Omnivory and grazer functional composition moderate cascading trophic effects in experimental Fucus vesiculosus habitats

We tested the relative strength of direct versus indirect effects of an aquatic omnivore depending on the functional composition of grazers by manipulating the presence of gastropod and amphipod grazers and omnivorous shrimp in outdoor mesocosms. By selectively preying upon amphipods and reducing their abundance by 70–80%, omnivorous shrimp favoured the dominance of gastropods. While gastropods were the main microalgal grazers, amphipods controlled macroalgal biomass in the experiment. However, strong predation on the amphipod by the shrimp had no significant indirect effects on macroalgal biomass, indicating that when amphipod abundances declined, complementary feeding by the omnivore on macroalgae may have suppressed a trophic cascade. Accordingly, in the absence of amphipods, the shrimp grazed significantly on green algae and thereby suppressed the diversity of the macroalgal community. Our experiment demonstrates direct consumer effects by an omnivore on both the grazer and producer trophic levels in an aquatic food web, regulated by prey availability.

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title: Life history, growth and production of Theodoxus fluviatilis in Lake Esrom, Denmark
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title: Strong bottom-up and top-down control of early life stages of macroalgae
abstract: In contrast to most pelagic primary producers, benthic macrophytes pass through morphologically distinct life stages, which can be subject to different ecological controls. Using factorial field experiments, we investigated how grazing pressure (three levels) and nutrient supply (four levels) interact in controlling the passage of marine macroalgae through an apparent recruitment ‘‘bottleneck’’ at the germling stage. In comparative experiments, we asked whether relative bottom-up and top-down effects on early life stages (,4 week germlings) vary (1) between the eutrophic Baltic Sea and the oligotrophic NW Atlantic, (2) across seasons in the NW Atlantic, and (3) among annual and perennial macroalgae. In both systems nutrient enrichment favored and grazers suppressed recruitment of green and brown annual algae; however, enrichment effects were much more pronounced in the Baltic, whereas grazer effects dominated in the NW Atlantic. Grazers induced a shift from grazer-susceptible green to more resistant brown algae in the Baltic without reducing total germling density. In the NW Atlantic, grazers strongly reduced overall recruitment rate throughout all seasons. Effects on perennials were similar in both systems with moderate losses to grazing and no effects of nutrient enrichment. Recruit densities and species composition shifted with season in the NW Atlantic. We conclude that the relative effects of grazers and nutrient enrichment depended on the nutrient status of the system, algal life history strategy, and season. Strong bottom-up and top-down controls shape benthic community composition before macroalgae reach visible size.

title: Biotic habitat complexity controls species diversity and nutrient effects on net biomass production.
abstract: Canopy-forming plants and algae commonly contribute to spatial variation in habitat complexity for associated organisms and thereby create a biotic patchiness of communities. In this study, we tested for interaction effects between biotic habitat complexity and resource availability on net biomass production and species diversity of understory macroalgae by factorial field manipulations of light, nutrients, and algal canopy cover in a subtidal rocky-shore community. Presence of algal canopy cover and/or artificial shadings limited net biomass production and facilitated species diversity. Artificial shadings reduced light to levels similar to those under canopy cover, and net biomass production was significantly and positively correlated to light availability. Considering the comparable and dependent experimental effects from shadings and canopy cover, the results strongly suggest that canopy cover controlled net biomass production and species diversity by limiting light and thereby limiting resource availability for community production. Canopy cover also controlled experimental nutrient effects by preventing a significant increase in net biomass production from nutrient enrichment recorded in ambient light (no shading). Changes in species diversity were mediated by changes in species dominance patterns and species evenness, where canopy cover and shadings facilitated slow-growing crust-forming species and suppressed spatial dominance by Fucus vesiculosus, which was the main contributor to net production of algal biomass. The demonstrated impacts of biotic habitat complexity on biomass production and local diversity contribute significantly to understanding the importance of functionally important species and biodiversity for ecosystem processes. In particular, this study demonstrates how loss of a dominant species and decreased habitat complexity change the response of the remaining assembly to resource loading. This is of potential significance for marine conservation since resource loading often promotes low habitat complexity and canopy species are among the first groups lost in degraded aquatic systems.

