Resilience of pond communities to extreme thermal regime shifts: an alpine–montane reciprocal transplant experiment

Abstract

Elevational gradients are ideal ecological venues for testing how communities respond to environmental changes associated with global warming. Recent warming rates have been shown to increase with elevation, hypothetically translating into heat waves that more adversely affect cold-adapted alpine communities than lower montane communities exposed to cold waves. We tested this hypothesis by conducting a replicated two-factor (source × elevation) experiment involving a reciprocal transplant of regional alpine and montane pond communities across two elevational sites (2345 m versus 1390 m asl). At each elevation, half of the mesocosms were inoculated with zooplankton and sedimentary egg-banks collected from alpine ponds while the other mesocosms were similarly seeded with zooplankton and sediment collected from montane ponds in 2016. After overwintering, the mesocosms were sampled for temperature and plankton during the ice-free periods of 2017 and 2018. The mesocosms at the low elevation were 8.0 ± 1.2 °C °C warmer than those at the high elevation. Elevation significantly affected total zooplankton biomass, whereas source effects were not significant. Elevation effects revealed that the temperature difference between sites affected the phenology of species within the assembled communities, but not their total biomass. Functional trait analysis revealed that the warmer environment selected for smaller body size and asexual reproduction as a warmer thermal regime stimulated several small parthenogenetic herbivores while suppressing larger, obligate sexual omnivores. Nevertheless, thermal regime shifts did not substantially affect the abundance-weighted mean functional identities of either transferred pond community, highlighting ponds as potential sources of functional insurance against increasingly extreme climate events.