Historically speaking, the collapse of societies as often been the result of environmental problems: deforestation, soil destruction, overhunting, overfishing, introduction of foreign species, human population growth, and the increased per capita impact of people. People's impact increases when they demand more of the products that affect the environment like multiple cars, TVs, air conditioners and even food. Damaging the environment makes us morally culpable. This concern is not some racist pretext advanced by citizens to deny entry to illegal aliens or legal immigrants of a different culture. It is a legitimate concern about the environment.
The natural resources of the U.S. and the world seemed inexhaustibly abundant to our forefathers and indeed they have been sufficient to sustain our society for more than two centuries. But the signs of incipient depletion become more obvious every day. Food prices are skyrocketing because of the demands of other countries for U.S. agricultural products, the tension between biomass for alternative energy purposes and the need for the same arable land to produce food, and, of course, the prices of the petroleum products needed to keep farm machinery operating.
Concern about these matters is not a sign of morally-culpable, blind or conscious selfishness or restrictionist sentiment, as some like to claim. It is wrongheaded and dangerous to invoke such pejorative and ad hominem expressions when the evidence is there for every intelligent person to see. It is disgusting that even some environmental organizations like the Sierra Club have yet to recognize that the biggest threat to the environment is people, too many of them. Unfettered population growth will easily exceed or overshadow any technical efforts to reduce pollution.
It is morally wrong for one people to supplant, dispossess, subjugate or exterminate another people as we, the Spaniards and countless others did to indigenous peoples in the Western Hemisphere. Is it happening again in the U.S. in the form of immigration's unarmed invasion?
We may be able to survive temporarily, in our self-inflicted resource depletion unless climate change produces a further resource depletion and if that caused by population growth is brought under control through secure borders, internal enforcement to create disincentives to future border violations, a dramatic reduction in the legal immigration quotas, an end to chain immigrations, and some reasonable limits on birthright citizenship.
(Some of these ideas and words are paraphrased from "Collapse" by Pulitzer Prize winning author, Dr. Jared Diamond.)