Modelling optimal behavioural strategies in structured populations using a novel theoretical framework.

Understanding complex behavioural patterns of organisms observed in nature can be facilitated using mathematical modelling. The conventional paradigm in animal behavior modelling consists of maximisation of some evolutionary fitness function. However, the definition of fitness of an organism or population is generally subjective, and using different criteria can lead us to contradictory model predictions regarding optimal behaviour. Moreover, structuring of natural populations in terms of individual size or developmental stage creates an extra challenge for theoretical modelling. Here we revisit and formalise the definition of evolutionary fitness to describe long-term selection of strategies in deterministic self-replicating systems for generic modelling settings which involve an arbitrary function space of inherited strategies. Then we show how optimal behavioural strategies can be obtained for different developmental stages in a generic von-Foerster stage-structured population model with an arbitrary mortality term. We implement our theoretical framework to explore patterns of optimal diel vertical migration (DVM) of two dominant zooplankton species in the north-eastern Black Sea. We parameterise the model using 7 years of empirical data from 2007-2014 and show that the observed DVM can be explained as the result of a trade-off between depth-dependent metabolic costs for grazers, anoxia zones, available food, and visual predation.

Major, trace, and rare-earth elements in the zooplankton of the Laptev Sea in relation to community composition.

We investigated the concentrations of major, trace, and rare-earth elements in zooplankton in relation to species community composition from the eastern part of the Lena Delta to the continental slope of the Laptev Sea in September 2015. The elemental composition of zooplankton inhabiting different areas demonstrated similarities. Cross-shelf changes were found for only 4 (Li, Zn, As, and U) of the 56 elements analyzed. Zinc was the only element concentration of which successively reduced across coastal-open ocean gradient. Considering own and literary data, we can assume that the cross-shelf decrease of zinc accumulation in zooplankton is a universal pattern, manifested in different climatic and biogeochemical environmental conditions and with different species compositions of the zooplankton community. Cross-shelf changes were also established for Li, As, and U. However, the concentrations of these elements increased along the gradient. We assume that increased As concentration in zooplankton across the shelf-continental slope gradient of the Laptev Sea is associated with a change in the feeding behavior of the species of the zooplankton community. However, a sharp increase in the concentrations of Li and U in zooplankton was associated with the appearance of Calanus copepods in the community. The average total concentration of rare-earth elements and yttrium in zooplankton of the Laptev Sea was 752.8 ng g-1 of dry weight. We found record high levels of rare-earth elements for zooplankton of the inner shelf, near the eastern part of the Lena Delta. From the inner shelf to the continental slope of the Laptev Sea, La, Ce, and Nd were the dominant rare-earth elements. The elemental composition of zooplankton in the Arctic Seas is characterized by a much lower content of major and trace elements in comparison with the zooplankton and total plankton of the ocean.

Revisiting the role of individual variability in population persistence and stability.

Populations often exhibit a pronounced degree of individual variability and this can be important when constructing ecological models. In this paper, we revisit the role of inter-individual variability in population persistence and stability under predation pressure. As a case study, we consider interactions between a structured population of zooplankton grazers and their predators. Unlike previous structured population models, which only consider variability of individuals according to the age or body size, we focus on physiological and behavioural structuring. We first experimentally demonstrate a high degree of variation of individual consumption rates in three dominant species of herbivorous copepods (Calanus finmarchicus, Calanus glacialis, Calanus euxinus) and show that this disparity implies a pronounced variation in the consumption capacities of individuals. Then we construct a parsimonious predator-prey model which takes into account the intra-population variability of prey individuals according to behavioural traits: effectively, each organism has a 'personality' of its own. Our modelling results show that structuring of prey according to their growth rate and vulnerability to predation can dampen predator-prey cycles and enhance persistence of a species, even if the resource stock for prey is unlimited. The main mechanism of efficient top-down regulation is shown to work by letting the prey population become dominated by less vulnerable individuals when predator densities are high, while the trait distribution recovers when the predator densities are low.