Humans have converted forest to agricultural and urban uses, exploited species, fragmented wildlands, changed the demographic structure of forests, altered habitat, degraded the environment with atmospheric and soil pollutants, introduced exotic pests and competitors, and domesticated favored species. None of these activities is new; perhaps with the exception of atmospheric pollution, they date back to prehistory. All have impacted genetic diversity (i.e., species diversity and genetic diversity within species) by their influence on the evolutionary processes of extinction, selection, drift, gene flow, and mutation, sometimes increasing diversity, as in the case of domestication, but often reducing it. Even in the absence of changes in diversity, mating systems were altered, changing the genetic structure of populations. Demographic changes (i.e., conversion of old-growth to younger, even-aged stands) influenced selection by increasing the incidence of disease. Introduction of exotic diseases, insects, mammalian herbivores, and competing vegetation has had the best-documented effects on genetic diversity, reducing both species diversity and intraspecific diversity. Deforestation has operated on a vast scale to reduce diversity by direct elimination of locally-adapted populations. Atmospheric pollution and global warming will be a major threat in the near future, particularly because forests are fragmented and migration is impeded. Past impacts can be estimated with reference to expert knowledge, but hard data are often lacking. Baselines are needed to quantify future impacts and provide an early warning of problems. Genetic inventories of indicator species can provide the baselines against which to measure changes in diversity.
