Viewpoint: a view on species additions and deletions and the balance of nature.

Abstract

Popular assumptions about ecosystem stability and the delicate balance of nature are found lacking when examined in terms of paleoecological, historical and current biochronological, and biogeographical sequences in a wide variety of environments. Species composition of vegetation varies continuously in time as well as space in the absence of acute perturbations. Species have been added to or removed from ecosystems without greatly affecting ecosystem function. Natural ecosystems exhibit greater stability (inertia) in physiognomic structure and functional processes than in species composition. For instance, creosotebush became dominant over many millions of hectares of the Chihuahuan, Sonoran, and Mojave Deserts over a short period of 11,000 years, but a limited number of generations precludes establishment of highly integrated and biologically regulated communities by co-evolution. Dramatic shifts in species composition of eastern deciduous forests of North America occurred in prehistory and continue into the present. Similar changes are noted in the constant assembling and reassembling of species in the purportedly ancient and stable forests of the tropics. Numerous introductions with few extinctions in the flora of California have increased species richness and probably diversity, and many recent additions are primary contributors to ecosystem productivity. Recognition that rangeland ecosystems persist in unstable rather than stable species compositions provides both a challenge and an opportunity for natural resource management. The challenge is to develop new management principles that incorporate nonequilibrium theory. The opportunity is the promotion of policies and regulations that more closely reflect reality.