  </tr>
    <tr>
      <th>variable</th>
      <td><p>a RasterLayer representing an environmental variable.</p></td>
    </tr>
    <tr>
      <th>range_colors</th>
      <td><p>vector of colors for borders of species ranges. If
<code>NULL</code>, the default, distinct levels of gray will be used.</p></td>
    </tr>
    <tr>
      <th>color_variable</th>
      <td><p>a color palette (a vector of continuous colors generated
by functions like heat.colors). If <code>NULL</code>, the default, rev(terrain.colors(