When Pacific salmon ( Oncorhynchus spp.) spawn and die, they deliver marine‐derived nutrient subsidies to freshwater and riparian ecosystems. These subsidies can alter the behavior, productivity, and abundance of recipient species and their habitats. Isotopes, such as nitrogen‐15 ( 15 N), are often used to trace the destination of marine‐derived nutrients in riparian habitats. However, few studies have tested for correlations between stable isotopes and physiological responses of riparian organisms. We examined whether increases in δ 15 N in terrestrial insect bodies adjacent to salmon spawning habitat translate to changes in percent nitrogen content and body size. This involved comparisons between distance from a salmon‐bearing river, marine‐derived nutrients in soils and insects, soil moisture content, and body size and nitrogen content in two common beetle families (Coleoptera: Curculionidae, Carabidae). As predicted, δ 15 N in riparian soils attenuated with distance from the river but was unaffected by soil moisture. This gradient was mirrored by δ 15 N in the herbivorous curculionid beetles, whereas carabid beetles, which feed at a higher trophic level and are more mobile, did not show discernable patterns in their δ 15 N content. Additionally, neither distance from the river nor body δ 15 N content was related to beetle body size. We also found that nitrogen‐15 was not correlated with total percent nitrogen in insect bodies, meaning that the presence of spawning salmon did not increase the percent nitrogen content of these insects. We conclude that while salmon‐derived nutrients had entered terrestrial food webs, the presence of δ 15 N alone did not indicate meaningful physiological changes in these insects in terms of percent nitrogen nor body size. While stable isotopes may be useful tracers of marine‐derived nutrients, they cannot necessarily be used as a proxy for physiologically important response variables.
Many studies have investigated plant-pollinator interactions using visit records of insects contacting floral reproductive organs. However, these studies may not reflect the effectiveness of visits, since factors such as visitor behaviour and the composition of pollen on their bodies may influence conspecific pollen transfer required for fertilisation in plants. Here we study how pollen transport to a generalist wildflower, Camassia quamash (Pursh) Greene (Asparagaceae), is influenced by the behaviour and body pollen of five functional visitor groups (Andrena Fabricius (Hymenoptera: Andrenidae)/Halictidae (Hymenoptera), Apis mellifera Linnaeus (Hymenoptera: Apidae), Bombus Latreille (Hymenoptera: Apidae), Osmia Panzer (Hymenoptera: Megachilidae), and Syrphidae (Diptera). We found that functional visitor groups differed in their behaviour (Bombus and Osmia were legitimate visitors, contacting both anthers and stigmas) and in the amount of conspecific pollen on their bodies (A. mellifera had the highest levels and Andrena/Halictidae the lowest). Conspecific pollen receipt by C. quamash stigmas was high (>80%), and best explained by visitor behaviour rather than the proportion of visitors with high amounts of conspecific body pollen. Our findings highlight the utility of pollen analyses for understanding pollinator effectiveness.
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