Community Gradients and Boundaries
It is often difficult or impossible to determine where one community ends and the next begins. Many communities, in fact, grade continuously into each other with no sharp boundaries. For example, if two forests, pine forest and spruce forest are nearby, one cannot see the boundaries between them. But if one moves from one end of the pine forest to the other end of spruce forest, one can observe difference in species composition between the two; yet one cannot demarcate a sharp boundary between these two forest communities.
There are, however, instances where sharp boundaries between the communities are seen, especially where the physical environment changes abruptly - for example, at the transition between aquatic and terrestrial habitats between distinct soil types, or between north-facing and south-facing slopes of a mountain.