Spatial models simulate processes that operate over an extended area rather than at one particular point. They are made up of a mosaic of unit models, called cells. A single cell can simulate just about anything in an ecosystem, from an individual tree to an entire forest type. "When you take a bunch of cell models and connect them together in a grid that covers some extended area, we call that a spatial model," Maxwell says.
Whether forest or fen, each spatial model cell contains the ensemble of processes that combine to form an ecosystem. To model these components, the University of Maryland researchers use a program called STELLA. The program makes modeling as intuitive as dropping icons onto an electronic desktop and drawing lines between them. (Although once the icons are linked, equations have to be installed in them to simulate ecosystem functions.) STELLA is no longer the only graphical programming tool of its kind, but it was the first and is still a favorite of many ecologists, Maxwell says.
STELLA models can simulate photosynthesis, soil chemistry, or any other process operating in a particular kind of landscape. But in isolation, they are like symphony musicians playing in individual sound-proof rooms. The musicians may all be virtuosos, but they can't play in harmony. To behave like an ecosystem, the STELLA models have to be able to "hear" each other.
That's
where the SME comes in. It provides a graphical setting for linking
STELLA models into an integrated cell -- that is, a functioning patch
of landscape. Each cell covers a small fraction of the total landscape.
A spatial model of a forest would include one cell type to represent
tree-covered ground; running in the background of the cell would be an
ensemble of STELLA models for the processes at work in a forest. Other
cell types might represent patches of open meadow or a lake. The cells
are linked together within the SME to create an entire functioning
landscape.
The SME also includes tools for visualizing and analyzing data from the model. This makes it possible to study the effects of changing a particular aspect of an ecosystem. For instance, researchers at the University of Maryland and the South Florida Water Management Agency have used the SME to construct a spatial model of the Florida Everglades. As the vast project to restore the 'Glades to better health progresses, researchers will be able to conduct "what if" experiments on the flow of water through the ecosystem -- for instance, determining what will happen if a dam or canal is removed.
An added boon to modelers is the fact that the SME archives its STELLA cells in a standard format called the Simulation Module Markup Language, which lets other modelers working with SME to draw on previous work. "You'll have this palette of components that you can choose from and plug in," Maxwell says. "You won't have to start from scratch every time. You'll have all these tools to work with that other ecologists have created for other ecosystems."
