Assuming that a finite biosphere can support infinite development seems logically indefensible, yet the concept of sustainable development has become a dominant conservation paradigm. The story of the endangered Florida Key deer (Odocoileus virginianus clavium) appears to support the legitimacy of sustainable development because Key deer numbers have increased 240% since 1970 while at the same time human numbers in their habitat increased nearly 10-fold. Because fawn mortality is considered the primary density-dependent factor regulating cervid populations as they approach K-carrying capacity, we hypothesized that changes in fawn demographics could elucidate the fallacy in assuming that development was sustainable on Big Pine Key. We determined and compared survival and range sizes for Key deer fawns between 1968–1972 (early urban development) and 1998–2002 (post-urban development). Fawn ranges (95% probability area, 149 to 33 ha) and core areas (50% probability area, 25 to 6 ha) decreased during this period of development while 6-month survival increased (0.47 to 0.96). All fawn mortality was due to anthropogenic causes; the positive relationship between fawn survival and development may be a function of isolating fawns from anthropogenic mortality. If this is true, the relationship is not sustainable because as ranges continue to shrink, they eventually will lack sufficient resources to support a fawn.