Salty, mild, and low plant biomass grasslands increase top‐heaviness of invertebrate trophic pyramids

Abstract

Aim: Multiple hypotheses predict how gradients of nutrient availability, plant biomass, and temperature shape trophic pyramids. We aim to disentangle the simultaneous influence of those factors and their indirect effects on trophic structure and individual trophic levels. Location: United States. Time period: 2017. Major taxa studied: Invertebrates. Methods: To examine differences in trophic pyramid shape and abundance within trophic levels and across ecological gradients, we conducted 54 standardized surveys of invertebrate communities in North American grasslands. We tested for the direct and indirect effects of plant biomass, temperature, sodium (Na), other essential elements (e.g. N, P, and K), and toxic heavy metals, (e.g. Ar and Pb) in plant tissue on both individual trophic levels, and trophic pyramid shape, estimated as the community trophic mean (CTM). Results: Plant sodium increased CTM, indicating that high plant sodium concentrations are associated with top-heavy invertebrate trophic pyramids. Sites with higher plant biomass had higher proportions of herbivores compared to higher trophic levels. Finally, increasing temperature resulted in more top-heavy trophic pyramids. Overall, plant biomass, temperature, and plant chemistry directly and indirectly affected the abundances within different trophic levels, highlighting the complexity of factors regulating trophic structure. Main conclusions: Trophic structure of grassland invertebrate communities is strongly influenced by plant sodium, plant biomass, and to a lesser extent, temperature. Grasslands occupy 30% of the Earth's terrestrial surface and are an imperiled ecosystem due to conversion to row crop agriculture. As biogeochemistry and temperature in the Anthropocene are increasingly modified, our results have considerable implications for the trophic structure of future grassland communities.

Tracking nutrients in space and time: Interactions between grazing lawns and drought drive abundances of tallgrass prairie grasshoppers

“…The only non‐grasshopper taxa to vary in abundance with year was the parasitoid Hymenoptera, which had higher abundances in 2019. This response may be due to higher growing season precipitation resulting in more plant aboveground biomass, and thus more habitat volume and our nectar for parasitoids (Post et al., 2000; Welti, Kuczynski, et al., 2020). Generally, non‐grasshopper taxa were only categorized to broad functional‐taxonomic groups and may vary compositionally in response to grazing lawns and drought at finer taxonomic levels.…”

Temperature drives variation in flying insect biomass across a German malaise trap network

“…While one long-term study of flying insects in the Netherlands found no evidence of higher rates of decline in larger species over the past two decades (Hallmann et al, 2020), larger-bodied species may have become rare earlier in the last century (Seibold et al, 2015). Climate and land-cover change may otherwise alter insect communities by favoring particular trophic levels (Welti, Kuczynski, et al, 2020), invasive (Ju et al, 2017), or pest species (Bernal & Medina, 2018). The lack of an overall unimodal relationship temperature may be a result of the coarse taxonomic (flying insects) and temporal (~2 week) sample resolution in comparison to other studies (e.g.…”

Dietary sodium levels affect grasshopper growth and performance

“…One consequence of this dynamic is that herbivores are often sodium limited—with shortfall leading to decreases in growth, survival, performance, and reproduction. This is true for both vertebrate and invertebrate herbivores (Jones and Hanson 1984, Hellgren and Pitts 1997, Welti et al 2019 b , 2020, Kaspari 2020). Invertebrate herbivores sate their sodium needs by puddling (Molleman 2010), opportunistic carnivory (Simpson et al 2006, Clay et al 2017), and seeking out more sodium‐rich plant tissue and exudates (Welti et al 2019 b , Kaspari et al 2020).…”