# Implement dispersal module for spatial ATN model

[SamHari1]

Summary

• Implemented density-independent (diffuse) and density-dependent (diffuse_density_dependent) dispersal functions on a 2D grid, based on Ryser et al. (2021)
• Wrote 5 behavioral tests comparing expected vs. observed dispersal dynamics, with output figures
• Added documentation, data needs, calibration formulas, and species trait data to support integration with the ATN

What was implemented

Functions (experiments/dispersal_experiments_design/functions/)

• diffuse.r — density-independent dispersal
• diffuse_density_dependent.r — density-dependent dispersal scaling with metabolism and net growth rate (Ryser formulation)
• run_diffusion_simulation.r — wrapper to run multi-step simulations
• enforce_boundary_conditions.r — reflective boundary handling

Tests (experiments/dispersal_experiments_design/tests/)

All tests simulate biomass spreading from a single central cell on a 51×51 grid over 50 time steps. The invasion front is tracked as the farthest cell above a threshold.

Test	Hypothesis	Outcome
01 — Sublinear dispersal	diffuse produces a sublinear distance-time relationship	Confirmed
02 — Equivalence	diffuse and diffuse_density_dependent agree when b=0 and max_disp_rate/2 = disp_rate	Confirmed
03 — Density-dependent faster	Density-dependent dispersal leads to faster initial spread	Same initial pace, but density-dependent shows sustained dispersal advantage — artifact of the threshold method for computing invasion front
04 — Metabolism comparison	High-metabolism species (hummingbird) disperses faster than low-metabolism species (black bear)	Confirmed
05 — Max dispersal comparison	Species with high a (0.8) disperse further than species with low a (0.2)	Confirmed

Documentation and data (experiments/dispersal_experiments_design/data/)

• a_calibration_formula.md — formulas to calibrate the maximum dispersal rate parameter a from species traits
• external_data_needs.md — required external data (species traits, allometric coefficients)
• SpeciesTraitsFull.csv — species trait dataset
• context/ryser2021_dispersal_summary.md — summary of the Ryser et al. (2021) dispersal model
• AGENTS.md — integration guidance for other teams

To review

• Review the 5 output figures in experiments/dispersal_experiments_design/figures/
• Check the test scripts match the hypotheses described in tests/README.md
• Confirm function signatures are compatible with the ATN integration interface described in experiments/dispersal_experiments_design/README.md