Revisiting the role of individual variability in population persistence and stability

Early developmental carry-over effects on exploratory behaviour and DNA methylation in wild great tits (Parus major)

Adverse, postnatal conditions experienced during development are known to induce lingering effects on morphology, behaviour, reproduction and survival. Despite the importance of early developmental stress for shaping the adult phenotype, it is largely unknown which molecular mechanisms allow for the induction and maintenance of such phenotypic effects once the early environmental conditions are released. Here we aimed to investigate whether lasting early developmental phenotypic changes are associated with post-developmental DNA methylation changes. We used a cross-foster and brood size experiment in great tit (Parus major) nestlings, which induced post-fledging effects on biometric measures and exploratory behaviour, a validated personality trait. We investigated whether these post-fledging effects are associated with DNA methylation levels of CpG sites in erythrocyte DNA. Individuals raised in enlarged broods caught up on their developmental delay after reaching independence and became more explorative as days since fledging passed, while the exploratory scores of individuals that were raised in reduced broods remained stable. Although we previously found that brood enlargement hardly affected the pre-fledging methylation levels, we found 420 CpG sites that were differentially methylated between fledged individuals that were raised in small versus large sized broods. A considerable number of the affected CpG sites were located in or near genes involved in metabolism, growth, behaviour and cognition. Since the biological functions of these genes line up with the observed post-fledging phenotypic effects of brood size, our results suggest that DNA methylation provides organisms the opportunity to modulate their condition once the environmental conditions allow it. In conclusion, this study shows that nutritional stress imposed by enlarged brood size during early development associates with variation in DNA methylation later in life. We propose that treatment-associated DNA methylation differences may arise in relation to pre- or post-fledging phenotypic changes, rather than that they are directly induced by the environment during early development.

Exploring the role of spatial and stoichiometric heterogeneity in the top-down control in eutrophic planktonic ecosystems

Understanding the impact of eutrophication on the dynamics of aquatic food webs, remains a long-term challenge in ecology. Mathematical models generally predict the destabilisation of such webs, under increasing eutrophication levels, with large oscillations of species densities that can result in their extinction. This is at odds with a number of ecological observations that show stable dynamics even for high nutrient loads. The apparent discrepancy between theory and observations is known as the Rosenzweig's 'paradox of enrichment' and various solutions to the problem have been proposed over the years. In this study, we explore the stabilisation of dynamics of a tri-trophic plankton model in a eutrophic environment which occurs as a result of interplay of space heterogeneity, ecological stoichiometry, and food taxis of predators. We build a variety of models of increasing complexity, to explore various scenarios of phytoplankton growth, zooplankton food-dependent vertical movement, and different stoichiometric limitations of zooplankton. We show that the synergy among the vertical gradient in phytoplankton growth, phytoplankton structuring in terms of their stoichiometric ratio, and food-dependent vertical movement of zooplankton, would result in a postponing of destabilisation of eutrophic systems as compared to a well-mixed system. Our approach reveals a high complexity of the bifurcation structure of the system when key model parameters, such as the degree of eutrophication and light shading, are varied. We find coexistence of limit cycles and stable equilibria and that the possibility of multiple attractors in the system can result in hysteresis phenomena when the nutrient load is manipulated. These results are relevant and should be taken into account in lake restoration programs.

Towards developing a general framework for modelling vertical migration in zooplankton

Diel vertical migration (DVM) of zooplankton is a widespread phenomenon in both oceans and lakes, and is generally considered to be the largest synchronized movement of biomass on Earth. Most existing mathematical models of DVM are based on the assumption that animals maximize a certain criterion such as the expected reproductive value, the venturous revenue, the ratio of energy gain/mortality or some predator avoidance function when choosing their instantaneous depth. The major shortcoming of this general point of view is that the predicted DVM may be strongly affected by a subjective choice of a particular optimization criterion. Here we argue that the optimal strategy of DVM can be unambiguously obtained as an outcome of selection in the underlying equations of genotype/traits frequency dynamics. Using this general paradigm, we explore the optimal strategy for the migration across different depths by zooplankton grazers throughout the day. To illustrate our ideas we consider four generic DVM models, each making different assumptions on the population dynamics of zooplankton, and demonstrate that in each model we need to maximize a particular functional to find the optimal strategy. Surprisingly, patterns of DVM obtained for different models greatly differ in terms of their parameters dependence. We then show that the infinite dimensional trait space of different zooplankton trajectories can be projected onto a low dimensional space of generalized parameters and the genotype evolution dynamics can be easily followed using this low-dimensional space. Using this space of generalized parameters we explore the influence of mutagenesis on evolution of DVM, and we show that strong mutagenesis allows the coexistence of an infinitely large number of strategies whereas for weak mutagenesis the selection results in the extinction of most strategies, with the surviving strategies all staying close to the optimal strategy in the corresponding mutagenesis-free system.

