populations 10 tings (Nowak & May 1992, 1993, Ellison 1993, Herz 1994, Lindgren & Nordahl 1994, Ferriere Michod 1996, Killingback & Doebeli 1996, Nalcamaru et al 1997, 1998, Nalcamaru & Iwasa 2005, 2006, van Baalen & Rand 1998, Yamamura et al 2004, Helbing & Wu 2008). Here most interactions occur among