Department of Mathematics, University of Fribourg, Fribourg, Switzerland.|Department of Mathematics, University of Fribourg, Fribourg, Switzerland.|Department of Biology, Unit of Ecology and Evolution, University of Fribourg, Fribourg, Switzerland.|Department of Biology, Unit of Ecology and Evolution, University of Fribourg, Fribourg, Switzerland.|Department of Mathematics, University of Fribourg, Fribourg, Switzerland.