Food Chain

Micronutrients such as vitamins are transferred from lower to higher trophic levels, but no general ecological concept describes the factors regulating this process. Here, we investigated thiamin (thiamine, vitamin B1), which is an example of a metabolically important water-soluble micronutrient. Thiamin is produced by organisms such as bacteria and phytoplankton, and all consumers, such as zooplankton and fish, rely on a continuous intake of thiamin through their diet and possibly from de novo-synthesized thiamin by gut microbiota. A deficiency in thiamin negatively affects reproduction in fish and bird populations worldwide. The aim of this study was to quantify thiamin transfer in a planktonic food web in response to thiamin and/or nutrient addition, using an outdoor mesocosm system (an approximately 1.9 m3 bag submerged in sea water). These estimates were then compared with literature data on thiamin concentrations at different trophic levels. The results showed that thiamin was rapidly taken up by phytoplankton in both the ambient and nutrient-amended treatments. However, large differences in thiamin concentrations in phytoplankton did not lead to any significant changes in community composition or abundance. Nitrogen addition led to changes in the abundance and community composition of picoplankton and phytoplankton but there were no additional major effects of thiamin addition. Differences in thiamin concentrations in phytoplankton were not detected at the next trophic level in zooplankton. Although the concentrations did not change, a greater abundance of some zooplankton taxa were developed in the thiamin treatments. Comparing the mesocosm results with literature data demonstrated a gradual reduction in thiamin concentrations along the food chain, with six percent of the concentration in producers occurring in top consumers (i.e., piscivorous fish). Overall, these observations illustrate the concept of trophic dilution of micronutrients where concentrations decrease along the food web from phytoplankton via zooplankton and planktivorous fish to piscivorous fish.

The stability of ecological communities has a profound impact on humans, ranging from individual health influenced by the microbiome to ecosystem services provided by fisheries. A long-standing goal of ecology is the elucidation of the interplay between biodiversity and ecosystem stability, with some ecologists warning of instability due to loss of species diversity while others arguing that greater diversity will instead lead to instability. Here, by considering a minimal two-level ecosystem with multiple predator and prey species, we show that stability does not depend on absolute diversity but rather on diversity differences between levels. We found that increasing diversity in either level first destabilizes but then stabilizes the community (i.e., a reentrant stability transition). We therefore find that it is the diversity difference between levels that is the key to stability, with the least stable communities having similar diversities in different levels. An analytical stability criterion is derived, demonstrating quantitatively that the critical diversity difference is determined by the correlation between how one level affects another and how it is affected in turn. Our stability criterion also applies to consumer-resource models with other forms of interaction such as cross-feeding. Finally, we show that stability depends on diversity differences in ecosystems with three trophic levels. Our finding of a nonmonotonic dependence of stability on diversity provides a natural explanation for the variety of diversity-stability relationships reported in the literature, and emphasizes the significance of level structure in predicting complex community behaviors.

The early radiation of dinosaurs remains a complex and poorly understood evolutionary event1-4. Here we use hundreds of fossils with direct evidence of feeding to compare trophic dynamics across five vertebrate assemblages that record this event in the Triassic-Jurassic succession of the Polish Basin (central Europe). Bromalites, fossil digestive products, increase in size and diversity across the interval, indicating the emergence of larger dinosaur faunas with new feeding patterns. Well-preserved food residues and bromalite-taxon associations enable broad inferences of trophic interactions. Our results, integrated with climate and plant data, indicate a stepwise increase of dinosaur diversity and ecospace occupancy in the area. This involved (1) a replacement of non-dinosaur guild members by opportunistic and omnivorous dinosaur precursors, followed by (2) the emergence of insect and fish-eating theropods and small omnivorous dinosaurs. Climate change in the latest Triassic5-7 resulted in substantial vegetation changes that paved the way for ((3) and (4)) an expansion of herbivore ecospace and the replacement of pseudosuchian and therapsid herbivores by large sauropodomorphs and early ornithischians that ingested food of a broader range, even including burnt plants. Finally, (5) theropods rapidly evolved and developed enormous sizes in response to the appearance of the new herbivore guild. We suggest that the processes shown by the Polish data may explain global patterns, shedding new light on the environmentally governed emergence of dinosaur dominance and gigantism that endured until the end-Cretaceous mass extinction.

Prey availability and predation risk drive animal distribution, movement, and foraging ecology, yet studies rarely analyse multiple predator-prey levels together. Understanding how predators optimise risk-reward tradeoffs is important for species conservation and management, especially in systems facing extreme ecosystem change. We examined how top-down (modelled polar bear habitat selection) and bottom-up (modelled fish diversity) processes influence the habitat selection, movement, and foraging behaviour of 26 ringed seals (greater than 70,000 dives and 10,000 locations over 877 seal days). Our results suggest that polar bears spatially restrict seal movements and reduce the time seals spend foraging, potentially decreasing foraging success. Seals were more likely to be present and dive longer in high-predation risk areas when prey diversity was high. Further, seal habitat selection models excluding polar bears overestimated core space use. These findings illustrate the dynamic tradeoffs that mesopredators make when balancing predation risk and resource acquisition.