Aquatic and terrestrial resources are not nutritionally reciprocal for consumers

Abstract

Aquatic and terrestrial ecosystems are connected through reciprocal fluxes of energy and nutrients that can subsidize consumers. Past research on reciprocal aquatic–terrestrial subsidies to consumers has generally focused on subsidy quantity while ignoring major differences in the nutritional composition of aquatic and terrestrial resources. Because aquatic resources contain substantially more highly unsaturated omega‐3 fatty acids (HUFAs) than terrestrial resources, aquatic subsidies may play a unique role by supplying these critical compounds to both aquatic and terrestrial consumers. Here, we first characterized nutritional quality in terms of HUFA content in aquatic and terrestrial insect prey. We then used bulk stable isotope analyses to estimate subsidy use by stream and riparian consumers coupled with compound‐specific stable isotope analyses, which allowed us to document consumer HUFA sources. Finally, in order to understand the nutritional importance of aquatic‐derived HUFAs for riparian consumers, we conducted manipulative diet experiments on Eastern Phoebe (Sayornis phoebe) chicks in the laboratory. Aquatic insects were significantly enriched in HUFAs, mainly in terms of eicosapentaenoic acid (EPA), compared with terrestrial insects. Stream fishes relied mainly upon aquatic resources, while insectivorous birds varied in their use of aquatic subsidies across sites. However, like stream fishes, Eastern Phoebe chicks received HUFAs from aquatic insects, even when they were heavily reliant upon terrestrial insects for their overall diet. In the laboratory, dietary HUFAs, such as those supplied by aquatic insects, increased the growth rate and condition of Eastern Phoebe chicks. This study demonstrates that aquatic and terrestrial subsidies are not nutritionally reciprocal from the perspective of consumers because aquatic resources are the main source of critical fatty acids for both stream and riparian consumers. It also confirms previous findings on the nutritional importance of HUFAs for riparian birds, demonstrating that an insectivorous riparian lifestyle influences avian nutritional needs. Finally, our findings raise the possibility that birds and other riparian insectivores may experience nutritional mismatches with terrestrial prey if they do not have access to high‐quality aquatic subsidies as a consequence of aquatic habitat degradation or shifts in consumer and resource phenology. A free Plain Language Summary can be found within the Supporting Information of this article.

Stream network geometry and the spatial influence of aquatic insect subsidies across the contiguous United States

“…Rarely have spatial subsidies been placed in a geographic context but as ecologists are tasked with addressing problems at increasingly large spatial scales, knowing where cross-ecosystems linkages are most important could inform management interventions and study designs to better understand local ecosystem functions (Loreau et al 2003, Turner and Chapin 2005, McCluney et al 2014. For example, the Lower Mississippi (VPU08) hydrologic region is relatively well dissected by streams and has a high number of confluences and narrow subbasins.…”

Nutritional hotspots? Prey from one ecosystem provide key fatty acids required for consumers in multiple ecosystems

“…Omega‐3 highly unsaturated fatty acids (HUFAs) are known to be highly important in consumer diets (Twining et al, ), and HUFAs have been shown to be more abundant in aquatic than terrestrial organisms (Hixson, Sharma, Kainz, Wacker, & Arts, ). Twining et al () analysed the relative abundance of the short‐chain precursor to HUFAs (ALA, alpha linolenic acid) versus HUFAs themselves within aquatic and terrestrial prey items, collected at three different study sites, and found that aquatic prey contained a much higher overall percentage of HUFAs, but not more ALA, compared to terrestrial prey. Thus, aquatic insect prey contained more HUFAs relative to terrestrial prey on a per capita basis at their study sites.…”

“…It makes sense that fish would get more HUFAs from aquatic prey if the quantity of prey ingested were the only important factor. But compound‐specific stable isotope analyses of four different fatty acids showed that both stream fish and riparian bird chicks relied on aquatic insects for HUFAs in their diets (Twining et al ). This demonstrates that, irrespective of the aquatic prey contribution to their overall diet, HUFAs came predominantly from aquatic insect prey.…”

“…Moreover, Eastern Phoebe chicks fed a high‐HUFA diet had mass growth rates 2–3 times greater than those fed a low‐HUFA diet. Body condition of chicks fed a high‐HUFA diet also increased more over time compared to that of chicks fed a low‐HUFA diet (Twining et al ).…”

“…Functional Ecology ALLEN over time compared to that of chicks fed a low-HUFA diet (Twining et al 2019).…”