In a given area, human activities usually cause the extinction of native species and the establishment of non-native species. A key conservation issue is whether non-native establishment tends to outpace native species extinction to produce a net gain in species richness. To determine this, empirical data must be accumulated at various scales. I show that, within the United States, the number of established non-native plant species per state does tend to outpace the number of extinct and threatened species per state. The net gain in plant species is strongly and positively correlated with human population density. Continuation of this trend predicts substantial gains in net plant species richness for all states in the United States as human population grows. This contrasts with freshwater fishes, where most states show a net loss of species diversity as extinct and threatened species exceed established non-native species. Changes in fish diversity do not correlate strongly with human population or non-native species but are largely driven by the decline of native fish species.