WHAT DETERMINES THE STRENGTH OF A TROPHIC CASCADE?

Trophic cascades have been documented in a diversity of ecological systems and can be important in determining biomass distribution within a community. To date, the literature on trophic cascades has focused on whether and in which systems cascades occur. Many biological (e.g., productivity:biomass ratios) and methodological (e.g., experiment size or duration) factors vary with the ecosystem in which data were collected, but ecosystem type, per se, does not provide mechanistic insights into factors controlling cascade strength. Here, we tested various hypotheses about why trophic cascades occur and what determines their magnitude using data from 114 studies that measured the indirect trophic effects of predators on plant community biomass in seven aquatic and terrestrial ecosystems. Using meta-analysis, we examined the relationship between the indirect effect of predator manipulation on plants and 18 biological and methodological factors quantified from these studies. We found, in contrast to predictions, that high system productivity and low species diversity do not consistently generate larger trophic cascades. A combination of herbivore and predator metabolic factors and predator taxonomy (vertebrate vs. invertebrate) explained 31% of the variation in cascade strength among all 114 studies. Within systems, 18% of the variation in cascade strength was explained with similar predator and herbivore characteristics. Within and across all systems, the strongest cascades occurred in association with invertebrate herbivores and endothermic vertebrate predators. These associations may result from a combination of true biological differences among species with different physiological requirements and bias among organisms studied in different systems. Thus, although cascade strength can be described by biological characteristics of predators and herbivores, future research on indirect trophic effects must further examine biological and methodological differences among studies and systems. © 2005 by the Ecological Society of America

Warming alters food web-driven changes in the CO2 flux of experimental pond ecosystems

Evidence shows the important role biota play in the carbon cycle, and strategic management of plant and animal populations could enhance CO(2) uptake in aquatic ecosystems. However, it is currently unknown how management-driven changes to community structure may interact with climate warming and other anthropogenic perturbations to alter CO(2) fluxes. Here we showed that under ambient water temperatures, predators (three-spined stickleback) and nutrient enrichment synergistically increased primary producer biomass, resulting in increased CO(2) uptake by mesocosms in early dawn. However, a 3°C increase in water temperatures counteracted positive effects of predators and nutrients, leading to reduced primary producer biomass and a switch from CO(2) influx to efflux. This confounding effect of temperature demonstrates that climate scenarios must be accounted for when undertaking ecosystem management actions to increase biosequestration

Landscape heterogeneity strengthens the relationship between β-diversity and ecosystem function.

Consensus has emerged in the literature that increased biodiversity enhances the capacity of ecosystems to perform multiple functions. However, most biodiversity/ecosystem function studies focus on a single ecosystem, or on landscapes of homogenous ecosystems. Here, we investigate how increased landscape-level environmental dissimilarity may affect the relationship between different metrics of diversity (α, β, or γ) and ecosystem function. We produced a suite of simulated landscapes, each of which contained four experimental outdoor aquatic mesocosms. Differences in temperature and nutrient conditions of the mesocosms allowed us to simulate landscapes containing a range of within-landscape environmental heterogeneities. We found that the variation in ecosystem functions was primarily controlled by environmental conditions, with diversity metrics accounting for a smaller (but significant) amount of variation in function. When landscapes were more homogeneous, α, β, and γ diversity was not associated with differences in primary production, and only γ was associated with changes in decomposition. In these homogeneous landscapes, differences in these two ecosystem functions were most strongly related to nutrient and temperature conditions in the ecosystems. However, as landscape-level environmental dissimilarity increased, the relationship between α, β, or γ and ecosystem functions strengthened, with β being a greater predictor of variation in decomposition at the highest levels of environmental dissimilarity than α or γ. We propose that when all ecosystems in a landscape have similar environmental conditions, species sorting is likely to generate a single community composition that is well suited to those environmental conditions, β is low, and the efficiency of diversity-ecosystem function couplings is similar across communities. Under this low β, the effect of abiotic conditions on ecosystem function will be most apparent. However, when environmental conditions vary among ecosystems, species sorting pressures are different among ecosystems, producing different communities among locations in a landscape. These conditions lead to stronger relationships between β and the magnitude of ecosystem functions. Our results illustrate that abiotic conditions and the homogeneity of communities influence ecosystem function expressed at the landscape scale

Does urbanization explain differences in interactions between an insect herbivore and its natural enemies and mutualists?

