IMMIGRATION AND THE DYNAMICS OF A PREDATOR-PREY INTERACTION IN BIOLOGICAL-CONTROL

1. A field experiment was conducted to determine if a predator-prey in teraction in biological control could be best described by single-popu lation or metapopulation dynamics. 2. A case of successful biological control on apple trees, the phytophagous mite, Panonychus ulmi. and it s predator, Typhlodromus pyri was selected, and two factors were manip ulated, the number of potentially interacting populations (spatial arr angement of trees) and the initial density of the prey. 3. Spatial arr angement of trees significantly affected immigration, with higher immi gration and higher turnover rates where there were more potentially in teracting populations. 4. Densities were higher and persistence was gr eater where immigration rates were higher. 5. Immigration slowed the t endency for natural and augmented density populations to converge to s imilar densities. 6. The patterns observed were consistent with the hy pothesis that T. pyri can sometimes drive P. ulmi populations to extin ction on individual small apple trees. 7. Increasing the number of int eracting populations and thus immigration rates slowed down the tenden cy to extinction, but it is not clear whether at the spatial scale of an orchard the ultimate outcome would still be extinction of P. ulmi o r if persistence is the rule.