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1. Subsidies of long‐chain polyunsaturated fatty acids from aquatic to terrestrial environments via amphibian emergence

   https://doi.org/10.1111/fwb.13266  

   Fritz, Kelley A. ; Whiles, Matt R. ; Trushenski, Jesse T. ( March 2019 , Freshwater Biology)

   Resource subsidies across aquatic‐terrestrial boundaries can alter predator distribution and biomass and elicit trophic cascades. Most studies have focused on the size of cross‐boundary fluxes, but the impact of a subsidy is also mediated by quality and relative abundance of similar resources in the recipient habitat.

   Long‐chain polyunsaturated fatty acids (LC‐PUFAs) are necessary for proper physiological function, are unevenly distributed across the landscape, and animals differ greatly in their ability to synthesize them from biochemical precursors, which creates the potential for limitation and increases their possible importance as a subsidy.

   We examined whole‐body LC‐PUFA content and export in eight species of emerging amphibian metamorphs across eight temporary ponds in a wetland complex. We found that whole‐body content and export of LC‐PUFAs varied across species, but were generally within the ranges of other amphibian studies and several freshwater fish and aquatic insects.

   Anurans exported higher amounts of LC‐PUFAs than salamanders, largely due to the higher emergence biomass of anurans. As such, the export of LC‐PUFAs closely mirrored the biomass export for each species.

   Larger ponds exported higher amounts of eicosapentaenoic acid and docosahexaenoic acid, but some smaller ponds exported more per unit wetted area, indicating the potential importance of small ponds at a landscape scale. Given their vital physiological roles, the uneven distribution of aquatic‐origin LC‐PUFAs could have far reaching effects on terrestrial predators.

    

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2. Aquatic and terrestrial resources are not nutritionally reciprocal for consumers

   https://doi.org/10.1111/1365-2435.13401  

   Twining, Cornelia W. ; Brenna, J. Thomas ; Lawrence, Peter ; Winkler, David W. ; Flecker, Alexander S. ; Hairston, Jr., Nelson G. ; Allen, ed., Daniel C. ( July 2019 , Functional Ecology)

   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 freePlain Language Summarycan be found within the Supporting Information of this article.

    

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3. Amphibian‐mediated nutrient fluxes across aquatic–terrestrial boundaries of temporary wetlands

   https://doi.org/10.1111/fwb.13130  

   Fritz, Kelley A. ; Whiles, Matt R. ( June 2018 , Freshwater Biology)

   Resource subsidies of energy and nutrients can be transported via physical forces, such as gravity, wind or water and biotic processes, such as animal migration or emigration. Migratory transport of nutrient subsidies may be associated with reproductive processes, such as the deposition of eggs or emigration of juveniles to adult habitats.

   We quantified net nutrient fluxes associated with amphibian migrations across aquatic–terrestrial boundaries of eight temporary ponds in Illinois,USA. We measured carbon (C), nitrogen (N) and phosphorus (P) subsidies from terrestrial environments to ponds, in the form of eggs, and the reciprocal subsidies from ponds to terrestrial environments via juvenile emigration.

   Juvenile emergence biomass did not follow patterns of egg biomass, as not all species bred successfully in each pond, resulting in variability in the magnitude of nutrient fluxes both across ponds and species.

   The terrestrial environment was not always the recipient system of net nutrient fluxes. Hydroperiod, trophic interactions and species composition explained some dynamics of N and P subsidies.

   Anthropogenic alterations that affect amphibian communities, such as habitat fragmentation and conversion for agriculture and urbanisation, could have large‐scale impacts on nutrient fluxes and connectivity of temporary pond–forest systems.

    

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4. Influence of fishes on macroinvertebrate communities and insect emergence production in intermittent stream refuges

   https://doi.org/10.1111/fwb.13509  

   Bonjour, Sophia M. ; Whiles, Matt R. ; Gido, Keith B. ( April 2020 , Freshwater Biology)

   Drying intermittent stream networks often have permanent water refuges that are important for recolonisation. These habitats may be hotspots for interactions between fishes and invertebrates as they become isolated, but densities and diversity of fishes in these refuges can be highly variable across time and space.

   Insect emergence from streams provides energy and nutrient subsidies to riparian habitats. The magnitude of such subsidies may be influenced by in‐stream predators such as fishes.

   We examined whether benthic macroinvertebrate communities, emerging adult insects, and algal biomass in permanent grassland stream pools differed among sites with naturally varying densities of fishes. We also manipulated fish densities in a mesocosm experiment to address how fishes might affect colonisation during recovery from hydrologic disturbance.

   Fish biomass had a negative impact on invertebrate abundance, but not biomass or taxa richness, in natural pools. Total fish biomass was not correlated with total insect emergence in natural pools, but orangethroat darter (Etheostoma spectabile) biomass was inversely correlated with emerging Chironomidae biomass and individual midge body size. The interaction in our models between predatory fish biomass and date suggested that fishes may also delay insect emergence from natural pools, altering the timing of aquatic–terrestrial subsidies.

   There was an increase over time in algal biomass (chlorophyll‐a) in mesocosms, but this did not differ among fish density treatments. Regardless, fish presence in mesocosms reduced the abundance of colonising insects and total invertebrate biomass. Mesocosm invertebrate communities in treatments without fishes were characterised by more Chironomidae, Culicidae, and Corduliidae.

   Results suggest that fishes influence invertebrates in habitats that represent important refuges during hydrologic disturbance, hot spots for subsidy exports to riparian food webs, and source areas for colonists during recovery from hydrologic disturbance. Fish effects in these systems include decreasing invertebrate abundance, shifting community structure, and altering patterns of invertebrate emergence and colonisation.

    

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5. Trait–environment relationships could alter the spatial and temporal characteristics of aquatic insect subsidies at the macrospatial scale

   https://doi.org/10.1111/ecog.05298  

   Kopp, Darin ; Allen, Daniel ( December 2020 , Ecography)

   null (Ed.)

   Ecological flows across ecosystem boundaries are typically studied at spatial scales that limit our understanding of broad geographical patterns in ecosystem linkages. Aquatic insects that metamorphose into terrestrial adults are important resource subsidies for terrestrial ecosystems. Traits related to their development and dispersal should determine their availability to terrestrial consumers. Here, we synthesize geospatial, aquatic biomonitoring and biological traits data to quantify the relative importance of several environmental gradients on the potential spatial and temporal characteristics of aquatic insect subsidies across the contiguous United States. We found the trait composition of benthic macroinvertebrate communities varies among hydrologic regions and could affect how aquatic insects transport subsidies as adults. Further, several trait–environment relationships were underpinned by hydrology. Large bodied taxa that could disperse further from the stream were associated with hydrologically stable conditions. Alternatively, hydrologically variable conditions were associated with multivoltine taxa that could extend the duration of subsidies with periodic emergence events throughout the year. We also found that anthropogenic impacts decrease the frequency of individuals with adult flight but potentially extend the distance subsidies travel into the terrestrial ecosystem. Collectively, these results suggest that natural and anthropogenic gradients could affect aquatic insect subsidies by changing the trait composition of benthic macroinvertebrate communities. The conceptual framework and trait–environment relationships we present shows promise for understanding broad geographical patterns in linkages between ecosystems. 

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