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Heikki Setälä - One of the best experts on this subject based on the ideXlab platform.
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Colonisation of newly established habitats by soil decomposer organisms: the effect of habitat corridors in relation to colonisation distance and habitat size
Applied Soil Ecology, 2005Co-Authors: Minna-liisa Rantalainen, Hannu Fritze, Jari Haimi, Taina Pennanen, Heikki SetäläAbstract:Abstract The aim of the present 2.5-year-long field experiment was to explore the ability of various members of the Detrital Food Web to colonise newly established habitat patches in field conditions, either in the presence or absence of habitat corridors. Patch size and distance to the “mainland” (colonisation source) were manipulated to explore the scale dependency of the corridor effects. Sterilised humus patches, embedded in mineral soil regarded as uninhabitable (or non-preferred) matrix for the soil organisms, functioned as newly established habitats. Intact forest soil served as the source of colonisers. Three kinds of patches were established: large ones situated at relatively long distance from the colonisation source, and small ones situated either at long or short distance from the source. Corridors, when present, connected the patches to the intact forest soil. The presence of corridors consistently increased the species richness of soil fungi in the patches. Also bacterial species richness, number of microarthropod taxa and abundance of enchytraeid worms were sometimes increased by the presence of corridors. The response of bacterial species was assumed to be indirect, caused by the response of enchytraeid worms functioning as dispersal agents for the bacteria. Colonisation of the patches by soil organisms was virtually independent of patch size and colonisation distance, both in the presence and absence of corridors. This can be related to the resource-poor or otherwise sub-optimal conditions in the initially sterile humus soil, restricting the number of species able to establish themselves in the patches and/or utilise the corridors. Nevertheless, the results indicate that habitat corridors can facilitate the colonisation of unpopulated habitats by organisms with greatly different life history traits.
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Vertical stratification and trophic interactions among organisms of a soil decomposer Food Web – a field experiment using 15N as a tool
European Journal of Soil Biology, 2002Co-Authors: Heikki Setälä, Tuula AarnioAbstract:Abstract In this field study, we explored the spatial segregation between the litter- and humus-inhabiting organisms of the Detrital Food Web using 15 N-isotope technique. The study was established in 11 × 11 m plots fertilized with 15 N-labelled urea. Ten years after urea application, soil samples were taken, both from the litter layer and the combined F+H layer. The samples were analysed for N content and the proportion of 15 N in (i) the residual organic matter in the litter and F+H layer (excluding microbes), (ii) microbial biomass, and (iii) various feeding guilds of soil fauna. The basal resource, soil microbes, and the fauna were more enriched with 15 N in the F+H layer than in the litter layer. In the litter layer, the 15 N enrichment of the expected Food source equalled the one of the consumers, whereas in the F+H layer all trophic groups, except microbes and small microbi-detritivores, showed a significantly lower 15 N enrichment than their expected Food source. The results indicate that large and mobile humus-inhabiting decomposers exploit the overlying litter layer as a feeding site, whereas the feeding of the more sedentary smaller organisms is restricted to the humus layer.
