Stream acidification and reduced aquatic prey availability are associated with dietary shifts in an obligate riparian Neotropical migratory songbird

Abstract

Streams and their surrounding riparian habitats are linked by reciprocal exchanges of insect prey essential to both aquatic and terrestrial consumers. Aquatic insects comprise a large proportion of total prey in riparian habitats and are opportunistically exploited by terrestrial insectivores; however, several species of songbirds are known to preferentially target aquatic prey via specialized foraging strategies. For these songbirds, reduced availability of aquatic insects via stream acidification may result in compensatory changes in provisioning during the nesting period, thereby influencing both adult and nestling diet composition. In this study, we used DNA metabarcoding to test the hypothesis that an obligate riparian Neotropical migratory songbird, the Louisiana Waterthrush (Parkesia motacilla), expands its diet to compensate for the loss of preferred aquatic prey taxa (primarily pollution-sensitive Ephemeroptera, Plecoptera, and Trichoptera) as a result of stream acidification. Our results revealed that both adult and nestling waterthrush exhibited an increase in dietary richness and niche breadth resulting from the consumption of terrestrial prey taxa in acidified riparian habitats. In contrast, compensatory dietary shifts were not observed in syntopic Neotropical migrant species known to primarily provision terrestrial prey taxa. In addition to providing support for our hypothesis that waterthrush compensate for stream acidification and aquatic prey limitations by expanding their diet, our findings highlight the vulnerability of Louisiana Waterthrush to anthropogenic disturbances that compromise stream quality or reduce the availability of pollution-sensitive aquatic insects.

Keywords: DNA metabarcoding; EPT; Louisiana Waterthrush; Macroinvertebrates; Next-generation sequencing; Resource subsidies.

Figure 1. Shifts in adult and nestling Louisiana Waterthrush (LOWA) diet in response to stream acidification.

(A) MOTU richness of adult (females, triangles; males, inverted triangles) and nestling (circles) diets increased significantly as mean territory pH declined ($X_{5,6}^2=10.80$; P = 0.001). Point shading indicates whether a fecal sample was collected from a territory with a pH ≤ (red) or > (blue) the median value of 6.68 (vertical dotted line). Gray shading represents the 95% confidence interval. (B) Unconstrained NMDS ordination (stress = 0.255) of adult (females, triangles; males, inverted triangles) and nestling (circles) diet composition at the MOTU level. Points represent the taxonomic composition of waterthrush diets and shading indicates that the individual occupied a territory with a pH ≤ (red) or > (blue) the median value of 6.68. Ellipses represent 95% confidence intervals (based on standard error) for group centroids and minimum convex polygons indicate the extent of dietary niche space for each group.

Figure 2. Shifts in adult and nestling Louisiana Waterthrush (LOWA) diet in response to reduced availability of EPT taxa during the period of nestling care.

(A) MOTU richness of adult (females, triangles; males, inverted triangles) and nestling (circles) diets increased significantly as percent EPT declined ($X_{4,5}^2=4.97$; P = 0.026). Point shading indicates whether a fecal sample was collected from a territory with a percent EPT ≤ (red) or > (blue) the median value of 17.7 (vertical dotted line). Gray shading represents the 95% confidence interval. (B) Unconstrained NMDS ordination (stress = 0.260) of adult (females, triangles; males, inverted triangles) and nestling (circles) diet composition at the MOTU level. Points represent the taxonomic composition of individual diets and shading indicates that the individual occupied a territory with a percent EPT ≤ (red) or > (blue) the median value of 17.7. Ellipses represent 95% confidence intervals (based on standard error) for group centroids and minimum convex polygons indicate the extent of dietary niche space for each group.

Figure 3. Effect of EPT availability on the occurrence of identified arthropod MOTUs (summarized by family) in the diets of adult and nestling Louisiana Waterthrush nestlings.

Only families with significant (P ≤ 0.05) increases or decreases in probability are reported. Short bars represent +/ −1 SE; long bars represent +/ −95% confidence intervals.