Trophic cascades in a temperate seagrass community

We assessed the relative importance of bottom–up and top–down processes in structuring an eelgrass community in Sweden, a system impacted both by eutrophication and overfishing. Using artificial seagrass as substrate, we manipulated nutrient levels and predator abundance in a full‐factorial cage‐experiment. The results revealed a seagrass community dominated by strong top–down processes controlling the aggregate biomass of mesograzers and macroalgae. In the absence of predators the large amphipod Gammarus locusta became very abundant resulting in a leaf community with low biomass of algae and smaller mobile fauna. One enclosed gobid fish predator reduced the abundance of adult G. locusta by >90%, causing a three to six times increase in the biomass of algae, smaller mesograzers and meiofauna. Numerous small predators in uncaged habitats reduced the biomass of G. locusta and other mesograzers by >95% in comparison to the fish treatment, further increasing the biomass of epiphytic algae and meiofauna. Although water column nutrient enrichment caused a temporal bloom of the filamentous macroalgae Ulva spp., no significant nutrient‐effects were found on the algal community at the end of the experiment. The only lasting nutrient‐effect was a significant increase in the biomass of G. locusta, but only in the absence of ambient predators. These results demonstrate that mesograzers can respond to enhanced food supply, increase their biomass and control the algal growth when predation rates are low. However, in the assessed system, high predation rates appear to make mesograzers functionally extinct, causing a community‐wide trophic cascade that promotes the growth of ephemeral algae. This top–down effect could penetrate down, despite a complex food‐web because the interaction strength in the community was strongly skewed towards two functionally dominant algal and grazer species that were vulnerable to consumption. These results indicate that overexploitation of gadoid fish may be linked to increased macroalgal blooms and loss of eelgrass in the area through a trophic cascade affecting the abundance of mesograzers.     

Trophic cascades: the primacy of trait-mediated indirect interactions

Trophic cascades are textbook examples of predator indirect effects on ecological systems. Yet there is considerable debate about their nature, strength and overall importance. This debate stems in part from continued uncertainty about the ultimate mechanisms driving cascading effects. We present a synthesis of empirical evidence in support of one possible ultimate mechanism: the foraging‐predation risk trade‐offs undertaken by intermediary species. We show that simple trade‐off behaviour can lead to both positive and negative indirect effects of predators on plant resources and hence can explain considerable contingency on the nature and strength of cascading effects among systems. Thus, predicting the sign and strength of indirect effect simply requires knowledge of habitat and resource use by prey with regard to predators’ presence, habitat use and hunting mode. The synthesis allows us to postulate a hypothesis for new conceptualization of trophic cascades which is to be viewed as an ultimate trade‐off between intervening species. In this context, different predators apply different rules of engagement based on their hunting mode and habitat use. These different rules then determine whether behavioural effects persist or attenuate at the level of the food chain.     

Unexpected resilience of a seagrass system exposed to global stressors

Despite a growing interest in identifying tipping points in response to environmental change, our understanding of the ecological mechanisms underlying nonlinear ecosystem dynamics is limited. Ecosystems governed by strong species interactions can provide important insight into how nonlinear relationships between organisms and their environment propagate through ecosystems, and the potential for environmentally mediated species interactions to drive or protect against sudden ecosystem shifts. Here, we experimentally determine the functional relationships (i.e., the shapes of the relationships between predictor and response variables) of a seagrass assemblage with well‐defined species interactions to ocean acidification (enrichment of CO₂) in isolation and in combination with nutrient loading. We demonstrate that the effect of ocean acidification on grazer biomass (Phyllaplysia taylori and Idotea resecata) was quadratic, with the peak of grazer biomass at mid‐pH levels. Algal grazing was negatively affected by nutrients, potentially due to low grazer affinity for macroalgae (Ulva intestinalis), as recruitment of both macroalgae and diatoms were favored in elevated nutrient conditions. This led to an exponential increase in macroalgal and epiphyte biomass with ocean acidification, regardless of nutrient concentration. When left unchecked, algae can cause declines in seagrass productivity and persistence through shading and competition. Despite quadratic and exponential functional relationships to stressors that could cause a nonlinear decrease in seagrass biomass, productivity of our model seagrass—the eelgrass (Zostera marina)‐ remained highly resilient to increasing acidification. These results suggest that important species interactions governing ecosystem dynamics may shift with environmental change, and ecosystem state may be decoupled from ecological responses at lower levels of organization.     

