Levels of Evidence in Studies of Competition, Predation, and Disease

The primary aim of a scientific investigation is to find the most likely model for a situation out of a host of alternative explanations. We usually start out with many ideas and end up with a few hypotheses but these are usually the ones that are most difficult to distinguish between. Our aim is to thin that down to one with no plausible alternatives. I am assuming that for this symposium our specific aim is to establish the existence and importance of one of the population processes (predation, competition, or disease) in influencing the numbers of a target species. I will review some of the various levels of evidence that are available to field ecologists faced with this kind of problem. Many sources of information can produce weak evidence at best, so I stress the role of statistical modelling and experimental design for efficiently distinguishing between biologically relevant hypotheses. In this context I discuss the concept of pseudoreplication and try to clear up some misconceptions that surround it.