Urbanization can alter the composition of arthropod communities. However, little is known about how urbanization affects ecological interactions. Using experimental colonies of the black bean aphid Aphis fabae Scopoli reared on Vicia faba L, we asked if patterns of predator-prey, host-parasitoid and ant-aphid mutualisms varied along an urbanization gradient across a large town in southern England. We recorded the presence of naturally occurring predators, parasitoid wasps and mutualistic ants together with aphid abundance. We examined how biotic (green areas and plant richness) and abiotic features (impervious surfaces and distance to town center) affected (1) aphid colony size, (2) the likelihood of finding predators, mutualistic ants and aphid mummies (indicating the presence of parasitoids), and (3) how the interplay among these factors affected patterns of parasitoid attack, predator abundance, mutualistic interactions and aphid abundance. The best model to predict aphid abundance was the number of mutualistic ants attending the colonies. Aphid predators responded negatively to both the proportion of impervious surfaces and to the number of mutualistic ants farming the colonies, and positively to aphid population size, whereas parasitized aphids were found in colonies with higher numbers of aphids and ants. The number of mutualistic ants attending was positively associated with aphid colony size and negatively with the number of aphid predators. Our findings suggest that for insect-natural enemy interactions, urbanization may affect some groups, while not influencing others, and that local effects (mutualists, host plant presence) will also be key determinants of how urban ecological communities are formed

Climate change and freshwater zooplankton: what does it boil down to?

Recently, major advances in the climate–zooplankton interface have been made some of which appeared to receive much attention in a broader audience of ecologists as well. In contrast to the marine realm, however, we still lack a more holistic summary of recent knowledge in freshwater. We discuss climate change-related variation in physical and biological attributes of lakes and running waters, high-order ecological functions, and subsequent alteration in zooplankton abundance, phenology, distribution, body size, community structure, life history parameters, and behavior by focusing on community level responses. The adequacy of large-scale climatic indices in ecology has received considerable support and provided a framework for the interpretation of community and species level responses in freshwater zooplankton. Modeling perspectives deserve particular consideration, since this promising stream of ecology is of particular applicability in climate change research owing to the inherently predictive nature of this field. In the future, ecologists should expand their research on species beyond daphnids, should address questions as to how different intrinsic and extrinsic drivers interact, should move beyond correlative approaches toward more mechanistic explanations, and last but not least, should facilitate transfer of biological data both across space and time

Producer Nutritional Quality Controls Ecosystem Trophic Structure

Trophic structure, or the distribution of biomass among producers and consumers, determines key ecosystem values, such as the abundance of infectious, harvestable or conservation target species, and the storage and cycling of carbon and nutrients. There has been much debate on what controls ecosystem trophic structure, yet the answer is still elusive. Here we show that the nutritional quality of primary producers controls the trophic structure of ecosystems. By increasing the efficiency of trophic transfer, higher producer nutritional quality results in steeper ecosystem trophic structure, and those changes are more pronounced in terrestrial than in aquatic ecosystems probably due to the more stringent nutritional limitation of terrestrial herbivores. These results explain why ecosystems composed of highly nutritional primary producers feature high consumer productivity, fast energy recycling, and reduced carbon accumulation. Anthropogenic changes in producer nutritional quality, via changes in trophic structure, may alter the values and functions of ecosystems, and those alterations may be more important in terrestrial ecosystems

Differential Levels of Stress Proteins (HSPs) in Male and Female Daphnia magna in Response to Thermal Stress: A Consequence of Sex-Related Behavioral Differences?

In two independent experiments, we compared: (1) water depth selection (and accompanying temperature selection) by male and female Daphnia magna under different kinds of environmental stress, including the presence of filamentous cyanobacteria, the risk of predation from fish, and the presence of toxic compounds; and (2) sex-dependent production of heat shock proteins (HSP60, 70, and 90) in response to a sudden change in temperature. Male D. magna selected deep water strata, which offer a relatively stable environment, and thereby avoided the threat of predation and the presence of toxic compounds in surface waters. Correlated with this behavior, males reduce their molecular defenses against stress, such as the production of heat shock proteins (HSPs), and do not maintain the physiological machinery that triggers an increase in HSP levels in response to stress. In contrast, female D. magna actively select habitats that offer optimal conditions for growth and production of offspring. Consequently, females are exposed to variable environmental conditions that may be associated with increased stress. To permit survival in these different habitats, D. magna females require molecular mechanisms to protect their cells from rapid changes in stress levels. Thus, they maintain high constitutive levels of the heat shock proteins from HSP 60, 70, and 90 families, and they have the potential to further enhance the production of the majority of these proteins under stress conditions. The results of this study indicate that the separate habitats selected by male and female D. magna result in different patterns of HSP production, leading us to hypothesize that that male and female Daphnia magna adopt different strategies to maximize the fitness of the species