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Reciprocal interactions between Scots pine and soil Food Web structure in the presence and absence of ectomycorrhiza
Oecologia, 2000Co-Authors: Heikki SetäläAbstract:Mycorrhizal plants are commonly believed to direct much more of their photosynthates into the soil than non-mycorrhizal plants. As the growth of most organisms of the Detrital Food Web is limited by energy, the flow of C through mycorrhizal plants into the below-ground milieu is widely assumed to nourish a variety of decomposer organisms in soils. In the current experiment, I explored whether some representatives of soil mesofauna, either fungivores or microbi-detritivores, derive benefit from the presence of ectomycorrhizal (EM) fungi growing on the roots of Scots pine (Pinus sylvestris). I also investigated whether the role of soil mesofauna in affecting pine growth depends on the presence of EM fungi in the pine rhizosphere. The study was established in microcosms with a mixture of raw humus and sand. The soil was defaunated, reinoculated with 10 species of soil bacteria and 11 species of saprophytic soil fungi, and pine seedlings, either infected or non-infected with four taxa of EM fungi, were planted in the microcosms. Five treatments with different Food Web configurations were established: (1) saprophytic microbes alone, (2) as (1) but with the omnivorous enchytraeid species Cognettia sphagnetorum present, (3) as (1) but with Collembola (Hypogastrura assimilis), (4) as (1) but with four species of oribatid mites (Acari) involved, and (5) as 1) but with C. sphagnetorum, H. assimilis and the Acari. The microcosms were incubated in a climate chamber with varying temperature and illumination regimes for two growing periods for the pine. After 60 weeks, pine biomass production was significantly greater in the mycorrhizal systems, the total biomass being 1.43 times higher in the presence than absence of EM fungi. Similarly, almost ten times more fungal biomass was detected on pine roots growing in the mycorrhizal than in the non-mycorrhizal systems. The presence of EM fungi was also associated with significantly lowered pH and percent organic matter of the soil. Despite the clearly larger biomass of both the pines and the fungi on the pine roots, neither the numbers nor biomasses of the mesofauna differed significantly between the EM and non-EM systems. The presence of Collembola and C. sphagnetorum had a positive influence on pine growth, particularly in the absence of EM fungi, whereas oribatid mites had no effects on pine growth. The complexity of the mesofaunal community was not related to the biomass production of the pines in a straightforward manner; for example, the complex systems with each faunal group present did not produce more pine biomass than the simple systems where C. sphagnetorum existed alone. The results of this experiment suggest that the short-term role of EM fungi in fuelling the Detrital Food Web is less significant than generally considered, but that their role as active decomposers and/or stimulators of the activity of saprophytic microbes can be more important than is often believed.
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Influence of Ectomycorrhiza on the Structure of Detrital Food Webs in Pine Rhizosphere
Oikos, 1999Co-Authors: Heikki Setälä, Pauliina Kulmala, Juha Mikola, Anna Mari MarkkolaAbstract:Trees infected with ectomycorrhizal (EM) fungi are known to direct several times more energy into the soil than non-ectomycorrhizal trees. Since energy-rich substrates are required by most biological processes, it is widely speculated that the flow of carbon from plants through EM mycelia into soils should have a wide range of effects in the below-ground milieu. In this study we investigated whether EM fungi of effects in the below-ground milieu. In this study we investigated whether EM fungi of Scots pine (Pinus sylvestris) seedlings exert a short-term influence on the biomass and composition of a Detrital Food Web consisting of up to three trophic levels. The experiment was conducted in microcosms with raw humus, perlite and sandy soil (1:1:1; vol.). The soil was defaunated. reinoculated with 10 species of soil bacteria. and 11 species of saprophytic soil fungi, and pine seedlings, either infected or non-infected with six taxa of EM fungi, were separately planted in the microcosms. Five different Food Webs were established: 1) saprophytic microbes alone. 2) as 1) but with two species of bacterial-feeding nematodes. 3) as 2) but two species of fungal-feeding nematodes included. 4) as 2) but with predatory nematodes, and 5) as 3) but with predatory nematodes. The microcosms were incubated in a climate chamber with varying illumination and temperature regimes for two annual cycles of the pine. After 37 weeks, ca 15 times more fungal biomass was detected on pine roots growing in the systems inoculated with EM fungi than in non-inoculated systems, and there was a significantly greater total biomass of pines in the EM systems. Despite this clearly enhanced autotroph production, neither the total nematode biomass nor the length of the Food chain were affected by the EM fungi. The presence of nematodes. independent of the composition of this fauna, enhanced the production of pine biomass significantly, but only in the absence of mycorrhizal fungi. Neither the species composition nor the trophic group architecture of the fauna influenced EM production or growth of EM infected pines, except for fungal-feeding nematodes which had a slight negative influence on the biomass of EM fungi. Our results infer that the role of EM fungi in affecting the amount of energy entering the soils is significant in the long run only, i.e. through increased litter production. We hypothesize that the short-term insensitivity of the decomposer Food Web to the presence of EM fungi reflects the nature of EM fungi as an efficient sink, preventing the additional production of plant-derived photosynthates from-reaching the organisms of the Detrital Food Web.
