Density-dependent population dynamics in clonal organisms: a modelling approach

1. Density dependence may act at several stages in an organisms life-cycle (e.g. on mortality. fecundity, etc.), but not all density-dependent process es necessarily regulate population size. In this paper I use a density mani pulation experiment to determine the effects of density on the transition r ates between different size classes of the clonal zoanthid Palythoa caesia Dana 1846. I then formulate a density-dependent matrix model of population dynamics of Palythoa, and perform a series of sensitivity analyses on the m odel to determine at what stage in the life-cycle regulation acts. 2. Seven of the 16 transition probabilities decreased with density, most of them being shrinkage (due to loss of tissue or fission) and stasis (the se lf-self transition) of medium and large colonies. The only probability to i ncrease was for the stasis of large colonies. Recruitment was quadratically dependent on density, peaking at intermediate densities. 3. Equilibrium cover in the model was 84% and was reached in approximate to 40 years. To determine which density-dependent transitions were involved i n population regulation, the strength of density dependence was varied in e ach independently. This sensitivity analysis showed that only changes in th e probabilities of large colonies remaining large and producing medium colo nies, were regulating. 4. These results suggest that regulation is primarily acting on fission of large colonies to produce intermediate-sized colonies. in combination with size specific growth rates. Fission rates decrease greatly with density, re sulting in a greater proportion of large colonies at high densities and lar ge colonies grow more slowly than small. Overall, this behaviour is very si milar to that of clonal plants which have a phalanx type life history.