The nature of species interactions shifts profoundly between time periods

Background/Question/Methods

Species interactions change through time, for example ontogenetically, successionally, and evolutionarily. They also change as environmental conditions change, both within years (seasonally) and between years (year effects).  The former are relatively well-studied, but the latter have received less attention.  For example, a review of ecological literature showed that only rarely are field experiments initiated repeatedly over multiple years, but when they are, more often than not the experimental effects are significantly different between years. In other words, initiating an experiment in a single year runs the risk of producing results that are more idiosyncratic than general, something that is better acknowledged for within-year studies.  In a world undergoing global change of various forms, these temporal effects are even more important to understand, and perhaps use as windows into an uncertain future. We summarize evidence of temporal effects from large-scale, long-term experiments in two very different ecosystems. Results from both systems and across multiple trophic levels indicate that temporal environmental variation can modify or even reverse the direction of species interactions.

Results/Conclusions

In the Kenya Long-term Exclosure Experiment (KLEE), we have shown that ecological relationships in several plant and animal systems shift between wet and dry seasons.  1) Wild ungulates compete with cattle in the dry season, but facilitate cattle in the wet season. 2) A bunchgrass, Pennisetum stramineum, suppresses its competitors in the wet season, but has facilitative effects in the dry season, and even then only in the presence of herbivory. 3) Herbivores facilitate plant diversity within the grassland understory, but only in wet periods following droughts.  In our Priority Effects and Year Effects in Ecological Restoration (PRYER) research in California, we are demonstrating that conditions in the year in which experiments are initiated can have profound effects on the results of these experiments. 1) There are distinct 'grass years' and 'forb years' in California grasslands. 2) In wet years native perennial grasses facilitate oak establishment (through suppression of exotic grasses), but in dry years with low exotic grass cover they compete with oak seedlings. 3) The strength of priority effects (arrival times of species) is different in different years, and these year effects differ across sites.