Species interactions mediate climate change impacts in rivers and meadows: time scales of effects

Background/Question/Methods There is uncertainty about what level(s) of biological organization must be understood in order to interpret and forecast ecological responses to climate change.  Projections based on physiological climate envelopes assume species will respond to altered climate individualistically. For forecasting ecological response to climate change, we do need better understanding of the physiological tolerances of cryptic life history stages that are “stored” through unfavorable periods (annual grass seeds, basal cells of attached algae), and their potential to recover and spread during favorable periods.  Interactions with competitors, mutualists, and enemies alter performances of organisms during good times and bad, however.  Such interactions can complicate or even reverse ecosystem responses to climate change forecast from individualistic projections.

Results/Conclusions We discuss results from field experiments in a meadow and a river food web (studied 10 and 20 years, respectively).  These sites are in the Angelo Coast Range Reserve of Northern California (39^o44’18” N, 123^o37’48”W), presently under a Mediterranean winter rain summer drought climate.  Leading climate models forecast increased rainfall and possible extension of rain into the early part of the current dry season for this region.  In both the river and the meadow, inter-specific interactions reversed the response of the dominant space-holding autotrophs to climate-associated factors (changes in timing of precipitation and in flood-drought regimes) over outcomes predicted from individualistic responses.  In the river, flood scour detaches and removes most of the dominant macroalgal (Cladophora) biomass, but also eliminates large, invulnerable grazers, allowing new Cladophora to proliferate over the early low-flow season without strong grazer limitation.  Despite losses to scour, Cladophora accumulates higher biomass following floods than during drought, when predator resistant grazers abound.  In the meadow, late (spring) rainfall should favor native forbs and perennial grasses that continue their growth into the warm season over exotic annual grasses that stop growth in March, regardless of moisture.  Early in the study (year 1) native forbs and grasses did benefit from the experimental extension of spring rain.  But as these altered conditions persisted across years, species interactions overshadowed individualistic responses.  Because spring rainfall also stimulated nitrogen-fixing forbs, annual grasses proliferated to densities that smothered forbs and perennial grasses.  We offer the speculative hypothesis that individualistic responses may govern ecological response to climate change over very short (physiological) and very long (biogeographical) time scales, whereas community-level interactions exert strong effects over intermediate (behavioral to demographic) scales, affecting contemporary ecosystem services and resilience.