title: Inducible resistance to herbivory in Fucus vesiculosus—duration, spreading and variation with nutrient availability
abstract: Inducible resistance in plants is a defense strategy for avoiding the negative conse- quences of herbivory on plant fitness. Since resistance is costly, the induction of resistance may depend on resource availability. Resource-based plant-herbivore hypotheses predict that the cost of producing carbon-based defensive metabolites will be higher when an excess of nutrients is available for growth. We tested experimentally the effect of nutrient availability on the occurrence, duration and within-alga spread of inducible resistance in the brown alga Fucus vesiculosus L. Based on feed- ing preference bioassays using the herbivore Idotea baltica (Pallas), simulated grazing of F. vesiculo- sus caused a clear, rapid, induced resistance that disappeared after 10 to 38 d. Furthermore, the induction of resistance does not spread to neighboring fronds or to growing tips above the point of simulated grazing. Induced resistance against isopod grazing, however, was not explained by increased production of phlorotannins, despite their putative role in the defense against herbivory. Phlorotannin production responded most strongly to nutrient enhancement, which reduced the phlorotannin concentration of the alga. Nutrient enhancement, however, did not affect the induction of resistance. The occurrence of induced resistance together with the lack of correlation between the phlorotannin concentration and the food preferences of herbivores imply that I. baltica is not deterred by the total quantity of phlorotannins; there may, however, be other, as yet unknown, rapidly induc- ing substances in F. vesiculosus that are capable of functioning as feeding deterrents to isopod graz- ers. Resistance may also arise as a side-effect of substances with a functional role, for instance in wound-healing processes. The localized nature and short duration of such deterrence imply that it may be inefficient as a general defense against herbivory, but may benefit the alga by dispersing future damage within the individual and avoiding the breakage of whole fronds.

title: Grazer identity is crucial for facilitating growth of the perennial brown alga Fucus vesiculosus
abstract: Grazer identity is crucial for facilitating growth of the perennial brown alga Fucus vesiculosus

title: Opposing effects of grazing and nutrients on diversity
abstract: Conceptual models predict counteractive effects of herbivores and nutrient enrichment on plant diversity and reversed effects of grazers under different nutrient regimes. I tested these hypotheses in 11 field experiments with periphyton communities in three different aquatic habitats (a highly eutrophic lake, an meso-eutrophic lake, and an meso-eutrophic part of the Baltic Sea coast) and in different seasons. Grazer access and nutrient supply were manipulated in a factorial design. Species richness and evenness were chosen as response variables. Both manipulated factors had significant and contrasting effects on diversity, with variable effect strength between sites and seasons. From the two aspects of diversity, evenness well reflected the changes in community composition. Fertilization tended to increase the dominance of few species and thus to decrease evenness, whereas grazers counteracted these effects by removing dominant life forms. The response of species richness was not as expected, since grazers decreased richness throughout, whereas nutrients had weaker effects but tended to increase richness. Species richness rather reflected changes in periphyton architecture. Grazers reduced algal richness presumably by co-consumption of rare species in the tightly connected periphyton assemblages, whereas enrichment may increase richness by providing more structure via increased dominance of filamentous species. Although grazer and nutrient effects on richness and evenness were opposing, there was no change in the effect of one factor by manipulation of the other.

title: Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure
abstract: Consumer and resource control of diversity in plant communities have long been treated as alternative hypotheses. However, experimental and theoretical evidence suggests that herbivores and nutrient resources interactively regulate the number and relative abundance of coexisting plant species. Experiments have yielded divergent and often contradictory responses within and among ecosystems, and no effort has to date reconciled this empirical variation within a general framework. Using data from 274 experiments from marine, freshwater, and terrestrial ecosystems, we present a cross-system analysis of producer diversity responses to local manipulations of resource supply and/or herbivory. Effects of herbivory and fertilization on producer richness differed substantially between systems: (i) herbivores reduced species richness in freshwater but tended to increase richness in terrestrial systems; (ii) fertilization increased richness in freshwater systems but reduced richness on land. Fertilization consistently reduced evenness, whereas herbivores increased evenness only in marine and terrestrial ecosystems. Producer community evenness and ecosystem productivity mediated fertilization and herbivore effects on diversity across ecosystems. Herbivores increased producer richness in more productive habitats and in producer assemblages with low evenness. These same assemblages also showed the strongest reduction in richness with fertilization, whereas fertilization increased (and herbivory decreased) richness in producer assemblages with high evenness. Our study indicates that system productivity and producer evenness determine the direction and magnitude of top-down and bottom-up control of diversity and may reconcile divergent empirical results within and among ecosystems.

