Estimated Total Numbers:
West Indian Manatee
(Trichechus manatus )1 1,000
Jaguar
(Panthera onca)2 600-1,000
Howler Monkey
(Alouatta pigra)3 10,000+
American Saltwater Crocodiles
(Crocodylus acutus)4 1,000
Morelets Crocodiles
(Crocodylus moreletii)5 8,800
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Sources
1 Belize Coastal Zone Management Authority, 2012
2 Alan R. Rabinowitz, 1991; some more recent estimates suggest the number of jaguars in Belize has declined
3 Reliable estimates of howler monkeys in all of Belize have not been found, although the population in the Baboon Sanctuary reportedly has grown to about 6,000, while the population near Monkey River is believed to have declined from over 11,000 to about 1,500 following Hurricane Iris in 2001
4 Platt & Thorbjarnarson, 2000 (population estimate excludes hatchlings)
5 Comisión Nacional para el Conocimiento y Uso de la Biodiversidad, Mexico, 2005 (population of all ages)
--Lan Sluder
Update: I'm told that a better current estimate (by the Panthera organization) of the jaguar population in Belize is 750, although the population is divided into two areas. About 500 in a single contiguous area is said to be a sustainable population.