Mangroves are unique tropical and subtropical trees and shrubs adapted to grow in saline coastal environments. These plants possess specialized adaptations, such as salt-excreting leaves and complex root systems, that allow them to thrive in intertidal zones where most other woody species cannot survive. Whether mangroves are an invasive species depends entirely on their geographic location. They are considered beneficial and native throughout their natural range, but they become problematic when introduced outside of those boundaries by humans.
The Criteria for Defining an Invasive Species
A species is classified as invasive only when it meets specific criteria. The first condition is that the species must be non-native, meaning it was introduced outside of its natural, historic range. Many non-native organisms, such as cultivated crops or livestock, do not become invasive. The second, and more defining, criterion is that the introduction must cause or be likely to cause economic or environmental harm, or harm to human health.
For a non-native species to be invasive, it must successfully overcome environmental barriers, establish a self-sustaining population, and spread rapidly enough to displace native species or fundamentally alter the ecosystem’s structure. Mangroves’ powerful ability to alter coastal hydrology and trap sediment is highly valued in their native range, but this same characteristic makes them environmentally damaging when introduced elsewhere.
Natural Range and Global Mangrove Biogeography
Mangroves inhabit sheltered coastlines in tropical and subtropical latitudes, typically between 30 degrees north and 30 degrees south of the equator. The global distribution of the roughly 70 species of true mangroves is divided into two major biogeographic regions. The Indo-West Pacific (IWP) region, stretching from East Africa to the Western Pacific, is characterized by high species diversity.
The Atlantic East Pacific (AEP) region covers the coasts of the Americas and West Africa, but contains a much lower diversity of species. Within these native habitats, mangroves are considered foundational species that provide crucial ecosystem services. Their dense, prop-rooted forests protect coastlines from erosion and storms, stabilize sediment, and create complex nursery habitats for various fish, crustaceans, and other wildlife.
Case Studies of Non-Native Mangrove Invasion
The primary case of mangrove invasiveness is found in the Hawaiian Archipelago, where no mangrove species were native prior to the 20th century. The red mangrove (Rhizophora mangle) was intentionally introduced to the island of Moloka‘i in 1902 to help stabilize coastal mudflats and control soil erosion caused by agricultural run-off. The introduction successfully established the species, which has since spread aggressively to nearly all the major islands, becoming an ecological invader.
The Rhizophora mangle propagules, which are buoyant and capable of long-distance dispersal, easily established themselves in the uncolonized intertidal zones of the islands. The lack of natural predators and diseases from their native range also contributed to their rapid growth and spread, a phenomenon known as the “enemy release hypothesis”. Today, the introduced mangroves are considered a noxious weed by local authorities due to their unchecked expansion into sensitive coastal environments.
Ecological Consequences of Non-Native Mangrove Expansion
Non-native mangroves expanding outside of their natural range cause negative ecological impacts by altering the structure of invaded habitats. One primary consequence is the displacement of native coastal ecosystems, which lack a natural defense against the mangroves’ dense, woody growth. In Hawaii, the invasive red mangrove has colonized and replaced native salt marshes, open mudflats, and traditional Hawaiian fishponds.
The tangled network of prop roots and pneumatophores traps massive amounts of fine sediment, which dramatically changes the hydrology and tidal flow of the area. This increased sedimentation and altered flow regime can smother and eliminate the unique, specialized flora and fauna of native habitats, such as the organisms found in anchialine pools. Furthermore, the dense canopy reduces habitat quality for endangered native waterbirds, including the Hawaiian stilt, which rely on open mudflats for foraging and nesting.