Intraspecific interactions can occur through many ways but the mechanisms can be broadly categorized as food exploitation and interference interactions. Identifying how intraspecific interactions impact life history is crucial to accurately predict how population density and structure influence dynamics. However, disentangling the effects of interference interactions from exploitation using experiments, is challenging for most biological systems.

Here we propose an approach that combines experiments with modeling to infer the pathways of intraspecific interactions in a system. First, a consumer-resource model is built without intraspecific interactions. Then, the model is parameterized by fitting it to life-history data from a first experiment in which food abundance was varied. Next, hypothesized scenarios of intraspecific interactions are incorporated into the model which is then used to predict life histories with increasing competitor density. Lastly, model predictions are compared against data from a second experiment which raised groups of competitors of different densities. This comparison allows us to infer the role of interference and exploitation in shaping life history.

We demonstrated the approach using the smaller tea tortrix Adoxophyes honmai across a range of temperature. We investigated five scenarios of interactions that included exploitation and three pathways for interference through some effects either on energetics to represent changes in ingestion or activity, or on mortality to model deadly interactions, or on mortality and ingestion to model cannibalism.

Overall, intraspecific interactions in tea tortrix are best explained by a high level of deadly interactions along with some level of interference that acts on energy such as escaping and blocking access to food. Deadly interactions increase with temperature while interference that acts on energy is strongest close to the optimal temperature for reproduction. Interestingly, exploitation is more important than interference at low competitor density.

The combination of mathematical modeling and experimentation allowed us to mechanistically characterize the intraspecific interactions in tea tortrix in a way that is readily incorporated into population-level mathematical models. The primary value of this approach, however, is that it can be applied to a much wider range of taxa than is possible with pure experimental approaches.

We designed an approach to infer the most likely pathways of intraspecific interactions that shape life histories in a studied system. The approach is in four steps that weave together theory and experiments. We demonstrated the approach with the smaller tea tortrix moth (Adoxophies honmai).

Step 1. Build base model

We first built the base model which is the baseline for the theoretical framework used later to predict how different pathways of intraspecific interactions influence life histories. The base model is a consumer-resource cohort model that assumes no intraspecific interactions – no food exploitation and no interference interactions. As such, the base model describes solely how vital rates are impacted by changes in food abundance.

Step 2. Parameterize base model (provided R script: Step2.r)

Most model parameters can be directly estimated from independent data but a few remained unknown. Unknown parameters were estimated by fitting the base model to the observed life-history traits in the food experiment (FoodExperiment.csv). The food experiment raised individuals in the absence of intraspecific interactions and exposed them to a wide range of food abundance.

Step 3. Incorporate intraspecific interactions in base model to predict their effects on life histories (provided R script: Step3.r)

In this step, the parameterized base model was modified to incorporate several hypothesized scenarios of intraspecific interactions. For each scenario, we predicted how intraspecific interactions impact life-history traits and stage-structure distributions for groups of competitors. 

Step 4. Test model predictions using experiment 

To evaluate the support for each hypothesis, we compared model predictions with data from the competition experiment (CompetitionExperiment.csv). The competition experiment measured the impact of intraspecific interactions (i.e. competitor density) on life-history traits and on the stage-structure of groups of competitors. The comparison of life-history data from experiment with model predictions allowed to infer the role of interference interactions and the one of food exploitation in shaping life histories, as well as the functional dependencies for interference interactions in the studied system.

Please refer to ReadMe file.