Modelling of predator–prey trophic interactions. Part II: Three trophic levels

A general class of lumped parameter models describing the local dynamics of a tri-trophic chain in a controlled environment is analyzed in detail. The trophic functions characterizing the interactions are defined only by some properties and allow us to treat both prey-dependent and ratio-dependent models in a unified manner. Conditions for existence and stability of extinction and coexistence equilibrium states are determined. Some peculiar aspects of the dynamics of the system depending on the bioecological parameters are presented, with particular attention to bistability situations, limit cycles and chaotic behaviours.     

Modelling of predator–prey trophic interactions. Part I: two trophic levels

A class of lumped parameter models to describe the local dynamics in a controlled environment of a two-trophic chain is considered. The class is characterized by a trophic function (functional response of predator to the abundance of prey) depending on the ratio of prey biomass x and a linear function of predator biomass y: f(qx/[(1-)k+y]), where q is the efficiency of the predation process, k is a reference biomass, and (01) specifies the predation model. The trophic function is defined only by some properties determining its shape. A stability analysis of the models has been performed by taking the parameters q and as bifurcation parameters: the regions in the (,q) plane of existence and stability of nonnegative equilibrium states and limit cycles are determined. This analysis shows that the behaviour of the models is qualitatively similar for 0<1 (in particular the null state is always a saddle point), while the value =1 gives rise to some kind of structural instability of the system (in particular the null state becomes an attractor for sufficiently high predation efficiency).     

Improving estimates of trophic shift (Δδtrophic) for diet reconstruction studies using enzyme activities

For diet reconstruction studies using stable isotopes, accurate estimates of trophic shift (Δδ_trophic) are necessary to get reliable results. Several factors have been identified which affect the trophic shift. The goal of the present experiment was to test whether measurements of the activities of enzymes could improve the accuracy of estimation of trophic shift in fish. Forty-eight Nile tilapia (Oreochromis niloticus) were fed under controlled conditions with two diets differing in their protein content (21 and 41%) each at four different levels (4, 8, 12 and 16 g kg^? 0.8 d^? 1). At the end of the feeding experiment, proximate composition, whole body δ¹³C and δ¹⁵N as well as the activities of enzymes involved in anabolism and catabolism were measured. Step-wise regression specified contributing variables for Δδ¹⁵N (malic enzyme, aspartate aminotransferase and protein content) and Δδ¹³C_{lipid-free material} (aspartate aminotransferase and protein content). Explained variation by using the significant main effects was about 70% for Δδ¹⁵N and Δδ¹³C_{lipid-free material}, respectively. The results of the present study indicate that enzyme activities are suitable indicators to improve estimates of trophic shift.     

Mycosporines from freshwater yeasts: a trophic cul-de-sac?

Mycosporine-like amino-acids (MAAs) are found in aquatic bacteria, algae, and animals. A related compound, the mycosporine-glutaminol-glucoside (myc-glu-glu), has recently been reported in freshwater yeasts. Although animals depend on other organisms as their source of MAAs, they can efficiently accumulate them in their tissues. In this work we assessed the potential transfer of the yeast mycosporine myc-glu-glu from the diet into the copepod Boeckella antiqua and the ciliate Paramecium bursaria. For this purpose, we performed experiments to study the feeding of B. antiqua and P. bursaria on the yeast Rhodotorula minuta and their ability to bioaccumulate myc-glu-glu. Bioaccumulation of myc-glu-glu in B. antiqua was assessed through long-term factorial experiments manipulating the diet (Chlamydomonas reinhardii and C. reinhardii + yeasts) and radiation exposure (PAR and PAR + UVR). Shorter term experiments were designed in the case of P. bursaria. The composition and concentration of MAAs in the diet and in the consumers were determined by HPLC analyses. Our results showed that even though both consumers ingested yeast cells, they were unable to accumulate myc-glu-glu. Moreover, when exposed to conditions that stimulated the accumulation of photoprotective compounds (i.e. UVR exposure), an increase in MAAs concentration occurred in copepods fed C. reinhardii plus yeasts as well as in those fed only C. reinhardii. This suggests that the copepods were able to modify their tissue concentrations of MAAs in response to environmental clues but also that the contribution of yeast mycosporines to total MAAs concentration was negligible.     

Terrestrial trophic cascades: how much do they trickle?