title: Consumers Control Diversity and Functioning of a Natural Marine Ecosystem
abstract: Our understanding of the functional consequences of changes in biodiversity has been hampered by several limitations of previous work, including limited attention to trophic interactions, a focus on species richness rather than evenness, and the use of artificially assembled communities.  In this study, we manipulated the density of an herbivorous snail in natural tide pools and allowed seaweed communities to assemble in an ecologically relevant and non-random manner. Seaweed species evenness and biomass-specific primary productivity (mg O2 h−1 g−1) were higher in tide pools with snails because snails preferentially consumed an otherwise dominant seaweed species that can reduce biomass-specific productivity rates of algal assemblages. Although snails reduced overall seaweed biomass in tide pools, they did not affect gross primary productivity at the scale of tide pools (mg O2 h−1 pool−1 or mg O2 h−1 m−2) because of the enhanced biomass-specific productivity associated with grazer-mediated increases in algal evenness.  Our results suggest that increased attention to trophic interactions, diversity measures other than richness, and particularly the effects of consumers on evenness and primary productivity, will improve our understanding of the relationship between diversity and ecosystem functioning and allow more effective links between experimental results and real-world changes in biodiversity.

title: Diversity effects beyond species richness: evidence from intertidal macroalgal assemblages
abstract: Several recent experimental studies have examined the effects of macroalgal diversity on community functioning using synthetic assemblages. However, their predictive relevance for nat- ural systems remains uncertain. Unlike terrestrial habitats, where observational studies in natural systems are profuse, studies on the relationship between diversity and ecosystem functioning in nat- ural marine ecosystems are scarce. In the present study, we explored how different components of biodiversity influence the performance of macroalgal assemblages in natural communities (intertidal boulders). Specifically, we examined the relationships between biomass, species richness, spatial aggregation and evenness and the productivity of macroalgal assemblages. We found the expected positive relationship for biomass and species richness. Additionally, we found significant statistical relationships between both spatial aggregation and evenness and some of the productivity-related variables analyzed: assemblages with a higher degree of spatial aggregation reduced their light capture and photosynthetic efficiency, while increasing evenness increased maximum net primary productivity. Although these patterns should be further tested using experimental approaches, obser- vational studies may provide valuable insights by revealing patterns usually overlooked by experi- mental approaches.

title: Effects of Pilayella littoralis on Fucus vesiculosus recruitment: implications for community composition
abstract: “Effects of Pilayella littoralis on Fucus vesiculosus recruitment: implications for community composition

title: STRONG IMPACTS OF GRAZING AMPHIPODS ON THE ORGANIZATION OF A BENTHIC COMMUNITY
abstract: Large brown seaweeds dominate coastal hard substrata throughout many of the world's oceans. In coastal North Carolina, USA, this dominance by brown seaweeds is facilitated by omnivorous fishes, which feed both on red and green algae and on herbivorous amphipods that graze brown algae. When fish are removed in the field, brown seaweeds are replaced by red seaweeds, and herbivorous amphipods are more abundant. Using an array of large (;4000 L) outdoor mesocosms, we tested three mechanistic hypotheses for this pattern: fish feeding facilitates brown algal dominance (1) by removing red and green algal competitors, (2) by removing amphipods and reducing their feeding on brown sea- weeds, or (3) through an interaction of these mechanisms. Our experiments revealed strong impacts of both fish and amphipods, and a key role for the interaction, in structuring this community. When both fish and amphipods were removed (the latter with dilute insecticide), space was rapidly dominated and held for 17 weeks by fast-growing, primarily filamentous green algae. In contrast, when either fish, amphipods, or both were present, green algae were cropped to a sparse turf, and space was more rapidly dominated by larger macroalgae. The impacts of amphipods and fish on late-successional macroalgal assemblages were comparable in magnitude, but different in sign: red seaweeds prevailed in the amphipod- dominated treatment, whereas browns dominated in the presence of fish. Laboratory feeding assays and amphipod densities in the tanks suggested that the significant effects of am- phipods were attributable largely, if not exclusively, to the single amphipod species Am- pithoe longimana, which fed heavily on brown macroalgae. Our experimental removal of red and green algae failed to enhance cover of brown algae significantly; however, the latter reached substantially lower cover in the grazer-removal treatment, where green algae were very abundant, than in the fish-only treatment, where green algae were sparse. Thus, our results support the third hypothesis: fish-mediated dominance of brown algae involves both suppression of grazing amphipods and removal of algal competitors. Although collective impacts of fish and amphipods on this benthic community were generally comparable in magnitude, impacts normalized to each grazer's aggregate biomass were consistently higher for amphipods than for fish, sometimes by 1-2 orders of magnitude. Thus, the impacts of grazing amphipods (specifically A. longimana) on the benthic community were both strong and disproportionate to their biomass. These experimental results imply that grazing am- phipods, which are ubiquitous in marine vegetation but poorly understood ecologically, may play important roles in the organization of benthic communities, particularly where predation pressure is low.

