Food-web interactions in extreme environments could provide useful insights into processes structuring communities because of the intense and severe pressures, both biotic and abiotic, at work. Despite the challenges, extremophile organisms often achieve very high abundance in such environments to the point where inverted Eltonian biomass pyramids form where standing biomass at upper trophic levels is much higher than that at lower levels. Such trophic arrangement, despite being potentially unstable, could be maintained by a combination of allochthonous energy subsidies and strong trophic interactions like cannibalism driven by resource shortages. We studied these possibilities in aquatic communities living in tree tip-up pools in the temperate rainforest of South Westland, New Zealand. These brown-water pools formed in the depressions of fallen trees, regularly have low pH, low dissolved oxygen, can dry for weeks at a time, and because of deep shade, have low primary production. Despite these severe conditions, predators in the pools, including Odonata and mudfish (Neochanna apoda), can be super abundant (e.g., 200 mudfish/m3). We measured trophic structure with size spectra, and quantified resource fluxes and standing biomass in tip-up pools (n=19) spanning a gradient in environmental extremes driven by pool drying.
Results/Conclusions
Size-spectra of aquatic organisms in the pools revealed highly top-heavy food webs characteristic of inverted biomass pyramids. The abundance of the aquatic top-predator, mudfish, was closely related to pool surface area to volume ratio, such that pools with more surface area contained more mudfish. Moreover, the abundance of aquatic primary consumers was very low. These findings all point to allochthonous resource subsidies, both leaves and invertebrates falling from the forest above, as supplying energy to the food web. Indeed fluxes of terrestrial invertebrates to pools can be high at some times and are a known food of mudfish. However, food webs were most top-heavy in pools experiencing the most extreme environmental conditions and in pools with mudfish compared to pools lacking mudfish. Cannibalism among mudfish and odonates and reciprocal intraguild predation, including odonates feeding on mudfish fry as well as mudfish feeding on odonates is probably common in pools. We predict that extreme conditions likely enhanced food-web inversion by increasing the strength of these interactions. Thus, although allochthonous energy makes inverted biomass pyramids possible, extreme environmental conditions driving strong biotic interactions may accentuate that trophic structure.