GDNF-deprived sympathetic neurons die via a novel nonmitochondrial pathway

The mitochondrial death pathway is triggered in cultured sympathetic neurons by deprivation of nerve growth factor (NGF), but the death mechanisms activated by deprivation of other neurotrophic factors are poorly studied. We compared sympathetic neurons deprived of NGF to those deprived of glial cell line-derived neurotrophic factor (GDNF). In contrast to NGF-deprived neurons, GDNF-deprived neurons did not die via the mitochondrial pathway. Indeed, cytochrome c was not released to the cytosol; Bax and caspase-9 and -3 were not involved; overexpressed Bcl-xL did not block the death; and the mitochondrial ultrastructure was not changed. Similarly to NGF-deprived neurons, the death induced by GDNF removal is associated with increased autophagy and requires multiple lineage kinases, c-Jun and caspase-2 and -7. Serine 73 of c-Jun was phosphorylated in both NGF- and GDNF-deprived neurons, whereas serine 63 was phosphorylated only in NGF-deprived neurons. In many NGF-deprived neurons, the ultrastructure of the mitochondria was changed. Thus, a novel nonmitochondrial caspase-dependent death pathway is activated in GDNF-deprived sympathetic neurons.     

Overcoming urban stream syndrome: Trophic flexibility confers resilience in a Hawaiian stream fish

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Understanding seagrass resilience in temperate systems: the importance of timing of the disturbance

Temperate seagrass meadows form valuable ecosystems in coastal environments and present a distinct seasonal growth. They are threatened by an increasing amount of stressors, potentially affecting their capacity to recover from disturbances. We hypothesized that their resilience to disturbances is affected by seasonal dynamics. Hence, we investigated the effect of the timing of the disturbance on seagrass Leaf Area Index (as a proxy for presence, or ‘visible’ status), recovery from disturbance (as a proxy for meadow resilience), and rhizome carbohydrates (as a proxy for longer term resilience) by a series of four disturbance-recovery field experiments spread over the growing season at two sites in Shandong Province, China. During the course of the growing season, we found the highest recovery at the start of the growing season, lowest recovery when Leaf Area Index peaked around mid-growing season, and intermediate recovery when Leaf Area Index decreased at the end of the growing season. Rhizome carbohydrates were not affected by disturbances during any of the four experimental periods and could not explain the low recovery during mid-growing season. The two sites differed in exposure and in the occurrence of incidents like a green tide and storms, which affected recovery. However, general patterns were similar; timing strongly influenced the indicator of meadow resilience and its correlation with presence during the two main seagrass growth phases. Our results emphasize the importance of carefully considering timing in the evaluation of seagrass resilience in temperate systems. Furthermore, our study implies that, to effectively protect seagrass beds, conservation management should aim at avoiding disturbances particularly during the peak of the growing season, when resilience is lowest.     

Export of proteins via a novel secretory pathway.

The intraerythrocytic location of the malaria parasite necessitates modification of the host cell. These alterations are mediated either directly or indirectly by parasite proteins exported to specific compartments within the host cell. However, little is known about how the parasite specifically targets proteins to locations beyond its plasma membrane. Mark Wiser, Norbert Lanners and Richard Bafford here propose an alternative secretory pathway for the export of parasite proteins into the host erythrocyte. The first step of this pathway is probably an endoplasmic reticulum (ER)-like organelle that is distinct from the normal ER. Possible mechanisms of protein trafficking in the infected erythrocyte are also discussed. The proposed ER-like organelle and alternative secretory pathway raise many questions about the cell biology of protein export and trafficking in Plasmodium.     

Trophic cascades in rocky shore tide pools: distinguishing lethal and nonlethal effects

The effects of predators on the density of their prey can have positive indirect effects on the abundance of the preys resource via a trophic cascade. This concept has strongly influenced contemporary views of how communities are structured. However, predators also can transmit indirect effects by inducing changes in prey traits. We show that the mere presence of predator risk cues can initiate a trophic cascade in rocky shore tide pools. In large (mean surface area =9 m²), natural tide pools, we manipulated crab density and their foraging ability to examine the relative importance of lethal (density-mediated) and non-lethal (trait-mediated) predator effects to algal community development. We found that perceived predation risk reduced snail density as much as the direct predation treatment, showing that green crab predation was not an important factor regulating local snail density. Instead, snail emigration away from resident crabs appears to be the most important factor regulating local snail density. As a result, the abundance of ephemeral green algae was similar in the predation risk and direct predation treatments, suggesting that the consumption of snails by crabs plays a minimal role in mediating the trophic cascade. Increased attention to trait-mediated effects that are transmitted by predator-induced changes in prey behavior may change our view of how predators exert their strong influence on community structure.     

