Clay-Turbid Interactions May Not Cascade—A Reminder for Lake Managers

Food web management is a frequently used lake restoration method, which aims to reduce phytoplankton biomass by strengthening herbivorous zooplankton through reduction of planktivorous fish. However, in clay‐turbid lakes several factors may reduce the effectivity of food web management. Increasing turbidity reduces the effectivity of fish predation and weakens the link between zooplankton and phytoplankton. Therefore, the effects of fish stock manipulations may not cascade to lower trophic levels as expected. Additionally, in clay‐turbid conditions invertebrate predators may coexist in high densities with planktivorous fish and negate the effects of fish reductions. For instance, in the stratifying regions of the clay‐turbid Lake Hiidenvesi, Chaoborus flavicans is the main regulator of cladocerans and occupies the water column throughout the day, although planktivorous Osmerus eperlanus is very abundant. The coexistence of chaoborids and fish is facilitated by a metalimnetic turbidity peak, which prevents efficient predation by fish. In the shallow parts of the lake, chaoborids are absent despite high water turbidity. We suggest that, generally, the importance of invertebrate predators in relation to vertebrate predators may change along turbidity and depth gradients. The importance of fish predation is highest in shallow waters with low turbidity. When water depth increases, the importance of fish in the top‐down regulation of zooplankton declines, whereas that of chaoborids increases, the change along the depth gradient being moderate in clear‐water lakes and steep in highly turbid lakes. Thus, especially deep clay‐turbid lakes may be problematic for implementing food web management as a restoration tool.     

The relevance of size efficiency to biomanipulation theory: a field test under hypertrophic conditions

The superiority of large zooplankton in suppressingphytoplankton growth has often been inferred from theSize Efficiency Hypothesis (S.E.H.). The S.E.H. hasoriginally been formulated to account for thecompetitive superiority of large to small zooplanktonunder food limiting conditions. Extrapolation of itspredictions to the suppression of phytoplankton byzooplankton under high food availability, should bedone with care. In an attempt to assess the relevanceof the S.E.H. to biomanipulation theory inhypertrophic systems, a fish exclosure experiment wascarried out in which the efficiency of two differentlystructured zooplankton communities in reducingphytoplankton biomass was examined. By inoculatingpart of the enclosures with laboratory grownDaphnia magna, a community dominated by this largecladoceran species could be compared with a communitymainly consisting of Bosmina and smallerDaphnia species. After the exclusion of fish, therewas an exponential increase of total zooplanktonbiomass. Phytoplankton growth was efficientlysuppressed to equal levels in both treatments, thoughthere was a difference in timing: chlorophyll-a levelsin the enclosures inoculated with D. magnadropped one week earlier than in non-inoculatedenclosures. The time-lag was even more pronounced whenlarge phytoplankton was considered. In accordance withthe S.E.H., the time lags could be explained bydifferences in population growth potential as well asby differences in zooplankton grazing rates(indirectly measured as the minimal zooplanktonbiomass needed to suppress phytoplankton growth) andfood particle size range.     

Feature-based attention: it is all bottom-up priming

Feature-based attention (FBA) enhances the representation of image characteristics throughout the visual field, a mechanism that is particularly useful when searching for a specific stimulus feature. Even though most theories of visual search implicitly or explicitly assume that FBA is under top-down control, we argue that the role of top-down processing in FBA may be limited. Our review of the literature indicates that all behavioural and neuro-imaging studies investigating FBA suffer from the shortcoming that they cannot rule out an effect of priming. The mere attending to a feature enhances the mandatory processing of that feature across the visual field, an effect that is likely to occur in an automatic, bottom-up way. Studies that have investigated the feasibility of FBA by means of cueing paradigms suggest that the role of top-down processing in FBA is limited (e.g. prepare for red). Instead, the actual processing of the stimulus is needed to cause the mandatory tuning of responses throughout the visual field. We conclude that it is likely that all FBA effects reported previously are the result of bottom-up priming.     

Observational constraints reduce model spread but not uncertainty in global wetland methane emission estimates

The recent rise in atmospheric methane (CH₄) concentrations accelerates climate change and offsets mitigation efforts. Although wetlands are the largest natural CH₄ source, estimates of global wetland CH₄ emissions vary widely among approaches taken by bottom-up (BU) process-based biogeochemical models and top-down (TD) atmospheric inversion methods. Here, we integrate in situ measurements, multi-model ensembles, and a machine learning upscaling product into the International Land Model Benchmarking system to examine the relationship between wetland CH₄ emission estimates and model performance. We find that using better-performing models identified by observational constraints reduces the spread of wetland CH₄ emission estimates by 62% and 39% for BU- and TD-based approaches, respectively. However, global BU and TD CH₄ emission estimate discrepancies increased by about 15% (from 31 to 36 TgCH₄ year⁻¹) when the top 20% models were used, although we consider this result moderately uncertain given the unevenly distributed global observations. Our analyses demonstrate that model performance ranking is subject to benchmark selection due to large inter-site variability, highlighting the importance of expanding coverage of benchmark sites to diverse environmental conditions. We encourage future development of wetland CH₄ models to move beyond static benchmarking and focus on evaluating site-specific and ecosystem-specific variabilities inferred from observations.     

From bottom-up to top-down control of invertebrate herbivores in a retrogressive chronosequence

In the long-term absence of disturbance, ecosystems often enter a decline or retrogressive phase which leads to reductions in primary productivity, plant biomass, nutrient cycling and foliar quality. However, the consequences of ecosystem retrogression for higher trophic levels such as herbivores and predators, are less clear. Using a post-fire forested island-chronosequence across which retrogression occurs, we provide evidence that nutrient availability strongly controls invertebrate herbivore biomass when predators are few, but that there is a switch from bottom-up to top-down control when predators are common. This trophic flip in herbivore control probably arises because invertebrate predators respond to alternative energy channels from the adjacent aquatic matrix, which were independent of terrestrial plant biomass. Our results suggest that effects of nutrient limitation resulting from ecosystem retrogression on trophic cascades are modified by nutrient-independent variation in predator abundance, and this calls for a more holistic approach to trophic ecology to better understand herbivore effects on plant communities.     

