Résumé
1. Movement and dispersal are critical processes for almost all organisms
in natural populations. Understanding their causes and consequences is
therefore of high interest. While both theoretical and empirical work
suggests that dispersal, more exactly emigration, is plastic and may be a
function of local population density, the functional relationship between
the underlying movement strategies and population density has received
less attention. 2. We here present evidence for the shape of this reaction
norm and are able to differentiate between three possible cues: the
relative number of individuals, the presence of metabolites (chemical
cues) and resource availability. 3. We performed microcosm experiments
with the ciliate model organism Tet-rahymena in order to understand the
plasticity of movement strategies with respect to local density while
controlling for possible confounding effects mediated by the availability
of different cues. In addition, we investigated how an Allee effect can
influence movement and dispersal plasticity. 4. Our findings suggest that
movement strategies in Tetrahymena are plastic and density-dependent. The
observed movement reaction norm was ushaped. This may be due to an Allee
effect which led to negative densitydependence at low population densities
and generally positive density-dependence at high population densities due
to local competition. This possibly adaptive density-dependent movement
strategy was likely mediated by chemical cues. 5. Our experimental work in
highly controlled conditions indicates that both environmental cues as
well as inherent population dynamics must be considered to understand
movement and dispersal.