Actin depolymerization via the beta-adrenoceptor in airway smooth muscle cells: a novel PKA-independent pathway

Actin is a majorfunctional and structural cytoskeletal protein that mediates suchdiverse processes as motility, cytokinesis, contraction, and control ofcell shape and polarity. While many extracellular signals are known tomediate actin filament polymerization, considerably less is known aboutsignals that mediate depolymerization of the actin cytoskeleton. Humanairway smooth muscle cells were briefly exposed to isoproterenol,forskolin, or the cAMP-dependent protein kinase A (PKA) agoniststimulatory diastereoisomer of adenosine 3',5'-cyclic monophosphate(Sp-cAMPS). Actin polymerization was measured by concomitantstaining of filamentous actin with FITC-phalloidin and globular actinwith Texas red DNase I. Isoproterenol, forskolin, or Sp-cAMPS inducedactin depolymerization, indicated by a decrease in the intensity offilamentous/globular fluorescent staining. The PKA inhibitor Rpdiastereomer of adenosine 3',5'-cyclic monophosphothioate(Rp-cAMPS) completely inhibited forskolin-stimulated depolymerization, whereas it only partially inhibitedisoproterenol-induced depolymerization. The protein tyrosine kinaseinhibitors genistein or tyrphostin A23 also partially inhibitedisoproterenol-induced actin depolymerization. In contrast, thecombination of Rp-cAMPS and either tyrosine kinase inhibitor had anadditive effect at inhibiting isoproterenol-induced actindepolymerization. These results suggest that both PKA-dependent and-independent pathways mediate actin depolymerization in human airwaysmooth muscle cells.

     

Integrating edge effects into studies of habitat fragmentation: a test using meiofauna in seagrass

Habitat fragmentation is thought to be an important process structuring landscapes in marine and estuarine environments, but effects on fauna are poorly understood, in part because of a focus on patchiness rather than fragmentation. Furthermore, despite concomitant increases in perimeter:area ratios with fragmentation, we have little understanding of how fauna change from patch edges to interiors during fragmentation. Densities of meiofauna were measured at different distances across the edges of four artificial seagrass treatments [continuous, fragmented, procedural control (to control for disturbance by fragmenting then restoring experimental plots), and patchy] 1 day, 1 week and 1 month after fragmentation. Experimental plots were established 1 week prior to fragmentation/disturbance. Samples were numerically dominated by harpacticoid copepods, densities of which were greater at the edge than 0.5 m into patches for continuous, procedural control and patchy treatments; densities were similar between the edge and 0.5 m in fragmented patches. For taxa that demonstrated edge effects, densities exhibited log-linear declines to 0.5 m into a patch with no differences observed between 0.5 m and 1 m into continuous treatments. In patchy treatments densities were similar at the internal and external edges for many taxa. The strong positive edge effect (higher densities at edge than interior) for taxa such as harpacticoid copepods implies some benefit of patchy landscapes. But the lack of edge effects during patch fragmentation itself demonstrates the importance of the mechanisms by which habitats become patchy.     

Trophic trickles and cascades in a complex food web: impacts of a keystone predator on stream community structure and ecosystem processes

