9522510
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.
53129649
META-ANALYSIS OF COD-SHRIMP INTERACTIONS REVEALS TOP-DOWN CONTROL IN OCEANIC FOOD WEBS
Here we present a meta-analytic approach to analyzing population interac- tions across the North Atlantic Ocean. We assembled all available biomass time series for a well-documented predator-prey couple, Atlantic cod (Gadus morhua) and northern shrimp (Pandalus borealis), to test whether the temporal dynamics of these populations are con- sistent with the ''top-down'' or the ''bottom-up'' hypothesis. Eight out of nine regions showed inverse correlations of cod and shrimp biomass supporting the ''top-down'' view. Exceptions occurred only close to the southern range limits of both species. Random-effects meta-analysis showed that shrimp biomass was strongly negatively related to cod biomass, but not to ocean temperature in the North Atlantic Ocean. In contrast, cod biomass was positively related to ocean temperature. The strength of the cod-shrimp relationship, how- ever, declined with increasing mean temperature. These results show that changes in predator populations can have strong effects on prey populations in oceanic food webs, and that the strength of these interactions may be sensitive to changes in mean ocean temperature. This means that the effects of overfishing in the ocean cascade down to lower trophic levels, as has been shown previously for lakes and coastal seas. In order to further investigate these processes, we establish a methodological framework to analyze species interactions from time series data.
3180571
Trophic cascades triggered by overfishing reveal possible mechanisms of ecosystem regime shifts
Large-scale transitions between alternative states in ecosystems are known as regime shifts. Once described as healthy and dominated by various marine predators, the Black Sea ecosystem by the late 20th century had experienced anthropogenic impacts such as heavy fishing, cultural eutrophication, and invasions by alien species. We studied changes related to these “natural experiments” to reveal the mechanisms of regime shifts. Two major shifts were detected, the first related to a depletion of marine predators and the second to an outburst of the alien comb jelly Mnemiopsis leidyi; both shifts were triggered by intense fishing resulting in system-wide trophic cascades. The complex nature of ecosystem responses to human activities calls for more elaborate approaches than currently provided by traditional environmental and fisheries management. This implies challenging existing practices and implementing explanatory models of ecosystem interactions that can better reconcile conservation and ecosystem management ideals.
22332630
Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean
Impacts of chronic overfishing are evident in population depletions worldwide, yet indirect ecosystem effects induced by predator removal from oceanic food webs remain unpredictable. As abundances of all 11 great sharks that consume other elasmobranchs (rays, skates, and small sharks) fell over the past 35 years, 12 of 14 of these prey species increased in coastal northwest Atlantic ecosystems. Effects of this community restructuring have cascaded downward from the cownose ray, whose enhanced predation on its bay scallop prey was sufficient to terminate a century-long scallop fishery. Analogous top-down effects may be a predictable consequence of eliminating entire functional groups of predators.
15046290
Multi-level trophic cascades in a heavily exploited open marine ecosystem
Anthropogenic disturbances intertwined with climatic changes can have a large impact on the upper trophic levels of marine ecosystems, which may cascade down the food web. So far it has been difficult to demonstrate multi-level trophic cascades in pelagic marine environments. Using field data collected during a 33-year period, we show for the first time a four-level community-wide trophic cascade in the open Baltic Sea. The dramatic reduction of the cod (Gadus morhua) population directly affected its main prey, the zooplanktivorous sprat (Sprattus sprattus), and indirectly the summer biomass of zooplankton and phytoplankton (top-down processes). Bottom-up processes and climate–hydrological forces had a weaker influence on sprat and zooplankton, whereas phytoplankton variation was explained solely by top-down mechanisms. Our results suggest that in order to dampen the occasionally harmful algal blooms of the Baltic, effort should be addressed not only to control anthropogenic nutrient inputs but also to preserve structure and functioning of higher trophic levels.
26122510
Declines in predatory fish promote bloom-forming macroalgae.
