Interactive effects of dispersal “type” and amount affect population dynamics, disease epidemics, and trait change in a zooplankton metapopulation

Background/Question/Methods

Levels of ecological and evolutionary divergence across a landscape are regulated by a combination of selection in local patches (which tends to increase divergence) and dispersal between patches (which tends to reduce divergence). A complicating factor in this relationship stems from an observation with growing empirical support; while dispersal and gene flow can constrain divergence between populations, so too can divergence constrain gene flow, if local adaptation to one habitat patch alters success in another. In this way, the levels of dispersal required to affect divergence between populations will depend strongly on the relative suitability of dispersers in their new habitat patch. We conducted a mesocosm experiment to test whether this interaction between dispersal “type” (relatively suitability of dispersing individuals) and amount (dispersal rate) affects population growth, disease dynamics, and trait change in the cladoceran Daphnia dentifera. Using host individuals from three naturally occurring populations, we established dispersal regimes where dispersing individuals were relatively more or relatively less suitable (in terms of susceptibility to a pathogen) than resident individuals in a habitat. We varied dispersal across a range of realistic rates, and quantified host population abundance, disease prevalence, and change in an important disease resistance-related trait over nine weeks.

Results/Conclusions

Daphnia abundance depended strongly on the interaction between dispersal “type” and amount. Resident populations of low (“L”) starting susceptibility numerically dominated at both high and low rates of dispersal. With intermediate dispersal rates, however, high susceptibility (“H”) resident population abundance exceeded “L” resident abundance. Infection prevalence showed the same interactive response to treatment as host abundance, suggesting that host density rather than individual susceptibility drove long-term disease dynamics. Average adult body size (related to disease susceptibility) also responded to treatment; “H” resident populations had higher initial body sizes, but changes through time for both resident populations differed by dispersal level. In other words, resident populations responded differently to dispersal from a common source, which altered population, disease, and trait dynamics over time. These results show that the effects of dispersal on resident populations depend on the relationship between resident individuals and dispersers (i.e. susceptibility to infection), as well as the rate at which dispersers enter a habitat. This interaction between relative suitability and dispersal rate has strong implications for levels of ecological and evolutionary divergence in regional metapopulations, which are governed by a complex eco-evolutionary feedback between local selection and regional dispersal.