Omnivory and food web dynamics

Risk Assessment and Characterization in Tuna Species of the Canary Islands According to Their Metal Content

Bioaccumulation is the process by which living organisms accumulate substances, such as pesticides, heavy metals, and other pollutants, from their environment. These substances can accumulate in the organism’s tissues over time, leading to potential health risks. Bioaccumulation can occur in both aquatic and terrestrial ecosystems, and can have a significant impact on the health of both humans and wildlife. The objective of this study is to find out if the concentrations of metals in the tuna species of the Canary Islands are suitable for human consumption and if they pose a health risk. Fifteen samples of Acanthocybium solandri, Katsuwonus pelamis, Thunnus albacares, Thunnus obesus and Thunnus thynnus present in canaries were analyzed. Ten grams of muscle were taken from each specimen and the metals Al, Cd, Cr, Cu, Fe, Li, Ni, Pb and Zn were determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The tuna species that presented more metals with a higher concentration compared to the others was T. thynnus, reaching up to 100 times more than the other studied species in Fe content with 137.8 ± 100.9 mg/Kg, which may be due to the fact that it is the largest species that reaches ages of more than fifteen years. The species Thunnus thynnus should not be suitable for commercialization according to the current legislation on the concentrations of Cd in blue fish, since 75% of the specimens studied exceeded the concentration legislated for Cd. A total of 40% of the studied specimens of this this species exceeded the legislated values for the concentration of Pb in oily fish meat, so this species must be monitored to ensure that it does not pose a risk to human health.

Influence of competition and intraguild predation between two candidate biocontrol parasitoids on their potential impact against Harrisia cactus mealybug, Hypogeococcus sp. (Hemiptera: Pseudococcidae)

When two or more parasitoid species, particularly candidates for biocontrol, share the same target in the same temporal window, a complex of behaviors can occur among them. We studied the type of interactions (competition and intraguild predation) that existed between the nymphal parasitoids Anagyrus cachamai and A. lapachosus (Hymenoptera: Encyrtidae), two candidate neoclassical biocontrol agents against the Puerto Rican cactus pest mealybug, Hypogeococcus sp. (Hemiptera: Pseudococcidae). The surrogate native congener host in Argentina, the cactus mealybug Hypogeococcus sp., was studied to predict which species should be released; in the case that both should be released, in which order, and their potential impact on host suppression. In the laboratory we conducted experiments where different densities of the host mealybug were exposed to naive females of A. cachamai and A. lapachosus sequentially in both directions. Experiments were analyzed by combining a series of competitive behavioral and functional response models. A fully Bayesian approach was used to select the best explaining models and calculate their parameters. Intraguild predation existed between A. cachamai, the species that had the greatest ability to exploit the resource, and A. lapachosus, the strongest species in the interference competition. The role that intraguild predation played in suppression of Hypogeococcus sp. indicated that a multiple release strategy for the two biocontrol agents would produce better control than a single release; as for the release order, A. lapachosus should be released first.

Dynamics of diffusive modified Previte-Hoffman food web model

This paper formulates and analyzes a modified Previte-Hoffman food web with mixed functional responses. We investigate the existence, uniqueness, positivity and boundedness of the proposed model's solutions. The asymptotic local and global stability of the steady states are discussed. Analytical study of the proposed model reveals that it can undergo supercritical Hopf bifurcation. Furthermore, analysis of Turing instability in spatiotemporal version of the model is carried out where regions of pattern creation in parameters space are obtained. Using detailed numerical simulations for the diffusive and non-diffusive cases, the theoretical findings are verified for distinct sets of parameters.

