Ideal free distribution in Daphnia? Are daphnids able to consider both the food patch quality and the position of competitors?

Abstract

The distribution of the planktonic crustacean Daphnia pulex was tested in a ring-formedflow-through chamber divided into eleven sections. Thedistribution of the animals under both homogeneousfood conditions and in a food gradient were studied.The distribution of repeated registrations of singleanimals was randomly distributed in a homogeneous,low food environment. Single daphnids exposed to afood gradient tended to reside during 88% of the timeat the highest or next highest food concentration,suggesting that daphnids can detect food gradients.When a group of approximately 100 co-occurring animalswere given low homogeneous food conditions, they alsotended to be randomly distributed. In one out of threecases, however, they were slightly less aggregatedthan expected from a random distribution. Exposed toa food gradient (0.5–0.0015 mg C l^-1), thedistribution of the daphnids approximated the idealfree distribution. A very high maximum density in thefood gradient (2 mg C l^-1) resulted in less strongaggregations than expected from the ideal freedistribution.