function fuel_design_tool($form, &$form_state) {
$form = array();
$form['#tree'] = TRUE;

if (empty($form_state[FIELDS_NUM])) {
    $form_state[FIELDS_NUM] = 1;
}

for ($i = 1; $i <= $form_state[FIELDS_NUM]; $i++) {

    $form['field'][$i] = array(
        '#type' => 'fieldset',
        '#title' => t('Field #@num', array('@num' => $i)),
        '#collapsible' => TRUE,
        '#collapsed' => FALSE,

    );

    $form['field'][$i][FORM_FIELD_SPECIES] = array(
        '#type' => 'select',
        '#required' => TRUE,
        '#title' => 'Choose Species',
        '#options' =>  _load_species1(),
        '#default_value' => 0,
        '#ajax' => array(
             'callback' => 'fuel_design_tool_callback',
             'wrapper' => 'fuel_design_tool-wrapper',
             'event' => 'change',
           )

       );

    $form['field'][$i][FORM_FIELD_DENSITY] = array(
        '#type' => 'textfield',
        '#prefix' => '<div id="fuel_design_tool-wrapper">',
        '#suffix' => '</div>',
        '#title' => t('Density'),

    );
    $form['add_field'] = array(
    '#type' => 'submit',
    '#value' => t('Add More Species'),
    '#submit' => array('fuel_design_tool_add_name'
    ),

     $form['submit_button'] = array(
     '#type' => 'submit',
     '#value' => t('Submit'),
     "#submit" => array("fuel_design_tool_submit"),
    );

     return $form;
 }

 function fuel_design_tool_callback($form, &$form_state)
  {
   for ($i = 1; $i <= $form_state[FIELDS_NUM]; $i++)
   {

   $form['field'][$i]['FORM_FIELD_DENSITY']['#value'] = $form_state['values']['field'] [$i]['FORM_FIELD_SPECIES'];
return  $form['field'][$i]['FORM_FIELD_DENSITY'];

 }

}