Intra-guild predation (IGP) can increase or decrease prey density depending on the strength of IGP

In consumer communities, intra-guild predation (IGP) is a commonly observed interaction that is widely believed to increase resource density. However, some recent theoretical work predicts that resource density should first decrease, and then increase as the strength of IGP increases. This occurs because weak to intermediate IGP increases the IG predator density more than it reduces the IG prey density, so that weak to intermediate IGP leads to the lowest resource density compared to weak or strong IGP. We test this prediction that basal resource density would first decrease and then increase as the strength of IGP increase. We used a well-studied system with two protozoa species engaged in IGP and three bacteria species as the basal resources. We experimentally manipulated the percentage of the IG prey population that was available to an IG predator as a proxy for IGP strength. We found that bacterial density first decreased (by ~25%) and then increased (by ~30%) as the strength of IGP increased. Using a modified version of a published IGP model, we were able to explain ~70% of the variation in protozoa and bacterial density. Agreement of the empirical results with model predictions suggests that IGP first increased the IG predator density by consuming a small proportion of the IG prey population, which in turn increased the summed consumer density and decreased the bacterial resource density. As IGP strength increased further, the IG predator became satiated by the IG prey, which then freed the bacterial resource from predation and thus increased bacterial density. Consequently, our work shows that IGP can indeed decrease or increase basal resource density depending on its strength. Consequently, the impacts of IGP on resource density is potentially more complex than previously thought.

Dynamics of an intraguild predation model with an adaptive IGpredator

In this paper, an intraguild predation model with an adaptive IGpredator is studied. IGpredator is assumed to adopt adaptive predation strategy to gain more fitness and the adaptive strength is variable. The existence and stability of the boundary equilibria and interior equilibrium are analyzed and it is found that the adaptive strength of IGpredator does not affect the stability of the boundary equilibria while it may change the stability of the interior equilibrium. Then we investigate numerically the effects of adaptive intraguild predation on the community structure along a gradient in environment productivity and find that it is possible for the appearance of the paradox of enrichment for intermediate speed of adaptivity. We also explore numerically how the dynamics of the adaptive system are affected by the adaptive strength of IGpredator. It is shown that the stationary coexistence of three species is stable when adaptation is strong and that a periodic solution with large amplitude appears when adaptation is weak, which implies that the adaptive activity of IGpredator to improve its fitness may lead to extinction of itself.

The interplay among prey preference, nutrient enrichment and stability in an omnivory system

Food webs usually display an intricate mix of trophic interactions where multiple prey are common. In this context omnivory has been the subject of intensive analysis regarding food web stability and structure. In a three species omnivory setting it is shown that the modeling of prey preference by the top predator may exert a strong influence on the short as well as on the long term dynamics of the respective food web. Clearly, this has implications concerning the stability and the structure of omnivory systems under disturbances such as nutrient enrichment.

From empirical patterns to theory: a formal metabolic theory of life

The diversity of life on Earth raises the question of whether it is possible to have a single theoretical description of the quantitative aspects of the organization of metabolism for all organisms. However, similarities between organisms, such as von Bertalanffy's growth curve and Kleiber's law on metabolic rate, suggest that mechanisms that control the uptake and use of metabolites are common to all organisms. These and other widespread empirical patterns in biology should be the ultimate test for any metabolic theory that hopes for generality. The present study (i) collects empirical evidence on growth, stoichiometry, feeding, respiration and energy dissipation and exhibits it as stylized biological facts; (ii) formalizes assumptions and propositions in a metabolic theory that is fully consistent with the Dynamic Energy Budget theory; and (iii) proves that these assumptions and propositions are consistent with the stylized facts.

Omnivory and the stability of food webs

The ecological concept of omnivory, feeding at more than a single trophic level, is formulated as an intermediate stage between any two of three classical three-dimensional species interaction systems-tritrophic chain, competition, and polyphagy. It is shown that omnivory may be either stabilizing or destabilizing, depending, in part, on the conditions of the parent systems from which it derives. It is further conjectured that the tritrophic to competition gradient cannot be entirely stable, that there must be an instability at some level of intermediate omnivory.

