The importance of inter‐individual variation in predicting species' responses to global change drivers

• Published in: Ecology and evolution 2019-04, Vol.9 (8), p.4327-4339
• Main Authors: Guscelli, Ella, Spicer, John I., Calosi, Piero
• Format: Article
• Language: English
• Subjects: Acids; acid–base regulation; acid–base status; Chemical analysis; individual variation; Mathematical models; Natural populations; ocean acidification; ocean warming; Organisms; Original Research; Parameters; Phenotypes; phenotypic variation; Phenotypic variations; Physiology; Population (statistical); Sea urchins; Seawater; Statistical analysis; Studies; Temperature effects; Water analysis
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.4810

Inter‐individual variation in phenotypic traits has long been considered as “noise” rather than meaningful phenotypic variation, with biological studies almost exclusively generating and reporting average responses for populations and species’ average responses. Here, we compare the use of an individual approach in the investigation of extracellular acid–base regulation by the purple sea urchin Paracentrotus lividus challenged with elevated pCO2 and temperature conditions, with a more traditional approach which generates and formally compares mean values. We detected a high level of inter‐individual variation in acid–base regulation parameters both within and between treatments. Comparing individual and mean values for the first (apparent) dissociation constant of the coelomic fluid for individual sea urchins resulted in substantially different (calculated) acid–base parameters, and models with stronger statistical support. While the approach using means showed that coelomic pCO2 was influenced by seawater pCO2 and temperature combined, the individual approach indicated that it was in fact seawater temperature in isolation that had a significant effect on coelomic pCO2. On the other hand, coelomic [HCO3−] appeared to be primarily affected by seawater pCO2, and less by seawater temperature, irrespective of the approach adopted. As a consequence, we suggest that individual variation in physiological traits needs to be considered, and where appropriate taken into account, in global change biology studies. It could be argued that an approach reliant on mean values is a “procedural error.” It produces an artefact, that is, a population's mean phenotype. While this may allow us to conduct relatively simple statistical analyses, it will not in all cases reflect, or take into account, the degree of (physiological) diversity present in natural populations. Not to Be Reproduced (La reproduction interdite): the mystic of individuality and the global change. The paradigm of individuality is central to modern biology. Darwin (1859) introduced the notion of variation as a prerequisite for natural selection to operate. Bennett states that “Real individuals are unique combination of traits, some above and some below average. It is time to recognize the uniqueness of the individual and to turn it to our advantage as biologists.” (Bennett, 1987). Inter‐individual variation in phenotypic traits is pervasive in life forms on Earth, and an individual is never the same as the n