Cristina Eisenberg's Huffington Post blog with permission. On April 22, 1970, at the urging of Wisconsin U.S. Senator Gaylord Nelson, 20 million Americans participated in the first Earth Day -- a nation-wide demonstration to advocate for a healthier planet. This marked the formal start of the modern environmental movement and heralded the beginning of a coordinated American effort to live more sustainably and ethically with nature. Indeed, the first Earth Day led to the passage of a slew of environmental laws, including the 1973 Endangered Species Act (ESA). In my previous blog post ("The ESA: Taking Noah's Ark into a Brave New World"), I discussed our naiveté in assuming that saving species threatened by extinction would be as simple as creating laws like the ESA designed to function like Noah's Ark. There were many things we didn't know back then that we know now. For example, during Earth Day 1970, as our nation protested environmental degradation, we'd become aware of the mere tip of the environmental damage iceberg. We didn't know then that we'd already set in motion human-caused environmental effects, such as climate change, that were causing a hemorrhage of extinction. And we didn't know that large carnivores could create ecosystems far more resilient to climate change than those that do not contain these keystone species. trophic cascades.