  Cases of what Sterelny (2003: 149ff) calls ‘cumulative  downstream niche construction’ occur when a generation of  animals modifies an environment that has already been modified by  earlier generations. A mountain thornbill's nest is an instance of  downstream niche construction (since its offspring are affected by  the thornbill's efforts). However, the construction is not  cumulative, since the nest is built anew each year. By contrast, a  rabbit warren extended and elaborated over several generations is an  instance of cumulative construction: successive generations of  offspring inherit an ever-more-complex niche and their other  behaviors are tuned accordingly in ever-more-complex ways. Tomasello, 1999 and Sterelny 2003 stress that niche construction, including  downstream niche construction, is not limited to the physical world:  animals make changes to their social and epistemic worlds as well.  For instance, chimpanzees live in groups (= construction of a social  niche) and dogs mark their territory (= a change in their epistemic  niche, relieving them of the necessity of remembering where the  boundaries of their territory are). Humans, says Sterelny, echoing a  theme of Tomasello 1999, are niche constructors “with a  vengeance” (2003:149) and many of the changes they make to  their physical, social and epistemic environments accumulate over  many generations (think of a city, a democracy, modern science, a  natural language). Such cumulative modifications allow for what  Tomasello calls a “ratchet effect”: a “cycle in  which an improvement is made, becomes standard for the group, and  then becomes a basis for further innovation.” (Sterelny 2003:  150-1)