Biomass conversion at population level

The conversion efficiency of prey biomass into predator biomass (or plant-biomass into herbivore-biomass) at the individual level is not constant. Measured by the yield factor, this conversion appears to depend on size, energy reserves, and a few species-specific compound parameters involving a variety of elements from the dynamic energy budget. This efficiency is studied here on the basts of a continuous time model for the energy budget of an individual organism. At the population level, the conversion of prey into predator biomass depends on harvesting processes. Included in harvesting is death by aging. In the absence of harvested individuals from a population of predators, a constant supply rate of prey results in a predator population of constant size. At equilibrium, this gives a conversion efficiency of zero. When the predator population is harvested, the conversion efficiency increases up to a maximum and then decreases to a threshold level, above which the population goes extinct. A practical implication is that it is possible that populations with substantial standing crops cannot stand sustained increased harvesting. For a well tested model for energy budgets of individuals, the conversion efficiency of prey biomass into predator biomass has been compared for two modes of prey selection by the predator: random predation and fixed age predation. Both strategies gave almost identical biomass conversion efficiencies. Propagation through eggs has been found to be a bit less efficient that propagation through binary fission. The way the efficiency depends on parameters of the individual energy budget is an important link between ecology and physiology, a fact which is highly relevant to areas of applied ecology, such as eco toxicology. The study of this relation is complicated by population oscillations induced by the harvesting process, but computer simulations show that an analysis of the conversion efficiency on the basis of stable age distributions is still valid

A cohort projection method to follow DEB-structured populations with periodic, synchronized and iteroparous reproduction

The need to follow structured populations, as opposed to unstructured ones, is well-recognized. The most detailed category of population models are the individual-based population models (IBMs), also called agent-based population models (ABMs). Their analysis is generally by simulation in time followed by statistical analysis of the numerical results. A less detailed method is the physiologically structured population model approach (PSPMs) leading originally to continuous-time partial differential equations (PDEs) for the p(opulation)-states such as number(-density) with respect to time and i(ndividual)-state such as age and/or size and later to a delay equation formulation. Their mathematical analysis and computational methods are generally complex. Discrete-time matrix population models (MPMs) are much simpler to analyse in all respects, but the applicability is limited due to stringent modelling assumptions made. We discuss here a class of models we call the Cohort Projection Models (CPMs), which were formerly introduced as a special case of PSPMs with pulsed reproduction. CPMs follow cohorts of identical individuals in a Lagrangian way of which the changes of their i-states such as, size, energy reserves and maturity, are described by age dependent ordinary differential equations (ODE)s from DEB theory. Simultaneously the p-states, such as number of individuals are described by time dependent ODEs obeying conservation laws. The population is subdivided in generations on the assumption that seasonal cycles synchronize reproduction events among cohorts and all eggs that are produced by different generations are the same. Feedback from the environment can be included via specification of food dynamics that accommodates competition. Temperature follows a specified periodic trajectory in time. This allows for the definition of a projection map of i-states and p-states, from one reproduction event to the next. The projection interval is typically one year for seasonal variability. The properties of the map can be studied using nonlinear dynamical system theory, such as existence and stability of fixed points and, thereby, the long-term dynamics of the food-population system. We demonstrate this using DEB parameter values from the Add-my-Pet (AmP) collection for over 2000 animal species, which were estimated from empirical data. CPMs are meant to match the relative simplicity of the analysis of MPMs with the realism of the DEB models for the dynamics of the population individuals

Understanding the dynamics of δ13C and δ15N in soft tissues of the bivalve Crassostrea gigas facing environmental fluctuations in the context of Dynamic Energy Budgets (DEB)

