With the accelerating loss of biodiversity there is increasing concern about how this loss may be affecting ecosystem processes, or services, that are of benefit to human well being. The limited studies that address the principal question directly, species numbers versus system function, are evaluated. Moreover, the degree of redundancy within systems, the ubiquity of keystone species, the tightness of species interactions from mutualisms to food webs, the resilience of systems to perturbation, the interactions of landscape units are explored, as is also how policy decisions are driven in this research area. This book brings together the disciplines of population biology and ecoysystem science, both directed toward evaluating the consequences of human-driven disruptions of natural systems.