Predator Dispersal Determines the Effect of Connectivity on Prey Diversity

Linking local communities to a metacommunity can positively affect diversity by enabling immigration of dispersal-limited species and maintenance of sink populations. However, connectivity can also negatively affect diversity by allowing the spread of strong competitors or predators. In a microcosm experiment with five ciliate species as prey and a copepod as an efficient generalist predator, we analysed the effect of connectivity on prey species richness in metacommunities that were either unconnected, connected for the prey, or connected for both prey and predator. Presence and absence of predator dispersal was cross-classified with low and high connectivity. The effect of connectivity on local and regional richness strongly depended on whether corridors were open for the predator. Local richness was initially positively affected by connectivity through rescue of species from stochastic extinctions. With predator dispersal, however, this positive effect soon turned negative as the predator spread over the metacommunity. Regional richness was unaffected by connectivity when local communities were connected only for the prey, while predator dispersal resulted in a pronounced decrease of regional richness. The level of connectivity influenced the speed of richness decline, with regional species extinctions being delayed for one week in weakly connected metacommunities. While connectivity enabled rescue of prey species from stochastic extinctions, deterministic extinctions due to predation were not overcome through reimmigration from predator-free refuges. Prey reimmigrating into these sink habitats appeared to be directly converted into increased predator abundance. Connectivity thus had a positive effect on the predator, even when the predator was not dispersing itself. Our study illustrates that dispersal of a species with strong negative effects on other community members shapes the dispersal-diversity relationship. When connections enable the spread of a generalist predator, positive effects of connectivity on prey species richness are outweighed by regional extinctions through predation

