1. Land Resources and Environmental Sciences Department, Montana State University, Bozeman, Montana, USA
2. Department of Biology, University of North Alabama, Florence, Alabama, USA
3. Department of Mathematical Sciences, Montana State University, Bozeman, Montana, USA
4. Department of Mathematics, Walla Walla University, College Place, Washington, USA
5. InterSystems Corporation, Cambridge, Massachusetts, USA
6. Department of Mathematics, Temple University, Philadelphia, Pennsylvania, USA