Ratio-dependent functional responses - tests with the zooplanktivore Mysis mixta

The response of a consumer to varying densities of its food resources is essential to consumer-resource interactions, yet the proper way to describe this interaction (the functional response) has been intensively debated. Traditionally, consumption has been assumed to be solely a function of resource abundance (prey dependent). However, it has lately been suggested that consumption depends on the ratio between resource and consumer abundances (ratio-dependent functional responses). In this paper, we experimentally tested if the consumer density influences food consumption rates. As the consumer, we used a dominant Baltic Sea zooplanktivore, the mysid shrimp Mysis mixta. The prey used were of 3 types; the large but slow Daphnia magna, the small and slow Artemia sp., and a natural assemblage of Baltic Sea copepods that exhibit strong escape responses. Our results show that intraspecific interactions occurred among the consumers, supporting the hypothesis that a functional response can be dependent on the consumer density.