The portability of foodweb dynamics: reassembling an Australian eucalypt–psyllid–bird association within California

Aims To evaluate the role of native predators (birds) within an Australian foodweb (lerp psyllids and eucalyptus trees) reassembled in California. Location Eucalyptus groves within Santa Cruz, California. Methods We compared bird diversity and abundance between a eucalyptus grove infested with lerp psyllids and a grove that was uninfested, using point counts. We documented shifts in the foraging behaviour of birds between the groves using structured behavioural observations. Additionally, we judged the effect of bird foraging on lerp psyllid abundance using exclosure experiments. Results We found a greater richness and abundance of Californian birds within a psyllid infested eucalyptus grove compared to a matched non‐infested grove, and that Californian birds modify their foraging behaviour within the infested grove in order to concentrate on ingesting psyllids. This suggests that Californian birds could provide indirect top‐down benefits to eucalyptus trees similar to those observed in Australia. However, using bird exclosure experiments, we found no evidence of top‐down control of lerp psyllids by Californian birds. Main conclusions We suggest that physiological and foraging differences between Californian and Australian pysllid‐eating birds account for the failure to observe top‐down control of psyllid populations in California. The increasing rate of non‐indigenous species invasions has produced local biotas that are almost entirely composed of non‐indigenous species. This example illustrates the complex nature of cosmopolitan native‐exotic food webs, and the ecological insights obtainable through their study.     

Hydrogen-Deuterium Exchange Coupled to Top- and Middle-Down Mass Spectrometry Reveals Histone Tail Dynamics before and after Nucleosome Assembly

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What is a gene?—Revisited

The dialectic discourse of the 'gene' as the unit of heredity deduced from the phenotype, whether an intervening variable or a hypothetical construct, appeared to be settled with the presentation of the molecular model of DNA: the gene was reduced to a sequence of DNA that is transcribed into RNA that is translated into a polypeptide; the polypeptides may fold into proteins that are involved in cellular metabolism and structure, and hence function. This path turned out to be more bewildering the more the regulation of products and functions were uncovered in the contexts of integrated cellular systems. Philosophers struggling to define a unified concept of the gene as the basic entity of (molecular) genetics confronted those who suggested several different 'genes' according to the conceptual frameworks of the experimentalists. Researchers increasingly regarded genes de facto as generic terms for describing their empiric data, and with improved DNA-sequencing capacities these entities were as a rule bottom-up nucleotide sequences that determine functions. Only recently did empiricists return to discuss conceptual considerations, including top-down definitions of units of function that through cellular mechanisms select the DNA sequences which comprise 'genomic-footprints' of functional entities.     

Nutrient limitation in detritus-based microcosms in Sarracenia purpurea

Most prior work on the role of top-down and bottom-up effects in aquatic communities has ignored the significant detrital component that occurs in natural systems. We investigated the effects of specific nutrients (carbon, phosphorus, and nitrogen), as well as a top predator (the mosquito Wyeomyia smithii), on the structure of the detritivore community found in the water-filled leaves of the pitcher plant Sarracenia purpurea. The concentrations of three nutrients and the presence of the predators were manipulated in a factorial design, while the response of the remaining community was quantified. Bacterial growth was found to be strongly carbon-limited and somewhat less limited by phosphorus and there was an interaction between the effects of the two nutrients. Neither carbon or phosphorus addition affected protozoan or rotifer abundance, and nitrogen had only a minor effect. The presence of the predator, however, significantly reduced the abundance of the four numerically dominant bacteriovores. There were no interactions between top-down and bottom-up effects; the strong direct reciprocal effects between adjacent trophic levels seem to be greatly attenuated as they are propagated farther up or down the food chain.     

Shorebirds, snails, and the amphipod (Corophium volutator) in the upper Bay of Fundy: top–down vs. bottom–up factors, and the influence of compensatory interactions on mudflat ecology

During their annual mid- to late-summer southward migration, Semipalmated Sandpipers (Calidris pusilla) feed intensively on the amphipod Corophium volutator on intertidal mudflats in the Bay of Fundy. Corophium, in turn, feed on diatoms and bacteria. Using a series of bird exclosures and fertilizer addition, we examined top–down and bottom–up effects, and investigated the presence of a trophic cascade in the mudflat community during the period when birds are abundant. Although both top–down and bottom–up forces were present in this system, neither transmitted beyond a single trophic link. Predation by shorebirds, which may be less size-selective than previously thought, reduced Corophium abundance in control plots by approximately 80% relative to exclosures, but most other species were unaffected. Shorebird predation did not result in an increase in diatom abundance, as predicted under the trophic cascade hypothesis. Fertilizer increased diatom abundance, but had no effect on Corophium abundance or bird predation, and little effect on other mudflat invertebrates. The only indirect effect observed was on mud snails (Ilyanassa obsoleta), which, by rapidly responding to changes in diatom abundance, compensated for both bird exclusion and fertilizer addition, and prevented the trophic cascade. This population response by snails, possibly stemming from competition with Corophium, probably contributed to the stability of the community. Our results provide an example of short-term compensation in a simple intertidal community, and highlight the importance of considering direct and indirect effects in community ecological studies. We conclude that while compensatory interactions that block trophic cascades may be more common in more complex ecosystems, they are not restricted to them.