title: Intraguild predation: The dynamics of complex trophic interactions.
abstract: There is a long-standing debate in ecology concerning the relative importance of competition and predation in determining community structure. Recently, a novel twist has been added with the growing recognition that potentially competing species are often engaged in predator-prey interactions. This blend of competition and predation is called intraguild predation (IGP). The study of IGP will lead to a reconsideration of many classical topics, such as niche shifts, species exclusion and cascading interactions in food webs. Theoretical models suggest that a variety of alternative stable states are likely in IGP systems, and that intermediate predators should tend to be superior in exploitative competition. Many field studies support these expectations. IGP is also important in applied ecological problems, such as the conservation of endangered species and fisheries management.

title: Species loss and ecosystem functioning : effects of species identity and community composition
abstract: Losing a single species from an ecosystem may have large effects on community and ecosystem properties, but this may depend on characteristics of the species and the ecosystem. We examined the effect of losing a single species on productivity and nitrogen retention in experimental grassland communities, concentrating on how these effects varied with the functional identity of the species lost and the diversity and composition of the community from which it was lost. In one experiment, we constructed random plant assemblages that varied in species richness to measure the effect of diversity alone on productivity and nitrogen retention. In another experiment, we constructed plant assemblages to assess the effects of deleting an individual plant species from assemblages differing in their functional and species richness and composition. On average, as species richness declined, productivity decreased but nitrogen retention was unaffected. However, the magnitude and direction of change in ecosystem functioning with declining diversity depended on the identity of the species deleted and the composition of the community from which it was deleted. The functional identity of a species predicted the type of impact its loss had on productivity, but not on nitrogen retention.

title: Partitioning selection and complementarity in biodiversity experiments
abstract: The impact of biodiversity loss on the functioning of ecosystems and their ability to provide ecological services has become a central issue in ecology. Several experiments have provided evidence that reduced species diversity may impair ecosystem processes such as plant biomass production. The interpretation of these experiments, however, has been controversial because two types of mechanism may operate in combination. In the ‘selection effect’, dominance by species with particular traits affects ecosystem processes. In the ‘complementarity effect’, resource partitioning or positive interactions lead to increased total resource use. Here we present a new approach to separate the two effects on the basis of an additive partitioning analogous to the Price equation in evolutionary genetics. Applying this method to data from the pan-European BIODEPTH experiment reveals that the selection effect is zero on average and varies from negative to positive in different localities, depending on whether species with lower- or higher-than-average biomass dominate communities. In contrast, the complementarity effect is positive overall, supporting the hypothesis that plant diversity influences primary production in European grasslands through niche differentiation or facilitation.

title: Grazer diversity effects on ecosystem functioning in seagrass beds
abstract: High plant species richness can enhance primary production, animal diversity, and invasion resistance. Yet theory predicts that plant and herbivore diversity, which often covary in nature, should have countervailing effects on ecosystem properties. Supporting this, we show in a seagrass system that increasing grazer diversity reduced both algal biomass and total community diversity, and facilitated dominance of a grazer-resistant invertebrate. In parallel with previous plant results, however, grazer diversity enhanced secondary production, a critical determinant of fish yield. Although sampling explained some diversity effects, only the most diverse grazer assemblage maximized multiple ecosystem properties simultaneously, producing a distinct ecosystem state. Importantly, ecosystem responses at high grazer diversity often differed in magnitude and sign from those predicted from summed impacts of individual species. Thus, complex interactions, often opposing plant diversity effects, arose as emergent consequences of changing consumer diversity, advising caution in extrapolating conclusions from plant diversity experiments to food webs.

title: THE ECOLOGICAL CONSEQUENCES OF VARIATION IN PLANTS AND PREY FOR AN OMNIVOROUS INSECT
abstract: We conducted a series of laboratory and field experiments to determine the effects of variation in plant quality and prey species on the survival, dispersal, and population size of a common, omnivorous insect. We also tested the hypothesis that plant feeding allows omnivorous “predators” to survive periods when prey are scarce. In addition, we compared the response of omnivores and strict predators to variation in plants and prey. We found that variation in plant parts (pods and leaves) and prey species (moth eggs and aphids) affected the survival, dispersal, and population size of big-eyed bugs, Geocoris punctipes, in Maryland lima beans. The survival of big-eyed bugs fed moth eggs was high and unaffected by the availability of lima bean pods or leaves as supplemental food. The survival of big-eyed bugs fed aphids, however, was relatively low and dramatically improved by the availability of pods. Big-eyed bugs fed only pods developed normally through the first and second instar, but development was arres...

