Increased searching and handling effort in tall swards lead to a Type IV functional response in small grazing herbivores |
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Authors: | Email author" target="_blank">Nicol?HeuermannEmail author Frank?van?Langevelde Sipke?E?van?Wieren Herbert?H?T?Prins |
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Institution: | (1) Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708 PB Wageningen, The Netherlands |
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Abstract: | 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 different-sized grazers. We argue that this Type IV functional
response is of more importance than previously thought. |
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