According to Webster's Unabridged Dictionary, one definition of community is a group of plants or animals living together in the same environment. Environment includes not only the obvious elements like sun exposure and soil type, but 'all the conditions, circumstances, and influences surrounding, and affecting the development of (a group of plants).' A monoculture can therefore be a community, but not one that is rich with diversity and able to sustain itself over time. A single whim of nature or an incorrect application of the wrong chemicals can wipe out a monoculture in an instant. In most plant communities, there is some diversity that contributes to the character of the area and the long-term survival of something green. What is it, exactly, that causes some communities to be teeming with diversity and others to consist of only a handful of members? We tend to define artificially created communities by use: rock garden, water garden, prairie garden. This may in fact be a simple way to combine defining the needs of a particular plant or plant community with the outcome or our desired landscape. We refer also to the 'population' of a community, and its stability. Plant communities are formed over time based on the availability of the elements each member needs for survival and growth. The fringes of a community, like the burgeoning suburbs of a city, may be either a gradual or an abrupt transition to an entirely different population. On a drive through your community or its surroundings, you've probably happened across a landscape that is a distinct contrast to its surroundings. It might be as dramatic as the Grand Canyon or the Badlands, or as simple as a tree-lined riparian swale or a pothole prairie. These plant communities stand out because they are so different from their surroundings, and they were formed because the environmental conditions and circumstances are markedly different as well. Other communities blend subtly together, the transitions almost sensed rather than seen. A journey south or north through eastern Nebraska's rolling hills and fields filled with crops or, occasionally, prairie, gives way to the almost treeless plains of South Dakota or Kansas. The short-grass prairie becomes the mixed grass prairie, and the mixed grass prairie transitions to sand sage prairie. Within any community, one or more members will be dominant. Through the process of succession, even a severely disturbed environment will eventually become revegetated and reach stability, although the plants that make up the climax vegetation may not be particularly desirable (or even native) species. Some of the most aggressive and tenacious herbaceous plants to colonize on disturbed sites are not found on the lists of good gardeners-and may be found on the noxious weed list instead. Fireweed, Epilobium angustifolium, is one such colonizer in the west. The annual snow-on-the-mountain, Euphorbia marginata, occupies the most dismally overgrazed pastures and gravel edges of country roads. And, of course, the devilish dandelion, brought to the United States by settlers that missed its yellow flowers, is the bane of any landscape in which it gets a toehold.