Dixon, P. M., Anthony R. Olsen, and B. Kahn. 1998. Measuring trends in ecological resources. Ecological Applications 8(2):225-227.
Questions about the existence or magnitude of trends in ecological resources are an important element of many current ecological and environmental issues. Some examples include: whether the number of lakes showing signs of stress from acidification is increasing, decreasing, or showing no change, whether amphibian or songbird populations are declining over large regions; and whether ecological communities are changing, as might be expected from global climate change. Good answers to these questions depend on using appropriate statistical designs to sample the environment and appropriate statistical techniques to estimate the trend. Although the concepts of trend and change appear simple, the ecological and statistical issues associated with good design and analysis can be quite complex. Part of the complexity is that "trend" is difficult to define precisely. Trend is often defined as long-term change in the mean, but what constitutes long term depends on the temporal scale of the study and the relevant ecological dynamics. It is also difficult to separate lot-term trends from other components of temporal variation, including multi-year cyclical variations, within-year seasonal variation, and erratic fluctuations. These components cannot be uniquely identified without using a model that precisely defines the temporal scale of each component. In practice, all four components may not be separated: instead, two or more (e.g., long-term trend and cyclical variation) are combined into a composite concept of annual temporal change.