In the Baltic Sea, increased dominance of ephemeral and bloom-forming algae is presently attributed to increased nutrient loads. Simultaneously, coastal predatory fish are in strong decline. Using field data from nine areas covering a 700-km coastline, we examined whether formation of macroalgal blooms could be linked to the composition of the fish community. We then tested whether predator or nutrient availability could explain the field patterns in two small-scale field experiments, by comparing joint effects on algal net production from nutrient enrichment with agricultural fertilizer and exclusion of larger predatory fish with cages. We also manipulated the presence of invertebrate grazers. The abundance of piscivorous fish had a strong negative correlation with the large-scale distribution of bloom-forming macroalgae. Areas with depleted top-predator communities displayed massive increases in their prey, small-bodied fish, and high covers of ephemeral algae. Combining the results from the two experiments showed that excluding larger piscivorous fish: (1) increased the abundance of small-bodied predatory fish; (2) changed the size distribution of the dominating grazers, decreasing the smaller gastropod scrapers; and (3) increased the net production of ephemeral macroalgae. Effects of removing top predators and nutrient enrichment were similar and additive, together increasing the abundance of ephemeral algae many times. Predator effects depended on invertebrate grazers; in the absence of invertebrates there were no significant effects of predator exclusion on algal production. Our results provide strong support for regional declines of larger predatory fish in the Baltic Sea promoting algal production by decreasing invertebrate grazer control. This highlights the importance of trophic interactions for ecosystem responses to eutrophication. The view emerges that to achieve management goals for water quality we need to consider the interplay between top-down and bottom-up processes in future ecosystem management of marine resources.
15734342
Cascading effects of overfishing marine systems
Profound indirect ecosystem effects of overfishing have been shown for coastal systems such as coral reefs and kelp forests. A new study from the ecosystem off the Canadian east coast now reveals that the elimination of large predatory fish can also cause marked cascading effects on the pelagic food web. Overall, the view emerges that, in a range of marine ecosystems, the effects of fisheries extend well beyond the collapse of fish exploited stocks.
21978740
Eutrophication and overfishing in temperate nearshore pelagic food webs: a network perspective
We investigated the effects of human activ- ities on the pelagic food web structure of nearshore marine ecosystems. Their generic structure was estab- lished on the basis of literature review and analyzed by qualitative structural network analysis. Two main issues were addressed: (1) the role of species capable of forming harmful algal blooms (HABs) and red tides (Noctiluca spp.), as well as the role of jellyfish, in eutrophicated sys- tems; (2) the contribution of human influences on food webs, focusing on bottom-up (increased nutrient loading) and top-down (overfishing) effects. Results suggest that HAB-forming species and Noctiluca stimulate the micro- bial network, but reduce higher trophic levels such as commercially important fish species. Jellyfish act as a buffer in eutrophicated and overfished systems, as they retain nutrients from the water column, but their blooms lead to a massive accumulation of large phytoplankton organisms. Anthropogenic nutrient enrichment favors undesirable species because of their specific position in the food web, and this crucial position may explain their far-reaching effects. Finally, while it appears that over- fishing of piscivorous fishes inhibited HABs and sup- ported blooms of diatoms and other large algae in the past, the present-day loss of planktivorous fishes acts syn- ergistically with nutrient enrichment in promoting HAB species, Noctiluca and jellyfish. These fundamental con- straints, which are inherent in the generic structure of pelagic food webs, thus largely determine community dynamics in marine coastal ecosystems.
27801798
Predicting ecological consequences of marine top predator declines.
Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how predators influence communities by inflicting mortality on prey and inducing behavioral modifications (risk effects). Both mechanisms are important in marine communities, and a sole focus on the effects of predator-inflicted mortality might severely underestimate the importance of predators. We outline direct and indirect consequences of marine predator declines and propose an integrated predictive framework that includes risk effects, which appear to be strongest for long-lived prey species and when resources are abundant. We conclude that marine predators should be managed for the maintenance of both density- and risk-driven ecological processes, and not demographic persistence alone.
