Revisiting Southern Toad ( Anaxyrus Terrestris ) Populations in Constructed Wetland Treatment Systems: Copper Tolerance or Potential Traps?

Copper (Cu) is an essential trace element but is often anthropogenically elevated to toxic concentrations, requiring organisms to adapt or perish. Wetland treatment systems (WTS) exemplify these conditions and are frequently colonized by amphibians. Previously, an acute Cu exposure study with southern toad (Anaxyrus terrestris) larvae from a WTS called A-01 exhibited high survival compared to reference populations, suggesting Cu tolerance. Conversely, embryos from another WTS called H-02 showed reduced survival in Cu treatments compared to reference populations, suggesting Cu intolerance. We conducted a chronic exposure trial with southern toads from A-01, H-02, and two reference wetlands 24 and 17 years since colonization of A-01 and H-02, respectively. We used nominal Cu concentrations of 0, 10, and 15 µg/L to assess survival, time-to and size-at metamorphosis throughout the embryonic and larval stage. Embryo survival did not differ by population, though the 15 µg/L treatment significantly reduced survival. However, in the 15 µg/L Cu treatment, the H-02 larval mortality risk was double that of the reference population (HR = 1.98; 95% CI [1.01-3.9], p = 0.047) and the odds of mortality were 3.3 times higher (Odds Ratio = 3.29, 95% CI [1.29-8.39], p = 0.013). In contrast, A-01 larval mortality did not diverge from the reference population. Our results indicate that 1) there is no evidence for Cu tolerance in either WTS population 2) the experimental approach for tolerance testing may impact outcomes, and 3) WTSs may have the potential to become ecological traps - an important consideration for amphibian conservation.