Individualised niches in a variable environment—Consequences for environmental change responses

Abstract Intraspecific trait variation (ITV) can be important for population performance in a variable and changing environment because individuals with different traits have different fitness responses. Furthermore, there are three mechanisms via which individuals can interact with their environment to potentially improve fitness: niche conformance, niche construction and niche choice (NC 3 ). These processes become increasingly important in the presence of environmental change, but there is still no mathematical modelling framework that would unite the effects of ITV and the NC 3 mechanisms. In this paper, we build a general model incorporating ITV and two of the NC 3 mechanisms (niche conformance and niche construction, NC 2 ) to investigate how they affect populations in a changing and variable environment. We quantify their effects on average individual fitness using non‐linear averaging approaches. Our method allows us to answer the question of what would have happened if individuals in the study system did not have ITV or did not perform NC 2 mechanisms. The answer to this question depends on the shapes of the fitness function and the NC 2 functions and can be estimated via a Taylor approximation. We apply the method to two case studies: great tits adjusting their laying date to yearly changes in vegetation green‐up and a host–parasite system in which the parasite changes its environment by immunodepression of the host. In the great tits, we found a slight negative effect of ITV and a slight positive effect of niche conformance on the population fitness. In the host–parasite system, we found ITV to have no effect without niche construction, but with niche construction, ITV decreased virulence. Also, niche construction had a strong negative effect on virulence. Our extension of non‐linear averaging theory, combining intraspecific and environmental variation, niche conformance and niche construction, allowed us to assess average population performance with those mechanisms at play. However, how well one can estimate such performance depends on the type of data available. This framework can be extended further to niche choice and evolution; therefore, including all processes that can change the match between individuals and their environment.