International audienceWe studied the dynamics of stable isotopes δ13C and δ15N of an opportunistic suspension feeder the Pacific oyster (Crassostrea gigas) to better understand the factors that influence the trophic enrichment (trophic-shift, Δ) between primary producers and consumers. Most of the previous studies on this topic do not quantify mass fluxes or isotopic discrimination phenomena in the organism, which are two pillars in isotope ecology. We used a dynamic energy budget (DEB) approach (Kooijman, 2010) to quantify i) the fluxes of elements and isotopes in C. gigas soft tissues and ii) the impact of the scaled feeding level, the organism mass and the isotopic ratio of food on the "trophic-shift" Δ, and isotope turnover in tissues. Calibration and parametrization modelling were based on data from the literature. We showed that a five-fold increase in scaled feeding level leads to a decrease of the trophic-shift value of 35% for carbon and 43% for nitrogen. This can be explained by the molecule selection for the anabolic and/or catabolic way. When f increases due to the reserve dynamic formulation in the standard DEB model, the half-life of the isotopic ratio tδ 1/2 in tissues also decreases from 13.1 to 7.9 d for δ13C and from 22.1 to 10.3 d for δ15N. Organism mass also affects the trophicshift value: an increase of the individual initial mass from 0.025 g to 0.6 g leads to an enrichment of 22% for δ13C and 21% for δ15N. For a large individual, these patterns show that a high structural volume has to be maintained. Another consequence of the mass effect is an increase of the half-life for δ13C from 6.6 to 12.0 d, and an increase of the half life for δ15N from 8.3 to 19.4 d. In a dynamic environment, the difference in the isotopic ratios between the individual tissues and the food (δ13CW − δ13CX) exhibits a range of variation of 2.02‰ for carbon and 3.03‰ for nitrogen. These results highlight the potential errors in estimating the contributions of the food sources without considering the selective incorporation of isotopes. We conclude that the dynamic energy budget model is a powerful tool to investigate the fate of isotopes in organisms

Comparative physiological energetics of Mediterranean and North Atlantic loggerhead turtles

Population of loggerhead turtles nesting in the Mediterranean Sea has probably evolved from the North Atlantic (NA) population, but is geographically and genetically distinct. We aggregated previously published and new unpublished data, and took two approaches to comparing these populations: an empirical one based on statistical analyses of morphological data, and a physiological one based on a Dynamic Energy Budget (DEB) model. We then analyzed causes of faster growth and maturation, but smaller size at puberty and ultimate size of the Mediterranean (MED) loggerhead turtles relative to their NA conspecifics. The empirical analysis shows that MED eggs, hatchlings, and nesting adults are consistently smaller in terms of length and mass. The physiological approach suggests physiological adaptations of the MED population to higher salinity and scarcer food availability. In particular, these adaptations include an increase in somatic maintenance needs, and a decrease in energy investment to reach and maintain sexual maturity. Our study therefore offers a mechanistic underpinning of previously observed but unexplained life-history traits, and showcases an application of DEB theory as a tool for comparative analysis of two distinct populations of the same species

Environmental effects on growth, reproduction, and life-history traits of loggerhead turtles

Understanding the relationship between the environmental conditions and life-history traits (such as growth, reproduction, and size at specific life stages) is important for understanding the population dynamics of a species and for constructing adaptable, relevant, and efficient conservation measures. For the endangered loggerhead turtle, characterizing effects of environmental conditions on the life-history traits is complicated by this species’ longevity, global distribution, and migratory way of life. Two significant environmental factors – temperature and available food – often account for most of observed intra-population variability in growth and reproduction rates, suggesting that those two factors determine the biological responses of an individual. Adopting this hypothesis, we simulate a range of the two environmental factors to quantify effects of changes in temperature and food availability on an individual’s physiology (energy investment into processes such as growth, maturation, and reproduction) and the resulting life-history traits. To represent an individual, we use a previously developed mechanistic dynamic energy budget (DEB) model for loggerhead turtles. DEB models rely on one of the empirically best validated general ecological theories, which captures rules of energy acquisition and utilization. We found that the ultimate size (length and mass) is primarily affected by food availability, whereas growth and maturation are primarily affected by temperature whilst also showing positive correlation with available food. Reproduction increases with both food availability and temperature because food availability determines energy investment into egg production, and temperature affects the rate of related processes (such as vitellogenesis). Length at puberty varies between simulated scenarios by only a small proportion, suggesting that inter-individual variability plays a larger role for length at puberty than the environmental factors do