85155900
Food Web Complexity and Community Dynamics
Food webs in nature have multiple, reticulate connections between a diversity of consumers and resources. Such complexity affects web dynamics: it first spreads the direct effects of consumption and productivity throughout the web rather than focusing them at particular "trophic levels." Second, consumer densities are often donor controlled with food from across the trophic spectrum, the herbivore and detrital channels, other habitats, life-history omnivory, and even trophic mutualism. Although consumers usually do not affect these resources, increased numbers often allow consumers to depress other resources to levels lower than if donor-controlled resources were absent. We propose that such donor-controlled and "multichannel" omnivory is a general feature of consumer control and central to food web dynamics. This observation is contrary to the normal practice of inferring dynamics by simplifying webs into a few linear "trophic levels," as per "green world" theories. Such theories do not accommodate common and dynamically important features of real webs such as the ubiquity of donor control and the importance and dynamics of detritus, omnivory, resources crossing habitats, life history, nutrients (as opposed to energy), pathogens, resource defenses, and trophic symbioses. We conclude that trophic cascades and top-down community regulation as envisioned by trophic-level theories are relatively uncommon in nature.
1820646
Omnivory as a Stabilizing Feature of Natural Communities
Omnivory—defined broadly as feeding on more than one trophic level—occupies a prominent position in discussions of food web architecture and dynamics, due in large part to an enduring conflict regarding omnivory's role in community dynamics. According to classical results from mathematical food web theory, omnivory destabilizes ecological communities, whereas more recent conceptual syntheses suggest that omnivory should be a strongly stabilizing factor in food webs. Working with an arthropod assemblage at Mount Saint Helens, I experimentally addressed this controversy using a two‐way factorial design that crossed a manipulation of the degree of omnivory with another “disturbance” manipulation that targeted a specific component of the assemblage. In this statistical design, significant interaction effects (i.e., how the community impacts of the disturbance varied with the degree of omnivory) identified key stabilizing or destabilizing influences of omnivory. Overall, my experimental results indicated that increasing the degree of omnivory stabilized community dynamics, in keeping with recent conceptual syntheses.
4420271
Weak trophic interactions and the balance of nature
Ecological models show that complexity usually destabilizes food webs,, predicting that food webs should not amass the large numbers of interacting species that are in fact found in nature. Here, using nonlinear models, we study the influence of interaction strength (likelihood of consumption of one species by another) on food-web dynamics away from equilibrium. Consistent with previous suggestions,, our results show that weak to intermediate strength links are important in promoting community persistence and stability. Weak links act to dampen oscillations between consumers and resources. This tends to maintain population densities further away from zero, decreasing the statistical chance that a population will become extinct (lower population densities are more prone to such chances). Data on interaction strengths in natural food webs indicate that food-web interaction strengths are indeed characterized by many weak interactions and a few strong interactions.
17617705
The functional role of biodiversity in ecosystems: incorporating trophic complexity.
Understanding how biodiversity affects functioning of ecosystems requires integrating diversity within trophic levels (horizontal diversity) and across trophic levels (vertical diversity, including food chain length and omnivory). We review theoretical and experimental progress toward this goal. Generally, experiments show that biomass and resource use increase similarly with horizontal diversity of either producers or consumers. Among prey, higher diversity often increases resistance to predation, due to increased probability of including inedible species and reduced efficiency of specialist predators confronted with diverse prey. Among predators, changing diversity can cascade to affect plant biomass, but the strength and sign of this effect depend on the degree of omnivory and prey behaviour. Horizontal and vertical diversity also interact: adding a trophic level can qualitatively change diversity effects at adjacent levels. Multitrophic interactions produce a richer variety of diversity-functioning relationships than the monotonic changes predicted for single trophic levels. This complexity depends on the degree of consumer dietary generalism, trade-offs between competitive ability and resistance to predation, intraguild predation and openness to migration. Although complementarity and selection effects occur in both animals and plants, few studies have conclusively documented the mechanisms mediating diversity effects. Understanding how biodiversity affects functioning of complex ecosystems will benefit from integrating theory and experiments with simulations and network-based approaches.
45088691
Trophic Cascades in a Formerly Cod-Dominated Ecosystem
Removal of top predators from ecosystems can result in cascading effects through the trophic levels below, completely restructuring the food web. Cascades have been observed in small-scale or simple food webs, but not in large, complex, open-ocean ecosystems. Using data spanning many decades from a once cod-dominated northwest Atlantic ecosystem, we demonstrate a trophic cascade in a large marine ecosystem. Several cod stocks in other geographic areas have also collapsed without recovery, suggesting the existence of trophic cascades in these systems.
