Oliver Krüger, Barbara A. Caspers, Nayden Chakarov, Alkistis Elliott-Graves, Marie Jolanda Kaiser, Peter Korsten, Joachim Kurtz, Caroline Müller, Holger Schielzeth, Tamas Szekely

Individuals differ. This seemingly trivial statement has nevertheless led to paradigm shifts, as three different fields of organismal biology have seen a marked change in key concepts over the past few decades. In animal behaviour, it has increasingly been realised that behavioural differences among individuals can be stable over time and across contexts, giving rise to the concept of animal personalities. In ecology, an increasing focus is likewise on the considerable variation in the ecological niche realised 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 an increasing awareness of the complexity with which individuals interact with the environment in producing unique phenotypes. Recent theoretical and empirical work has highlighted that the fitness landscape is rather complex, with multiple fitness peaks. It depends on the individual with its genotype, in interaction with its specific environment, which local or global fitness peak is attainable. Over the past 15 years, the need for more integrated conceptual frameworks transcending disciplines has been voiced ever more strongly. Whereas initially the ecological time scale was deemed to be fundamentally different from the evolutionary one, this notion has recently been replaced by a more integrative one, where evolution can indeed happen over ecological time scales. While in each of the three fields behaviour, ecology, and evolution, the concept of individualisation has contributed to major scientific progress, sufficient cross-fertilisation is lacking. Here, we propose a new level of conceptual unification: the individualised niche. By merging the niche concept with the fitness and animal personality concepts, new explanatory power for both ecological and evolutionary processes emerges.