Data from: Understanding spatial distributions: Negative density-dependence in prey causes predators to trade-off prey quantity with quality

Negative density-dependence is generally studied within a single trophic level, thereby neglecting its effect on higher trophic levels. The ‘functional response’ couples a predator's intake rate to prey density. Most widespread is a type II functional response, where intake rate increases asymptotically with prey density; this predicts the highest predator densities at the highest prey densities. In one of the most stringent tests of this generality to date, we measured density and quality of bivalve prey (edible cockles Cerastoderma edule) across 50 km² of mudflat, and simultaneously, with a novel time-of-arrival methodology, tracked their avian predators (red knots Calidris canutus). Because of negative density-dependence in the individual quality of cockles, the predicted energy intake rates of red knots declined at high prey densities (a type IV, rather than a type II functional response). Resource-selection modelling revealed that red knots indeed selected areas of intermediate cockle densities where energy intake rates were maximized given their phenotype-specific digestive constraints (as indicated by gizzard mass). Because negative density-dependence is common, we question the current consensus and suggest that predators commonly maximize their energy intake rates at intermediate prey densities. Prey density alone may thus poorly predict intake rates, carrying capacity and spatial distributions of predators. The data package contains 7 datafiles - spatial rasters: The coordinate reference system is EPSG:32631 - WGS 84 / UTM zone 31N - Data file for resource selection analyses corresponding to Fig. 4 and Table S2 and S3 This file contains data for the resource selection analyses. Each row represents a used or available residence patch. The different columns are: tagID (the bird's tag number), gizzard_mass (the measured gizzard mass in g), X (X-coordinate of residence patch in m), Y (Y-coordinate of residence patch in m), RT (duration of residence patch in h), present (1 indicates a used residence patch, and 0 indicates an available residence patch), density (cockle density at these coordinates in numbers per square meter), AFDMflesh (relative AFDMflesh at these coordinates), IR (predicted intake rate at these coordinates in mg AFDMflesh per second), IR_avg_gizzard (predicted intake rate at these coordinates with an average gizzard mass in mg AFDMflesh per second), IR_ind_gizzard (standardised predicted intake rates at these coordinates given an individual's measured gizzard mass), and weights (statistical weight in resource selection model). - Data for the analyses of density dependence in flesh and shell mass of cockles corresponding to Fig. 2A and Table S1. This file contains data for the analyses of density dependence in flesh and shell mass of cockles. Each row represents a measurement of relative flesh or shell mass. The different columns are: sampling_station (sampling station of cockle measurements), density (cockle density at this sampling station), relative_AFDMflesh (proportional deviation in AFDMflesh compared to the average of cockles with identical length), relative_DMshell (proportional deviation in DMshell compared to the average of cockles with identical length), length (length of the cockle in mm)