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composition and trophic structure of Detrital Food Web in ant nest mounds of formica aquilonia and in the surrounding forest soil
Oikos, 1998Co-Authors: Jouni Laakso, Heikki SetäläAbstract:Community composition and Food Web structure of soil decomposer biota in relation to various habitat properties were compared between upper parts of red wood ant (Formica aquilonia) nest mounds and the adjacent forest soil. For a description of trophic structure of the decomposer community in the two habitats, soil decomposers were classified into 14 trophic groups. Classification of the taxa into three habitat preference categories resulted in a clear division of the fauna into either soil or nest specialists, relatively few taxa falling between these two groups. A large majority of the nest specialists belonged to a non-myrmecophilous soil decomposer fauna so far largely overlooked in studies on ant-invertebrate associations. Trophic organisation of the nest mound community differed clearly from that in the soil by having considerably larger biomass at the base of the Food Web, and less large predators -other than ants - at the top of the Web. Contrary to forest soils. the clear dominance of bacterial feeding microfauna over the fungal feeding microfauna in the nest mounds suggests that most of the energy passing through the Food Web is channelled through a bacterial-based Food-Web compartment in the nest mounds. Relatively constant temperature and moisture in the nest surface, continuous energy input by the ants to the nests, and ant-induced reduction in predation pressure on macropredators are suggested to be responsible for the development of the typical decomposer community structure in the nest mounds. Thus, the Food-Web dynamics in ant nest mounds represent an interesting case in which the behaviour of an invertebrate species (i.e. the ant) has a potential to control the development of a system-level organisation. The high biomass of microbi-detritivorous animals, especially earthworms, in the nest mounds suggests that the activities of the decomposer fauna may feed back to the structure of nest mound and indirectly alter the performance of the ant colony.
Jean-françois Carrias - One of the best experts on this subject based on the ideXlab platform.
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Understorey environments influence functional diversity in tank‐bromeliad ecosystems
Freshwater Biology, 2012Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Céline Leroy, Jean-françois CarriasAbstract:1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad Food Webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad Food Webs in samples taken from 171 tank-bromeliads. 3. We observed a gradient of detritus-based to algal-based Food Webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad Food Webs in exposed areas, whereas filter-feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun-exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank-bromeliads, which grow in a broad range of ecological conditions, promote aquatic Food Web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus-based systems in the literature, we show that algal production can support a non-Detrital Food Web in these systems.
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Understorey environments influence functional diversity in tank-bromeliad ecosystems
Freshwater Biology, 2012Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Céline Leroy, Jean-françois CarriasAbstract:1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad Food Webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad Food Webs in samples taken from 171 tank-bromeliads. 3. We observed a gradient of detritus-based to algal-based Food Webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad Food Webs in exposed areas, whereas filter-feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun-exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank-bromeliads, which grow in a broad range of ecological conditions, promote aquatic Food Web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus-based systems in the literature, we show that algal production can support a non-Detrital Food Web in these systems.
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Are algae relevant to the detritus-based Food Web in tank-bromeliads?
PLoS ONE, 2011Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Olivier Roux, Anne-hélène Le Jeune, Céline Leroy, Jean-françois CarriasAbstract:We assessed the occurrence of algae in five species of tank-bromeliads found in contrasting environmental sites in a Neotropical, primary rainforest around the Nouragues Research Station, French Guiana. The distributions of both algal abundance and biomass were examined based on physical parameters, the morphological characteristics of bromeliad species and with regard to the structure of other aquatic microbial communities held in the tanks. Algae were retrieved in all of the bromeliad species with mean densities ranging from ∼10(2) to 10(4) cells/mL. Their biomass was positively correlated to light exposure and bacterial biomass. Algae represented a tiny component of the Detrital Food Web in shaded bromeliads but accounted for up to 30 percent of the living microbial carbon in the tanks of Catopsis berteroniana, located in a highly exposed area. Thus, while nutrient supplies are believed to originate from wind-borne particles and trapped insects (i.e., allochtonous organic matter), our results indicate that primary producers (i.e., autochtonous organic matter) are present in this insectivorous bromeliad. Using a 24-h incubation of size-fractionated and manipulated samples from this plant, we evaluated the impact of mosquito foraging on algae, other microorganisms and rotifers. The prey assemblages were greatly altered by the predation of mosquito larvae. Grazing losses indicated that the dominant algal taxon, Bumilleriopsis sp., like protozoa and rotifers, is a significant part of the diet of mosquito larvae. We conclude that algae are a relevant functional community of the aquatic Food Web in C. berteroniana and might form the basis of a complementary non-Detrital Food Web.