19203503
Consequences of omnivory for trophic interactions on a salt marsh shrub.
Although omnivory is common in nature, its impact on trophic interactions is variable. Predicting the food web consequences of omnivory is complicated because omnivores can simultaneously produce conflicting direct and indirect effects on the same species or trophic level. We conducted field and laboratory experiments testing the top-down impacts of an omnivorous salt marsh crab, Armases cinereum, on the shrub Iva frutescens and its herbivorous and predatory arthropod fauna. Armases is a "true omnivore," consuming both Iva and arthropods living on Iva. We hypothesized that Armases would benefit Iva through a top-down trophic cascade, and that this benefit would be stronger than the direct negative effect of Armases on Iva. A field experiment on Sapelo Island, Georgia (USA), supported this hypothesis. Although Armases suppressed predators (spiders), it also suppressed herbivores (aphids), and benefited Iva, increasing leaf number, and reducing the proportion of dead shoots. A one-month laboratory experiment, focusing on the most common species in the food web, also supported this hypothesis. Armases strongly suppressed aphids and consumed fewer Iva leaves if aphids were available as an alternate diet. Armases gained more body mass if they could feed on aphids as well as on Iva. Although Armases had a negative effect on Iva when aphids were not present, Armases benefited Iva if aphids were present, because Armases controlled aphid populations, releasing Iva from herbivory. Although Armases is an omnivore, it produced strong top-down forces and a trophic cascade because it fed preferentially on herbivores rather than plants when both were available. At the same time, the ability of Armases to subsist on a plant diet allows it to persist in the food web when animal food is not available. Because omnivores feed on multiple trophic levels, their effects on food webs may differ from those predicted by standard trophic models that assume that each species feeds only on a single trophic level. To better understand the complexity of real food webs, the variable feeding habits and feeding preferences of different omnivorous species must be taken into consideration.
86327847
Cascading effects of predator diversity and omnivory in a marine food web
Over-harvesting, habitat loss and exotic invasions have altered predator diversity and composition in a variety of communities which is predicted to affect other trophic levels and ecosystem functioning. We tested this hypothesis by manipulating predator identity and diversity in outdoor mesocosms that contained five species of macroalgae and a macroinvertebrate herbivore assemblage dominated by amphipods and isopods. We used five common predators including four carnivores (crabs, shrimp, blennies and killifish) and one omnivore (pinfish). Three carnivorous predators each induced a strong trophic cascade by reducing herbivore abundance and increasing algal biomass and diversity. Surprisingly, increasing predator diversity reversed these effects on macroalgae and altered algal composition, largely due to the inclusion and performance of omnivorous fish in diverse predator assemblages. Changes in predator diversity can cascade to lower trophic levels; the exact effects, however, will be difficult to predict due to the many complex interactions that occur in diverse food webs.
84991750
Relative consumer sizes and the strengths of direct and indirect interactions in omnivorous feeding relationships
Omnivory (the consumption of resources from more than one trophic level) is widespread in nature and has the potential to produce a richness of indirect effects. Nevertheless, its effects on popula ...
32310077
Effects of grazer richness and composition on algal biomass in a closed and open marine system.
Most natural local systems exchange organisms with a regional pool of species through migration and dispersal. Such metacommunity processes of interconnected multispecies assemblages are likely to affect local dynamics of both species and processes. We present results from an artificial marine outdoor rock pool system in which we investigated the factors of (1) local grazer richness and composition, and (2) connectivity of local patches to a regional species pool, and their effects on algal biomass. Local species richness of six grazers was manipulated in both open and closed pools, which were embedded in a regional species pool containing all six grazers. Grazer richness showed significant net biodiversity effects on grazing in the closed, but not in the open, system. Grazer composition, on the other hand, showed significant effects on grazing in both open and closed systems, depending on which species were initially present. The two most efficient grazers were able to compensate for less efficient grazers in species mixtures, hence ensuring the function of grazing. The efficiency of top-down control of algal biomass in open systems thus depends on which particular species are lost. Further, differences in grazing between the open and closed system changed over time due to temporal dynamics in grazer composition. The results emphasize the importance of including system connectivity in experimental designs to allow an extrapolation of biodiversity ecosystem-functioning relationships to natural systems.