Individual variation in functional response parameters is explained by body size but not by behavioural types in a poeciliid fish

Functional responses are per-capita feeding rate models whose parameters often scale with individual body size but the parameters may also be further influenced by behavioural traits consistently differing among individuals, i.e. behavioural types or animal personalities. Behavioural types may intrinsically lead to lower feeding rates when consistently shy, inactive and easily stressed individuals cannot identify or respond to risk-free environments or need less food due to lower metabolic rates linked to behaviour. To test how much variation in functional response parameters is explained by body size and how much by behavioural types, we estimated attack rate and handling time individually for differently sized female least killifish (Heterandria formosa) and repeatedly measured behavioural traits for each individual. We found that individual fish varied substantially in their attack rate and in their handling time. Behavioural traits were stable over time and varied consistently among individuals along two distinct personality axes. The individual variation in functional responses was explained solely by body size, and contrary to our expectations, not additionally by the existing behavioural types in exploration activity and coping style. While behavioural trait-dependent functional responses may offer a route to the understanding of the food web level consequences of behavioural types, our study is so far only the second one on this topic. Importantly, our results indicate in contrast to that previous study that behavioural types do not per se affect individual functional responses assessed in the absence of external biotic stressors.

Revisiting the Stability of Spatially Heterogeneous Predator-Prey Systems Under Eutrophication

We employ partial integro-differential equations to model trophic interaction in a spatially extended heterogeneous environment. Compared to classical reaction-diffusion models, this framework allows us to more realistically describe the situation where movement of individuals occurs on a faster time scale than on the demographic (population) time scale, and we cannot determine population growth based on local density. However, most of the results reported so far for such systems have only been verified numerically and for a particular choice of model functions, which obviously casts doubts about these findings. In this paper, we analyse a class of integro-differential predator-prey models with a highly mobile predator in a heterogeneous environment, and we reveal the main factors stabilizing such systems. In particular, we explore an ecologically relevant case of interactions in a highly eutrophic environment, where the prey carrying capacity can be formally set to 'infinity'. We investigate two main scenarios: (1) the spatial gradient of the growth rate is due to abiotic factors only, and (2) the local growth rate depends on the global density distribution across the environment (e.g. due to non-local self-shading). For an arbitrary spatial gradient of the prey growth rate, we analytically investigate the possibility of the predator-prey equilibrium in such systems and we explore the conditions of stability of this equilibrium. In particular, we demonstrate that for a Holling type I (linear) functional response, the predator can stabilize the system at low prey density even for an 'unlimited' carrying capacity. We conclude that the interplay between spatial heterogeneity in the prey growth and fast displacement of the predator across the habitat works as an efficient stabilizing mechanism. These results highlight the generality of the stabilization mechanisms we find in spatially structured predator-prey ecological systems in a heterogeneous environment.

Natural variation in cold tolerance in the nematode Pristionchus pacificus: the role of genotype and environment

Low temperature is a primary determinant of growth and survival among organisms and almost all animals need to withstand temperature fluctuations in their surroundings. We used the hermaphroditic nematode Pristionchus pacificus to examine variation in cold tolerance in samples collected from 18 widespread locations. Samples were challenged by exposure to both direct and gradual low temperature after culture in the laboratory at 20°C. A short-term acclimation treatment was also applied to assess cold tolerance following a pre-exposure cold treatment. Finally, genotype-by-environment (G × E) analysis was performed on a subset of samples cultured at two additional temperatures (15°C and 25°C). P. pacificus displayed a high degree of natural variation in cold tolerance, corresponding to the presence of three distinct phenotypic classes among samples: cold tolerant, non-cold tolerant, cold tolerant plastic. Survival of gradual cold exposure was significantly higher than survival of direct exposure to low temperature and a cold exposure pre-treatment significantly enhanced cold tolerance in some samples. By focusing on a sub-set of well-sampled locations from tropical La Réunion Island, we found evidence of significant effects of genotype and environment on cold tolerance, and we also showed that, within the different Réunion locations sampled, all three phenotypic classes are generally well represented. Taken together, our results show that P. pacificus exhibits a highly plastic tolerance to cold exposure that may be partly driven by differential trait sensitivity in diverse environments.