Olivier Brouard - One of the best experts on this subject based on the ideXlab platform.
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Understorey environments influence functional diversity in tank‐bromeliad ecosystems
Freshwater Biology, 2012Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Céline Leroy, Jean-françois CarriasAbstract:1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad Food Webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad Food Webs in samples taken from 171 tank-bromeliads. 3. We observed a gradient of detritus-based to algal-based Food Webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad Food Webs in exposed areas, whereas filter-feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun-exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank-bromeliads, which grow in a broad range of ecological conditions, promote aquatic Food Web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus-based systems in the literature, we show that algal production can support a non-Detrital Food Web in these systems.
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Understorey environments influence functional diversity in tank-bromeliad ecosystems
Freshwater Biology, 2012Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Céline Leroy, Jean-françois CarriasAbstract:1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad Food Webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad Food Webs in samples taken from 171 tank-bromeliads. 3. We observed a gradient of detritus-based to algal-based Food Webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad Food Webs in exposed areas, whereas filter-feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun-exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank-bromeliads, which grow in a broad range of ecological conditions, promote aquatic Food Web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus-based systems in the literature, we show that algal production can support a non-Detrital Food Web in these systems.
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Are algae relevant to the detritus-based Food Web in tank-bromeliads?
PLoS ONE, 2011Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Olivier Roux, Anne-hélène Le Jeune, Céline Leroy, Jean-françois CarriasAbstract:We assessed the occurrence of algae in five species of tank-bromeliads found in contrasting environmental sites in a Neotropical, primary rainforest around the Nouragues Research Station, French Guiana. The distributions of both algal abundance and biomass were examined based on physical parameters, the morphological characteristics of bromeliad species and with regard to the structure of other aquatic microbial communities held in the tanks. Algae were retrieved in all of the bromeliad species with mean densities ranging from ∼10(2) to 10(4) cells/mL. Their biomass was positively correlated to light exposure and bacterial biomass. Algae represented a tiny component of the Detrital Food Web in shaded bromeliads but accounted for up to 30 percent of the living microbial carbon in the tanks of Catopsis berteroniana, located in a highly exposed area. Thus, while nutrient supplies are believed to originate from wind-borne particles and trapped insects (i.e., allochtonous organic matter), our results indicate that primary producers (i.e., autochtonous organic matter) are present in this insectivorous bromeliad. Using a 24-h incubation of size-fractionated and manipulated samples from this plant, we evaluated the impact of mosquito foraging on algae, other microorganisms and rotifers. The prey assemblages were greatly altered by the predation of mosquito larvae. Grazing losses indicated that the dominant algal taxon, Bumilleriopsis sp., like protozoa and rotifers, is a significant part of the diet of mosquito larvae. We conclude that algae are a relevant functional community of the aquatic Food Web in C. berteroniana and might form the basis of a complementary non-Detrital Food Web.
David H Wise - One of the best experts on this subject based on the ideXlab platform.
