Set the modelObject of the action dropdown ('Run', 'Fly', 'Fruit') when you click on your link (animals, birds, and trees,).
add(new AjaxLink("run"){
private static final long serialVersionUID = 1L;
@Override
public void onClick(AjaxRequestTarget target)
{
actionDropdown.setModelObject("Run");
target.addComponent(actionDropdown);
}
});
This should change whats selected in the action dropdown but also leave the other options available to be selected.