Another complication is that environmental conditions across an ecoregion boundary may change very gradually, e.g. the prairie-forest transition in the midwestern United States, making it difficult to identify an exact dividing boundary.
另一个问题是，某个生态区边界的环境状况的变化可能会极为缓慢，例如在美国中西部地区的森林-草原过渡使其很难确定一个具体的边界。