Manatees are definitively not an invasive species. These large, slow-moving aquatic mammals are protected native inhabitants of the Americas and West Africa. The West Indian manatee, which includes the Florida manatee subspecies, is a long-established resident, having evolved within these ecosystems. While increased visibility might lead to public confusion, their presence is not the result of a harmful invasion.
Understanding the Criteria for Invasive Species
The classification of a species as “invasive” is based on two precise scientific criteria that must both be met. An invasive species must first be non-native, or alien, to the ecosystem it is now found in, meaning it did not originate there naturally. Secondly, its introduction must cause or be likely to cause environmental or economic harm, or harm to human health.
The distinction between a non-native and an invasive species is significant, as not all non-native organisms are considered harmful. Domesticated livestock, for instance, are non-native to North America but serve an agricultural purpose without severely disrupting the natural ecosystem. A species is only deemed invasive if it outcompetes native species, reduces biodiversity, or permanently alters habitats in a detrimental way.
Manatees: Native Range and Ecological Role
Manatees belong to the order Sirenia, and the West Indian manatee (Trichechus manatus) is native to the southeastern United States, the Caribbean, and along the northern coast of South America. The Florida manatee subspecies is found primarily in coastal and inland waters throughout Florida, while the Antillean manatee ranges from Mexico down to Brazil.
These animals are primary herbivores, consuming vast quantities of aquatic vegetation, including sea grasses and freshwater plants. By grazing, manatees play a part in maintaining the health of shallow coastal waters and estuaries, preventing vegetation from becoming overgrown. Their feeding habits and subsequent nutrient excretion support the productivity of seagrass meadows, benefiting the wide range of marine life that relies on these environments. The Florida manatee is recognized as a threatened species under the Endangered Species Act, confirming its intrinsic value to the ecosystem.
Factors Driving Manatee Movement and Range Expansion
The public perception that manatees are “new” to certain areas often stems from their seasonal movements and population recovery. Manatees are cold-intolerant mammals, requiring water temperatures above 68°F to avoid cold stress syndrome. During winter months, they undertake seasonal migrations to natural warm-water refuges, such as Florida’s spring systems, or to artificial sources like the warm water discharges from power plants.
Conservation efforts have led to a significant population increase in the Florida manatee over the past few decades, resulting in more sightings and a wider distribution during warmer months. During the summer, individuals may travel far outside the core wintering range, with some observed as far north as Massachusetts or as far west as Texas. Partial migration, where some individuals extend their range poleward, is becoming a more common mechanism for range expansion as water temperatures rise due to climate change.