Complex eco-evolutionary dynamics induced by the coevolution of predator–prey movement strategies

Evolutionary Ecology - The coevolution of predators and prey has been the subject of much empirical and theoretical research that produced intriguing insights into the interplay of ecology and...     

Self-Organized Shape and Frontal Density of Fish Schools

Models of swarming (based on avoidance, alignment and attraction) produce patterns of behaviour also seen in schools of fish. However, the significance of such similarities has been questioned, because some model assumptions are unrealistic [e.g. speed in most models is constant with random error, the perception is global and the size of the schools that have been studied is small (up to 128 individuals)]. This criticism also applies to our former model, in which we demonstrated the emergence of two patterns of spatial organization, i.e. oblong school form and high frontal density, which are supposed to function as protection against predators. In our new model we respond to this criticism by making the following improvements: individuals have a preferred ‘cruise speed’ from which they can deviate in order to avoid others or to catch up with them. Their range of perception is inversely related to density, with which we take into account that high density limits the perception of others that are further away. Swarm sizes range from 10 to 2000 individuals. The model is three‐dimensional. Further, we show that the two spatial patterns (oblong shape and high frontal density) emerge by self‐organization as a side‐effect of coordination at two speeds (of two or four body lengths per second) for schools of sizes above 20. Our analysis of the model leads to the development of a new set of hypotheses. If empirical data confirm these hypotheses, then in a school of real fish these patterns may arise as a side‐effect of their coordination in the same way as in the model.     

The strength of competition among individual trees and the biomass-density trajectories of the cohort

We simulated the self-thinning of Rhizophora mangle mangrove forests with the spatially explicit simulation model KiWi. This model is an application of the field-of-neighbourhood (FON) approach, which describes an individual tree by a competition function defined on the zone of influence (ZOI) around the stem. The FON causes growth depression of the trees involved. Sustained growth depression results in tree death. We propose a subdivision of the biomass density trajectories (bdt), obtained during the thinning process, into four segments related to characteristic shapes of the stem diameter distribution of the cohort. A positive skewness of the stem diameter distribution, indicating that the majority of the individuals are small and hindered in growth, is necessary for the occurrence of a linear segment within the bdt, the so-called 'self-thinning line'. This segment is the third bdt segment according to our classification. It is reached when the positive skewness of the stem diameter distribution is maximal and ends when the skewness reaches its second zero transition. The thinning line is therefore linked to the homogenisation process, which forces the symmetry of the stem distribution. We show that the ongoing search for a universal slope for the linear segment of the biomass-density trajectory (bdt) cannot succeed, since it is species-specific and may also be site-dependent. The slope increases with increasing competition strength of the individuals. Nevertheless, the lower limit of the slope is pre-defined by geometrical constraints and modified by the actual strength of the neighbourhood competition. Although the simulations were all carried out with growth parameters of the mangrove species Rhizophora mangle, our results should be qualitatively valid and form a plausible theoretical framework to account for different biomass-density trajectories.     

Diffusion and Topological Neighbours in Flocks of Starlings: Relating a Model to Empirical Data

Moving in a group while avoiding collisions with group members causes internal dynamics in the group. Although these dynamics have recently been measured quantitatively in starling flocks (Sturnus vulgaris), it is unknown what causes them. Computational models have shown that collective motion in groups is likely due to attraction, avoidance and, possibly, alignment among group members. Empirical studies show that starlings adjust their movement to a fixed number of closest neighbours or topological range, namely 6 or 7 and assume that each of the three activities is done with the same number of neighbours (topological range). Here, we start from the hypothesis that escape behavior is more effective at preventing collisions in a flock when avoiding the single closest neighbor than compromising by avoiding 6 or 7 of them. For alignment and attraction, we keep to the empirical topological range. We investigate how avoiding one or several neighbours affects the internal dynamics of flocks of starlings in our computational model StarDisplay. By comparing to empirical data, we confirm that internal dynamics resemble empirical data more closely if flock members avoid merely their single, closest neighbor. Our model shows that considering a different number of interaction partners per activity represents a useful perspective and that changing a single parameter, namely the number of interaction partners that are avoided, has several effects through selforganisation.     

Time-Resolved Surface-Enhanced IR-Absorption Spectroscopy of Direct Electron Transfer to Cytochrome c Oxidase from R.?sphaeroides

The aggregation of α-synuclein is thought to play a role in the death of dopamine neurons in Parkinson’s disease (PD). Alpha-synuclein transitions itself through an aggregation pathway consisting of pathogenic species referred to as protofibrils (or oligomer), which ultimately convert to mature fibrils. The structural heterogeneity and instability of protofibrils has significantly impeded advance related to the understanding of their structural characteristics and the amyloid aggregation mystery. Here, we report, to our knowledge for the first time, on α-synuclein protofibril structural characteristics with cryo-electron microscopy. Statistical analysis of annular protofibrils revealed a constant wall thickness as a common feature. The visualization of the assembly steps enabled us to propose a novel, to our knowledge, mechanisms for α-synuclein aggregation involving ring-opening and protofibril-protofibril interaction events. The ion channel-like protofibrils and their membrane permeability have also been found in other amyloid diseases, suggesting a common molecular mechanism of pathological aggregation. Our direct visualization of the aggregation pathway of α-synuclein opens up fresh opportunities to advance the understanding of protein aggregation mechanisms relevant to many amyloid diseases. In turn, this information would enable the development of additional therapeutic strategies aimed at suppressing toxic protofibrils of amyloid proteins involved in neurological disorders.     

Time-Resolved Surface-Enhanced IR-Absorption Spectroscopy of Direct Electron Transfer to Cytochrome c Oxidase from R. sphaeroides

Time-resolved surface-enhanced IR-absorption spectroscopy triggered by electrochemical modulation has been performed on cytochrome c oxidase from Rhodobacter sphaeroides. Single bands isolated from a broad band in the amide I region using phase-sensitive detection were attributed to different redox centers. Their absorbances changing on the millisecond timescale could be fitted to a model based on protonation-dependent chemical reaction kinetics established previously. Substantial conformational changes of secondary structures coupled to redox transitions were revealed.