Ecological Physiology

Environmental stress plays an important role in determining ecosystem functioning and structure. In estuarine areas both tidal and seasonal salinity changes may cause osmotic stress on predators, affecting their behaviour and survival. The interaction between these predators and their prey may affect performance, thus influencing predator impact on prey populations. The common starfish, Asterias rubens, inhabits estuarine areas, such as the Dutch Wadden Sea, that exhibit large seasonal variation in salinity (10 – 32 PSU). In those areas A. rubens exerts top down control on its prey, thus representing an important shellfish predator. This predation may impact on cultured and natural shellfish populations. However, the effects of osmotic stress on A. rubens performance may influence its effect on prey. Although the effect of salinity in A. rubens survival has been extensively studied, the impact on its predation behaviour and acclimation capacity remains unclear. In this study, we analyse the performance of A. rubens preying on mussels (Mytilus edulis) after a salinity decrease and monitor its acclimation capacity over a period of 22 days. Our experiments demonstrated that salinity affected performance by reducing feeding activity and altering size prey selection. Moreover, as acclimation occurred, A. rubens predation performance improved in all sub-lethal treatments. We conclude that osmotic stress caused by decreasing salinity potentially influences A. rubens distribution, abundance, and potential impact on prey populations. However the magnitude of the change in salinity (from 31 to a minimum of 10 PSU) and its timescale (3 weeks) mediate this effect.

The concept of biological adaptation was closely connected to some mathematical and engineering ideas from the very beginning. Cannon’s homeostasis is, in its essence, automatic stabilisation of the body.  Selye discovered the phases and limits of adaptation to harmful conditions. His model of General Adaptation Syndrome (GAS) states that an event that threatens an organism’s well-being leads to a three-stage bodily response: Alarm – Resistance – Exhaustion. He concluded from his experiments that the regulatory mechanisms need some “adaptation resource” (adaptability) and demonstrated that the adaptability decreases in the course of adaptation. This adaptability is a hypothetical extensive variable, adaptation energy. In GAS, the adaptation resource is spending for continuous neutralisation of a harmful factor which affects the organism. In addition, Selye demonstrated that the adaptability is spending for training: a rat was trained for resistance to one factor but lost the ability to train for resistance to another factor. There is a fundamental difference between the resource (adaptation energy) spending in GAS and the resource pending for training. The second type is the change of the resistivity landscape. The domain of values of the harmful factors, where the organism can survive, is changing. The volume of this domain can be extended but not for free, its extension requires adaptation energy. Logarithm of this volume is an entropy, and we can call it the “adaptation entropy”. Thus, analysis of Selye’s experiments and physiological hypotheses lead us to the notion of adaptation entropy and, in combination with adaptation energy, to the “adaptation free energy”. We present a new family of dynamical models of physiological adaptation based on the notion of the adaptation free energy. This is a new class of the “top-down” thermodynamic models for physiology.

We study the dynamics of correlation and variance in systems under the load of environmental factors. A universal effect in ensembles of similar systems under the load of similar factors is described: in crisis, typically, even before obvious symptoms of crisis appear, correlation increases, and, at the same time, variance (and volatility) increases too. This effect is supported by many experiments and observations of groups of humans, mice, trees, grassy plants, and on financial time series.

A general approach to the explanation of the effect through dynamics of individual adaptation of similar non-interactive individuals to a similar system of external factors is developed. Qualitatively, this approach follows Selye’s idea about adaptation energy.

We study ensembles of similar systems under load of environmental factors. The phenomenon of adaptation has similar properties for systems of different nature. Typically, when the load increases above some threshold, then the adapting systems become more different (variance increases), but the correlation increases too. If the stress continues to increase then the second threshold appears: the correlation achieves maximal value, and start to decrease, but the variance continue to increase. In many applications this second threshold is a signal of approaching of fatal outcome. This effect is supported by many experiments and observation of groups of humans, mice, trees, grassy plants, and on financial time series.

A general approach to explanation of the effect through dynamics of adaptation is developed. H. Selye introduced “adaptation energy” for explanation of adaptation phenomena. We formalize this approach in factors – resource models and develop hierarchy of models of adaptation. Different organization of interaction between factors (Liebig’s versus synergistic systems) lead to different adaptation dynamics. This gives an explanation to qualitatively different dynamics of correlation under different types of load and to some deviation from the typical reaction to stress. In addition to the “quasistatic” optimization factor – resource models, dynamical models of adaptation are developed, and a simple model (three variables) for adaptation to one factor load is formulated explicitly.