In addition to posing questions that are relevant to resource management and that investigate mechanisms, scientists should be concerned with the development of research questions that are tractable. Asking why certain species are present at a particular place and time forces the investigator to rely on correlation. In contrast, asking why species are not present (e.g., in locations that appear suitable) forces the investigator to search for constraints, and therefore mechanisms (e.g., DeStefano and
McCloskey 1997). Although Harper (1977, 1982) presented a compelling case for tractable, mechanistic research focused on applied ecological issues two decades ago, the underwhelming response by ecologists indicates that his message bears repeating.