Effects of estuarine acidification on predator–prey interactions

Short-term experiments exposing calcifying organisms to acidification have revealed altered growth and strength of their exoskeletons. We tested the hypothesis that multi-generational exposure to sustained estuarine acidification from runoff from acid sulphate soils (ASS) would: (1) reduce the shell strength of sessile or relatively immobile wild benthic invertebrates and (2) as a consequence render these invertebrates that rely on armour for anti-predator defence more susceptible to generalist benthic predators. First, we compared the force required to break the exoskeletons of Saccostrea glomerata, Bembicium auratum, and Heloecius cordiformis be tween replicate south-east Australian mangrove forests close to (acidified) and away from (reference) major ASS outflow drains. Second, we assessed differences in the susceptibility of oysters from acidified and reference forests to predation by the generalist muricid gastropod Morula marginalba. Mollusc shells were significantly weaker at ASS-affected than at reference sites, but the strength of crab carapaces was not influenced by acidification. Oysters from acidified sites were consumed by M. marginalba at a faster rate than oysters from reference sites in choice and no-choice experiments because M. marginalba required less time to drill through weaker shells. Many other predators such as crabs are generalist feeders that consume prey at rates inversely proportional to their shell strength. Hence, in the absence of effects of acidification on the ability of these predators to consume prey, molluscs at acidified sites may also be more susceptible to other such predators. This study highlights how human stressors can rapidly alter predator-prey interactions that have evolved over many years.