Ecological surprises by an invasive species and environmental warming

Background/Question/Methods

Non-additive ecological responses to multiple stressors are pervasive and limit our ability to predict the cumulative effects of global change. For example, climate warming and invasive sportfish are concurrent stressors expected to generate ecological surprises in naturally fishless cold-water ecosystems.  If both stressors operate via the same size-selective mechanism favoring smaller species that are co-tolerant of predation and warming, then this redundancy should elicit antagonistic responses by prey communities.  However, net community responses to global change also likely depend on phenology, or the timing of exposure to various stressors.  Therefore, we hypothesized that although size-selective predation followed by warming dampens their net functional impact on prey communities, the reverse order of exposure generates a synergistic scenario in which warming first stimulates predation by invertebrates that are then subsequently suppressed by introduced planktivorous fish.  To test these hypotheses, we performed an 42-d outdoor mesocosm (1000 L) experiment during summer 2011 in which plankton communities from five fishless mountain lakes were each exposed to predation by non-native rainbow trout (F) and/or warming (W) in six treatment combinations: an unstressed control, two sequential exposures (F then W, W then F), two single-stressor exposures (F, W), and one simultaneous exposure (FW).

Results/Conclusions

Our preliminary results reveal that introduction of exotic fish and warming by an average of 3.6¢ªC synergistically increased final zooplankton biomass. Increased biomass under both stressors likely occurred because species that were tolerant of fish predation due to their small size and/or agility also have temperature-sensitive generation times that are greatly reduced with warming. Thus, warming likely enabled tolerant species to reproduce rapidly when released from competition and predation by larger invertebrates through size-selective fish predation. Moreover, warming enhanced the fertilizing effect of exotic fish on primary producers, thereby increasing potential food resource availability to these tolerant grazers.  However, net ecological effects of these two stressors did not differ substantially as a consequence of order of exposure. A potential explanation for the lack of importance of stressor order is the degree of asymmetry detected between the relatively overwhelming direct effects of predation and the weak direct influences of warming. These findings suggest that ecological surprises are more likely to occur in perturbed systems where equity exists among the individual effects of the different regional and local stressors.