56069737
Trophic interactions in Zostera marina beds along the Swedish coast
We compared eelgrass Zostera marina communities in 3 regions in Sweden believed to be affected by eutrophication and overfishing, to determine whether bottom-up or top-down processes control the biomass of epiphytic macroalgae and grazers. Nitrogen and carbon isotope signatures were analyzed to explore the food webs and to identify the grazing species feeding on filamentous algae and/or eelgrass. Mixing model (IsoSource version 1.3.1) analysis of the isotope signatures indicated that the amphipods Gammarus locusta and Microdeutopus gryllotalpa fed primarily on filamentous algae and that only 2 small gastropod species consumed eelgrass. Moreover, the grass shrimp Palaemon elegans and F adspersus were ca. 1 trophic level above amphipods and algae, but according to the mixing model played different trophic roles in the different areas. The highest biomass of filamentous algae was found in the west coast beds housing grazers with the lowest biomass and mean size (predominantly G. locusta and M. gryllotalpa, 0.5 to 3 mm). In contrast, the Baltic Sea beds had low algal biomass, but the grazers (mostly G. locusta and Idotea baltica) had higher biomass and were significantly larger (mean size ca. 10 mm). An overall negative correlation was found between grazer biomass and biomass of filamentous algae. The significantly smaller grazers and absence of isopod grazers on the west coast may be due to substantial consumption by small predatory fish. This supports the suggestions that, in Swedish eelgrass beds, grazers are top-down controlled, and overexploitation. of large predators and eutrophication play an important role in the recent increases in algal biomass.
54948454
Trophic role of the omnivorous grass shrimp Palaemon elegans in a Swedish eelgrass system
Algal grazers are thought to play a critical role in preventing overgrowth by ephemeral algae in seagrass beds. We studied the interaction between the omnivorous grass shrimp Palaemon elegans, the algal grazer Gammarus locusta and the filamentous green alga Ulva sp. (syn. Entero- morpha sp.) to assess whether the shrimp has a positive or negative effect on the growth of ephemeral algae in eelgrass beds on the Swedish west coast. Laboratory experiments demonstrated that P. ele- gans were effective predators on gammarids 5 mm TL, but negatively affected when G. locusta was smaller. In a field cage experiment, enclosed P. elegans decreased the abundance of gammarids by on average 68%, causing a trophic cascade that increased the biomass of filamentous Ulva sp. 7-fold. These results suggest that P. elegans on the Swedish northwest coast feed mainly on amphipods, including the algal grazer G. locusta, resulting in a positive net trophic effect on the growth of filamentous Ulva sp. This study indicates that P. ele- gans may be an important link between overexploitation of cod in northwest Sweden and the recent increase of algal mats and loss of eelgrass in the area.
43530463
Consumers affect prey biomass and diversity through resource partitioning.
Consumer presence and nutrient availability can have contrasting and interactive effects on plant diversity. In a factorial experiment, we manipulated two levels of nutrient supply and the presence of two moderately specialized grazers in different combinations (no grazers, two species in monoculture, and both in combination). We tested how nutrients and grazers regulated the biomass of marine coastal epiphytes and the diversity of algal assemblages, based on the prediction that the effect of consumers on prey diversity depends on productivity and consumer specialization. Nutrient enrichment increased the epiphytic load, while monocultures of single grazer species partly prevented epiphyte growth. However, only the presence of two species with complementary feeding preferences effectively prevented epiphyte overgrowth. The epiphytes comprised micro- and macroalgal species, and the diversity of these algal assemblages differed, depending on grazer identity. For the microalgae, diversity was reduced by nutrient addition when grazer control was inefficient, but not when specialist microalgal grazers were present. Macroalgal diversity was reduced in ambient water with specialist macroalgal grazers compared to the treatment with inefficient ones. These results indicate that grazer composition and productivity are crucial in determining whether consumer pressure will have a positive or negative effect on algal diversity.
