Trophic differences regulate grassland food webs: herbivores track food quality and predators select for habitat volume

Regulatory mechanisms of nitrogen homeostasis in insect growth and development

Nitrogen is an essential element for the synthesis of proteins, nucleic acids, and various other critical biological molecules in insects. The maintenance of nitrogen homeostasis in insects is achieved through a balance of dietary intake, metabolic conversion, and excretion. Insects primarily acquire nitrogen from their diet, which is subsequently metabolized into amino acids, proteins, and other vital biomolecules following digestion and absorption. Excess nitrogen is excreted in forms such as uric acid, allantoin, allantoic acid, urea, and ammonia. Disruptions in nitrogen regulation can result in ammonia toxicity and abnormal production or excretion of nitrogenous metabolites, including uric acid, ultimately impairing insect development and survival. This review examines the mechanisms underlying nitrogen homeostasis in insects, with a focus on the intricate regulatory roles of carbohydrate metabolism, amino acid metabolism, uric acid metabolism, urea and polyamine metabolism, ammonia transport pathways, and symbiotic interactions. By elucidating these processes, this review aims to enhance our understanding of insect nutritional metabolism and developmental biology, while offering novel perspectives for the development of more effective pest management strategies.

Soil nutrient limitation controls trophic cascade effects of micro-food web-derived ecological functions in degraded agroecosystems

INTRODUCTION

Soil nutrient supply drives the ecological functions of soil micro-food webs through bottom-up and top-down mechanisms in degraded agroecosystems. Nutrient limitation responds sensitively to variations in degraded agroecosystems through restoration practices, such as legume intercropping.

OBJECTIVES

This study examined the effects of legume intercropping on trophic cascade dynamics through resource supply in degraded purple soil ecosystems.

METHODS

A field experiment was conducted with three plantation types: Camellia oleifera monoculture (CK), C. oleifera-Arachis hypogaea (peanut) intercropping (CP), and C. oleifera-Senna tora intercropping (CS). Using soil nutrient limitation as a premise, modified by legume intercropping, we assessed the biodiversity of soil biotic taxa, analysed their community composition, and applied partial least squares path modelling (PLS-PM) to link trophic cascade with ecological functions.

RESULTS

Legume intercropping altered the abundance of biotic taxa, leading to changes in biotic diversity and microbial life strategies. The PLS-PM results indicated that legume intercropping enhanced bacterial diversity by aggravating soil P limitation, which subsequently increased protist consumer diversity and omnivore-predator nematode abundance through a bottom-up effect. Omnivore-predator nematodes and protist consumers indirectly influenced soil P metabolism, down-regulated through bacteria in the top-down effect. We observed high consistency between the untargeted metabolomic analysis and soil nutrient limitations. These findings indicate that soil micro-food web structure and function responded sensitively to legume intercropping in degraded ecosystems.

CONCLUSION

The results highlight the role of soil nutrient limitation in shaping micro-food webs and suggest that soil P limitation controls the down-regulation of soil P-related ecological functions through bottom-up and top-down effects.

Electrolytes on the prairie: How urine-like additions of Na and K shape the dynamics of a grassland food web

The electrolytes Na and K both function to maintain water balance and membrane potential. However, these elements work differently in plants-where K is the primary electrolyte-than in animals-where ATPases require a balanced supply of Na and K. Here, we use monthly factorial additions of Na and K to simulate bovine urine inputs and explore how these electrolytes ramify through a prairie food web. Against a seasonal trend of increasing grass biomass and decreasing water and elemental tissue concentrations, +K and +Na plots boosted water content and, when added together, plant biomass. Compared to control plots, +Na and +K plots increased element concentrations in above-ground plant tissue early in summer and decreased them in September. Simultaneously, invertebrate abundance on Na and K additions were sequentially higher and lower than control plots from June to September and were most suppressed when grass was most nutrient rich. K was the more effective plant electrolyte, but Na frequently promoted similar changes in grass ionomes. The soluble/leachable ions of Na and K showed significant ability to shape plant growth, water content, and the 15-element ionome, with consequences for higher trophic levels. Grasslands with high inputs of Na and K-via large mammal grazers or coastal aerosol deposition-likely enhance the ability of plants to adjust their above-ground ionomes, with dramatic consequences for the distribution of invertebrate consumers.

How and why grasshopper community maturation rates are slowing on a North American tall grass prairie

Invertebrate growth rates have been changing in the Anthropocene. We examine rates of seasonal maturation in a grasshopper community that has been declining annually greater than 2% a year over 34 years. As this grassland has experienced a 1°C increase in temperature, higher plant biomass and lower nutrient densities, the community is maturing more slowly. Community maturation had a nutritional component: declining in years/watersheds with lower plant nitrogen. The effects of fire frequency were consistent with effects of plant nitrogen. Principal components analysis also suggests associated changes in species composition-declines in the densities of grass feeders were associated with declines in community maturation rates. We conclude that slowed maturation rates-a trend counteracted by frequent burning-likely contribute to long-term decline of this dominant herbivore.

Trophic differences regulate grassland food webs: herbivores track food quality and predators select for habitat volume

The impacts of altered biogeochemical cycles on ecological systems are likely to vary with trophic level. Predicting how these changes will affect ecological food webs is further complicated by human activities, which are simultaneously altering the availability of macronutrients like nitrogen (N) and phosphorus (P), and micronutrients such as sodium (Na). Here we contrast three hypotheses that predict how increasing nutrient availability will shape grassland food webs. We conducted a distributed factorial fertilization experiment (N and P crossed with NaCl) across four North American grasslands, quantifying the responses of aboveground plant biomass and volume, plant tissue and soil elemental concentrations, as well as the abundance of five arthropod functional groups. Fertilization with N and P increased plant biomass and foliar N and P concentrations in grasses but not forbs. Fertilization with Na had no effect on plant biomass but increased foliar Na concentrations. Consistent with the nutrient limitation hypothesis, we found strong evidence of nutrient limitation for insect herbivores across the four sites with sucking (phloem and xylem feeding) herbivores increasing in abundance with NP fertilization and chewing herbivores increasing in response to both Na and NP fertilization, and a trend for increased response of arthropods to lower plant nutrient availability. We found no evidence for an interaction of NaCl and NP on arthropod abundance as predicted by the serial colimitation hypothesis. Finally, consistent with the ecosystem size hypothesis, predator and parasitoid abundances increased with plant volume, but not fertilization. Our results suggest these functional group-specific responses to changes in plant nutrients and structure are key to predicting the future of grassland food webs in an era with increasing use of N and P fertilizers, and increasing terrestrial inputs of Na from road salt, saline irrigation water, and aerosols due to rising sea levels.