Chalk streams are among the most species-rich and productive of all temperate ecosystems. Despite this, a few keystone species have the potential to exert disproportionately powerful effects on community structure and ecosystem processes. Two of these are the bullhead Cottus gobio , a small benthic fish that is an extremely abundant, voracious predator, and the freshwater shrimp Gammarus pulex , which dominates the prey assemblage and is the principal detritivore. Field experiments detected a bullhead– Gammarus –detritus trophic cascade, with detrital processing rates slowed dramatically in the presence of the predator. In addition, survey data also revealed strong negative density-dependence between bullhead and brown trout, adding a further link in the cascade. However, although bullhead also depressed the abundance of a dominant grazer, the snail Potamopyrgus antipodarum , there was no cascading effect upon algal production, suggesting that autochthonous inputs were not controlled by top–down effects. This skewed effect of the predator upon autochthonous versus allochthonous basal resources stresses the need to consider both pathways of energy flux into the food web, whereas many previous studies have potentially overemphasized the importance of predator–herbivore–primary producer cascades. The wider community food web contained 142 species and 1383 feeding links. This complex network exhibited "small world" properties, such as high clustering (unlike many other food webs) and shortest path lengths between species were small (in common with many other food webs). In particular, each of the four members of the detrital cascade could be connected to any other species by three links or fewer. Our data revealed that powerful cascading effects can be imbedded within even very complex ecological networks.     

Trophic cascades in an invaded ecosystem: native keystone predators facilitate a dominant invader in an estuarine community

Determining how various factors contribute to the invasibility of systems is essential for both understanding community formation and informing management of natural areas. Research demonstrating that predators can provide biotic resistance to invasions by consuming invasive species has led to the presence of healthy predator populations being associated with reduced invasion potential of ecosystems. However, predators structure communities in many ways and their presence could also potentially facilitate invasions if they decrease populations of native species that compete with or consume an invader. We considered these two impacts of predators on invasion by analyzing the effects of two keystone predators (Pisaster spp. and Enhydra lutris nereis) on two foundation species (a native mussel Mytilus californianus and the invasive exotic bryozoan Watersipora subtorquata, a putative competitor for space with Mytilus californianus). Both native predators were found to facilitate the invasion of the exotic bryozoan, and the rate of invasion was highest when both predators were present. Facilitation of W. subtorquata occurred via indirect mechanisms that partly involved the removal of a competitor (mussels) via predation. These results illustrate that although predators can provide biotic resistance to invasion, healthy predator populations do not always confer this advantage and in fact may facilitate invasions. Therefore, implementation of management actions to enhance populations of top predators could also potentially increase the invasibility of some ecosystems.     

Habitat cascades: the conceptual context and global relevance of facilitation cascades via habitat formation and modification

The importance of positive interactions is increasingly acknowledged in contemporary ecology. Most research has focused on direct positive effects of one species on another. However, there is recent evidence that indirect positive effects in the form of facilitation cascades can also structure species abundances and biodiversity. Here we conceptualize a specific type of facilitation cascade-the habitat cascade. The habitat cascade is defined as indirect positive effects on focal organisms mediated by successive facilitation in the form of biogenic formation or modification of habitat. Based on a literature review, we demonstrate that habitat cascades are a general phenomenon that enhances species abundance and diversity in forests, salt marshes, seagrass meadows, and seaweed beds. Habitat cascades are characterized by a hierarchy of facilitative interactions in which a basal habitat former (typically a large primary producer, e.g., a tree) creates living space for an intermediate habitat former (e.g., an epiphyte) that in turn creates living space for the focal organisms (e.g., spiders, beetles, and mites). We then present new data on a habitat cascade common to soft-bottom estuaries in which a relatively small invertebrate provides basal habitat for larger intermediate seaweeds that, in turn, generate habitat for focal invertebrates and epiphytes. We propose that indirect positive effects on focal organisms will be strongest when the intermediate habitat former is larger and different in form and function from the basal habitat former. We also discuss how humans create, modify, and destroy habitat cascades via global habitat destruction, climatic change, over-harvesting, pollution, or transfer of invasive species. Finally, we outline future directions for research that will lead to a better understanding of habitat cascades.     

A novel functional link between MAP kinase cascades and the Ras/cAMP pathway that regulates survival

