Habitat loss and fragmentation reduce population sizes and increase isolation between populations. To better understand how functional connectivity is affected by habitat modification over large scales, we here applied a meta-population framework to the Brazilian Atlantic Forest, a highly degraded and fragmented biodiversity hotspot. Other studies have used mainly hypothetical or estimated dispersal values for connectivity calculation which may not be reflective of species requirements. Here, we collated dispersal values for 45 species of birds, 5 mammals and 4 insects and found that 50% of the Atlantic Forest species can cross only up to 150 m of open gaps between forest patches. Because of the high levels of fragmentation, the median size of a functionally connected network of fragments in the Brazilian Atlantic Forest only decreased from 15 ha to 14 ha when the crossable distance considered was reduced from 150 m to 0 m. We show that for species solely reliant on native forest habitat, a large proportion of the remaining Atlantic Forest fragments represent many small and isolated populations with few large connected areas. Our results support further evidence that for future management and restoration to be successful, existing connectivity must be vastly improved to provide forest areas large enough to support viable populations.