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Long-term resource addition to a Detrital Food Web yields a pattern of responses more complex than pervasive bottom-up control
PeerJ, 2017Co-Authors: Kendra L. Lawrence, David H WiseAbstract:Background Theory predicts strong bottom-up control in detritus-based Food Webs, yet field experiments with detritus-based terrestrial systems have uncovered contradictory evidence regarding the strength and pervasiveness of bottom-up control processes. Two factors likely leading to contradictory results are experiment duration, which influences exposure to temporal variation in abiotic factors such as rainfall and affects the likelihood of detecting approach to a new equilibrium; and openness of the experimental units to immigration and emigration. To investigate the contribution of these two factors, we conducted a long-term experiment with open and fenced plots in the forest that was the site of an earlier, short-term experiment (3.5 months) with open plots (Chen & Wise, 1999) that produced evidence of strong bottom-up control for 14 taxonomic groupings of primary consumers of fungi and detritus (microbi-detritivores) and their predators. Methods We added artificial high-quality detritus to ten 2 × 2-m forest-floor plots at bi-weekly intervals from April through September in three consecutive years (Supplemented treatment). Ten comparable Ambient plots were controls. Half of the Supplemented and Ambient plots were enclosed by metal fencing. Results Arthropod community structure (based upon 18 response variables) diverged over time between Supplemented and Ambient treatments, with no effect of Fencing on the multivariate response pattern. Fencing possibly influenced only ca. 30% of the subsequent univariate analyses. Multi- and univariate analyses revealed bottom-up control during Year 1 of some, but not all, microbi-detritivores and predators. During the following two years the pattern of responses became more complex than that observed by Chen & Wise (1999). Some taxa showed consistent bottom-up control whereas others did not. Variation across years could not be explained completely by differences in rainfall because some taxa exhibited negative, not positive, responses to Detrital supplementation. Discussion Our 3-year experiment did not confirm the conclusion of strong, pervasive bottom-up control of both microbi-detritivores and predators reported by Chen & Wise (1999). Our longer-term experiment revealed a more complex pattern of responses, a pattern much closer to the range of outcomes reported in the literature for many short-term experiments. Much of the variation in responses across studies likely reflects variation in abiotic and biotic factors and the quality of added detritus. Nevertheless, it is also possible that long-term resource enhancement can drive a community towards a new equilibrium state that differs from what would have been predicted from the initial short-term responses exhibited by primary and secondary consumers.
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Long-term resource addition to a Detrital Food Web yields a pattern of responses more complex than pervasive bottom-up control
2017Co-Authors: Kendra L. Lawrence, David H WiseAbstract:Background. Theory predicts strong bottom-up control in detritus-based Food Webs, yet field experiments with detritus-based terrestrial systems have uncovered contradictory evidence regarding the strength and pervasiveness of bottom-up control processes. Two factors likely leading to contradictory results are experiment duration, which influences exposure to temporal variation in abiotic factors such as rainfall and affects the likelihood of detecting approach to a new equilibrium; and openness of the experimental units to immigration and migration. To investigate the contribution of these two factors, we conducted a long-term experiment with open and fenced plots in the forest that was the site of an earlier, short-term experiment (3.5 months) with open plots (Chen & Wise 1999) that produced evidence of strong bottom-up control for 14 taxonomic groupings of primary consumers of litter and fungi (microbi-detritivores) and their predators. Methods. We added artificial high-quality detritus to ten 2 x 2-m forest-floor plots at bi-weekly intervals from April through September in three consecutive years (Supplemented treatment). Ten comparable Ambient plots were controls. Half of the Supplemented and Ambient plots were enclosed by metal fencing. Results. Arthropod community structure (based upon 18 response variables) diverged over time between Supplemented and Ambient treatments, with no effect of Fencing on the multivariate response pattern. Fencing possibly influenced only ca. 20% of the subsequent univariate analyses. Multi- and univariate analyses revealed bottom-up control by fall of Year 1 of some, but not all, microbi-detritivores and predators. During the following two years the pattern of responses became more complex than that observed by Chen & Wise (1999). Some taxa showed consistent bottom-up control whereas many did not. Variation across years could not be explained completely by differences in rainfall because some taxa exhibited negative, not positive, responses to Detrital supplementation. Discussion. Our 3-yr experiment did not confirm the conclusion of strong, pervasive bottom-up control of microbi-detritivores and predators reported by Chen and Wise (1999). Our longer-term experiment revealed a more complex pattern of responses, a pattern much closer to the range of outcomes reported in the literature for many short-term experiments. Much of the variation in responses across studies likely reflects variation in factors such as rainfall and the quality of added detritus. Nevertheless, it is also possible that long-term resource enhancement can drive a community towards a new equilibrium state that differs from what would have been predicted from the initial short-term responses exhibited by primary and secondary consumers.