In mammalian cells, Ras regulates multiple effectors, including activators of mitogen-activated protein kinase (MAPK) cascades, phosphatidylinositol-3-kinase, and guanine nucleotide exchange factors (GEFs) for RalGTPases. In S. cerevisiae, Ras regulates the Kss1 MAPK cascade that promotes filamentous growth and cell integrity, but its major function is to activate adenylyl cyclase and control proliferation and survival ([; see Figure S1 in the Supplemental Data available with this article online). Previous work hints that the mating Fus3/Kss1 MAPK cascade cross-regulates the Ras/cAMP pathway during growth and mating, but direct evidence is lacking. Here, we report that Kss1 and Fus3 act upstream of the Ras/cAMP pathway to regulate survival. Loss of Fus3 increases cAMP and causes poor long-term survival and resistance to stress. These effects are dependent on Kss1 and Ras2. Activation of Kss1 by a hyperactive Ste11 MAPKKK also increases cAMP, but mating receptor/scaffold activation has little effect and may therefore insulate the MAPKs from cross-regulation. Catalytically inactive Fus3 represses cAMP by blocking accumulation of active Kss1 and by another function also shared by Kss1. The conserved RasGEF Cdc25 is a likely control point, because Kss1 and Fus3 complexes associate with and phosphorylate Cdc25. Cross-regulation of Cdc25 may be a general way that MAPKs control Ras signaling networks.     

The song must go on: resilience of the songbird vocal motor pathway

Stereotyped sequences of neural activity underlie learned vocal behavior in songbirds; principle neurons in the cortical motor nucleus HVC fire in stereotyped sequences with millisecond precision across multiple renditions of a song. The geometry of neural connections underlying these sequences is not known in detail though feed-forward chains are commonly assumed in theoretical models of sequential neural activity. In songbirds, a well-defined cortical-thalamic motor circuit exists but little is known the fine-grain structure of connections within each song nucleus. To examine whether the structure of song is critically dependent on long-range connections within HVC, we bilaterally transected the nucleus along the anterior-posterior axis in normal-hearing and deafened birds. The disruption leads to a slowing of song as well as an increase in acoustic variability. These effects are reversed on a time-scale of days even in deafened birds or in birds that are prevented from singing post-transection. The stereotyped song of zebra finches includes acoustic details that span from milliseconds to seconds--one of the most precise learned behaviors in the animal kingdom. This detailed motor pattern is resilient to disruption of connections at the cortical level, and the details of song variability and duration are maintained by offline homeostasis of the song circuit.     

Osteocyte hypoxia: a novel mechanotransduction pathway

Bone is a unique tissue in which to examine mechanotransductiondue to its essential role in weight bearing. Within bone, the osteocyteis an ideal cellular mechanotransducer candidate. Because osteocytesreside distant from the blood supply, their metabolic needs are met bya combination of passive diffusion and enhanced diffusion, arising whenthe tissue is loaded during functional activity. Therefore, wehypothesized that depriving a bone of mechanical loading (and thuseliminating diffusion enhanced by loading) would rapidly induceosteocyte hypoxia. Using the avian ulna model of disuse osteopenia, wefound that 24 h of unloading results in significant osteocyte hypoxia(8.4 ± 1.8%) compared with control levels (1.1 ± 0.5%;P = 0.03). Additionally, we present preliminary data suggesting that a brief loading regimen is sufficient to rescue osteocytes from this fate. The rapid onset of the observed osteocyte hypoxia, the inhibition of hypoxia by brief loading, and thecellular consequences of oxygen deprivation are suggestive of a novelmechanotransduction pathway with implications across organ systems.

     

Trophic cascades in tropical rainforests: Effects of vertebrate predator exclusion on arthropods and plants in Papua New Guinea

Insect herbivores have the potential to consume large amounts of plant tissue in tropical forests, but insectivorous vertebrates effectively control their abundances, indirectly increasing plant fitness accordingly. Despite several studies already sought understanding of the top-down effects on arthropod community structure and herbivory, such studies of trophic cascades in old tropics are underrepresented, and little attention was paid to top-down forces in various habitats. Therefore, we examine how flying insectivorous vertebrates (birds and bats) impact arthropods and, consequently, affect herbivore damage of leaves in forest habitats in Papua New Guinea. In a 3-month long predator exclosure experiment conducted at four study sites across varying elevation and successional stage, we found that vertebrate predators reduced arthropod density by ∼52%. In addition, vertebrate predators decreased the mean body size of arthropods by 26% in leaf chewers and 47% in non-herbivorous arthropods but had only a small effect on mesopredators and sap suckers. Overall, the exclusion of vertebrate predators resulted in a ~ 41% increase in leaf damage. Our results, across different types of tropical forests in Papua New Guinea, demonstrate that flying vertebrate insectivores have a crucial impact on plant biomass, create a selective pressure on larger and non-predatory prey individuals and they prey partition with mesopredators.