Blinded by the stink: Eutrophication impairs the anti-predator responses of freshwater gastropods

Animal phenotypes often depend on predation risk, and these trait shifts have important ecological consequences. Because adaptive trait shifts require reliable cues for predation risk, environmental factors which constrain sensory perception will have a large influence on ecological systems. We investigated how the ability of freshwater gastropods to detect predators depends on ecosystem productivity. Snails, like most aquatic animals, use waterborne chemical signals to detect and avoid predators. We hypothesized that predator recognition would be impaired at high pH because the structure of the amino acids used to detect predators is pH dependent. Eutrophication of ponds and lakes is associated with high pH, as nutrients additions stimulate plant productivity, which in turn depletes bicarbonate.

Two experiments were conducted in which we evaluated how perception of risk depends on pH. One study, conducted in outdoor mesocosms, manipulated ecosystem productivity with nutrient additions, and midafternoon pH ranged from 8.5 to 9.8. In behavioral assays, controlling for food availability, snails showed strong avoidance of predators at a pH < 9.0 but no avoidance at higher pH. In another study, conducted indoors, we manipulated pH directly with buffers. Again, snails showed strong avoidance at low pH (8.3), but no avoidance at high pH (9.8). Thus, in eutrophic conditions, the ability of snails to detect predators is severely impaired. Given the diversity of freshwater animals that depend upon perception of chemical cues, it is likely that eutrophication affects the sensory abilities of many other freshwater taxa. This discovery is of considerable applied interest, as it represents an important but unknown consequence of nutrient pollution. It is also of large interest to ecologists in general, many of whom have expended much effort in recent years studying chemically-mediated behavior without any appreciation of the contingency of such behavior on environmental conditions.