85848182
A comparative biodiversity study of the associated fauna of perennial fucoids and filamentous algae
Abstract Anthropogenic activities worldwide have contributed to vegetation changes in many coastal areas, changes that may in turn affect faunal and algal assemblages in the involved ecosystems. In the northernmost part of the Baltic Sea the salinity is extremely low (3–4) and the only structurally complex alga present is Fucus radicans . Since in this area F. radicans is living at its salinity tolerance limit, it is potentially very sensitive to environmental changes. Any change in salinity could thus alter the overall algal community, changing it to one dominated solely by filamentous algae. To determine the importance of F. radicans to the associated faunal community, we examined differences between the 2 main vegetation types present, i.e., F. radicans and filamentous algae, in the Kronoren marine reserve in the northernmost part of the Baltic Sea. A similar study was conducted in the Asko area in the northern Baltic Proper, where the more-investigated Fucus vesiculosus is the only large fucoid present. The biomass of associated fauna was significantly higher in both the F. radicans and F. vesiculosus than in the filamentous algal vegetation at some, but not all, sites. The F. radicans community also displayed a greater diversity of associated fauna in 3 of 5 investigated Kronoren sites, whereas no difference in diversity was detected between F. vesiculosus and the filamentous algal vegetations in the Asko sites. Furthermore, the F. radicans community displayed a different faunal community, being the only investigated algal community with a faunal community dominated by K -strategy species, according to abundance–biomass comparison curves. This pattern may be due to the low epiphytic load on these Fucus plants. In contrast, the F. vesiculosus community, as well as the algal communities with no Fucus in both areas, had high biomasses of filamentous algae and an invertebrate fauna dominated by Chironomidae, occurring in great abundance but only with a low biomass. ANOSIM analyses of faunal composition demonstrated a significant difference between the 2 vegetation types in both areas, largely due to greater abundance of Gammarus spp. and Theodoxus fluviatilis in the fucoid vegetation. Differences observed between the F. radicans and filamentous algal vegetation types were generally more pronounced than those between F. vesiculosus and nearby filamentous algal vegetation. These observations may be due to abiotic factors that differ between the 2 investigated areas, factors such as depth distribution, wave action and eutrophication level. This study has demonstrated that the less-investigated F. radicans may be as important as the larger F. vesiculosus for the associated faunal assemblages. At the same time, the limited extent of F. radicans at shallower depths makes F. radicans vegetation potentially more vulnerable to anthropogenic changes, as declines in fucoid vegetation are usually first manifested in populations at their lower depth limits, whereas shallow populations are less affected.
13512544
A cross-system synthesis of consumer and nutrient resource control on producer biomass.
Nutrient availability and herbivory control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that (i) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems and (ii) bottom-up and top-down forces are interdependent, with statistical interactions that synergize or dampen relative influences on producer biomass. We used simple dynamic models to review ecological mechanisms that generate independent vs. interactive responses of community-level biomass. We calibrated these mechanistic predictions with the metrics of factorial meta-analysis and tested their prevalence across freshwater, marine and terrestrial ecosystems with a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients. Our analysis showed that producer community biomass increased with fertilization across all systems, although increases were greatest in freshwater habitats. Herbivore removal generally increased producer biomass in both freshwater and marine systems, but effects were inconsistent on land. With the exception of marine temperate rocky reef systems that showed positive synergism of nutrient enrichment and herbivore removal, experimental studies showed limited support for statistical interactions between nutrient and herbivory treatments on producer biomass. Top-down control of herbivores, compensatory behaviour of multiple herbivore guilds, spatial and temporal heterogeneity of interactions, and herbivore-mediated nutrient recycling may lower the probability of consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently multiple producer resources (e.g. nitrogen, phosphorus, light), multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates) and multiple trophic levels; and - in addition to measuring producer biomass - assess the responses of species diversity, community composition and nutrient status.
