Pecenko, N. 1997. Visitors as an environmental enrichment. In: Proceedings on the International Conference on Environmental Enrichment. Holst, B. (ed), 325-33. Copenhagen Zoo, Frederiksberg, DK.

Because of the influence in behaviour, we can say that visitors are a kind of environmental enrichment. .... Sometimes visitors have a negative effect (increase of aggressive behaviour, begging for food) and in some cases human interactions with animals are a possible source of additional stress.