The role of historical contingency (year effects) in restoration and community ecology

Ambient conditions following a disturbance can affect the long-term species composition of the resultant plant community. In particular, interannual variations in climatic conditions may affect the developmental trajectory of a plant community. Year effects (the year of restoration implementation) are reported to have profound impacts on the success of many restoration projects, but have yet to be formally explored. More generally, although ecologists fully understand that ecological systems vary from year to year, they have not generally asked whether the results of their own experiments are influenced by year effects (the year of experimental manipulation). We summarize a review of several hundred experimental studies recently published in five leading ecology journals. Fewer than 5% of these initiated repeat experiments in more than one year. The great majority of the studies initiated in multiple years found significant year effects. More importantly, however, 83% of those studies that found significant year effects also found significant treatment by year interactions, in which experimental effects in one year disappeared or were even reversed when carried out in a different year. This is sobering news for field ecology, but also a strong impetus to initiate experiments in multiple years. We describe an experiment explicitly designed to investigate how year effects interact with manipulative treatments within a grassland restoration context.