Edge

The edges of systems are where many interactions occur between species. 
In practice, many Permaculture strategies seek to increase the ratio of edge in relation to the rest of the system so that interaction is encouraged. 
In fact, a well designed permaculture may seem as though there are no edges at all, with each area blending smoothly into the next.
Planting in a mosaic or tiled pattern (tesselation) or nesting one species within a group of a different species (annidation) are two ways to increase edge harmonics. For example, tomatoes tend to grow well with borage, so annidation would work well with those two species.
Another strategy for increasing edge ratio overall is to create an undulating (wavy) edge on a pond or a garden. This increases the size of the zone in which dynamic interactions will take place between species.
In ponds, this type of edge can take the form of "chinampas," which are fingers of earth separated by water, and may be planted with appropriate species (perhaps some which require constant irrigation, or food plants for fish).