Although more consensus is now emerging on the magnitude and frequency of cascading trophic effects in aquatic communities, the debate over their terrestrial counterparts continues. We used meta-analysis to analyze field experiments on trophic cascades in terrestrial arthropod-dominated food webs to evaluate the overall magnitude of trophic cascades and conditions affecting their occurrence and strength. We found extensive support for the presence of trophic cascades in terrestrial communities. In the majority of experiments, predator removal led to increased densities of herbivorous insects and higher levels of plant damage. Cascades in which removing predators led to decreased herbivory also were detected but were less frequent and weaker, suggesting a predominantly three-trophic-level behavior of arthropod-dominated terrestrial food webs. Despite the clear evidence that cascades often decreased plant damage, residual effects of predation produced either no or only minimal changes in overall plant biomass. Agricultural systems and natural communities exhibited similarly strong effects of predation on herbivore abundance. However, resulting effects on plant damage and community-wide effects of trophic cascades on plant biomass usually were highly variable, and only in the managed agricultural systems did predators occasionally have strong indirect effects on plant biomass. Our meta-analysis suggests that the effects of trophic cascades on the biomass of primary producers are weaker in terrestrial than aquatic food webs.     

Does asymmetric specialization differ between mutualistic and trophic networks?

Recently, plant–pollinator networks have been found to be highly structured in a nested pattern in which specialists interact with generalist species. This structure is often assumed to be particular to mutualistic interactions in opposition to the compartmentalized pattern expected for antagonistic networks. We investigated the presence of asymmetric specialization in a data set assembled from the literature of 20 highly resolved plant–insect herbivore networks and compared them with 24 plant–pollinator networks. Our results indicate that these two types of networks differ, but not in the way it is generally assumed. Asymmetric specialization is present in plant–herbivore networks even if it appears less frequently than in plant–pollinator networks. Indeed, mean and median percentages of species showing asymmetric specialisation in herbivory webs are 33% and 14% respectively, compared to 57% and 60% in pollination webs. Furthermore, the amount of asymmetry is linked with species diversity and not to connectance in plant-pollinator networks whereas the opposite pattern is found in plant–herbivore networks. Our results offer promising perspectives for understanding both the mechanisms that structure ecological communities and their impact on community dynamics depending on the type of interaction.     

Trophic levels and trophic dynamics: A consensus emerging?

There are three clearly different views on trophic levels. The systems-ecological tradition sees trophic levels as relatively discrete and well-defined units whose interactions cannot be derived from interactions between constituent populations. The reductionist population-ecological tradition sees trophic levels as inappropriate abstractions that cannot be used in formulating predictive theories. The tradition of trophic dynamics sees the first three trophic levels of autotroph-based ecosystems as reasonable abstractions, useful in formulating predictive theories, but devoid of properties that could not be directly extrapolated from those of constituent populations. Recent literature suggests that the first two schools are converging towards the viewpoints of the third, though the latter has also been modified by the interaction.     

Parasite effects on host’s trophic and isotopic niches

Wild animals are usually infected with parasites that can alter their hosts’ trophic niches in food webs as can be seen from stable isotope analyses of infected versus uninfected individuals. The mechanisms influencing these effects of parasites on host isotopic values are not fully understood. Here, we develop a conceptual model to describe how the alteration of the resource intake or the internal resource use of hosts by parasites can lead to differences of trophic and isotopic niches of infected versus uninfected individuals and ultimately alter resource flows through food webs. We therefore highlight that stable isotope studies inferring trophic positions of wild organisms in food webs would benefit from routine identification of their infection status.     

Insect endosymbionts: manipulators of insect herbivore trophic interactions?

Throughout their evolutionary history, insects have formed multiple relationships with bacteria. Although many of these bacteria are pathogenic, with deleterious effects on the fitness of infected insects, there are also numerous examples of symbiotic bacteria that are harmless or even beneficial to their insect host. Symbiotic bacteria that form obligate or facultative associations with insects and that are located intracellularly in the host insect are known as endosymbionts. Endosymbiosis can be a strong driving force for evolution when the acquisition and maintenance of a microorganism by the insect host results in the formation of novel structures or changes in physiology and metabolism. The complex evolutionary dynamics of vertically transmitted symbiotic bacteria have led to distinctive symbiont genome characteristics that have profound effects on the phenotype of the host insect. Symbiotic bacteria are key players in insect–plant interactions influencing many aspects of insect ecology and playing a key role in shaping the diversification of many insect groups. In this review, we discuss the role of endosymbionts in manipulating insect herbivore trophic interactions focussing on their impact on plant utilisation patterns and parasitoid biology.     

