Increased searching and handling effort in tall swards lead to a Type IV functional response in small grazing herbivores

• Published in: Oecologia Vol. 166; no. 3; pp. 659 - 669
• Main Authors: Heuermann, Nicol, van Langevelde, Frank, van Wieren, Sipke E., Prins, Herbert H. T.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer 01-07-2011; Springer-Verlag; Springer Nature B.V
• Subjects: Animal and plant ecology; Animal physiology; Animal, plant and microbial ecology; Animals; Aquatic birds; Biological and medical sciences; Biomass; Biomedical and Life Sciences; Bites; Body mass; Body Weight; Branta canadensis; density; Ecology; Feeding Behavior; Food intake; Foraging; foraging behavior; Functional responses; Fundamental and applied biological sciences. Psychology; Geese; Geese - physiology; General aspects; grass stems; Grasses; Grazing; height; Herbivores; Hydrology/Water Resources; instantaneous intake rate; Leaves; Life Sciences; mammalian herbivores; Mass; Nutrient content; Plant Sciences; PLANT-ANIMAL INTERACTIONS; Plant-Animal interactions - Original Paper; Poaceae; Population dynamics; quality; Resource partitioning; serengeti; Swallowing; Sward; Intake rate; Body mass; Geese; Bite mass; Grassland; Herbivorous; Functional response; Body weight; Anatidae; Heavy metal; Mass; Vertebrata; Branta canadensis; Lead; Aves
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s00442-010-1894-8

Understanding the functional response of species is important in comprehending the species' population dynamics and the functioning of multi-species assemblages. A Type II functional response, where instantaneous intake rate increases asymptotically with sward biomass, is thought to be common in grazers. However, at tall, dense swards, food intake might decline due to mechanical limitations or if animals selectively forage on the most nutritious parts of a sward, leading to a Type IV functional response, especially for smaller herbivores. We tested the predictions that bite mass, cropping time, swallowing time and searching time increase, and bite rate decreases with increasing grass biomass for different-sized Canada geese (Branta canadensis) foraging on grass swards. Bite mass indeed showed an increasing asymptotic relationship with grass biomass. At high biomass, difficulties in handling long leaves and in locating bites were responsible for increasing cropping, swallowing, and searching times. Constant bite mass and decreasing bite rate caused the intake rate to decrease at high sward biomass after reaching an optimum, leading to a Type IV functional response. Grazer body mass affected maximum bite mass and intake rate, but did not change the shape of the functional response. As grass nutrient contents are usually highest in short swards, this Type IV functional response in geese leads to an intake rate that is maximised in these swards. The lower grass biomass at which intake rate was maximised allows resource partitioning between differentsized grazers. We argue that this Type IV functional response is of more importance than previously thought.