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Subsidy from the Detrital Food Web, but not microhabitat complexity, affects the role of generalist predators in an aboveground herbivore Food Web
Oikos, 2008Co-Authors: Klaus Birkhofer, David H Wise, Stefan ScheuAbstract:The activity and density of generalist predators, such as carabid beetles, rove beetles and spiders, may increase in response to: (1) increased availability of prey from the belowground subsystem and/or (2) enhanced complexity of aboveground vegetation. Organic farming practices support decomposer populations and enhance habitat complexity due to an increased weed density. A response by generalist predators to such below- or aboveground changes could affect predation rates on herbivores in the aboveground Food Web. We tested this hypothesis in a replicated field experiment conducted in a winter wheat field, where increased predator activity could lead to improved control of herbivorous pests. In a crossed design, we increased and lowered densities of decomposer prey, and manipulated vegetation complexity using artificial plants in order to examine the effect of structural complexity in isolation from effects of plant-attracted additional prey. Isotomid Collembola exhibited lowest activity-densities (AD) in plots treated with soil insecticide and had gradually increasing AD in untreated plots and plots receiving Detrital subsidies. Carabid beetles and cursorial spiders did not respond to increased availability of isotomid prey, and they unexpectedly displayed higher AD in the structurally less-complex plots. Aphid density mirrored the positive response of isotomids to Detrital subsidies, suggesting that aphids benefited from reduced predation due to predators switching to abundant prey in the decomposer subsystem. The absence of a numerical response by surface-active predators apparently strengthened this indirect effect of isotomids on aphids. Our results suggest that indirect predator-mediated prey-prey interactions can reduce beneficial effects of Detrital subsidies on pest suppression. We further demonstrated that generalist predators may not per se benefit from structural complexity. Both results document the challenges associated with management practices that support generalist predators, as these measures may not necessarily improve herbivore suppression.
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Using stable isotopes to reveal shifts in prey consumption by generalist predators.
Ecological applications : a publication of the Ecological Society of America, 2006Co-Authors: David H Wise, Denise M. Moldenhauer, Juraj HalajAbstract:The effectiveness of generalist predators in biological control may be diminished if increased availability of alternative prey causes individual predators to decrease their consumption of crop pests. Farming practices that enhance densities of microbidetritivores in the Detrital Food Web can lead to increased densities of generalist predators that feed on pest species. The ability to predict the net biocontrol impact of increased predator densities depends upon knowing the extent to which individual predators may shift to Detrital prey and feed less on crop pests when prey of the detritus-based Food Web are more abundant. We addressed this question by comparing ratios of stable isotopes of carbon (delta13C) and nitrogen (delta15N) in generalist ground predators and two types of prey (crop pests and microbidetritivores) in replicated 8 x 8 m cucurbit gardens subjected to one of two treatments: a Detrital subsidy or no addition of detritus (control). Small sheet-Web spiders (Linyphiidae) and small wolf spiders (Lycosidae) had delta13C values similar to those of Collembola in both the Detrital and control treatments, indicating that small spiders belong primarily to the Detrital Food Web. In control plots the larger generalist predators had delta13C values similar to those of the major insect pests, consistent with their known effectiveness as biocontrol agents. Adding detritus may have caused delta13C of one species of large wolf spider to shift toward that of the microbi-detritivores, although evidence is equivocal. In contrast, another large wolf spider displayed no shift in delta13C in the Detrital treatment. Thus, stable isotopes revealed which generalist predators will likely continue to feed on pest species in the presence of greater densities of alternative prey.
