1. Department of Biology, University of Pisa, CoNISMa, Via Derna 1, Pisa, Italy
2. Dipartimento di Science Botaniche, Ecologiche e Geologiche, Università di Sassari, Via Piandanna 4, 07100, Sassari, Italy
3. Department of Mathematics, 1523 Greene Street, University of South Carolina, Columbia, SC 29208, USA
4. Mathematical Biosciences Institute, Jennings Hall, 1735 Neil Avenue Columbus, OH 43210, USA