample is thereby provided to the idea that it is always important to use accurate environmental change distributions in artificial life models.

Hybrid Model of the Collapse of the Commercial Crab Paralithodes camtschaticus (Decapoda, Lithodidae) Population of the Kodiak Archipelago.

Degradation of commercial populations remains a frequent phenomenon even with the use of methods of stocktaking and control of production volume. In fish farming, the concept of "overfishing" is used with and the signs of this condition are well known. However, the processes leading to the degradation of reserves develope in various ways. According to the theory of nonlinear dynamical systems, several types of crisis development can be classified. Of particular interest are the phenomena of collapse, that is, variants of a rapid decline in numbers, which are unexpected for the organizations controlling the fishery. Immediately before the collapse, the state of the stock can be assessed as relatively stable and it may experience fluctuations. Contrary to expectations, there was no rapid recovery after a rapid reduction in cod, whitefish Coregonus clupeaformis of the Great Lakes, halibut and other valuable species. This paper considers a hybrid model for the collapse of red king crab Paralithodes camtschaticus stocks of the Kodiak archipelago of Alaska with unusual distinctive oscillating dynamics. The computational scenario in a hybrid system with survival and growth equations considers the logic of decision-making in operation management. The scenario differs in that after the fall of catches, the crab population goes into the sporadic fluctuations that do not have a regular character and are not characteristic of the population. The collapse itself occurs after a long interval of fishing while the population is in an unstable mode. The analysis shows that a long species life cycle is not a decisive factor for eliminating the risk of a collapse scenario. The presence of reserve generations does not change the situation qualitatively, the efficiency of their reproduction in crab and cod off the coast of Labrador turned out to be unexpectedly low. The status of stocks of large predators that require seasonal moratoriums on fishing must be regularly checked.

Phase transitions in biology: from bird flocks to population dynamics.

Phase transitions are an important and extensively studied concept in physics. The insights derived from understanding phase transitions in physics have recently and successfully been applied to a number of different phenomena in biological systems. Here, we provide a brief review of phase transitions and their role in explaining biological processes ranging from collective behaviour in animal flocks to neuronal firing. We also highlight a new and exciting area where phase transition theory is particularly applicable: population collapse and extinction. We discuss how phase transition theory can give insight into a range of extinction events such as population decline due to climate change or microbial responses to stressors such as antibiotic treatment.

Population collapse of habitat-forming species in the Mediterranean: a long-term study of gorgonian populations affected by recurrent marine heatwaves.

Understanding the resilience of temperate reefs to climate change requires exploring the recovery capacity of their habitat-forming species from recurrent marine heatwaves (MHWs). Here, we show that, in a Mediterranean highly enforced marine protected area established more than 40 years ago, habitat-forming octocoral populations that were first affected by a severe MHW in 2003 have not recovered after 15 years. Contrarily, they have followed collapse trajectories that have brought them to the brink of local ecological extinction. Since 2003, impacted populations of the red gorgonian Paramuricea clavata (Risso, 1826) and the red coral Corallium rubrum (Linnaeus, 1758) have followed different trends in terms of size structure, but a similar progressive reduction in density and biomass. Concurrently, recurrent MHWs were observed in the area during the 2003-2018 study period, which may have hindered populations recovery. The studied octocorals play a unique habitat-forming role in the coralligenous assemblages (i.e. reefs endemic to the Mediterranean Sea home to approximately 10% of its species). Therefore, our results underpin the great risk that recurrent MHWs pose for the long-term integrity and functioning of these emblematic temperate reefs.

Density dependence and the spread of invasive big-headed ants (Pheidole megacephala) in an East African savanna.