Stoichiometry and food-chain dynamics

Traditional models of chemostat systems looking at interactions between predator, prey and nutrients have used only a single currency, such as energy or nitrogen. In reality, growth of autotrophs and heterotrophs may be limited by various elements, e.g. carbon, nitrogen, phosphorous or iron. In this study we develop a dynamic energy budget model chemostat which has both carbon and nitrogen as currencies, and examine how the dual availability of these elements affects the growth of phytoplankton, trophic transfer to zooplankton, and the resulting stability of the chemostat ecosystem. Both species have two reserve pools to obtain a larger metabolic flexibility with respect to changing external environments. Mineral nitrogen and carbon form the base of the food chain, and they are supplied at a constant rate. In addition, the biota in the chemostat recycle nutrients by means of respiration and excretion, and organic detritus is recycled at a fixed rate. We use numerical bifurcation analysis to assess the model's dynamic behavior. In the model, phytoplankton is nitrogen limited, and nitrogen enrichment can lead to oscillations and multiple stable states. Moreover, we found that recycling has a destabilizing effect on the food chain due to the increased repletion of mineral nutrients. We found that both carbon and nitrogen enrichment stimulate zooplankton growth. Therefore, we conclude that the concept of single-element limitation may not be applicable in an ecosystem context.

Adaptive omnivory and species coexistence in tri-trophic food webs

The commonness of omnivory in natural communities is puzzling, because simple dynamic models of tri-trophic systems with omnivory are prone to species extinction. In particular, the intermediate consumer is frequently excluded by the omnivore at high levels of enrichment. It has been suggested that adaptive foraging by the omnivore may facilitate coexistence, because the intermediate consumer should persist more easily if it is occasionally dropped from the omnivore's diet. We explore theoretically how species permanence in tri-trophic systems is affected if the omnivore forages adaptively according to the "diet rule", i.e., feeds on the less profitable of its two prey species only if the more profitable one is sufficiently rare. We show that, compared to systems where omnivory is fixed, adaptive omnivory may indeed facilitate 3-species persistence. Counter to intuition, however, facilitation of 3-species coexistence requires that the intermediate consumer is a more profitable prey than the basal resource. Consequently, adaptive omnivory does not facilitate persistence of the intermediate consumer but enlarges the persistence region of the omnivore towards parameter space where a fixed omnivore would be excluded by the intermediate consumer. Overall, the positive effect of adaptive omnivory on 3-species persistence is, however, small. Generally, whether omnivory is fixed or adaptive, 3-species permanence is most likely when profitability (=conversion efficiency into omnivores) is low for basal resources and high for intermediate consumers.

Dynamics of Compartmented and Reticulate Food Webs in Relation to Energetic Flows

Using simple food webs, we address how the interactions of food web structure and energetic flows influence dynamics. We examine the effect of food web topologies with equivalent energetics (i.e., trophic interactions are equivalent at each trophic level), following which we vary energetic flows to include weak and strong interactions or nonequivalent energetics. In contrast to some work (Pimm 1979), we find that compartmented webs are more stable than reticulate webs. However, we find that nonequivalent energetics can stabilize previously unstable reticulate structures. It is not only weak flows that can be stabilizing but also the arrangement of the flows that emphasizes stabilizing mechanisms. We find that the main stabilizing mechanism is asynchrony, where structures and energetic arrangements that decrease synchrony such as internal segregation or competition will stabilize dynamics. Since compartments allow prey dynamics to behave somewhat independently, compartmentation readily promotes stability. In addition, these results can be scaled from simple food webs to more complex webs with many interacting subsystems so that linking weak subsystems to strong ones can stabilize dynamics. We show that food web dynamics are determined not only by topology but also the arrangement of weak and strong energetic flows.

Population dynamic theory of size-dependent cannibalism

Cannibalism is characterized by four aspects: killing victims, gaining energy from victims, size-dependent interactions and intraspecific competition. In this review of mathematical models of cannibalistic populations, we relate the predicted population dynamic consequences of cannibalism to its four defining aspects. We distinguish five classes of effects of cannibalism: (i) regulation of population size; (ii) destabilization resulting in population cycles or chaos; (iii) stabilization by damping population cycles caused by other interactions; (iv) bistability such that, depending on the initial conditions, the population converges to one of two possible stable states; and (v) modification of the population size structure. The same effects of cannibalism may be caused by different combinations of aspects of cannibalism. By contrast, the same combination of aspects may lead to different effects. For particular cannibalistic species, the consequences of cannibalism will depend on the presence and details of the four defining aspects. Empirical evidence for the emerged theory of cannibalism is discussed briefly. The implications of the described dynamic effects of cannibalism are discussed in the context of community structure, making a comparison with the community effects of intraguild predation.