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

1. Most ectotherms follow the temperature-size rule (TSR): in cold environments individuals grow slowly but reach a large asymptotic length. Intraspecific competition can induce plastic changes of growth rate and asymptotic length and competition may itself be modulated by temperature.
2. Our aim is to disentangle the joint effects of temperature and intraspecific competition on growth rate and asymptotic length.
3. We used two distinct clonal lineages of the Collembola Folsomia candida, to describe thermal reaction norms of growth rate, asymptotic length and reproduction over 6 temperatures between 6°C and 29°C. In parallel, we measured the long-term size-structure and dynamics of populations reared under the same temperatures to measure growth rates and asymptotic lengths in populations and to quantify the joint effects of competition and temperature on these traits.
4. We show that intraspecific competition modulates the temperature-size rule. In dense populations there is a direct negative effect of temperature on asymptotic length, but there is no temperature dependence of the growth rate, the dominant factor regulating growth being competition. The two lineages responded differently to the joint effects of temperature and competition on growth and asymptotic size and these genetic differences have marked effects on population structure along our temperature gradient.
5. Our results reinforce the idea that the TSR response of ectotherms can be modulated by biotic and abiotic stressors when studied in non-optimal laboratory experiments. Untangling complex interactions between environment and demography will help to understand how size will respond to environmental change and how climate change may influence population size structure.
Methods
These datasets come from two experiments made in the laboratory. In the first one we have raised isolated collembola from two lineages (labelled TO and HA) from the species Folsomia candida from birth to death and have recorded their growth and reproduction. This has been done at six different temperatures. We have used the growth trajectories of each individual to measure its growth rate and its asymptotic body length.

In the second set of experiment we have raised populations of the same lineages from the same species at the same six temperatures. We have counted and measured the size of each individual every week or so to describe the dynamics of the population size structure over time.

From these dynamics we have measured the growth rate and asymptotic size of some cohorts and the average adult and juvenile densities at the time of these measurements.