Organisms interact with their environments in various ways. We present a conceptual framework that distinguishes three mechanisms of organism–environment interaction. We call these NC3 mechanisms: niche construction, in which individuals make changes to the environment; niche choice, in which individuals select an environment; and niche conformance, in which individuals adjust their phenotypes in response to the environment. Each of these individual-level mechanisms affects an individual's phenotype–environment match, its fitness, and its individualized niche, defined in terms of the environmental conditions under which the individual can survive and reproduce. Our framework identifies how individuals alter the selective regimes that they and other organisms experience. It also places clear emphasis on individual differences and construes niche construction and other processes as evolved mechanisms. The NC3 mechanism framework therefore helps to integrate population-level and individual-level research.

No abstract

We here develop a concept of an individualized niche in analogy to Hutchison’s population-level concept of the ecological niche. We consider the individualized (ecological) niche as the range of environmental conditions under which a particular individual has an expected lifetime reproductive success of ≥ 1. Our concept has primarily an ecological function, as it refers to the match of an individual phenotype to its contemporary environment (niche fit) while we discuss evolutionary fitness as an evaluative parameter of this fit. We address four specific challenges that occur when scaling the niche down from populations to individuals. In particular, we discuss (1) the consequences of uniqueness of individuals in a population and the corresponding lack of statistical replication, (2) the dynamic nature of individualized niches and how they can be studied either as time-slice niches, as prospective niches or as trajectory-based niches, (3) the dimensionality of the individualized niche, that is greater than the population niche due to the additional dimensions of intra-specific niche space, (4) how the boundaries of individualized niche space can be defined by expected lifetime reproductive success and how expected reproductive success can be inferred by marginalizing fitness functions across phenotypes or environments. We frame our discussion in the context of recent interest in the causes and consequences of individual differences in animal behavior.

mean value) is the factor that influences the cooperative-antagonistic outcome. Heritable trait differences and phenotypic plasticity are sources of phenotypic variation among individuals, and both the degree of heritability and plasticity of the trait involved may determine whether shifts between cooperation and antagonism are likely to be short-term (i.e. context-dependent) or lead to more persistent shifts (e.g. mutualism breakdown). To guide future research, we describe knowledge gaps and divergences between empirical and theoretical literature, highlighting the value of applying evidence synthesis methods in ecology and evolution.

The study of consistent individual differences in behaviour has become an important focus in research on animal behaviour. Behavioural phenotypes are typically measured through standardized testing paradigms and one frequently used paradigm is the novel object test. In novel object tests, animals are exposed to new (unknown) objects and their reaction is quantified. When repeating trials to assess the temporal consistency of individual differences, researchers face the dilemma of whether to use the same or different ‘novel’ objects, since the same stimulus can result in habituation, while exposure to different objects can result in context-dependent responses. We performed a quantitative assessment of 254 effect sizes from 113 studies on novel-object trials to evaluate the properties of this testing paradigm, in particular the effect of object novelty and time interval between novel-object trials on estimates of individual consistency. We found an increase of sample sizes and an increase of estimates of repeatabilities with time. The vast majority of short-term studies (<one month) used different novel objects, while long-term studies (>one month) used either the same or different novel objects about equally often. The average estimate for individual consistency was r = 0.47 (short-term r = 0.52, long-term r = 0.44). Novelty, time interval between trials and their interaction together explained only 3% of the total heterogeneity. Overall, novelobject trials reliably estimate individual differences in behaviour, but results were very heterogeneous even within the same study species, suggesting susceptibility to unknown details in testing conditions. Most studies that measure novel-object responses in association with food label the trait as neophobia, while novel-object trials in a neutral context are labelled variously as boldness/shyness, exploratory behaviour or neophobia/neophilia. Neophobia/neophilia is also the term most specific to novel object presentations. To avoid ambiguity, we suggest object neophobia/neophilia as the most specific label for novel-object responses.