From communities to individuals

Most organisms on Earth live a life where they are exposed to multiple and variable threats. In order to maximise survival they need to be able to perceive and respond to these threats. Two common threats that crustacean zooplankton are faced with are predation and ultraviolet radiation (UVR). To cope with these threats zooplankton use different strategies such as diel vertical migration and the accumulation of photoprotective compounds. In this thesis I study the threat responses of zooplankton when exposed to predation and UVR. I explore both interand intraspecific differences in the response to these threat situations. In addition to field data I also developed a new technique for tracking zooplankton using fluorescent nanoparticles, which I use to track the individual responses of zooplankton. I investigate if accumulation of photoprotective pigmentation may affect the spatial distribution of species in nature and how individual size may affect the migratory behaviour of zooplankton. In addition I also explore how previous exposure to threats affect the behavioural responses and if some of the observed behavioural variance could be explained by consistent individual differences in behaviour. In summary, I show that even closely related species may show large differences in their response to UVR. Intraspecific differences in pigmentation could not be related to the spatial distribution of either Daphnia or calanoid copepods; however, both Daphnia and copepods were found to perform size structured migration. I also demonstrate that previous experiences of a threat can affect how the organism responds. Some evidence for consistent individual differences in the behavioural response of Daphnia to UVR is presented, suggesting that individual based studies could add further to our knowledge about behavioural responses in zooplankton. Given the variability both with respect to interand intraspecific differences in UVR response we need to keep these differences in mind when postulating new, more comprehensive theories explaining the behaviour of zooplankton exposed to everyday threats.