DOI: 10.5061/dryad.d75hq
title: Data from: Understanding spatial distributions: negative density-dependence in prey causes predators to trade-off prey quantity with quality

description: 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.
description: spatial raster with
 cockle densities corresponding to Fig3AThe coordinate reference
 system is EPSG:32631 - WGS 84 / UTM zone 31N, and cockle density is
 presented in numbers per square meter.resource_landscape-Fig3A_cockle_density.tifSpatial raster with relative AFDMflesh corresponding to Fig3BThe coordinate reference system is EPSG:32631 - WGS 84 / UTM zone 31N. Relative AFDMflesh is presented as the ratio of AFDMflesh to average AFDMflesh for cockles of identical length (see main article).resource_landscape-Fig3B_relative_AFDMflesh.tifspatial raster with predicted intake rates corresponding to Fig3CThe coordinate reference system is EPSG:32631 - WGS 84 / UTM zone 31N, and predicted intake rates are presented in mg AFDMflesh per second. These intake rates correspond to foragers without a digestive constraint.resource_landscape-Fig3C_IR.tifspatial raster with predicted intake rates for knots with an average gizzard mass corresponding to Fig3DThe coordinate reference system is EPSG:32631 - WGS 84 / UTM zone 31N, and predicted intake rates are presented in mg AFDMflesh per second. These intake rates are for digestively constraint red knots with an average gizzard mass (7g).resource_landscape-Fig3D_IR_avg_gizzard_mass.tifSpatial raster with prey quality that is used to calculate the digestive constraintThe coordinate reference system is EPSG:32631 - WGS 84 / UTM zone 31N, and prey quality is presented as the ratio of AFDMflesh to DMshell. A red knot's digestive constraint can be calculated as q × 0.05 × G^2, where q is prey quality and G is gizzard mass (see main article).resource_landscape-prey_quality_for_calculating_digestive_constraint.tifData file for resource selection analyses corresponding to Fig. 4 and Table S2 and S3This 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).resource_selection_of_knots.csvData for the analyses of density dependence in flesh and shell mass of cockles corresponding to Fig. 2A and Table S1This 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).density_dependence_in_cockles.csv