85216965
Life history, growth and production of Theodoxus fluviatilis in Lake Esrom, Denmark
Abstract A population of Theodoxus fluviatilis L in the littoral zone of Lake Esrom was investigated from November 1977 to February 1979. The population was sampled every month in the winter period and twice during the rest of the year. Biomass was estimated as ash-free dry weight (AFDW) of the organic matter both of the soft parts of the animal and the shell itself. The relation between AFDW ( c ) and shell length ( l ) was log  c =2.9509×log ( l )−1.7120. The population comprised more than 1 year-class, which could be separated by shell length, by a narrow band on the shells and the growth of algae on the shell. The life cycle lasted 2 1 2 - 3 years. The oldest animals had a shell length of 7.0–7.5 mm. A few individuals who were estimated to be 3 1 2 years had a shell length up to 8.6 mm. Population density varied between 575 and 2115 individuals m −2 on the stony substratum. The average was 1160 individuals m −2 . Mortality was low during the summer period. In winter many animals died due to the effect of ice and stormy weather on the stony substratum. Growth of the animals was estimated from the shell length. Maximum growth was observed from May to August with no growth during the winter. Egg capsules were found on the stones all year round. New capsules were found from late May to the middle of November. Most freshly laid capsules were observed in May–June and August–September. Capsules from the late summer hatched in spring and capsules laid in the spring hatched in August–September. The average annual net production for the whole population was estimated by three methods. The Allen curve method gave 1.895 AFDW m −2 , the growth-increment method gave 1.784 mg AFDW m −2 and the Hynes method 2.284 mg AFDW m −2 . Corresponding estimated P/B ratios were 1.29, 1.30 and 1.57. Annual net-production of the four investigated year-classes was 16 mg AFDW m −2  year −1 for 1975, 224 mg AFDW m −2  year −1 for 1976, 1.258 mg AFDW m −2  year −1 for 1977 and 287 mg AFDW m −2  year −1 for 1978. P/B ratios for the three oldest year-classes were, respectively, 0.32, 0.50 and 1.67. A comparison with other investigations on gastropod life cycles, reproduction and P/B ratios is made and differences discussed. Variations are correlated to temperature, and food quality and quantity.
11727147
Strong bottom-up and top-down control of early life stages of macroalgae
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.
22329291
Biotic habitat complexity controls species diversity and nutrient effects on net biomass production.
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.
17249868
Inducible resistance to herbivory in Fucus vesiculosus—duration, spreading and variation with nutrient availability
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.
54503424
Grazer identity is crucial for facilitating growth of the perennial brown alga Fucus vesiculosus
Grazer identity is crucial for facilitating growth of the perennial brown alga Fucus vesiculosus
84386229
Opposing effects of grazing and nutrients on diversity
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.
8496385
Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure
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.
14327955
Consumers Control Diversity and Functioning of a Natural Marine Ecosystem
Background 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. Methodology and Principal Findings 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. Significance 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.
54052643
Diversity effects beyond species richness: evidence from intertidal macroalgal assemblages
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.
54654824
Effects of Pilayella littoralis on Fucus vesiculosus recruitment: implications for community composition
“Effects of Pilayella littoralis on Fucus vesiculosus recruitment: implications for community composition
54598097
STRONG IMPACTS OF GRAZING AMPHIPODS ON THE ORGANIZATION OF A BENTHIC COMMUNITY
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.
205080265
Intraguild predation: The dynamics of complex trophic interactions.
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.
84170092
Species loss and ecosystem functioning : effects of species identity and community composition
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.
205018808
Partitioning selection and complementarity in biodiversity experiments
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.
86547253
Grazer diversity effects on ecosystem functioning in seagrass beds
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.
86707310
THE ECOLOGICAL CONSEQUENCES OF VARIATION IN PLANTS AND PREY FOR AN OMNIVOROUS INSECT
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...
18183926
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.
87627144
Why Are Parts of the World Green? Multiple Factors Control Productivity and the Distribution of Biomass
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.
11744558
Predator interactions, mesopredator release and biodiversity conservation.
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.
2114852
Network structure and biodiversity loss in food webs: robustness increases with connectance
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.
16159889
Cascading effects from predator removal depend on resource availability in a benthic food web
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.