title: Trophic cascades in a temperate seagrass community
abstract: 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.

title: Why Are Parts of the World Green? Multiple Factors Control Productivity and the Distribution of Biomass
abstract: This paper evaluates the multiple factors that determine the production of plant biomass and its distribution among producers and various trophic groups of con_ ?^)^ Osumers. In rough order of their importance, water and nutrient availability, factors 0*S that deter herbivores (plant defenses, environmental heterogeneity and disturbance, nutrient stoichiometry), and consumption by herbivores appear to be the most universal determinants of the production and distribution of plant biomass. In some times and places, indirect effects from enemies of herbivores (predators, parasites, parasitoids and pathogens) propagate through the food web to influence plant biomass, in a manner somewhat consistent with green world and exploitation ecosystem mechanisms. I discuss why such food web dynamics appear to be much more important in water than on land. The only demonstrated cases of communitylevel trophic cascades occur in water. Although species-level cascades are moderately frequent on land, community-level cascades rarely or never occur.

title: Predator interactions, mesopredator release and biodiversity conservation.
abstract: There is growing recognition of the important roles played by predators in regulating ecosystems and sustaining biodiversity. Much attention has focused on the consequences of predator-regulation of herbivore populations, and associated trophic cascades. However apex predators may also control smaller 'mesopredators' through intraguild interactions. Removal of apex predators can result in changes to intraguild interactions and outbreaks of mesopredators ('mesopredator release'), leading in turn to increased predation on smaller prey. Here we provide a review and synthesis of studies of predator interactions, mesopredator release and their impacts on biodiversity. Mesopredator suppression by apex predators is widespread geographically and taxonomically. Apex predators suppress mesopredators both by killing them, or instilling fear, which motivates changes in behaviour and habitat use that limit mesopredator distribution and abundance. Changes in the abundance of apex predators may have disproportionate (up to fourfold) effects on mesopredator abundance. Outcomes of interactions between predators may however vary with resource availability, habitat complexity and the complexity of predator communities. There is potential for the restoration of apex predators to have benefits for biodiversity conservation through moderation of the impacts of mesopredators on their prey, but this requires a whole-ecosystem view to avoid unforeseen negative effects. 'Nothing has changed since I began. My eye has permitted no change. I am going to keep things like this.' From 'Hawk Roosting', by Ted Hughes.

title: Network structure and biodiversity loss in food webs: robustness increases with connectance
abstract: Food-web structure mediates dramatic effects of biodiversity loss including secondary and ‘cascading’ extinctions. We studied these effects by simulating primary species loss in 16 food webs from terrestrial and aquatic ecosystems and measuring robustness in terms of the secondary extinctions that followed. As observed in other networks, food webs are more robust to random removal of species than to selective removal of species with the most trophic links to other species. More surprisingly, robustness increases with food-web connectance but appears independent of species richness and omnivory. In particular, food webs experience ‘rivet-like’ thresholds past which they display extreme sensitivity to removal of highly connected species. Higher connectance delays the onset of this threshold. Removing species with few trophic connections generally has little effect though there are several striking exceptions. These findings emphasize how the number of species removed affects ecosystems differently depending on the trophic functions of species removed.

title: Cascading effects from predator removal depend on resource availability in a benthic food web
abstract: We tested joint effects of predator loss and increased resource availability on the grazers’ trophic level and the propagation of trophic interactions in a benthic food web by excluding larger predatory fish from cages and manipulating nutrients in the coastal zone of the Baltic Sea. The combination of nutrient enrichment and excluding larger predators induced an increase in medium-sized predatory fish (three-spined stickleback). The meso-predator fish in turn did not change the total abundance of the invertebrate herbivores, but did cause a substantial shift in their community composition towards the dominance of gastropods by reducing amphipods by 40–60%, while gastropods were left unchanged. The shift in grazer composition generated a 23 times higher producer biomass, but only under nutrient enrichment. Our results show that top-predator declines can substantially shift the species composition at the grazers’ level, but that cascading effects on producers by a trophic cascade strongly depend on resource availability.