Drought-Resistance Is a Threshold Function of Habitat Size and Population Density in a Fish Metapopulation

The increased frequency of extreme droughts predicted under climate warming and increasing human water consumption seriously threatens freshwater organisms. Because smaller habitats dry more frequently and severely during droughts, habitat size (HS) may be a key determinant of population resistance to such change. Using Cormack-Jolly-Seber survival models, we investigated the relationship between HS and drought resistance in populations of forest-dwelling brown mudfish (Neochanna apoda), during a 1 in 70 year drought in New Zealand. We expected an HS threshold below which survival would decline rapidly, because of the interaction between increasing population density and physiological stress as HS shrinks. Drought survival depended on pool depth; populations in pools deeper than 230 mm were the most drought resistant, and survivorship declined steeply in shallower pools. Survivorship was driven by a strong density-dependent interaction with pool depth, supporting our threshold response hypothesis. Reduced drought resistance in smaller habitats suggests that habitat shrinkage caused by intensified human water consumption will minimise population resistance to climate change. Nevertheless, the habitat-size drought-resistance threshold we found may be useful for setting limits of allowable land-use change so that the combined threats of global warming and land-use change can be managed effectively.