A dynamic energy budget model of Fenneropenaeus chinensis with applications for aquaculture and stock enhancement

Dynamic energy budget (DEB) theory provides a framework for quantifying metabolic processes and biological rates. DEB models have been widely applied to aquaculture species, but this type of model has great potential for application to fisheries for stock assessment and enhancement. The shrimp Fenneropenaeus chinensis, widely distributed along the coast of China and Korea, is the most important fisheries and aquaculture species in China. With the AmP method, DEB parameters were estimated for the population along the coast of China. The parameter estimation achieved an overall goodness of fit with MRE of 0.131 and SMSE of 0.178. In comparison with similar species, the values of a few main parameters are relatively high including reserve capacity (Em), somatic maintenance (ṗM) and allocation fraction to growth and somatic maintenance (κ). This may reflect an adaptation to variation of environmental conditions. The model can predict the physiological behaviours including respiration, ammonia excretion and feeding rates reasonably well. It shows overall capability to predict the growth and reproduction with acceptable confidence in three main geographic regions. There are clear differences between the female and male with much faster growth rate of the former. Validations of the model have shown that it can adequately predict growth of the shrimp in both its natural distribution waters and land-based culture systems. This study provides important information for further development of modeling tools which can contribute to estimating the carrying capacity for stock enhancement and optimizing production from integrated multi-trophic aquaculture

A dynamic energy budget model for small yellow croaker Larimichthys polyactis:Parameterisation and application in its main geographic distribution waters

Global fishery resources have generally declined, although the rapidity has decreased, and trends have reversed in specific instances following judiciously well-informed intervention. Appropriate management is necessary for the optimum utilisation of fish stocks. Small yellow croaker Larimichthys polyactis is an important fisheries species in the northwest Pacific Ocean, and a major decline in its abundance has been attributed to overfishing and environmental changes. For understanding energetics of small yellow croaker in responses to varying environmental conditions, a dynamic energy budget (DEB) model was applied to this species in the Yellow and East China Seas. The model was parameterised with the observed data including age-at-birth, age-since-birth-at-puberty, life span, fecundity, age-length and length-weight relationship. The model parameterisation has achieved an acceptable goodness of fit, with both low mean relative error and symmetric mean squared error of 0.083. Applications of the model have shown that it can reasonably reproduce the energetics of small yellow croaker in its main biogeographic distribution. Growth simulations in both length and weight were within the range of observations. The simulated fecundity matched the observation reasonably well. We discuss the need for further improvement of the model and additional collection of environmental data

Life in the slow lane? A dynamic energy budget model for the western swamp turtle, Pseudemydura umbrina

Dynamic energy budget (DEB) theory provides a generalised way to quantify how an organism assimilates and utilizes energy throughout its life cycle. Over 800 DEB models have been created to date, typically under the assumption of constant food supply. The Critically Endangered, semi-aquatic western swamp turtle occupies an ephemeral wetland environment in which food resources fluctuate from abundant to absent with the seasonal filling and drying of swamps. Approximately six months of each year are spent in aestivation underground when the swamps are dry and conditions are warm. We estimated DEB parameters for the western swamp turtle with the explicit incorporation of these seasonal fluctuations in food and temperature. A metabolic depression factor was applied during the aestivation stage, without which turtles lost both mass and length, and reserves were insufficient to reach puberty. The swamp turtle had a very high Arrhenius temperature, being almost 2.5-fold greater than that of the other Testudine species for which there are DEB models (typical Arrhenius temperatures are around 8000 K; western swamp turtle is 19,371 K). It also had the second highest somatic maintenance costs of the reptiles in the DEB ‘Add My Pet’ collection, and the highest for Testudines. We explore these results in context of the “waste to hurry” hypothesis, which we suggest may apply for this species. We also consider how a DEB model for this species might be applied in its future conservation and management