Conclusions
All species have complex interactive effects on ecosystems. Humans, with their
unrivalled capacity for ecosystem engineering, have outsized effects and add even greater complexity and novelty by acting both as individual agents of change and collectively as human systems with adaptive social learning networks. A single human being can intentionally transform a pristine forest to pasture using fire and livestock or unintentionally by introducing an invasive species. Human systems can sustain cities in the desert and convert factories to woodlands. Yet human transformation of terrestrial ecology is always incomplete: some native species flourish even in the mostly densely populated cities.