Nitrogen and Phosphorus Availability Mediates the Effects of Bottom-Up Limitation on Orb-Weaver Spider Traits in an Amazonian Rainforest

Abstract Nutrient availability is a key driver of terrestrial ecosystem functioning and food web structure. In central Amazonian forests, phosphorus (P) and cations (Ca 2+ , Mg 2+ , K + ) are significant constraints for plants, potentially constraining the propagation of nutrients across trophic levels. Because predator life-history traits can be strongly linked to nutrient demands, these limitations may affect predators across trophic webs, although this mechanism remains poorly investigated in terrestrial ecosystems. To address this gap, we tested whether alleviating nutrient limitation affects predator traits using the Amazon Fertilisation Experiment (AFEX), a large-scale factorial manipulation of P, cations, and the less-limiting nutrient nitrogen (N), established in 2017. To evaluate consumer responses, we sampled two abundant understory orb-weaving spiders, Spilasma duodecimguttata and Hingstepeira folisecens , quantifying body size and fat reserves as indicators of growth and nutritional condition. We observed that individuals of S. duodecimguttata were smaller in N-only plots but, conversely, larger in plots with added N and P. In contrast, H. folisecens body size was unaffected, and fat mass did not differ in either species. These divergent responses likely reflect differences in foraging strategy and microhabitat use : S. duodecimguttata builds low-cost horizontal webs near the ground, where prey communities may be more directly influenced by soil nutrient dynamics, whereas H. folisecens forages higher above the ground, likely experiencing fewer direct effects of nutrient addition on prey. Our results indicate that, beyond nutrient limitation, N and P nutritional imbalances in tropical forests can trigger species-specific responses in life-history traits, with potential implications across food web dynamics.