Powerful fish in poor environments: Energetic trade-offs drive distribution and abundance in an extremophile forest-dwelling fish

Abstract

For many species, distribution and abundance is driven by a trade-off between abiotic and biotic stress tolerance (i.e. physical stress versus competition or predation stress). This trade- off may be caused by metabolic rate differences in species such that slow metabolic rates increase abiotic tolerance but decrease biotic tolerance. I investigated how metabolic rate differences were responsible for an abiotic-biotic tolerance trade-off in brown mudfish (Neochanna apoda) and banded kokopu (Galaxias fasciatus), that drives the allopatric distribution of these fish in podocarp swamp-forest pools. Brown mudfish and banded kokopu distribution across 65 forest pools in Saltwater forest, Westland National Park, New Zealand was almost completely allopatric. Mudfish were restricted to pools with extreme abiotic stress including hypoxia, acidity and droughts because of kokopu predation in benign pools. This meant the mudfish realised niche was only a small fraction of their large fundamental niche, which was the largest out of sixteen freshwater fish species surveyed in South Island West Coast habitats. Thus mudfish had a large fundamental to realised niche ratio because of strong physiological stress tolerance but poor biotic stress tolerance compared to other fish. A low metabolic capacity in mudfish compared to kokopu in terms of resting and maximum metabolic rates and aerobic scope explained the strong mudfish tolerance to extreme abiotic stress, but also their sensitivity to biotic stress by more powerful kokopu in benign pools, and hence their allopatric distribution with kokopu. Despite being restricted to extreme physical stress, mudfish populations were, in fact, more dense than those of kokopu, because of low individual mudfish resting metabolic rates, which would cause resources to be divided over more individuals. Distribution and abundance in mudfish and kokopu were therefore driven by an abiotic-biotic tolerance trade-off caused by a physiological trade-off between having slow or fast metabolic rates, respectively. The negative relationship between species resting metabolic rates and their tolerance to abiotic stress provides a way of estimating the impact of human induced environmental change that can either increase or decrease habitat harshness. Thus species with low metabolic rates, like mudfish, will be negatively affected by human induced environmental change that removes abiotic habitat stress and replaces it with benign conditions. My evidence shows that extreme stressors provide a protective habitat supporting high mudfish biomass with significant conservation value that should be maintained for the long-term persistence of mudfish populations.