Spatial Invasion Simulator

Model Description
Software
Case Study
Restoration Example
Run the Spatial Invasion Simulator Model

Model Description

The Spatial Invasion Simulator (SIS) is a spatially explicit, two-species lattice model. To explore alternative restoration and invasion scenarios, the two species are designated as Exotic and Native.

Individuals of each species occupy discrete cells on a square lattice. At each time step individuals have some probability of mortality (100% if they are annuals). Unoccupied cells are colonized by either species, depending on their dispersal mode, their abundance, and the positive soil feedback state generated by the previous cell occupant.

Dispersal can be either local (nearest 8 neighbors) or global (all individuals in the lattice can send propagules to all cells). Boundaries are absorbing and the lattice is updated synchronously.

Positive soil feedback decreases the probability of the other species establishment and survival. The strength of the feedback increases with time of occupancy of the species up to the maximum value.

The Model assumes the following:

The environment is homogenous (there is no variation in the geography, soil type, etc.) and that species are identical in their environmental requirements.
Net feedback is positive (or zero, i.e., positive feedback effects are greater than or equal to negative feedback effects).
There is no long-term seedbanking: all propagules come from the previous time step.

Alternative management scenario options include:

Solarizing the soil (resetting the initial soil feedback state).
Control of the exotic species (increasing exotic mortality).
Augmenting native seed production (decreasing exotic/native seed ratio).

Including an external source of exotic seeds (one edge of the lattice provides a persistent source of seeds).

Software

The model was written and developed C. Parameters are set in the web-form and the model is executed through a command-line call via Perl-CGI scripts. The source code is open source in accordance with the Open Source Initiative definition.

SIS source code

SIS compiled code

Perl script

The model outputs four text files (initial and final lattices, species abundance over time, and a log file). A script written in R (http://www.r-project.org) creates a PDF file (click here for example) containing plots of the output and the log information. Adobe Reader is required to view the file.

R plotting code

The code for the original prototype, developed in R, is also available. Note that lattices larger than 10x10, and longer iterations can be very slow to run.

Prototype R Model

Acknowledgments: We are very grateful to Margaret Royall for web design and to Harry Mangalam, University of California Irvine Network & Academic Computing Services for his help with the web interface development. This project was supported by a grant from the Center for Invasive Plant Managment (CIPM).

Disclaimer: this software is provided soley for educational use; the authors accept no responsibility for decisions based on model results. Restoration and invasive plant management are best performed within an ecologically-based scientific framework.