Supercolonial ants are among the largest cooperative units in nature, attaining extremely high densities. How these densities feed back into their population growth rates and how abundance and extrinsic factors interact to affect their population dynamics remain open questions. We studied how local worker abundance and extrinsic factors (rain, tree density) affect population growth rate and spread in the invasive big-headed ant, which is disrupting a keystone mutualism between acacia trees and native ants in parts of East Africa. We measured temporal changes in big-headed ant (BHA) abundance and rates of spread over 20 months along eight transects, extending from areas behind the front with high BHA abundances to areas at the invasion front with low BHA abundances. We used models that account for negative density dependence and incorporated extrinsic factors to determine what variables best explain variation in local population growth rates. Population growth rates declined with abundance, however, the strength of density dependence decreased with abundance. We suggest that weaker density dependence at higher ant abundances may be due to the beneficial effect of cooperative behavior that partially counteracts resource limitation. Rainfall and tree density had minor effects on ant population dynamics. BHA spread near 50 m/year, more than previous studies reported and comparable to rates of spread of other supercolonial ants. Although we did not detect declines in abundance in areas invaded a long time ago (> 10 years), continued monitoring of abundance at invaded sites may help to better understand the widespread collapse of many invasive ants.

When very slow is too fast - collapse of a predator-prey system.

Critical transitions or regime shifts are sudden and unexpected changes in the state of an ecosystem, that are usually associated with dangerous levels of environmental change. However, recent studies show that critical transitions can also be triggered by dangerous rates of environmental change. In contrast to classical regime shifts, such rate-induced critical transitions do not involve any obvious loss of stability, or a bifurcation, and thus cannot be explained by the linear stability analysis. In this work, we demonstrate that the well-known Rosenzweig-MacArthur predator-prey model can undergo a rate-induced critical transition in response to a continuous decline in the habitat quality, resulting in a collapse of the predator and prey populations. Rather surprisingly, the collapse occurs even if the environmental change is slower than the slowest process in the model. To explain this counterintuitive phenomenon, we combine methods from geometric singular perturbation theory with the concept of a moving equilibrium, and study critical rates of environmental change with dependence on the initial state and the system parameters. Moreover, for a fixed rate of environmental change, we determine the set of initial states that undergo a rate-induced population collapse. Our results suggest that ecosystems may be more sensitive to how fast environmental conditions change than previously assumed. In particular, unexpected critical transitions with dramatic ecological consequences can be triggered by environmental changes that (i) do not exceed any dangerous levels, and (ii) are slower than the natural timescales of the ecosystem. This poses an interesting research question whether regime shifts observed in the natural world are predominantly rate-induced or bifurcation-induced.

Disease, contagious cannibalism, and associated population crash in an omnivorous bug, Geocoris pallens.

Disease and cannibalism are two strongly density-dependent processes that can suppress predator populations. Here we show that California populations of the omnivorous predatory bug Geocoris pallens are subject to infection by a pathogen, as yet unidentified, that elicits elevated expression of cannibalism. Laboratory experiments showed that the pathogen is moderately virulent, causing flattened abdomens, elevated nymphal mortality, delayed development, and reduced body size of adult females. Infection furthermore increases the expression of cannibalism. Field populations of Geocoris spp. declined strongly in association with sharp increases in the expression of egg cannibalism by adult G. pallens. Increased cannibalism was accompanied by a strongly bimodal distribution of cannibalism expression, with some females (putatively uninfected) expressing little cannibalism and others (putatively infected) consuming most or all of the eggs present. Highly cannibalistic females did not increase their consumption of Ephestia cautella moth eggs, suggesting that the high cannibalism phenotype reflected a specific loss of restraint against eating conspecifics. Highly cannibalistic females also often exhibited reduced egg laying, consistent with a virulent pathogen; less frequently, more cannibalistic females exhibited elevated egg laying, suggesting that cannibalism might also facilitate recycling of nutrients in eggs. Elevated cannibalism was not correlated with reduced prey availability or elevated field densities of G. pallens. Geocoris pallens population crashes appear to reflect the combined consequences of direct virulence-adverse pathogen effects on the infected host's physiology-and indirect virulence-mortality of both infected and uninfected individuals due to elevated cannibalism expression by infected individuals.

Mechanism for microbial population collapse in a fluctuating resource environment.

