Inferring the relative importance of navigation and landscape effects for migratory populations using agent-based models

Seasonal migration is a vital behavior for numerous animal species, offering ecological services such as seed dispersal, pollination, and resource influxes. However, accurately describing and predicting the spatial dynamics of migrating populations remains a challenge due to the scale of the behavior and the different navigation strategies employed by the animals to complete it. The goal of this study is two-fold: (1) to develop an agent-based model (ABM) to simulate the final spatial distributions of migratory animals using two navigation strategies—true navigation and vector navigation—and (2) to use a case study of migratory monarch butterflies to determine the value of comparing model-generated spatial patterns with empirical observations. True navigators are hypothesized to use a combination of location and directional information to reach specific destinations, while vector navigators follow consistent directions throughout their journey, without altering these directions with respect to their current location. Therefore, it is logical to assume that true navigators would be more accurate in reaching specific destinations than vector navigators. Our model was designed to test this assumption by investigating the spatial distribution of migrants traveling across landscapes characterized by water bodies and elevational gradients. While true navigators were indeed more likely to reach their destinations than vector navigators on homogeneous landscapes, our model revealed that heterogeneous landscapes can successfully channel vector navigators to their destinations. Applying the model to monarch butterflies, we found that simulated distributions based on vector navigation closely matched observed overwintering locations. This not only demonstrates the utility of our model in generating spatial patterns that can be compared to empirical data, but also reveals that vector navigation across a heterogeneous landscape is a plausible strategy that would allow western monarch butterflies to reach their overwintering sites. The analysis also highlighted limitations in the observed data, such as biases introduced by preferential sampling, emphasizing the need for improved data collection methodologies. This study demonstrates the utility of ABMs in linking hypothesized navigation processes to observed migratory patterns, offering a framework for investigating and distinguishing navigation strategies in varying migratory species.