Apparent Maladaptive Oviposition Site Choice of Cope's Gray Treefrogs (Hyla chrysoscelis) When Offered an Array of Pond Conditions
The ability of organisms to respond adaptively to anthropogenic environmental change is behaviorally mediated, and recent studies indicate that anthropogenic acidification impairs behavioral responses by impacting olfactory abilities of aquatic organisms. The effect on behavior of other stressors, such as plant secondary compounds, in concert with low pH, has not been investigated. In this study we sought to a) determine whether the oviposition site choices of adult female Cope's Gray Treefrogs (Hyla chrysoscelis) correspond with the pH and tannin conditions that maximize tadpole survival and performance in the laboratory and b) investigate the impacts of mildly acidic conditions, with and without the added stress of tannins, on the survival, development, and antipredator behavior of this frog's tadpoles. We conducted a field oviposition experiment to determine adult female site choice and reared tadpoles in acidic and tannic conditions to investigate survival and antipredator behaviors. Female oviposition site choice did not correspond with conditions that maximize offspring survival. Acidity did not reduce embryonic (pH = 4.5, 5.5) or larval (pH = 5.5) survival. Tadpole mortality was highest in tannic treatment, yet this treatment received the second most eggs in the oviposition experiment. Some aspects of tadpole antipredator behavior in mildly acidic conditions suggested impaired predator recognition, though this difference was not statistically significant. Tannic conditions appear to have the greatest negative effect on tadpole fitness, and adult females appear to respond maladaptively when offered pools with a tannin concentration likely to be created by some invasive exotic wetland plants.
Mean (±SE) number of eggs laid in mesocosm pools (300 L) by Cope's Gray Treefrogs (Hyla chrysoscelis) was affected by water treatment (P = 0.007). Significantly more eggs were oviposited per night in the combined acidic and tannic (AT, n = 5) treatment than in the control (C, pH ∼7.0, n = 10) and acidic (A, pH = 5.5, n = 5) treatments, but not more than in the tannic treatment (T, tannic acid = 11 mg/L added at outset, n = 5). Columns with dissimilar letters above them are significantly different. Pools were monitored for 30 nights during the start of the breeding season (May–June 2017). For ease of interpretation the untransformed dependent variable values are shown.
Hatching success (%±SE) of Cope's Gray Treefrog (Hyla chrysoscelis) eggs reared in acidic (A, pH = 5.5, n = 4) and heavily acidic (HA, pH = 4.5, n = 4) water conditions did not differ from eggs reared in the control conditions (C, pH ∼7.0, n = 4; P = 0.48) in the laboratory. Each 8 L treatment aquarium contained 50 eggs collected from untreated pools at the University of Mississippi Field Station in June of 2016. pH was adjusted daily as necessary. Eggs that did not hatch after seven days were considered non-viable.
Survivorship curves for Cope's Gray Treefrog (Hyla chrysoscelis) tadpoles reared in the laboratory for 44 days in the following treatments: control (C, pH ∼7.0, n = 3), acidic (A, pH = 5.5, n = 3), tannic (T, n = 3), and acidic and tannic combined (AT, n = 3), each beginning with 25 eggs collected from untreated pools at the University of Mississippi Field Station in July of 2017. Daily survival probability drops suddenly for tadpoles in the tannic treatment starting on day 17 (P < 0.001). pH was corrected daily as needed. Tannic acid concentration was not measured over the course of the experiment, but was refreshed weekly (11 mg/L added at outset, followed by weekly alternations of a 50% water change with 11 mg/L tannic acid and a 100% water change with 11 mg/L).
(A) Mean (±SE) activity (time in motion) of Hyla chrysoscelis tadpoles was not significantly affected by the presence of water-borne cues from a common dragonfly predator (Anax junius; dark bars: cues absent; light bars: cues present) that had been fed tadpoles, although less reduction in activity did occur in all non-control treatments as predicted. Tadpoles were reared under control treatment (pH ∼7.0) water conditions and then observed for antipredator responses under the following water conditions: control (C, pH ∼7.0, n = 12), acidic (A, pH = 5.5, n = 12), tannic (T, 11 mg/L, n = 12), and acidic and tannic combined (A&T, n = 12). (B) Mean (±SE) activity (time in motion) of Hyla chrysoscelis tadpoles was significantly decreased in the presence of water-borne cues from a common fish predator (Notemigonus crysoleucas; dark bars: cues absent; light bars: cues present; P = 0.007) that had been fed tadpoles but was not affected by acid or tannin treatments. Tadpoles were observed for antipredator behavior under the same water treatment conditions as their rearing treatment, as follows: control (C, pH ∼7.0, n = 20), acidic (A, pH = 5.5, n = 20), and acidic and tannic combined (A&T, n = 14). In both experiments, tadpoles varied in Gosner stages from 28–35 at the time of testing. Behavioral trial duration was 20 min, and each individual was tested only once.
Contributor Notes
Associate Editor: J. D. Litzgus.