Managing trade-offs through gene regulation is believed to confer resilience to a microbial community in a fluctuating resource environment. To investigate this hypothesis, we imposed a fluctuating environment that required the sulfate-reducer Desulfovibrio vulgaris to undergo repeated ecologically relevant shifts between retaining metabolic independence (active capacity for sulfate respiration) and becoming metabolically specialized to a mutualistic association with the hydrogen-consuming Methanococcus maripaludis Strikingly, the microbial community became progressively less proficient at restoring the environmentally relevant physiological state after each perturbation and most cultures collapsed within 3-7 shifts. Counterintuitively, the collapse phenomenon was prevented by a single regulatory mutation. We have characterized the mechanism for collapse by conducting RNA-seq analysis, proteomics, microcalorimetry, and single-cell transcriptome analysis. We demonstrate that the collapse was caused by conditional gene regulation, which drove precipitous decline in intracellular abundance of essential transcripts and proteins, imposing greater energetic burden of regulation to restore function in a fluctuating environment.

Relation between stability and resilience determines the performance of early warning signals under different environmental drivers.

Shifting patterns of temporal fluctuations have been found to signal critical transitions in a variety of systems, from ecological communities to human physiology. However, failure of these early warning signals in some systems calls for a better understanding of their limitations. In particular, little is known about the generality of early warning signals in different deteriorating environments. In this study, we characterized how multiple environmental drivers influence the dynamics of laboratory yeast populations, which was previously shown to display alternative stable states [Dai et al., Science, 2012]. We observed that both the coefficient of variation and autocorrelation increased before population collapse in two slowly deteriorating environments, one with a rising death rate and the other one with decreasing nutrient availability. We compared the performance of early warning signals across multiple environments as "indicators for loss of resilience." We find that the varying performance is determined by how a system responds to changes in a specific driver, which can be captured by a relation between stability (recovery rate) and resilience (size of the basin of attraction). Furthermore, we demonstrate that the positive correlation between stability and resilience, as the essential assumption of indicators based on critical slowing down, can break down in this system when multiple environmental drivers are changed simultaneously. Our results suggest that the stability-resilience relation needs to be better understood for the application of early warning signals in different scenarios.

Fishing amplifies forage fish population collapses.

Forage fish support the largest fisheries in the world but also play key roles in marine food webs by transferring energy from plankton to upper trophic-level predators, such as large fish, seabirds, and marine mammals. Fishing can, thereby, have far reaching consequences on marine food webs unless safeguards are in place to avoid depleting forage fish to dangerously low levels, where dependent predators are most vulnerable. However, disentangling the contributions of fishing vs. natural processes on population dynamics has been difficult because of the sensitivity of these stocks to environmental conditions. Here, we overcome this difficulty by collating population time series for forage fish populations that account for nearly two-thirds of global catch of forage fish to identify the fingerprint of fisheries on their population dynamics. Forage fish population collapses shared a set of common and unique characteristics: high fishing pressure for several years before collapse, a sharp drop in natural population productivity, and a lagged response to reduce fishing pressure. Lagged response to natural productivity declines can sharply amplify the magnitude of naturally occurring population fluctuations. Finally, we show that the magnitude and frequency of collapses are greater than expected from natural productivity characteristics and therefore, likely attributed to fishing. The durations of collapses, however, were not different from those expected based on natural productivity shifts. A risk-based management scheme that reduces fishing when populations become scarce would protect forage fish and their predators from collapse with little effect on long-term average catches.

Catch composition and aspects of the biology of sharks caught by Thai commercial fisheries in the Andaman Sea.

