Individualised niches: an integrative conceptual framework across behaviour, ecology, and evolution.

Individuals differ. While seemingly trivial, this insight has nevertheless led to paradigm shifts, as three key fields of organismal biology have seen marked changes in key concepts over the past few decades. In animal behaviour, it has become increasingly recognised that behavioural differences among individuals can be stable over time and across contexts, giving rise to the concept of animal personalities. In ecology, attention has similarly shifted towards variation in the ecological niches occupied by species, populations and individuals, giving rise to the concept of niche specialisation or individual niche variation. In evolutionary biology, where individual variation has always been central, there is a growing awareness of the complex and dynamic ways in which individuals interact with the environment to produce unique phenotypes. Additionally, recent theoretical and empirical research suggests that fitness landscapes are not only complex, with multiple fitness peaks, but might even be more accurately described as constantly shifting 'fitness seascapes', where the fitness peak that an individual can reach - whether local or global - depends on its genotype and its interaction with the environment. Moreover, the previous distinction between ecological and evolutionary timescales is being replaced by a more integrative view that recognises that evolution can occur on ecological timeframes. These shifting perspectives over the past two decades underscore the need for a more integrated conceptual framework that transcends disciplines. While in behaviour, ecology and evolution, the concept of individualisation has contributed to major scientific progress, sufficient cross-fertilisation is still lacking. Here, we propose a new conceptual unification: the individualised niche. By merging the niche concept with the fitness concept, new explanatory power for both ecological and evolutionary processes emerges.