Floodplain ecosystem dynamics under extreme dry and wet phases in semi-arid Australia

Ecological networks are a cornerstone of ecological theory, offering an integrated approach to understanding food webs and ecosystem dynamics required for restoration and conservation ecology. We investigated ecological network dynamics in a large floodplain undergoing extreme variation in water availability, with drought and subsequent flooding representing a resource pulse. We used structural equationmodels to quantify ecological network dynamics for the Lowbidgee floodplain (Australia), based on surveys over 5years while the floodplain transitioned from extremely dry (2009, 2010) to wet (2011) and post-wet (2013, 2014) conditions. We identified significant associations of species and trophic guilds with inundation at the site and floodplain scale, which allowed us to quantify the strength of biotic interactions within the network and the stability of interactions under differing patterns of resource availability. At the floodplain scale, most taxa responded strongly in distribution and abundance to the 2011 resource pulse, a widespread flood, but this response did not persist during subsequent years of moderate floods. In contrast, fish species, both native and exotic, responded strongly only in the post-wet period. At the fine spatial scale (i.e., sites), complex responses were observed, with only waterbirds, frogs and tadpoles positively associated with inundation, while fish species showed a range of associations with fine-scale inundation. Biotic interactions within sites, across all trophic guilds, were predominately overridden by inundation and water temperature, mediated by strong associations with aquatic vegetation. Stratifying the ecological network to dry, wet and post-wet periods highlighted varying associations of taxa with fine-scale inundation, generally responding synchronously to resource pulses, with relatively weak biotic interactions. Associations with site-scale inundation were strongest during the post-wet period for fishes and frogs. Only Litoria spp. (Hylidae) tadpoles, waterbirds and aquatic vegetation had positive associations with site-scale inundation during the dry period. We conclude that responses of trophic guilds are largely dependent on the way they interact with their environment at particular spatial and temporal scales. Our investigation of this ecological network reinforced the importance of hydrological drivers over biotic interactions, with clear implications for the management of environmental flows, particularly in systems recovering from long-term flow alteration. Management efforts should focus environmental flows to promote specialist species (e.g., southern bell frog, Litoria raniformis) and Murray hardyhead (Craterocephalus fluviatilis: Atherinidae), waterbirds and aquatic vegetation over the more generalist fish species that have established because of the loss of the natural flow regime.