Catch composition, landing patterns and biological aspects of sharks caught by commercial fishing fleet operating in the Andaman Sea were recorded from landing sites in Ranong province of Thailand over a period of 1 year. Of the 64 species previously reported in the existing Thailand checklist, only 17 species were recorded in this study. Shark landings from the Andaman Sea appear now to be dominated largely by bamboo sharks Chiloscyllium spp. (Hemiscylliidae), which contribute c. 65% of the total number of sharks recorded. The carcharhinid sharks comprised c. 30·5% to the total catch, while the remaining c. 4·5% of landings comprised sharks from the families Squalidae, Stegostomatidae, Sphyrnidae and Triakidae. The catch composition is remarkably different from the previous landing survey in 2004, in that the current study found noticeable declines in landings of slow-growing, late- maturing and low-fecundity species (especially sphyrnid and carcharhinid species). The absences of many species and changes in life-stage composition suggest that the populations of these groups may be close to collapse. The results from this study emphasize the urgency for additional research and monitoring efforts and also the need for management incentives in order to manage shark fisheries effectively in the Andaman Sea.

Boom-bust dynamics in biological invasions: towards an improved application of the concept.

Boom-bust dynamics - the rise of a population to outbreak levels, followed by a dramatic decline - have been associated with biological invasions and offered as a reason not to manage troublesome invaders. However, boom-bust dynamics rarely have been critically defined, analyzed, or interpreted. Here, we define boom-bust dynamics and provide specific suggestions for improving the application of the boom-bust concept. Boom-bust dynamics can arise from many causes, some closely associated with invasions, but others occurring across a wide range of ecological settings, especially when environmental conditions are changing rapidly. As a result, it is difficult to infer cause or predict future trajectories merely by observing the dynamic. We use tests with simulated data to show that a common metric for detecting and describing boom-bust dynamics, decline from an observed peak to a subsequent trough, tends to severely overestimate the frequency and severity of busts, and should be used cautiously if at all. We review and test other metrics that are better suited to describe boom-bust dynamics. Understanding the frequency and importance of boom-bust dynamics requires empirical studies of large, representative, long-term data sets that use clear definitions of boom-bust, appropriate analytical methods, and careful interpretations.

Invasions Slow Down or Collapse in the Presence of Reactive Boundaries.

Motivated by the propagation of thin bacterial films around planar obstacles, this paper considers the dynamics of travelling wave solutions to the Fisher-KPP equation [Formula: see text] in a planar strip [Formula: see text], [Formula: see text]. We examine the propagation of fronts in the presence of a mixed boundary condition (also referred to as a 'partially absorbing' or 'reactive' boundary) [Formula: see text], with [Formula: see text], at [Formula: see text]. The presence of boundary conditions of this kind leads to the development of front solutions that propagate in x but contain transverse structure in y. Motivated by the observation that the speed of propagation in the Fisher-KPP equation is determined (for exponentially decaying initial conditions) by the behaviour at the leading edge, we analyse the linearised Fisher-KPP equation in order to estimate the speed of the stable travelling front, a function of the width L and the imposed boundary conditions. For wide strips the speed estimate based on the linearised equation agrees well with the results of numerical simulations. For narrow channels numerical simulations indicate that the stable front propagates more slowly, and for sufficiently small L or sufficiently large [Formula: see text] the front speed falls to zero and the front collapses. The reason for the collapse is the non-existence, far behind the front, of a stable positive equilibrium solution u(x, y). While existence of these equilibrium states can be demonstrated via phase plane arguments, the investigation of stability is similar to calculations of critical patch sizes carried out in similar ecological models.

The functional extinction of Andean megafauna.

Controversy exists over the cause and timing of the extinction of the Pleistocene megafauna. In the tropical Andes, deglaciation and associated rapid climate change began ~8,000 years before human arrival, providing an opportunity to separate the effects of climate change from human hunting on megafaunal extinction. We present a paleoecological record spanning the last 25,000 years from Lake Pacucha, Peru (3,100 m elevation). Fossil pollen, charcoal, diatoms, and the dung fungus Sporormiella, chronicle a two-stage megaherbivore population collapse. Sporormiella abundance, the proxy for megafaunal presence, fell sharply at ~21,000 years ago, but rebounded prior to a permanent decline between ~16,800 and 15,800 years ago. This two-stage decline in megaherbivores resulted in a functional extinction by ~15,800 years ago, 3,000 years earlier than known human occupation of the high Andes. Declining megaherbivore populations coincided with warm, wet intervals. Climatic instability and megafaunal population collapse triggered an ecological cascade that resulted in novel floral assemblages, and increases in woody species, fire frequency, and plant species that were sensitive to trampling. Our data revealed that Andean megafaunal populations collapsed due to positive feedbacks between habitat quality and climate change rather than human activity.

