Data from: Field measurements give biased estimates of functional response parameters, but help explain foraging distributions

1. Mechanistic insights and predictive understanding of the spatial distributions of foragers are typically derived by fitting either field measurements on intake rates and food abundance, or observations from controlled experiments, to functional response models. It has remained unclear, however, whether and why one approach should be favoured above the other, as direct comparative studies are rare. 2. The field measurements required to parameterize either single or multi-species functional response models are relatively easy to obtain, except at sites with low food densities and at places with high food densities, as the former will be avoided and the second will be rare. Also, in foragers facing a digestive bottleneck, intake rates (calculated over total time) will be constant over a wide range of food densities. In addition, interference effects may further depress intake rates. All of this hinders the appropriate estimation of parameters such as the ‘instantaneous area of discovery’ and the handling time, using a type II functional response model also known as ‘Holling's disc equation’. 3. Here we compare field- and controlled experimental measurements of intake rate as a function of food abundance in female bar-tailed godwits Limosa lapponica feeding on lugworms Arenicola marina. 4. We show that a fit of the type II functional response model to field measurements predicts lower intake rates (about 2.5 times), longer handling times (about 4 times) and lower ‘instantaneous areas of discovery’ (about 30 to 70 times), compared with measurements from controlled experimental conditions. 5. In agreement with the assumptions of Holling's disc equation, under controlled experimental settings both the instantaneous area of discovery and handling time remained constant with an increase in food density. The field data, however, would lead us to conclude that although handling time remains constant, the instantaneous area of discovery decreased with increasing prey densities. This will result into highly underestimated sensory capacities when using field data. 6. Our results demonstrate that the elucidation of the fundamental mechanisms behind prey detection and prey processing capacities of a species necessitates measurements of functional response functions under the whole range of prey densities on solitary feeding individuals, which is only possible under controlled conditions. Field measurements yield ‘consistency tests’ of the distributional patterns in a specific ecological context.

1. 对觅食者空间分布的机制性解析与预测性认知，通常通过将取食率与食物丰度的野外实测数据，或是受控实验观测结果，拟合至功能响应模型（functional response model）而获得。然而，由于直接对比研究较为匮乏，目前仍不清楚两种方法中哪一种更具优势，以及其优势来源为何。2. 若要对单物种或多物种功能响应模型进行参数化，所需的野外实测数据通常较易获取，但食物密度极低或极高的站点除外——前者会被觅食者规避，后者则较为罕见。此外，当觅食者遭遇消化瓶颈时，以总时长计算的取食率会在较宽的食物密度范围内保持恒定。除此之外，干扰效应还可能进一步降低取食率。上述种种因素，均会阻碍利用II型功能响应模型（type II functional response model，亦称为霍林圆盘方程Holling's disc equation）对‘瞬时发现面积（instantaneous area of discovery）’与‘处理时间（handling time）’等参数进行准确估算。3. 本研究针对以沙蚕（Arenicola marina）为食的雌性斑尾塍鹬（Limosa lapponica），对比了其取食率随食物丰度变化的野外实测与受控实验数据。4. 研究结果显示，相较于受控实验条件下的实测数据，将II型功能响应模型拟合至野外实测数据后，预测得到的取食率更低（约为实验值的2.5倍）、处理时间更长（约为实验值的4倍），且瞬时发现面积更小（约为实验值的30至70倍）。5. 符合霍林圆盘方程的理论假设：在受控实验环境下，瞬时发现面积与处理时间均随食物密度升高保持恒定。但野外实测数据却会让我们得出相反结论：尽管处理时间仍保持恒定，但瞬时发现面积会随猎物密度升高而降低。若直接使用野外数据，将会极大低估物种的感知能力。6. 本研究结果表明，若要阐明某一物种猎物探测与猎物处理能力背后的核心机制，需对单独觅食个体在全猎物密度范围内的功能响应函数进行实测，而这仅能在受控实验条件下实现。野外实测数据则可用于在特定生态背景下对分布模式进行‘一致性检验’。