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From individuals to populations : How intraspecific competition shapes thermal reaction norms

Environmental variations can lead to changes in life history trait distributions in natural popula- tions, and these changes can be difficult to predict because of the numerous interactions be- tween various ecological processes. Specifically, ambient temperature is known to influence life history traits in ectotherms, most of which follow the temperature-size rule : in cold environments individuals grow more slowly but reach a larger size than in warmer environments. These traits (growth rate and body size) can also be altered by density through intraspecific competition which can slow-down growth and affect adult sizes. In turn, the strength of intraspecific competition may itself be modulated by temperature.
We investigate here the joint effects of temperature and density dependence on growth rates and body lengths using two genetically distant strains of the Collembola Folsomia candida. We first study isolated individuals to describe the direct effects of temperature on growth rates and adult sizes over a large thermal range (6°-29°C). In parallel, we reared collembola populations under the same temperatures during one to two and a half years. We examined the structures and long-term dynamics of these populations to quantify the joint effects of temperature and intraspecific com- petition on these traits.
In populations, collembolans’ growth rates are slowed down and become almost insensitive to temperature variations. Asymptotic lengths are also negatively affected in populations. As op- posed to growth, the temperature-induced adult body lengths decrease is stronger in populations. Additionally, we show that these effects are modulated by genetic variation in life histories as different investment in growth or fecundity affect both the temperature response, the population response and their joint effect.
Our findings illustrate that, in populations, life history traits are plastically shaped by biotic and abiotic factors and that the complex phenotypic responses to these factors can vary according to the studied trait and to the genetic origin of the population.

Key-words : Body length, Collembola, Demography, Density-dependence, Growth Trajectory, Plasticity, Population Dynamic, Temperature Size Rule.

Publication :
François Mallard (ancien membre), Monique Avnaim1, Romain Péronnet, Thomas Tully.

Contact :
TULLY Thomas, Maître de conférence, SU.
iEES Paris (sous tutelle de Sorbonne université, CNRS, INRA, IRD, P7, UPEC)
Adresse : Institut d’Ecologie et des Sciences de l’Environnement de Paris, UMR 7618
Sorbonne Université
Tour 44 - 45 - 2e étage, 216
4, place Jussieu, 75005 PARIS
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