Risk assessment of inbreeding and outbreeding depression in a captive-breeding program.

Captive-breeding programs can be implemented to preserve the genetic diversity of endangered populations such that the controlled release of captive-bred individuals into the wild may promote recovery. A common difficulty, however, is that programs are founded with limited wild broodstock, and inbreeding can become increasingly difficult to avoid with successive generations in captivity. Program managers must choose between maintaining the genetic purity of populations, at the risk of inbreeding depression, or interbreeding populations, at the risk of outbreeding depression. We evaluate these relative risks in a captive-breeding program for 3 endangered populations of Atlantic salmon (Salmo salar). In each of 2 years, we released juvenile F(1) and F(2) interpopulation hybrids, backcrosses, as well as inbred and noninbred within-population crosstypes into 9 wild streams. Juvenile size and survival was quantified in each year. Few crosstype effects were observed, but interestingly, the relative fitness consequences of inbreeding and outbreeding varied from year to year. Temporal variation in environmental quality might have driven some of these annual differences, by exacerbating the importance of maternal effects on juvenile fitness in a year of low environmental quality and by affecting the severity of inbreeding depression differently in different years. Nonetheless, inbreeding was more consistently associated with a negative effect on fitness, whereas the consequences of outbreeding were less predictable. Considering the challenges associated with a sound risk assessment in the wild and given that the effect of inbreeding on fitness is relatively predictable, we suggest that risk can be weighted more strongly in terms of the probable outcome of outbreeding. Factors such as genetic similarities between populations and the number of generations in isolation can sometimes be used to assess outbreeding risk, in lieu of experimentation.

Photo-acclimatory thresholds anticipate sudden shifts in seagrass ecosystem state under reduced light conditions.

Seagrass ecosystems usually respond in a nonlinear fashion to increasing pressures and environmental changes. Feedback mechanisms operating at the ecosystem level and involving multiple interactions among the seagrass meadow, its associated community and the physical environment are known to play a major role in such nonlinear responses. Phenotypic plasticity may also be important for buffering these ecological thresholds (i.e., regime shifts) as many physiological processes show nonlinear responses to gradual environmental changes, conferring the appearance of resistance before the effects at the organism and population levels are visible. However, the potential involvement of plant plasticity in driving catastrophic shifts in seagrass ecosystems has not yet been assessed. In this study, we conducted a manipulative 6-month light-gradient experiment in the field to capture nonlinearities of the physiological and population responses of the seagrass Cymodocea nodosa to gradual light reduction. The aim was to explore if and how the photo-acclimatory responses of shaded plants are translated to the population level and, hence, to the ecosystem level. Results showed that the seagrass population was rather stable under increasing shading levels through the activation of multilevel photo-acclimative responses, which are initiated with light reduction and modulated in proportion to shading intensity. The activation of photo-physiological and metabolic compensatory responses allowed shaded plants to sustain nearly constant plant productivity (metabolic carbon balance) along a range of shading levels before losing linearity and starting to decline. The species then activated plant- and meadow-scale photo-acclimative responses and drew on its energy reserves (rhizome carbohydrates) to confer additional population resilience. However, when the integration of all these buffering mechanisms failed to counterbalance the effects of extreme light limitation, the population collapsed, giving place to a phase shift from vegetated to bare sediments with catastrophic ecosystem outcomes. Our findings evidence that ecological thresholds in seagrass ecosystems under light limitation can be explained by the role of species' compensatory responses in modulating population-level responses. The thresholds of these plastic responses anticipate the sudden loss of seagrass meadows with the potential to be used as early warning indicators signalling the imminent collapse of the ecosystem, which is of great value for the real-world management of seagrass ecosystems.