Over the past four decades population numbers of Cardisoma guanhumi in Puerto Rico have been declining, and the effects of land-use/land-cover (LULC) changes on their distribution and habitat have never been investigated. Our objectives were 1) to develop a spatially explicit distribution model using Geographic Information System for C. guanhumi in the San Juan Bay Estuary (SJBE) and Jobos Bay Estuary (JBE) and 2) to determine if population declines are due to reduction in distribution and habitat loss. Photointerpretation from 1936, 1971/1977, and 2002 for both our study sites illustrated drastic changes to LULC. There was 100 % loss of agriculture and 182 % increase in urban development and 77 % loss in agriculture and 650 % increase in urban development for SJBE and JBE respectively. The conversion of land use from agriculture to urban development significantly reduced the areas available for land crab habitat. We suspect that the agricultural activities in Puerto Rico provided novel habitat for C. guanhumi during the 1930's to 1970's resulting in higher population densities and inland migration. With the collapse of the agricultural industry in Puerto Rico population numbers of C. guanhumi consequently declined. Therefore, we conclude that the observed declines are due not only to overexploitation as is commonly suggested but also due to the reduction in their distribution and habitat loss. Our study provides explicit spatial information for the management and conservation of the few remaining habitats of C. guanhumi in Puerto Rico.