These questions are the purview of conservation biology and the purview of conservation genetics.
Lets take a look at the numbers, based on survey estimates from 1991 - 2016. The population grew from 1267 to 6200 individuals. Overall that’s a growth rate of 6.6% per year; that’s astounding considering 8% of the known population died in 2016 (520 known deaths). Comparative measure; the estimated human population growth rate is 1.1%, which means the Manatee growth rate is 6 times that of the human population. Skeptic alarm bells start ringing.
Let me graph this:
Manatees typically produce a single young. To maintain this growth rate - simplifying the numbers a little, this means 900 females are pregnant (Manatees have a 12 month gestation period). Manatees also nurse their young for 12 to 18 months, at least 900 females are then nursing. This adds up to about 2700 individual females occupied in/with offspring.
I don’t know the female to male ratio of Florida manatees but lets assume a 50:50 ratio as Ronald Fisher and mammalian genetics predict (now known as Fisher’s principle). This means only 400 female manatees are not occupied with offspring, seems a little fishy. Let me make it more fishy. Sexual maturity in Manatees takes about 5 years. Surely many individuals are too young to breed, and some individuals are too old, and some may not breed for other reasons. Then the 2700 females actively partaking in child oriented duties is 87% of the females. Eight-seven percent of the females are supposed to be sexual mature, healthy, and active with offspring at some stage of development.
Seems more than a little fishy. I can you hear the car-talk guys now…BOOOOOOOOGUS.
No wonder the article from Wikipedia citing the U.S. Marine Mammal Commission from 1999 states: “Accurate population estimates of the Florida manatee (T. manatus) are difficult. They have been called scientifically weak due to widely varying counts from year to year, some areas showing increases, others decreases and little strong evidence of increases except in two areas.”
All of this to say, what we really don’t have is enough information.
I’m not quite done yet: One growing aspect of conservation biology is taking into account the genetic diversity of the species of interest. Its not just about having enough members in the population but having enough genetic diversity in the population. What is the genetic diversity of Florida Manatees? Two studies suggest the genetic diversity of the Florida Manatees is low (studies done in 1988 and 2012 listed below), other studies (listed below as well) have shown very little or no gene flow among Manatee subpopulations in the America’s and even lower genetic diversity among Puerto Rican and Mexican populations than found in Florida.
The inbreeding coefficient, one measure of genetic diversity, was as high as 5.2 %, that's higher than second cousins. So most manatee's are breeding with close relatives. This level was found to be higher than that of other mammals (2005 study by Garner et al. below).
The FWS is/has downgraded protections on the Manatee. why on earth should we downgrade the protections of this marvelous beast when the reality is that we really don’t know how protected the Florida population is from extinction? Lowering extinction risk is the purpose of the Endangered Species Act after all. Protections for the Manatee, by the way, may very well aid other Florida species - acting as an umbrella species of sorts.
Is it silly or negligent that the U.S. Fish and Wildlife Service delist the Manatee? Okay, I’m being a bit extreme, the proposal by FWS would change the status of manatee to threatened rather than endangered (one step “better”) and so many protections for the species would be retained.
Next we need to chat about the Vaquita porpoises, the most endangered marine mammal on the planet which, may already be extinct.
Mentions and further readings:
Fisher, R.A. 1930 The Genetical Theory of Natural Selection, Clarendon Press, Oxford.
Garner, et al. 2005. Patterns of genetic diversity and its loss in mammalian populations. Conservation Biology 19:1215–1221.
Hunter et al. 2010. Low genetic variation and evidence of limited dispersal in the regionally important Belize manatee. Animal Conservation 13:592–602.
Nourisson et al. 2011. Evidence of two genetic clusters of manatees with low genetic diversity in Mexico and implications for their conservation. Genetica 139:833–842.
McClenaghan and O’Shea. 1988. Genetic variability in the Florida manatee (Trichechus manatus). Journal of Mammalogy 69:481–488.
Tucker, et al. 2012. Journal of Mammalogy, 93(6):1504–1511.