Body size decline is predicted to be one of the major responses to rising temperatures, however, the ecological consequences of such body size change are not well understood. Studies often expose organisms to elevated temperatures over relatively short periods of time to gauge responses. However, this ignores the possibility of thermal adaptation which may modulate any effect of body size change, even over contemporary time scales. We conducted a mesocosm experiment in which we examined the ecological effects of ‘large’ and ‘small’ body size distributions of two thermally divergent populations. We used Gambusia affinis (mosquitofish), a globally invasive freshwater fish, to test whether individuals from ‘hot’ and ‘cool’ populations had different ecological effects at both body sizes. We measured a suite of ecological response variables ranging from zooplankton community composition to greenhouse gas emissions over one month. We aimed to 1) understand the ecological effect of shifting body size and 2) to understand whether thermal adaptation alters the ecological role of body size.
Results/Conclusions
Ecological responses to different body size distributions varied in directionality among parameters, with some unresponsive to body size change, some showing divergent responses to body size change between populations, and some showing similar response to size in both populations. For example, zooplankton biomass was unresponsive to body size change but was responsive to source population, and both zooplankton size and primary production declined with increasing body size. This decrease in primary production in ‘large’ treatments was reflected in carbon dioxide emissions from the mesocosms, where treatments with larger individuals had lower mass carbon dioxide efflux overnight. Overall, our data suggest that increasing temperatures and decreasing body sizes are likely to alter ecosystem function.