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.

Two is better than one: Coupling DNA metabarcoding and stable isotope analysis improves dietary characterizations for a riparian‐obligate, migratory songbird

“…Although the present study was limited to a single primer set (ZBJ; Zeale et al, 2011), we agree with the growing body of work recommending that researchers use multiple primer sets (Forsman et al, 2021), potentially targeting multiple genes (Alberdi et al, 2018), as such approaches tend to detect a greater number of prey taxa and provide more taxonomically precise prey characterizations. Nevertheless, as our DNA‐based analyses were focused on broad taxonomic classification (i.e., family or order level) and our results largely agree with past work employing both DNA‐based (Trevelline et al, 2016; Trevelline, Nuttle, Hoenig, et al, 2018; Trevelline, Nuttle, Porter, et al, 2018) and morphological methods (Eaton, 1958; though see Craig, 1987), we are confident that the present study provides an accurate taxonomic representation of the arthropod diet of nestling waterthrush.…”

“…Although we did not find a significant correlation between isotopic niche breadth and either Aquatic Diptera or Lepidoptera prey contribution, we did observe significant correlations for both the Ephemeroptera and Plecoptera and Terrestrial Diptera prey groups, in negative and positive directions, respectively. The observed shrinking of functional dietary niche breadth in response to increased Ephemeroptera and Plecoptera prey contribution and niche expansion in response to an increase in Terrestrial Diptera prey contribution provides further evidence that breeding waterthrush preferentially provision pollution‐intolerant aquatic prey to their young but widen their trophic niche when such prey are not readily available (Trevelline, Nuttle, Porter, et al, 2018).…”

“…In addition to the impacts that adults may incur, limitations in aquatic arthropod prey may also explain why nestlings raised on streams devoid of pollution-intolerant aquatic arthropods exhibited stunted physiological development (Mulvihill et al, 2008). Recent studies have shown that aquatic arthropod prey possess much higher concentrations of long-chain polyunsaturated fatty acids (Hixson et al, 2015), and that consumption of these aquatic-derived fatty acids are critical for the rapid development of nestling songbirds (Twining et al, 2016(Twining et al, , 2018. As prey composition represents an important facet of a species' life history with far-reaching consequences, our findings suggest an imperative need for researchers to study both the qualitative and the quantitative aspects of a species' diet to ensure our understanding of a species' trophic ecology is as complete as possible.…”

“…Waterthrush are riparian‐obligate, migratory wood‐warblers (family: Parulidae) which are hypothesized to specialize on pollution‐intolerant aquatic arthropod prey, (e.g., Ephemeroptera and Plecoptera) due to their preference for the high‐quality breeding territories that support these prey (Mulvihill et al, 2008) and the frequent detection of these taxa in their diets (Trevelline et al, 2016; Trevelline, Nuttle, Hoenig, et al, 2018; Trevelline, Nuttle, Porter, et al, 2018). However, recent DNA‐based evidence has found that terrestrial prey groups, such as Lepidoptera, are detected in nestling waterthrush diets as frequently as pollution‐intolerant, aquatic prey (Trevelline et al, 2016; Trevelline, Nuttle, Hoenig, et al, 2018; Trevelline, Nuttle, Porter, et al, 2018), causing some to question if waterthrush are more generalist than once believed (Bryant et al, 2020). Because deviations from a species' fundamental dietary niche can have lasting impacts on reproductive success and population stability (Narango et al, 2018), effective species conservation may be reliant upon a strong understanding of species' prey preferences—especially for avian insectivores, a group undergoing unprecedented population declines (Rosenberg et al, 2019), probably in response to equally drastic population declines of their arthropod prey (Hallmann et al, 2017).…”

Dietary DNA metabarcoding reveals seasonal trophic changes among three syntopic freshwater trout species

“…Seasonal fluctuations in prey availability contribute to the immense complexity within an ecosystem's food web and highlight the effects that prey variability has on trophic dynamics throughout the year. In temperate riparian forests, aquatic prey biomass reaches its peak in early spring as adult macroinvertebrates emerge from the stream to reproduce, thus providing energetic subsidies to terrestrial insectivores (Nakano & Murakami, 2001), such as breeding migratory songbirds (Macdade, Rodewald, & Hatch, 2011) and their offspring (Trevelline, Nuttle, Porter, et al., 2018). Throughout the later spring and summer months, when terrestrial invertebrate populations peak (Baxter et al., 2005), aquatic predators rely heavily on these prey taxa to supplement their diets (Johnson & Ringler, 1980).…”