Rutas tróficas en macrozooplancton del Lago de Tota - Boyacá, Colombia

1 recurso en línea (56 páginas) : ilustraciones, figuras, tablas.In aquatic ecosystems inputs of matter and energy present in greater or lesser extent depending on the basin, given the origin and quality of these resources, you can alter the balance in metabolism or interactions in the food web. Using stable isotopes of carbon and nitrogen, the link between potential sources of funds and use established by macrozooplankton species in Lake Big and Lake Fellow the Lake Tota sectors. Trophic possible ways the role of each species in the proposed network, as well. In general, most of this study, most species have a bond with the autotrophic pathway, being the phytoplankton the food resource base for the food web, commonly found impoverished carbon signals. Particularly, the analysis showed a variation in the source or resource for two species (Daphnia laevis and Bosmina (Bosmina) cf. longirostris), with an enrichment in its signal to 19.92 ‰ -20.34 ‰ δ13C, which can be given by plasticity in their food supply, and even if Boeckella gracilis having a signal indicating a specialization in its appeal with carbon impoverished -30 to -26 ‰ values δ13C while nitrogen values are enriched with an average value 20.21 ‰ in δ15N. To set the trophic fractionation means establishing three groups, the first (herbivores) consisting of (D. laevis, B. (Bosmina) and Ceriodaphnia pulchella cf. longirostris), the second (omnivore) which is (Daphnia pulex, Macrocyclops sp. and Cyclopoida), and a third group (secondary consumer) where is the only calanoido B. gracilis. In comparing the temporal variation is observed that there is a significant change in signals δ13C and δ15N of some species in relation to the influence of sources and allochthonous or autochthonous origin, spatial variation was not significant. It is concluded that the macrozooplanton Lake Tota, has a strong link with the autotrophic pathway and pelagic trophic levels have 3 sections, plus the primary producers and the food web in general tends to have an amplitude of trophic niche.En los ecosistemas acuáticos se presentan aportes de materia y energía, en mayor o menor proporción dependiendo de su cuenca, dado el origen y calidad de estos recursos, se puede alterar el balance en el metabolismo o las interacciones en la red trófica. Utilizando los isotopos estables de Carbono y Nitrógeno, se estableció el vínculo entre fuentes potenciales de recursos y el uso por las especies del macrozooplancton en los sectores Lago grande y Lago chico del lago de Tota (Boyacá). Así, se propusieron las posibles vías tróficas y el rol de cada especie en la red. En general, en la mayor parte de este estudio, las especies tuvieron un vínculo marcado con la vía autotrófica, siendo el fitoplancton el recurso alimenticio base para la red trófica, encontrando comúnmente señales empobrecidas de carbono. Particularmente, el análisis mostró una variación en la fuente o recurso para dos especies (Daphnia laevis y Bosmina (Bosmina) cf. longirostris), con un enriquecimiento en su señal de 19.92‰ δ13C a -20.34‰ δ13C, lo que se puede dar por una plasticidad en su fuente alimenticia, e incluso el caso de Boeckella gracilis que tiene una señal que indica una especialización en su recurso con valores empobrecidos de carbono -30 a -26‰ δ13C mientras que los valores de nitrógeno son enriquecidos con un valor promedio de 20.21‰ en δ15N. Para establecer el fraccionamiento trófico medio se formaron tres grupos, el primero (herbívoros) conformado por (D. laevis, B. (Bosmina) cf. longirostris y Ceriodaphnia pulchella), el segundo (omnívoro) en el que se encuentra (Daphnia pulex, Macrocyclops sp. y Cyclopoida), y un tercer grupo (Consumidor secundario) donde esta B. gracilis el único calanoido. En la comparación de la variación temporal, se observa que hay un cambio significativo en las señales de δ13C y δ15N de algunas especies en relación a la influencia de las fuentes y su origen alóctono o autóctono, la variación espacial no fue significativa. Se concluye que el macrozooplanton del lago de Tota, tiene un fuerte vínculo con la vía autotrófica y que los niveles tróficos pelágicos tienen 3 eslabones, más los productores primarios, y la red trófica en general tiende a tener una amplitud del nicho trófico. En los ecosistemas acuáticos se presentan aportes de materia y energía, en mayor o menor proporción dependiendo de su cuenca, dado el origen y calidad de estos recursos, se puede alterar el balance en el metabolismo o las interacciones en la red trófica. Utilizando los isotopos estables de Carbono y Nitrógeno, se estableció el vínculo entre fuentes potenciales de recursos y el uso por las especies del macrozooplancton en los sectores Lago grande y Lago chico del lago de Tota (Boyacá). Así, se propusieron las posibles vías tróficas y el rol de cada especie en la red. En general, en la mayor parte de este estudio, las especies tuvieron un vínculo marcado con la vía autotrófica, siendo el fitoplancton el recurso alimenticio base para la red trófica, encontrando comúnmente señales empobrecidas de carbono. Particularmente, el análisis mostró una variación en la fuente o recurso para dos especies (Daphnia laevis y Bosmina (Bosmina) cf. longirostris), con un enriquecimiento en su señal de 19.92‰ δ13C a -20.34‰ δ13C, lo que se puede dar por una plasticidad en su fuente alimenticia, e incluso el caso de Boeckella gracilis que tiene una señal que indica una especialización en su recurso con valores empobrecidos de carbono -30 a -26‰ δ13C mientras que los valores de nitrógeno son enriquecidos con un valor promedio de 20.21‰ en δ15N. Para establecer el fraccionamiento trófico medio se formaron tres grupos, el primero (herbívoros) conformado por (D. laevis, B. (Bosmina) cf. longirostris y Ceriodaphnia pulchella), el segundo (omnívoro) en el que se encuentra (Daphnia pulex, Macrocyclops sp. y Cyclopoida), y un tercer grupo (Consumidor secundario) donde esta B. gracilis el único calanoido. En la comparación de la variación temporal, se observa que hay un cambio significativo en las señales de δ13C y δ15N de algunas especies en relación a la influencia de las fuentes y su origen alóctono o autóctono, la variación espacial no fue significativa. Se concluye que el macrozooplanton del lago de Tota, tiene un fuerte vínculo con la vía autotrófica y que los niveles tróficos pelágicos tienen 3 eslabones, más los productores primarios, y la red trófica en general tiende a tener una amplitud del nicho trófico.Bibliografía: páginas 46-56.MaestríaMagíster en Ciencias Biológica

Grazer diversity effects in an eelgrass–epiphyte–microphytobenthos system

The dramatic loss of biodiversity and its consequences for ecosystem processes have been of considerable interest in recent ecological studies. However, the complex and interacting processes influencing diversity effects in multitrophic systems are still poorly understood. We used an experimental eelgrass system to study the effects of changing richness of three consumer species on the biomass, diversity and taxonomic composition of both epiphytic and benthic microalgal assemblages. After 1 week, consumer richness enhanced the grazing impact on epiphyte biomass relative to single consumer treatments and a positive effect of consumer richness on prey diversity was found. Moreover, strong effects of consumer species identity on taxonomic composition were found in both microalgal assemblages. However, the effects of consumer richness were not consistent over time. The consequences of high nutrient availability seemed to have masked consumer richness effects