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IMPACT OF A Detrital SUBSIDY ON TROPHIC CASCADES IN A TERRESTRIAL GRAZING Food Web
Ecology, 2002Co-Authors: Juraj Halaj, David H WiseAbstract:Theory predicts that allochthonous energy subsidies can modify the strength of trophic cascades in Food Webs. We tested the hypothesis that a Detrital subsidy would enhance trophic cascades in the grazing Food Web of an agroecosystem in which generalist predators, wolf spiders and carabid beetles, feed both on detritivores and herbivores. We crossed detritus-addition with predator-removal treatments in replicated 8 × 8 m open and fenced plots of cucumbers and squash. Collembola, a major detritivore, became 2–4 times more abundant in detritus-addition than control plots; densities of other detritivores also increased. Although the Detrital subsidy initially increased ground beetle numbers and produced a 2–3-fold increase in wolf spider densities, we found no consistent evidence of strong trophic cascades measured as changes in fruit yield in either detritus-addition or predator-removal treatments. This result contrasts with predator-removal experiments in previous years in the same study area, which revealed strong cascades affecting cucumber yield. We hypothesize that (1) competition between cucurbits and fungi for nitrogen in the detritus-addition plots, (2) unusually high herbivore densities in the spring cucumber crop, and low densities on the summer squash crop, (3) changes in patterns of intraguild predation in the detritus-addition plots, and (4) perhaps a switch by generalist predators to prey of the Detrital Food Web all may have contributed to preventing the Detrital subsidy from benefiting primary producers via an enhanced trophic cascade.
Céline Leroy - One of the best experts on this subject based on the ideXlab platform.
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Understorey environments influence functional diversity in tank‐bromeliad ecosystems
Freshwater Biology, 2012Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Céline Leroy, Jean-françois CarriasAbstract:1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad Food Webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad Food Webs in samples taken from 171 tank-bromeliads. 3. We observed a gradient of detritus-based to algal-based Food Webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad Food Webs in exposed areas, whereas filter-feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun-exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank-bromeliads, which grow in a broad range of ecological conditions, promote aquatic Food Web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus-based systems in the literature, we show that algal production can support a non-Detrital Food Web in these systems.
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Understorey environments influence functional diversity in tank-bromeliad ecosystems
Freshwater Biology, 2012Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Céline Leroy, Jean-françois CarriasAbstract:1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad Food Webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad Food Webs in samples taken from 171 tank-bromeliads. 3. We observed a gradient of detritus-based to algal-based Food Webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad Food Webs in exposed areas, whereas filter-feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun-exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank-bromeliads, which grow in a broad range of ecological conditions, promote aquatic Food Web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus-based systems in the literature, we show that algal production can support a non-Detrital Food Web in these systems.
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Are algae relevant to the detritus-based Food Web in tank-bromeliads?
PLoS ONE, 2011Co-Authors: Olivier Brouard, Laurent Pelozuelo, Bruno Corbara, Regis Cereghino, Alain Dejean, Olivier Roux, Anne-hélène Le Jeune, Céline Leroy, Jean-françois CarriasAbstract:We assessed the occurrence of algae in five species of tank-bromeliads found in contrasting environmental sites in a Neotropical, primary rainforest around the Nouragues Research Station, French Guiana. The distributions of both algal abundance and biomass were examined based on physical parameters, the morphological characteristics of bromeliad species and with regard to the structure of other aquatic microbial communities held in the tanks. Algae were retrieved in all of the bromeliad species with mean densities ranging from ∼10(2) to 10(4) cells/mL. Their biomass was positively correlated to light exposure and bacterial biomass. Algae represented a tiny component of the Detrital Food Web in shaded bromeliads but accounted for up to 30 percent of the living microbial carbon in the tanks of Catopsis berteroniana, located in a highly exposed area. Thus, while nutrient supplies are believed to originate from wind-borne particles and trapped insects (i.e., allochtonous organic matter), our results indicate that primary producers (i.e., autochtonous organic matter) are present in this insectivorous bromeliad. Using a 24-h incubation of size-fractionated and manipulated samples from this plant, we evaluated the impact of mosquito foraging on algae, other microorganisms and rotifers. The prey assemblages were greatly altered by the predation of mosquito larvae. Grazing losses indicated that the dominant algal taxon, Bumilleriopsis sp., like protozoa and rotifers, is a significant part of the diet of mosquito larvae. We conclude that algae are a relevant functional community of the aquatic Food Web in C. berteroniana and might form the basis of a complementary non-Detrital Food Web.