19: Interaction Effects (Effect Modification)

Author

Affiliation

Smit, A. J.

University of the Western Cape

1 Introduction

Following the conceptual introduction in Chapter 17, this chapter provides a deeper dive into interaction effects. Interactions are fundamental to ecology, as the effect of one environmental factor often depends on another. Here, we focus on how to correctly specify, interpret, and visualize interaction terms in a regression model.

2 Key Concepts

• Interpreting Interaction Terms: Moving beyond the “p-value” of an interaction to understand what the coefficient of an interaction term actually means.
• Conditional Effects: Understanding that when an interaction is present, the effect of one predictor is conditional on the value of the other. Main effects can no longer be interpreted in isolation.
• Biological Justification: Emphasizing that interactions should be included in a model based on ecological theory (e.g., the interactive effects of temperature and nutrient availability on growth).
• Plotting Interactions: Visualizing conditional effects is the most effective way to understand and communicate what an interaction means.

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