Complex interactive effects of pathogens and an herbicide on Pacific treefrog (Pseudacris regilla) hosts
The association between the fungal pathogen Batrachochytrium dendrobatidis (Bd) and amphibian population declines has sometimes been described as a classic case of a novel pathogen causing extreme effects in naïve host populations. Nevertheless, the disease caused by Bd, amphibian chytridiomycosis, is influenced by many environmental factors. Certain species of water molds (Phylum Oomycota) in the genera Saprolegnia and Achlya are common in moist soil and aquatic habitats and are pathogenic to amphibians. Yet they have not received much attention as possible cofactors contributing to chytridiomycosis. We tested the influence of the widely used, glyphosate-based herbicide Roundup and a water mold (Achlya sp.) on Bd infection. Roundup has direct negative effects on Bd as well as amphibians, so its effects on chytridiomycosis are difficult to predict. Moreover, Achlya, like other water molds, produces zoospores that could compete with Bd zoospores for space, but Achlya infection could weaken amphibian defenses against Bd. We employed a 3 × 2 × 2 fully factorial laboratory experiment that exposed Pacific treefrog (Pseudacris regilla) larvae to three ecologically realistic levels of field-formulated Roundup (0, 1, and 2 parts per million of active ingredient), two Achlya treatments (present and absent), and two Bd treatments (present and absent).
Roundup diminished survival of treefrog larvae, but survival was not affected by Achlya or Bd. Furthermore, no signs of Achlya infection were detected. Nevertheless, both Achlya and Roundup influenced Bd infection. Achlya alone elevated the proportion of Bd-exposed larvae infected with Bd, suggesting that Achlya acted as a synergistic cofactor that facilitated the establishment of Bd infection. However, there was an Achlya × Roundup interaction consistent with the Achlya effect decreasing in magnitude as Roundup concentration increased and disappearing at the highest Roundup concentration. In addition, Achlya was associated with higher average Bd load, although this relationship was not statistically significant. Roundup alone did not significantly influence the proportion of larvae infected with Bd, however, Roundup decreased the average Bd load among Bd-exposed larvae in a linear manner as Roundup concentration increased. Our study shows complex interactive effects of two pathogens and an abiotic stressor on amphibian hosts. Achlya and Roundup both interacted with Bd, but in different ways, and Roundup appeared to mediate the influence of Achlya on Bd. Based on our experiment, the first to demonstrate a water mold-Bd synergism, we propose further investigation of water molds as cofactors in diseases associated with amphibian population declines.