Trace metals in barnacles:the significance of trophic transfer

Barnacles have very high accumulated trace metal body concentrations that vary with local trace metal bioavailabilities and represent integrated measures of the supply of bioavailable metals. Pioneering work in Chinese waters in Hong Kong highlighted the potential value of barnacles (particularly Balanus amphitrite) as trace metal biomonitors in coastal waters, identifying differences in local trace metal bioavailabilities over space and time. Work in Hong Kong has also shown that although barnacles have very high rates of trace metal uptake from solution, they also have very high trace metal assimilation efficiencies from the diet. High assimilation efficiencies coupled with high ingestion rates ensure that trophic uptake is by far the dominant trace metal uptake route in barnacles, as verified for cadmium and zinc. Kinetic modelling has shown that low efflux rate constants and high uptake rates from the diet combine to bring about accumulated trace metal concentrations in barnacles that are amongst the     

How does global change affect the strength of trophic interactions?

Recent research has generally shown that a small change in the number of species in a food web can have consequences both for community structure and ecosystem processes. However ‘change’ is not limited to just the number of species in a community, but might include an alteration to such properties as precipitation, nutrient cycling and temperature, all of which are correlated with productivity. Here we argue that predicted scenarios of global change will result in increased plant productivity. We model three scenarios of change using simple Lotka–Volterra dynamics, which explore how a global change in productivity might affect the strength of local species interactions and detail the consequences for community and ecosystem level stability. Our results indicate that (i) at local scales the average population size of consumers may decline because of poor quality food resources, (ii) that the strength of species interactions at equilibrium may become weaker because of reduced population size, and (iii) that species populations may become more variable and may take longer to recover from environmental or anthropogenic disturbances. At local scales interaction strengths encompass such properties as feeding rates and assimilation efficiencies, and encapsulate functionally important information with regard to ecosystem processes. Interaction strengths represent the pathways and transfer of energy through an ecosystem. We examine how such local patterns might be affected given various scenarios of ‘global change’ and discuss the consequences for community stability and ecosystem functioning.

Zusammenfassung

Die neuere Forschung hat im Allgemeinen gezeigt, dass eine kleine Veränderung in der Zahl der Arten in einem Nahrungsnetz sowohl Konsequenzen für die Gemeinschaftsstruktur als auch für die Ökosystemprozesse haben kann. „Wandel“ist jedoch nicht nur auf die Anzahl der Arten in einer Gemeinschaft beschränkt, sondern kann auch Veränderungen von Eigenschaften wie Niederschlag, Nährstoffkreisläufe und Temperatur beinhalten, die alle mit der Produktivität korreliert sind. Hier argumentieren wir, dass vorhergesagte Szenarios des globalen Wandels in einer erhöhten Pflanzenproduktivität resultieren werden. Wir modellieren drei Szenarien des Wandels unter der Verwendung einfacher Lotka–Volterra–Dynamiken, die erkunden, wie ein globaler Wandel in der Produktivität die Stärke von lokalen Arteninteraktionen beeinflusst, und wir beschreiben detailliert die Konsequenzen für die Gemeinschaft und die Stabilität des Ökosystemlevels. Unsere Ergebnisse deuten darauf hin, (i) dass auf einer lokalen Skala die durchschnittliche Populationsgröße der Konsumenten aufgrund der geringen Qualität der Nahrungsressourcen abnehmen könnte, (ii) dass die Stärke der Arteninteraktionen im Gleichgewicht aufgrund der reduzierten Populationsgröße schwächer werden könnte und (iii) dass die Populationen der Arten variabler werden und länger brauchen könnten, um sich von umweltbedingten oder anthropogenen Störungen zu erholen. Auf lokalen Skalen umfassen Interaktionsstärken Eigenschaften wie Fraßraten und Assimilationseffizienzen und enthalten wichtige funktionelle Information in Bezug auf Ökosystemprozesse. Interaktionsstärken repräsentieren die Energiewege und den Energietransfer in einem Ökosystem. Wir untersuchen, wie solche lokalen Muster unter der Voraussetzung verschiedener gegebener Szenarios des „globalen Wandels“beeinflusst werden könnten und diskutieren die Konsequenzen für die Gemeinschaftsstabilität und Ökosystemfunktion.     

Can current velocity mediate trophic cascades in a mountain stream?

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